HTTP Message Signatures

HTTP Message Signatures Amazon

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Bespoke Engineering

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Digital Bazaar

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Applications and Real-Time HTTP PKI This document describes a mechanism for creating, encoding, and verifying digital signatures or message authentication codes over components of an HTTP message. This mechanism supports use cases where the full HTTP message may not be known to the signer, and where the message may be transformed (e.g., by intermediaries) before reaching the verifier. This document also describes a means for requesting that a signature be applied to a subsequent HTTP message in an ongoing HTTP exchange. About This Document Status information for this document may be found at . Discussion of this document takes place on the HTTP Working Group mailing list (), which is archived at . Working Group information can be found at . Source for this draft and an issue tracker can be found at .

Introduction Message integrity and authenticity are security properties that are critical to the secure operation of many HTTP applications. Application developers typically rely on the transport layer to provide these properties, by operating their application over . However, TLS only guarantees these properties over a single TLS connection, and the path between client and application may be composed of multiple independent TLS connections (for example, if the application is hosted behind a TLS-terminating gateway or if the client is behind a TLS Inspection appliance). In such cases, TLS cannot guarantee end-to-end message integrity or authenticity between the client and application. Additionally, some operating environments present obstacles that make it impractical to use TLS (such as presentation of client certificates from a browser), or to use features necessary to provide message authenticity. Furthermore, some applications require the binding of a higher-level application-specific key to the HTTP message, separate from any TLS certificates in use. Consequently, while TLS can meet message integrity and authenticity needs for many HTTP-based applications, it is not a universal solution. Additionally, many applications need to be able to generate and verify signatures despite incomplete knowledge of the HTTP message as seen on the wire, due to the use of libraries, proxies, or application frameworks that alter or hide portions of the message from the application at the time of signing or verification. These applications need a means to protect the parts of the message that are most relevant to the application without having to violate layering and abstraction. Finally, object-based signature mechanisms such as require the intact conveyance of the exact information that was signed. When applying such technologies to an HTTP message, elements of the HTTP message need to be duplicated in the object payload either directly or through inclusion of a hash. This practice introduces complexity since the repeated information needs to be carefully checked for consistency when the signature is verified. This document defines a mechanism for providing end-to-end integrity and authenticity for components of an HTTP message by use of a detached signature on HTTP messages. The mechanism allows applications to create digital signatures or message authentication codes (MACs) over only the components of the message that are meaningful and appropriate for the application. Strict canonicalization rules ensure that the verifier can verify the signature even if the message has been transformed in many of the ways permitted by HTTP. The signing mechanism described in this document consists of three parts:

A common nomenclature and canonicalization rule set for the different protocol elements and other components of HTTP messages, used to create the signature base. ()
Algorithms for generating and verifying signatures over HTTP message components using this signature base through application of cryptographic primitives. ()
A mechanism for attaching a signature and related metadata to an HTTP message, and for parsing attached signatures and metadata from HTTP messages. To facilitate this, this document defines the "Signature-Input" and "Signature" fields. ()

This document also provides a mechanism for negotiation the use of signatures in one or more subsequent messages via the "Accept-Signature" field (). This optional negotiation mechanism can be used along with opportunistic or application-driven message signatures by either party. The mechanisms defined in this document are important tools that can be used to an overall security mechanism for an application. This toolkit provides some powerful capabilities, but does not sufficient in creating an overall security story. In particular, the requirements in and the security considerations in are of high importance to all implementors of this specification. For example, this specification does not define a means to directly cover HTTP message content (defined in ), but relies on the specification to provide a hash of the message content, as discussed in .

Conventions and Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. The terms "HTTP message", "HTTP request", "HTTP response", "target URI", "gateway", "header field", "intermediary", "request target", "trailer field", "sender", "method", and "recipient" are used as defined in . For brevity, the term "signature" on its own is used in this document to refer to both digital signatures (which use asymmetric cryptography) and keyed MACs (which use symmetric cryptography). Similarly, the verb "sign" refers to the generation of either a digital signature or keyed MAC over a given signature base. The qualified term "digital signature" refers specifically to the output of an asymmetric cryptographic signing operation. This document uses the following terminology from to specify data types: List, Inner List, Dictionary, Item, String, Integer, Byte Sequence, and Boolean. This document defines several string constructions using and uses the following ABNF rules: VCHAR, SP, DQUOTE, LF. This document uses the following ABNF rules from : sf-string, inner-list, parameters. This document uses the following ABNF rules from and : field-content, obs-fold, obs-text. In addition to those listed above, this document uses the following terms:

HTTP Message Signature:: A digital signature or keyed MAC that covers one or more portions of an HTTP message. Note that a given HTTP Message can contain multiple HTTP Message Signatures.
Signer:: The entity that is generating or has generated an HTTP Message Signature. Note that multiple entities can act as signers and apply separate HTTP Message Signatures to a given HTTP Message.
Verifier:: An entity that is verifying or has verified an HTTP Message Signature against an HTTP Message. Note that an HTTP Message Signature may be verified multiple times, potentially by different entities.
HTTP Message Component:: A portion of an HTTP message that is capable of being covered by an HTTP Message Signature.
Derived Component:: An HTTP Message Component derived from the HTTP message through the use of a specified algorithm or process. See .
HTTP Message Component Name:: A string that identifies an HTTP Message Component's source, such as a field name or derived component name.
HTTP Message Component Identifier:: The combination of an HTTP Message Component Name and any parameters that uniquely identifies a specific HTTP Message Component in respect to a particular HTTP Message Signature and the HTTP Message it applies to.
HTTP Message Component Value:: The value associated with a given component identifier within the context of a particular HTTP Message. Component values are derived from the HTTP Message and are usually subject to a canonicalization process.
Covered Components:: An ordered set of HTTP message component identifiers for fields () and derived components () that indicates the set of message components covered by the signature, never including the @signature-params identifier itself. The order of this set is preserved and communicated between the signer and verifier to facilitate reconstruction of the signature base.
Signature Base:: The sequence of bytes generated by the signer and verifier using the covered components set and the HTTP Message. The signature base is processed by the cryptographic algorithm to produce or verify the HTTP Message Signature.
HTTP Message Signature Algorithm:: A cryptographic algorithm that describes the signing and verification process for the signature, defined in terms of the HTTP_SIGN and HTTP_VERIFY primitives described in .
Key Material:: The key material required to create or verify the signature. The key material is often identified with an explicit key identifier, allowing the signer to indicate to the verifier which key was used.
Creation Time:: A timestamp representing the point in time that the signature was generated, as asserted by the signer.
Expiration Time:: A timestamp representing the point in time after which the signature should no longer be accepted by the verifier, as asserted by the signer.
Target Message:: The HTTP message to which an HTTP Message Signature is applied.
Signature Context:: The data source from which the HTTP Message Component Values are drawn. The context includes the target message and any additional information the signer or verifier might have, such as the full target URI of a request or the related request message for a response.

The term "Unix time" is defined by , Section 4.16. This document contains non-normative examples of partial and complete HTTP messages. Some examples use a single trailing backslash \ to indicate line wrapping for long values, as per . The \ character and leading spaces on wrapped lines are not part of the value.

Requirements HTTP permits and sometimes requires intermediaries to transform messages in a variety of ways. This can result in a recipient receiving a message that is not bitwise-equivalent to the message that was originally sent. In such a case, the recipient will be unable to verify integrity protections over the raw bytes of the sender's HTTP message, as verifying digital signatures or MACs requires both signer and verifier to have the exact same signature base. Since the exact raw bytes of the message cannot be relied upon as a reliable source for a signature base, the signer and verifier have to independently create the signature base from their respective versions of the message, via a mechanism that is resilient to safe changes that do not alter the meaning of the message. For a variety of reasons, it is impractical to strictly define what constitutes a safe change versus an unsafe one. Applications use HTTP in a wide variety of ways and may disagree on whether a particular piece of information in a message (e.g., the message content, the method, or a particular header field) is relevant. Thus, a general purpose solution needs to provide signers with some degree of control over which message components are signed. HTTP applications may be running in environments that do not provide complete access to or control over HTTP messages (such as a web browser's JavaScript environment), or may be using libraries that abstract away the details of the protocol (such as the Java HTTPClient library). These applications need to be able to generate and verify signatures despite incomplete knowledge of the HTTP message.

HTTP Message Transformations As mentioned earlier, HTTP explicitly permits and in some cases requires implementations to transform messages in a variety of ways. Implementations are required to tolerate many of these transformations. What follows is a non-normative and non-exhaustive list of transformations that could occur under HTTP, provided as context:

Re-ordering of fields with different field names ().
Combination of fields with the same field name ().
Removal of fields listed in the Connection header field ().
Addition of fields that indicate control options ().
Addition or removal of a transfer coding ().
Addition of fields such as Via () and Forwarded ().
Conversion between different versions of the HTTP protocol (e.g., HTTP/1.x to HTTP/2, or vice-versa).
Changes in casing (e.g., "Origin" to "origin") of any case-insensitive components such as field names, request URI scheme, or host.
Changes to the request target and authority that when applied together do not result in a change to the message's target URI, as defined in .

Additionally, there are some transformations that are either deprecated or otherwise not allowed, but still could occur in the wild. These transformations can still be handled without breaking the signature, and include things such as:

Use, addition, or removal of leading or trailing whitespace in a field value.
Use, addition, or removal of obs-fold in field values ().

We can identify these types of transformations as ones that should not prevent signature verification, even when performed on message components covered by the signature. Additionally, all changes to components not covered by the signature should not prevent signature verification. Some examples of these kinds of transformations, and the effect they have on the message signature, are found in . Other transformations, such as parsing and re-serializing the field values of a covered component or changing the value of a derived component, can cause a signature to no longer validate against a target message. Applications of this specification need to take care to ensure that the transformations expected by the application are adequately handled by the choice of covered components.

Application of HTTP Message Signatures HTTP Message Signatures are designed to be a general-purpose tool applicable in a wide variety of circumstances and applications. In order to properly and safely apply HTTP Message Signatures, an application or profile of this specification MUST specify at least all of the following items:

The set of component identifiers and signature parameters that are expected and required to be included in the covered components list. For example, an authorization protocol could mandate that the Authorization field be covered to protect the authorization credentials and mandate the signature parameters contain a created parameter, while an API expecting semantically relevant HTTP message content could require the Content-Digest field defined in to be present and covered as well as mandate a value for tag that is specific to the API being protected.
The expected structured field types () of any required or expected covered component fields or parameters.
A means of retrieving the key material used to verify the signature. An application will usually use the keyid parameter of the signature parameters () and define rules for resolving a key from there, though the appropriate key could be known from other means such as pre-registration of a signer's key.
The set of allowable signature algorithms to be used by signers and accepted by verifiers.
A means of determining that the signature algorithm used to verify the signature is appropriate for the key material and context of the message. For example, the process could use the alg parameter of the signature parameters () to state the algorithm explicitly, derive the algorithm from the key material, or use some pre-configured algorithm agreed upon by the signer and verifier.
A means of determining that a given key and algorithm used for a signature are appropriate for the context of the message. For example, a server expecting only ECDSA signatures should know to reject any RSA signatures, or a server expecting asymmetric cryptography should know to reject any symmetric cryptography.
A means of determining the context for derivation of message components from an HTTP message and its application context. While this is normally the target HTTP message itself, the context could include additional information known to the application through configuration, such as an external host name.
If binding between a request and response is needed using the mechanism in , all elements of the request and response message that would be required to provide properties of such a binding.
The error messages and codes that are returned from the verifier to the signer when the signature is invalid, the key material is inappropriate, the validity time window is out of specification, a component value cannot be calculated, or any other errors in the signature verification process. For example, if a signature is being used as an authentication mechanism, an HTTP status code of 401 Unauthorized or 403 Forbidden could be appropriate. If the response is from an HTTP API, a response with an HTTP status code of 400 Bad Request could include details as described in , such as an indicator that the wrong key material was used.

When choosing these parameters, an application of HTTP message signatures has to ensure that the verifier will have access to all required information needed to re-create the signature base. For example, a server behind a reverse proxy would need to know the original request URI to make use of the derived component @target-uri, even though the apparent target URI would be changed by the reverse proxy (see also ). Additionally, an application using signatures in responses would need to ensure that clients receiving signed responses have access to all the signed portions of the message, including any portions of the request that were signed by the server using the related-response parameter. The details of this kind of profiling are the purview of the application and outside the scope of this specification, however some additional considerations are discussed in . In particular, when choosing the required set of component identifiers, care has to be taken to make sure that the coverage is sufficient for the application, as discussed in and . This specification defines only part of a full security system for an application. When building a complete security system based on this tool, it is important to perform a security analysis of the entire system of which HTTP Message Signatures is a part. Historical systems, such as , can provide inspiration and examples of how to apply similar mechanisms in a secure and trustable fashion.

HTTP Message Components In order to allow signers and verifiers to establish which components are covered by a signature, this document defines component identifiers for components covered by an HTTP Message Signature, a set of rules for deriving and canonicalizing the values associated with these component identifiers from the HTTP Message, and the means for combining these canonicalized values into a signature base. The signature context for deriving these values MUST be accessible to both the signer and the verifier of the message. The context MUST be the same across all components in a given signature. For example, it would be an error to use a the raw query string for the @query derived component but combined query and form parameters for the @query-param derived component. For more considerations of the message component context, see . A component identifier is composed of a component name and any parameters associated with that name. Each component name is either an HTTP field name () or a registered derived component name (). The possible parameters for a component identifier are dependent on the component identifier, and the HTTP Signature Component Parameters registry cataloging all possible parameters is defined in . Within a single list of covered components, each component identifier MUST occur only once. One component identifier is distinct from another if the component name differs, or if any of the parameters differ for the same component name. Multiple component identifiers having the same component name MAY be included if they have parameters that make them distinct, such as "foo";bar and "foo";baz. The order of parameters MUST be preserved when processing a component identifier (such as when parsing during verification), but the order of parameters is not significant when comparing two component identifiers for equality checks. That is to say, "foo";bar;baz cannot be in the same message as "foo";baz;bar, since these two component identifiers are equivalent, but a system processing one form is not allowed to transform it into the other form. The component value associated with a component identifier is defined by the identifier itself. Component values MUST NOT contain newline (\n) characters. Some HTTP message components can undergo transformations that change the bitwise value without altering the meaning of the component's value (for example, when combining field values). Message component values must therefore be canonicalized before they are signed, to ensure that a signature can be verified despite such intermediary transformations. This document defines rules for each component identifier that transform the identifier's associated component value into such a canonical form. The following sections define component identifier names, their parameters, their associated values, and the canonicalization rules for their values. The method for combining message components into the signature base is defined in .

HTTP Fields The component name for an HTTP field is the lowercased form of its field name as defined in . While HTTP field names are case-insensitive, implementations MUST use lowercased field names (e.g., content-type, date, etag) when using them as component names. The component value for an HTTP field is the field value for the named field as defined in . The field value MUST be taken from the named header field of the target message unless this behavior is overridden by additional parameters and rules, such as the req and tr flags, below. For most fields, the field value is an ASCII string as recommended by , and the component value is exactly that string. Other encodings could exist in some implementations, and all non-ASCII field values MUST be encoded to ASCII before being added to the signature base. The bs parameter defined in provides a method for wrapping such problematic field values. Unless overridden by additional parameters and rules, HTTP field values MUST be combined into a single value as defined in to create the component value. Specifically, HTTP fields sent as multiple fields MUST be combined using a single comma (",") and a single space (" ") between each item. Note that intermediaries are allowed to combine values of HTTP fields with any amount of whitespace between the commas, and if this behavior is not accounted for by the verifier, the signature can fail since the signer and verifier will see a different component value in their respective signature bases. For robustness, it is RECOMMENDED that signed messages include only a single instance of any field covered under the signature, particularly with the value for any list-based fields serialized using the algorithm below. This approach increases the chances of the field value remaining untouched through intermediaries. Where that approach is not possible and multiple instances of a field need to be sent separately, it is RECOMMENDED that signers and verifiers process any list-based fields taking all individual field values and combining them based on the strict algorithm below, to counter possible intermediary behavior. When the field in question is a structured field of type List or Dictionary, this effect can be accomplished more directly by requiring the strict structured field serialization of the field value, as described in . Note that some HTTP fields, such as Set-Cookie (), do not follow a syntax that allows for combination of field values in this manner (such that the combined output is unambiguous from multiple inputs). Even though the component value is never parsed by the message signature process and used only as part of the signature base in , caution needs to be taken when including such fields in signatures since the combined value could be ambiguous. The bs parameter defined in provides a method for wrapping such problematic fields. See for more discussion of this issue. If the correctly combined value is not directly available for a given field by an implementation, the following algorithm will produce canonicalized results for list-based fields:

Create an ordered list of the field values of each instance of the field in the message, in the order that they occur (or will occur) in the message.
Strip leading and trailing whitespace from each item in the list. Note that since HTTP field values are not allowed to contain leading and trailing whitespace, this will be a no-op in a compliant implementation.
Remove any obsolete line-folding within the line and replace it with a single space (" "), as discussed in . Note that this behavior is specific to HTTP/1.1 and does not apply to other versions of the HTTP specification which do not allow internal line folding.
Concatenate the list of values together with a single comma (",") and a single space (" ") between each item.

The resulting string is the component value for the field. Note that some HTTP fields have values with multiple valid serializations that have equivalent semantics, such as allow case-insensitive values that intermediaries could change. Applications signing and processing such fields MUST consider how to handle the values of such fields to ensure that the signer and verifier can derive the same value, as discussed in . Following are non-normative examples of component values for header fields, given the following example HTTP message fragment: The following example shows the component values for these example header fields, presented using the signature base format defined in : Empty HTTP fields can also be signed when present in a message. The canonicalized value is the empty string. This means that the following empty header, with (SP) indicating a single trailing space character before the empty field value: Is serialized by the signature base generation algorithm with an empty string value following the colon and space added after the content identifier. Any HTTP field component identifiers MAY have the following parameters in specific circumstances, each described in detail in their own sections:

sf: A boolean flag indicating that the component value is serialized using strict encoding of the structured field value ().
key: A string parameter used to select a single member value from a Dictionary structured field ().
bs: A boolean flag indicating that individual field values are encoded using Byte Sequence data structures before being combined into the component value ().
req: A boolean flag for signed responses indicating that the component value is derived from the request that triggered this response message and not from the response message directly. Note that this parameter can also be applied to any derived component identifiers that target the request ().
tr: A boolean flag indicating that the field value is taken from the trailers of the message as defined in . If this flag is absent, the field value is taken from the headers of the message as defined in ().

Multiple parameters MAY be specified together, though some combinations are redundant or incompatible. For example, the sf parameter's functionality is already covered when the key parameter is used on a dictionary item, since key requires strict serialization of the value. The bs parameter, which requires the raw bytes of the field values from the message, is not compatible with use of the sf or key parameters, which require the parsed data structures of the field values after combination. Additional parameters can be defined in the HTTP Signature Component Parameters registry established in .

Strict Serialization of HTTP Structured Fields If the value of an HTTP field is known by the application to be a structured field type (as defined in or its extensions or updates), and the expected type of the structured field is known, the signer MAY include the sf parameter in the component identifier. If this parameter is included with a component identifier, the HTTP field value MUST be serialized using the formal serialization rules specified in (or the applicable formal serialization section of its extensions or updates) applicable to the type of the HTTP field. Note that this process will replace any optional internal whitespace with a single space character, among other potential transformations of the value. If multiple field values occur within a message, these values MUST be combined into a single List or Dictionary structure before serialization. If the application does not know the type of the field, or the application does not know how to serialize the type of the field, the use of this flag will produce an error. As a consequence, the signer can only reliably sign fields using this flag when the verifier's system knows the type as well. For example, the following dictionary field is a valid serialization: If included in the signature base without parameters, its value would be: However, if the sf parameter is added, the value is re-serialized as follows: The resulting string is used as the component value in .

Dictionary Structured Field Members If a given field is known by the application to be a Dictionary structured field, an individual member in the value of that Dictionary is identified by using the parameter key and the Dictionary member key as a String value. If multiple field values occur within a message, these values MUST be combined into a single Dictionary structure before serialization. An individual member value of a Dictionary Structured Field is canonicalized by applying the serialization algorithm described in on the member_value and its parameters, not including the dictionary key itself. Specifically, the value is serialized as an Item or Inner List (the two possible values of a Dictionary member), with all parameters and possible sub-fields serialized using the strict serialization rules defined in (or the applicable section of its extensions or updates). Each parameterized key for a given field MUST NOT appear more than once in the signature base. Parameterized keys MAY appear in any order in the signature base, regardless of the order they occur in the source Dictionary. If a Dictionary key is named as a covered component but it does not occur in the Dictionary, this MUST cause an error in the signature base generation. Following are non-normative examples of canonicalized values for Dictionary structured field members given the following example header field, whose value is known by the application to be a Dictionary: The following example shows canonicalized values for different component identifiers of this field, presented using the signature base format discussed in : Note that the value for key="c" has been re-serialized according to the strict member_value algorithm, and the value for key="d" has been serialized as a Boolean value.

Binary-wrapped HTTP Fields If the value of the HTTP field in question is known by the application to cause problems with serialization, particularly with the combination of multiple values into a single line as discussed in , the signer SHOULD include the bs parameter in a component identifier to indicate the values of the field need to be wrapped as binary structures before being combined. If this parameter is included with a component identifier, the component value MUST be calculated using the following algorithm:

Let the input be the ordered set of values for a field, in the order they appear in the message.
Create an empty List for accumulating processed field values.
For each field value in the set:
1. Strip leading and trailing whitespace from the field value. Note that since HTTP field values are not allowed to contain leading and trailing whitespace, this will be a no-op in a compliant implementation.
2. Remove any obsolete line-folding within the line and replace it with a single space (" "), as discussed in . Note that this behavior is specific to and does not apply to other versions of the HTTP specification.
3. Encode the bytes of the resulting field value as a Byte Sequence. Note that most fields are restricted to ASCII characters, but other octets could be included in the value in some implementations.
4. Add the Byte Sequence to the List accumulator.
The intermediate result is a List of Byte Sequence values.
Follow the strict serialization of a List as described in and return this output.

For example, the following field with internal commas prevents the distinct field values from being safely combined: In our example, the same field can be sent with a semantically different single value: Both of these versions are treated differently by the application. However, if included in the signature base without parameters, the component value would be the same in both cases: However, if the bs parameter is added, the two separate instances are encoded and serialized as follows: For the single-instance field above, the encoding with the bs parameter is: This component value is distinct from the multiple-instance field above, preventing a collision which could potentially be exploited.

Trailer Fields If the signer wants to include a trailer field in the signature, the signer MUST include the tr boolean parameter to indicate the value MUST be taken from the trailer fields and not from the header fields. For example, given the following message: The signer decides to add both the Trailer header field as well as the Expires trailer to the signature base, along with the status code derived component: If a field is available as both a header and trailer in a message, both values MAY be signed, but the values MUST be signed separately. The values of header fields and trailer fields of the same name MUST NOT be combined for purposes of the signature. Since trailer fields could be merged into the header fields or dropped entirely by intermediaries as per , it is NOT RECOMMENDED to include trailers in the signature unless the signer knows that the verifier will have access to the values of the trailers as sent.

Derived Components In addition to HTTP fields, there are a number of different components that can be derived from the control data, signature context, or other aspects of the HTTP message being signed. Such derived components can be included in the signature base by defining a component name, possible parameters, message target, and the derivation method for its component value. Derived component names MUST start with the "at" @ character. This differentiates derived component names from HTTP field names, which cannot contain the @ character as per . Processors of HTTP Message Signatures MUST treat derived component names separately from field names, as discussed in . This specification defines the following derived components:

@method: The method used for a request. ()
@target-uri: The full target URI for a request. ()
@authority: The authority of the target URI for a request. ()
@scheme: The scheme of the target URI for a request. ()
@request-target: The request target. ()
@path: The absolute path portion of the target URI for a request. ()
@query: The query portion of the target URI for a request. ()
@query-param: A parsed and encoded query parameter of the target URI for a request. ()
@status: The status code for a response. ()

Additional derived component names are defined in the HTTP Signature Derived Component Names Registry. () Derived component values are taken from the context of the target message for the signature. This context includes information about the message itself, such as its control data, as well as any additional state and context held by the signer or verifier. In particular, when signing a response, the signer can include any derived components from the originating request by using the request-response parameter.

request:: Values derived from and results applied to an HTTP request message as described in . If the target message of the signature is a response, using the req parameter allows a request-targeted derived component to be included in the signature (see ).
response:: Values derived from and results applied to an HTTP response message as described in .

A derived component definition MUST define all target message types to which it can be applied. Derived component values MUST be limited to printable characters and spaces and MUST NOT contain any newline characters. Derived component values MUST NOT start or end with whitespace characters.

Method The @method derived component refers to the HTTP method of a request message. The component value is canonicalized by taking the value of the method as a string. Note that the method name is case-sensitive as per . While conventionally standardized method names are uppercase , no transformation to the input method value's case is performed. For example, the following request message: Would result in the following @method component value: And the following signature base line:

Target URI The @target-uri derived component refers to the target URI of a request message. The component value is the full absolute target URI of the request, potentially assembled from all available parts including the authority and request target as described in . For example, the following message sent over HTTPS: Would result in the following @target-uri component value: And the following signature base line:

Authority The @authority derived component refers to the authority component of the target URI of the HTTP request message, as defined in . In HTTP/1.1, this is usually conveyed using the Host header, while in HTTP/2 and HTTP/3 it is conveyed using the :authority pseudo-header. The value is the fully-qualified authority component of the request, comprised of the host and, optionally, port of the request target, as a string. The component value MUST be normalized according to the rules in . Namely, the host name is normalized to lowercase and the default port is omitted. For example, the following request message: Would result in the following @authority component value: And the following signature base line: The @authority derived component SHOULD be used instead of signing the Host header directly, see .

Scheme The @scheme derived component refers to the scheme of the target URL of the HTTP request message. The component value is the scheme as a lowercase string as defined in . While the scheme itself is case-insensitive, it MUST be normalized to lowercase for inclusion in the signature base. For example, the following request message requested over plain HTTP: Would result in the following @scheme component value: And the following signature base line:

Request Target The @request-target derived component refers to the full request target of the HTTP request message, as defined in . The component value of the request target can take different forms, depending on the type of request, as described below. For HTTP/1.1, the component value is equivalent to the request target portion of the request line. However, this value is more difficult to reliably construct in other versions of HTTP. Therefore, it is NOT RECOMMENDED that this component be used when versions of HTTP other than 1.1 might be in use. The origin form value is combination of the absolute path and query components of the request URL. For example, the following request message: Would result in the following @request-target component value: And the following signature base line: The following request to an HTTP proxy with the absolute-form value, containing the fully qualified target URI: Would result in the following @request-target component value: And the following signature base line: The following CONNECT request with an authority-form value, containing the host and port of the target: Would result in the following @request-target component value: And the following signature base line: The following OPTIONS request message with the asterisk-form value, containing a single asterisk * character: Would result in the following @request-target component value: And the following signature base line:

Path The @path derived component refers to the target path of the HTTP request message. The component value is the absolute path of the request target defined by , with no query component and no trailing ? character. The value is normalized according to the rules in . Namely, an empty path string is normalized as a single slash / character. Path components are represented by their values before decoding any percent-encoded octets, as described in the simple string comparison rules in . For example, the following request message: Would result in the following @path component value: And the following signature base line:

Query The @query derived component refers to the query component of the HTTP request message. The component value is the entire normalized query string defined by , including the leading ? character. The value is read using the simple string comparison rules in . Namely, percent-encoded octets are not decoded. For example, the following request message: Would result in the following @query component value: And the following signature base line: The following request message: Would result in the following @query component value: And the following signature base line: Just like including an empty path component, the signer can include an empty query component to indicate that this component is not used in the message. If the query string is absent from the request message, the component value is the leading ? character alone: Resulting in the following signature base line:

Query Parameters If a request target URI uses HTML form parameters in the query string as defined in the "application/x-www-form-urlencoded" section of , the @query-param derived component allows addressing of individual query parameters. The query parameters MUST be parsed according to the "application/x-www-form-urlencoded parsing" section of , resulting in a list of (nameString, valueString) tuples. The REQUIRED name parameter of each component identifier contains the encoded nameString of a single query parameter as a String value. The component value of a single named parameter is the encoded valueString of that single query parameter. Several different named query parameters MAY be included in the covered components. Single named parameters MAY occur in any order in the covered components, regardless of the order they occur in the query string. The value of the name parameter and the component value of a single named parameter are calculated by the following process:

Parse the nameString or valueString of the named query parameter defined by the "application/x-www-form-urlencoded parsing" section of , which is the value after percent-encoded octets are decoded.
Encode the nameString or valueString using the "percent-encode after encoding" process defined by the "application/x-www-form-urlencoded serializing" section of , which results in an string.
Output the ASCII string,

Note that the component value does not include any leading ? characters, equals sign =, or separating & characters. Named query parameters with an empty valueString have an empty string as the component value. Note that due to inconsistencies in implementations, some query parameter parsing libraries drop such empty values. If a query parameter is named as a covered component but it does not occur in the query parameters, this MUST cause an error in the signature base generation. For example for the following request: Indicating the baz, qux and param named query parameters would result in the following @query-param component values: baz: batman qux: an empty string param: value And the following signature base lines, with (SP) indicating a single trailing space character before the empty component value: This derived component has some limitations. Specifically, the algorithm in only supports query parameters using percent-escaped UTF-8 encoding. Other encodings are not supported. Additionally, multiple instances of a named parameter are not reliably supported in the wild. If a parameter name occurs multiple times in a request, the named query parameter MUST NOT be included. If multiple parameters are common within an application, it is RECOMMENDED to sign the entire query string using the @query component identifier defined in . The encoding process allows query parameters that include newlines or other problematic characters in their values, or with alternative encodings such as using the plus character to represent spaces. For the query parameters in this message: The resulting values are encoded as follows: If the encoding were not applied, the resultant values would be: This base string contains characters that violate the constraints on component names and values, and is therefore invalid.

Status Code The @status derived component refers to the three-digit numeric HTTP status code of a response message as defined in . The component value is the serialized three-digit integer of the HTTP status code, with no descriptive text. For example, the following response message: Would result in the following @status component value: And the following signature base line: The @status component identifier MUST NOT be used in a request message.

Signature Parameters HTTP Message Signatures have metadata properties that provide information regarding the signature's generation and verification, consisting of the ordered set of covered components and the ordered set of parameters including a timestamp of signature creation, identifiers for verification key material, and other utilities. This metadata is represented by a special message component in the signature base for signature parameters, and it is treated slightly differently from other message components. Specifically, the signature parameters message component is REQUIRED as the last line of the signature base, and the component identifier MUST NOT be enumerated within the set of covered components for any signature, including itself. The signature parameters component name is @signature-params. The signature parameters component value is the serialization of the signature parameters for this signature, including the covered components ordered set with all associated parameters. These parameters include any of the following:

created: Creation time as an Integer UNIX timestamp value. Sub-second precision is not supported. Inclusion of this parameter is RECOMMENDED.
expires: Expiration time as an Integer UNIX timestamp value. Sub-second precision is not supported.
nonce: A random unique value generated for this signature as a String value.
alg: The HTTP message signature algorithm from the HTTP Signature Algorithms registry, as a String value.
keyid: The identifier for the key material as a String value.
tag: An application-specific tag for the signature as a String value. This value is used by applications to help identify signatures relevant for specific applications or protocols.

Additional parameters can be defined in the HTTP Signature Metadata Parameters Registry. Note that there is no general ordering to the parameters, but once an ordering is chosen for a given set of parameters, it cannot be changed without altering the signature parameters value. The signature parameters component value is serialized as a parameterized Inner List using the rules in as follows:

Let the output be an empty string.
Determine an order for the component identifiers of the covered components, not including the @signature-params component identifier itself. Once this order is chosen, it cannot be changed. This order MUST be the same order as used in creating the signature base ().
Serialize the component identifiers of the covered components, including all parameters, as an ordered Inner List of String values according to and append this to the output. Note that the component identifiers can include their own parameters, and these parameters are ordered sets. Once an order is chosen for a component's parameters, the order cannot be changed.
Determine an order for any signature parameters. Once this order is chosen, it cannot be changed.
Append the parameters to the Inner List in order according to , skipping parameters that are not available or not used for this message signature.
The output contains the signature parameters component value.

Note that the Inner List serialization from is used for the covered component value instead of the List serialization from in order to facilitate parallelism with this value's inclusion in the Signature-Input field, as discussed in . This example shows the serialized component value for the parameters of an example message signature: Note that an HTTP message could contain multiple signatures, but only the signature parameters used for a single signature are included in a given signature parameters entry.

Signing Request Components in a Response Message When a request message results in a signed response message, the signer can include portions of the request message in the signature base by adding the req parameter to the component identifier.

req: A boolean flag indicating that the component value is derived from the request that triggered this response message and not from the response message directly.

This parameter can be applied to both HTTP fields and derived components that target the request, with the same semantics. The component value for a message component using this parameter is calculated in the same manner as it is normally, but data is pulled from the request message instead of the target response message to which the signature is applied. Note that the same component name MAY be included with and without the req parameter in a single signature base, indicating the same named component from both the request and response message, respectively. The req parameter MAY be combined with other parameters as appropriate for the component identifier, such as the key parameter for a dictionary field. For example, when serving a response for this request: This would result in the following unsigned response message: The server signs the response with its own key, including the @status code and several header fields in the covered components. While this covers a reasonable amount of the response for this application, the server additionally includes several components derived from the original request message that triggered this response. In this example, the server includes the method, authority, path, and content digest from the request in the covered components of the response. The Content-Digest for both the request and the response are included under the response signature. For the application in this example, the query is deemed not to be relevant to the response and is therefore not covered. Other applications would make different decisions based on application needs as discussed in . The signature base for this example is: The signed response message is: Note that the ECDSA algorithm in use here is non-deterministic, meaning a different signature value will be created every time the algorithm is run. The signature value provided here can be validated against the given keys, but newly-generated signature values are not expected to match the example. See . Since the component values from the request are not repeated in the response message, the requester MUST keep the original message component values around long enough to validate the signature of the response that uses this component identifier parameter. In most cases, this means the requester needs to keep the original request message around, since the signer could choose to include any portions of the request in its response, according to the needs of the application. Since it is possible for an intermediary to alter a request message before it is processed by the server, applications need to take care not to sign such altered values as the client would not be able to validate the resulting signature. It is additionally possible for a server to create a signed response in response to a signed request. For example, this signed request: The server could choose to sign portions of this response, including several portions of the request resulting in this signature base: And the following signed response: Note that the ECDSA algorithm in use here is non-deterministic, meaning a different signature value will be created every time the algorithm is run. The signature value provided here can be validated against the given keys, but newly-generated signature values are not expected to match the example. See . Applications signing a response to a signed request SHOULD sign all of the components of the request signature value to provide sufficient coverage and protection against a class of collision attacks, as discussed in . The server in this example has included all components listed in the Signature-Input of the client's signature on the request in the response signature, in addition to components of the response. While it is syntactically possible to include the Signature and Signature-Input fields of the request message in the signature components of a response a message using this mechanism, this practice is NOT RECOMMENDED. This is because signatures of signatures do not provide transitive coverage of covered components as one might expect and the practice is susceptible to several attacks as discussed in . An application that needs to signal successful processing or receipt of a signature would need to carefully specify alternative mechanisms for sending such a signal securely. The response signature can only ever cover what is included in the request message when using this flag. Consequently, if an application needs to include the message content of the request under the signature of its response, the client needs to include a means for covering that content, such as a Content-Digest field. See the discussion in for more information. The req parameter MUST NOT be used for any component in a signature that targets a request message.

Creating the Signature Base The signature base is a string containing the canonicalized HTTP message components covered by the signature. The input to the signature base creation algorithm is the ordered set of covered component identifiers and their associated values, along with any additional signature parameters discussed in . Component identifiers are serialized using the strict serialization rules defined by . The component identifier has a component name, which is a String Item value serialized using the sf-string ABNF rule. The component identifier MAY also include defined parameters which are serialized using the parameters ABNF rule. The signature parameters line defined in follows this same pattern, but the component identifier is a String Item with a fixed value and no parameters, and the component value is always an Inner List with optional parameters. Note that this means the serialization of the component name itself is encased in double quotes, with parameters following as a semicolon-separated list, such as "cache-control", "@authority", "@signature-params", or "example-dictionary";key="foo". The output is the ordered set of bytes that form the signature base, which conforms to the following ABNF: To create the signature base, the signer or verifier concatenates together entries for each component identifier in the signature's covered components (including their parameters) using the following algorithm. All errors produced as described immediately MUST fail the algorithm with no signature output base output.

Let the output be an empty string.
For each message component item in the covered components set (in order):
1. Check that the component identifier (including its parameters) has not already been added to the signature base. If this happens, produce an error.
2. Append the component identifier for the covered component serialized according to the component-identifier ABNF rule. Note that this serialization places the component name in double quotes and appends any parameters outside of the quotes.
3. Append a single colon :
4. Append a single space " "
5. Determine the component value for the component identifier.
  - If the component identifier has a parameter that is not understood, produce an error.
  - If the component identifier has several incompatible parameters, such as bs and sf, produce an error.
  - If the component identifier contains the req parameter and the target message is a request, produce an error.
  - If the component identifier contains the req parameter and the target message is a response, the context for the component value is the related request message of the target response message. Otherwise, the context for the component value is the target message.
  - If the component name starts with an "at" character (@), derive the component's value from the message according to the specific rules defined for the derived component, as in , including processing of any known valid parameters. If the derived component name is unknown or the value cannot be derived, produce an error.
  - If the component name does not start with an "at" character (@), canonicalize the HTTP field value as described in , including processing of any known valid parameters. If the field cannot be found in the message, or the value cannot be obtained in the context, produce an error.
6. Append the covered component's canonicalized component value.
7. Append a single newline \n
Append the signature parameters component () according to the signature-params-line rule as follows:
1. Append the component identifier for the signature parameters serialized according to the component-identifier rule, i.e. the exact value "@signature-params" (including double quotes)
2. Append a single colon :
3. Append a single space " "
4. Append the signature parameters' canonicalized component value as defined in , i.e. an Inner List structured field value with parameters
Produce an error if the output string contains any non-ASCII () characters.
Return the output string.

If covered components reference a component identifier that cannot be resolved to a component value in the message, the implementation MUST produce an error and not create a signature base. Such situations are included but not limited to:

The signer or verifier does not understand the derived component name.
The component name identifies a field that is not present in the message or whose value is malformed.
The component identifier includes a parameter that is unknown or does not apply to the component identifier to which it is attached.
The component identifier indicates that a structured field serialization is used (via the sf parameter), but the field in question is known to not be a structured field or the type of structured field is not known to the implementation.
The component identifier is a dictionary member identifier that references a field that is not present in the message, is not a Dictionary Structured Field, or whose value is malformed.
The component identifier is a dictionary member identifier or a named query parameter identifier that references a member that is not present in the component value, or whose value is malformed. E.g., the identifier is "example-dict";key="c" and the value of the Example-Dict header field is a=1, b=2, which does not have the c value.

In the following non-normative example, the HTTP message being signed is the following request: The covered components consist of the @method, @authority, and @path derived components followed by the Content-Digest, Content-Length, and Content-Type HTTP header fields, in order. The signature parameters consist of a creation timestamp of 1618884473 and a key identifier of test-key-rsa-pss. Note that no explicit alg parameter is given here since the verifier is known by the application to use the RSA PSS algorithm based on the identified key. The signature base for this message with these parameters is:

Non-normative example Signature Base Note that the example signature base here, or anywhere else within this specification, does not include the final newline that ends the displayed example.

HTTP Message Signatures An HTTP Message Signature is a signature over a string generated from a subset of the components of an HTTP message in addition to metadata about the signature itself. When successfully verified against an HTTP message, an HTTP Message Signature provides cryptographic proof that the message is semantically equivalent to the message for which the signature was generated, with respect to the subset of message components that was signed.

Creating a Signature Creation of an HTTP message signature is a process that takes as its input the signature context (including the target message) and the requirements for the application. The output is a signature value and set of signature parameters that can be communicated to the verifier by adding them to the message. In order to create a signature, a signer MUST follow the following algorithm:

The signer chooses an HTTP signature algorithm and key material for signing from the set of potential signing algorithms. The set of potential algorithms is determined by the application and is out of scope for this document. The signer MUST choose key material that is appropriate for the signature's algorithm, and that conforms to any requirements defined by the algorithm, such as key size or format. The mechanism by which the signer chooses the algorithm and key material is out of scope for this document.
The signer sets the signature's creation time to the current time.
If applicable, the signer sets the signature's expiration time property to the time at which the signature is to expire. The expiration is a hint to the verifier, expressing the time at which the signer is no longer willing to vouch for the safety of the signature.
The signer creates an ordered set of component identifiers representing the message components to be covered by the signature, and attaches signature metadata parameters to this set. The serialized value of this is later used as the value of the Signature-Input field as described in .
- Once an order of covered components is chosen, the order MUST NOT change for the life of the signature.
- Each covered component identifier MUST be either an HTTP field in the signature context or a derived component listed in or the HTTP Signature Derived Component Names registry.
- Signers of a request SHOULD include some or all of the message control data in the covered components, such as the @method, @authority, @target-uri, or some combination thereof.
- Signers SHOULD include the created signature metadata parameter to indicate when the signature was created.
- The @signature-params derived component identifier MUST NOT be listed in the list of covered component identifiers. The derived component is required to always be the last line in the signature base, ensuring that a signature always covers its own metadata and the metadata cannot be substituted.
- Further guidance on what to include in this set and in what order is out of scope for this document.
The signer creates the signature base using these parameters and the signature base creation algorithm. ()
The signer uses the HTTP_SIGN primitive function to sign the signature base with the chosen signing algorithm using the key material chosen by the signer. The HTTP_SIGN primitive and several concrete applications of signing algorithms are defined in .
The byte array output of the signature function is the HTTP message signature output value to be included in the Signature field as defined in .

For example, given the HTTP message and signature parameters in the example in , the example signature base is signed with the test-key-rsa-pss key in and the RSA PSS algorithm described in , giving the following message signature output value, encoded in Base64:

Non-normative example signature value Note that the RSA PSS algorithm in use here is non-deterministic, meaning a different signature value will be created every time the algorithm is run. The signature value provided here can be validated against the given keys, but newly-generated signature values are not expected to match the example. See .

Verifying a Signature Verification of an HTTP message signature is a process that takes as its input the signature context (including the target message, particularly its Signature and Signature-Input fields) and the requirements for the application. The output of the verification is either a positive verification or an error. In order to verify a signature, a verifier MUST follow the following algorithm:

Parse the Signature and Signature-Input fields as described in and , and extract the signatures to be verified.
1. If there is more than one signature value present, determine which signature should be processed for this message based on the policy and configuration of the verifier. If an applicable signature is not found, produce an error.
2. If the chosen Signature value does not have a corresponding Signature-Input value, produce an error.
Parse the values of the chosen Signature-Input field as a parameterized Inner List to get the ordered list of covered components and the signature parameters for the signature to be verified.
Parse the value of the corresponding Signature field to get the byte array value of the signature to be verified.
Examine the signature parameters to confirm that the signature meets the requirements described in this document, as well as any additional requirements defined by the application such as which message components are required to be covered by the signature. ()
Determine the verification key material for this signature. If the key material is known through external means such as static configuration or external protocol negotiation, the verifier will use that. If the key is identified in the signature parameters, the verifier will dereference this to appropriate key material to use with the signature. The verifier has to determine the trustworthiness of the key material for the context in which the signature is presented. If a key is identified that the verifier does not know, does not trust for this request, or does not match something preconfigured, the verification MUST fail.
Determine the algorithm to apply for verification:
1. Start with the set of allowable algorithms known to the application. If any of the following steps selects an algorithm that is not in this set, the signature validation fails.
2. If the algorithm is known through external means such as static configuration or external protocol negotiation, the verifier will use this algorithm.
3. If the algorithm can be determined from the keying material, such as through an algorithm field on the key value itself, the verifier will use this algorithm.
4. If the algorithm is explicitly stated in the signature parameters using a value from the HTTP Signature Algorithms registry, the verifier will use the referenced algorithm.
5. If the algorithm is specified in more than one location, such as through static configuration and the algorithm signature parameter, or the algorithm signature parameter and from the key material itself, the resolved algorithms MUST be the same. If the algorithms are not the same, the verifier MUST fail the verification.
Use the received HTTP message and the parsed signature parameters to re-create the signature base, using the algorithm defined in . The value of the @signature-params input is the value of the Signature-Input field for this signature serialized according to the rules described in . Note that this does not include the signature's label from the Signature-Input field.
If the key material is appropriate for the algorithm, apply the appropriate HTTP_VERIFY cryptographic verification algorithm to the signature, recalculated signature base, key material, signature value. The HTTP_VERIFY primitive and several concrete algorithms are defined in .
The results of the verification algorithm function are the final results of the cryptographic verification function.

If any of the above steps fail or produce an error, the signature validation fails. For example, verifying the signature with the key sig1 of the following message with the test-key-rsa-pss key in and the RSA PSS algorithm described in : With the additional requirements that at least the method, authority, path, content-digest, content-length, and content-type be signed, and that the signature creation timestamp is recent enough at the time of verification, the verification passes.

Enforcing Application Requirements The verification requirements specified in this document are intended as a baseline set of restrictions that are generally applicable to all use cases. Applications using HTTP Message Signatures MAY impose requirements above and beyond those specified by this document, as appropriate for their use case. Some non-normative examples of additional requirements an application might define are:

Requiring a specific set of header fields to be signed (e.g., Authorization, Digest).
Enforcing a maximum signature age from the time of the created time stamp.
Rejection of signatures past the expiration time in the expires time stamp. Note that the expiration time is a hint from the signer and that a verifier can always reject a signature ahead of its expiration time.
Prohibition of certain signature metadata parameters, such as runtime algorithm signaling with the alg parameter when the algorithm is determined from the key information.
Ensuring successful dereferencing of the keyid parameter to valid and appropriate key material.
Prohibiting the use of certain algorithms, or mandating the use of a specific algorithm.
Requiring keys to be of a certain size (e.g., 2048 bits vs. 1024 bits).
Enforcing uniqueness of the nonce parameter.
Requiring an application-specific value for the tag parameter.

Application-specific requirements are expected and encouraged. When an application defines additional requirements, it MUST enforce them during the signature verification process, and signature verification MUST fail if the signature does not conform to the application's requirements. Applications MUST enforce the requirements defined in this document. Regardless of use case, applications MUST NOT accept signatures that do not conform to these requirements.

Signature Algorithms An HTTP Message signature MUST use a cryptographic digital signature or MAC method that is appropriate for the key material, environment, and needs of the signer and verifier. This specification does not strictly limit the available signature algorithms, and any signature algorithm that meets these basic requirements MAY be used by an application of HTTP message signatures. Each signing method HTTP_SIGN takes as its input the signature base defined in as a byte array (M), the signing key material (Ks), and outputs the signature output as a byte array (S): S ]]> Each verification method HTTP_VERIFY takes as its input the re-generated signature base defined in as a byte array (M), the verification key material (Kv), and the presented signature to be verified as a byte array (S) and outputs the verification result (V) as a boolean: V ]]> The following sections contain several common signature algorithms and demonstrate how these cryptographic primitives map to the HTTP_SIGN and HTTP_VERIFY definitions here. Which method to use can be communicated through the explicit algorithm signature parameter alg defined in , by reference to the key material, or through mutual agreement between the signer and verifier. Signature algorithms selected using the alg parameter MUST use values from the HTTP Signature Algorithms registry.

RSASSA-PSS using SHA-512 To sign using this algorithm, the signer applies the RSASSA-PSS-SIGN (K, M) function defined in with the signer's private signing key (K) and the signature base (M) (). The mask generation function is MGF1 as specified in with a hash function of SHA-512 . The salt length (sLen) is 64 bytes. The hash function (Hash) SHA-512 is applied to the signature base to create the digest content to which the digital signature is applied. The resulting signed content byte array (S) is the HTTP message signature output used in . To verify using this algorithm, the verifier applies the RSASSA-PSS-VERIFY ((n, e), M, S) function using the public key portion of the verification key material ((n, e)) and the signature base (M) re-created as described in . The mask generation function is MGF1 as specified in with a hash function of SHA-512 . The salt length (sLen) is 64 bytes. The hash function (Hash) SHA-512 is applied to the signature base to create the digest content to which the verification function is applied. The verifier extracts the HTTP message signature to be verified (S) as described in . The results of the verification function indicate if the signature presented is valid. Note that the output of RSA PSS algorithms are non-deterministic, and therefore it is not correct to re-calculate a new signature on the signature base and compare the results to an existing signature. Instead, the verification algorithm defined here needs to be used. See . Use of this algorithm can be indicated at runtime using the rsa-pss-sha512 value for the alg signature parameter.

RSASSA-PKCS1-v1_5 using SHA-256 To sign using this algorithm, the signer applies the RSASSA-PKCS1-V1_5-SIGN (K, M) function defined in with the signer's private signing key (K) and the signature base (M) (). The hash SHA-256 is applied to the signature base to create the digest content to which the digital signature is applied. The resulting signed content byte array (S) is the HTTP message signature output used in . To verify using this algorithm, the verifier applies the RSASSA-PKCS1-V1_5-VERIFY ((n, e), M, S) function using the public key portion of the verification key material ((n, e)) and the signature base (M) re-created as described in . The hash function SHA-256 is applied to the signature base to create the digest content to which the verification function is applied. The verifier extracts the HTTP message signature to be verified (S) as described in . The results of the verification function indicate if the signature presented is valid. Use of this algorithm can be indicated at runtime using the rsa-v1_5-sha256 value for the alg signature parameter.

HMAC using SHA-256 To sign and verify using this algorithm, the signer applies the HMAC function with the shared signing key (K) and the signature base (text) (). The hash function SHA-256 is applied to the signature base to create the digest content to which the HMAC is applied, giving the signature result. For signing, the resulting value is the HTTP message signature output used in . For verification, the verifier extracts the HTTP message signature to be verified (S) as described in . The output of the HMAC function is compared bytewise to the value of the HTTP message signature, and the results of the comparison determine the validity of the signature presented. Use of this algorithm can be indicated at runtime using the hmac-sha256 value for the alg signature parameter.

ECDSA using curve P-256 DSS and SHA-256 To sign using this algorithm, the signer applies the ECDSA algorithm defined in using curve P-256 with the signer's private signing key and the signature base (). The hash SHA-256 is applied to the signature base to create the digest content to which the digital signature is applied, (M). The signature algorithm returns two integer values, r and s. These are both encoded as big-endian unsigned integers, zero-padded to 32-octets each. These encoded values are concatenated into a single 64-octet array consisting of the encoded value of r followed by the encoded value of s. The resulting concatenation of (r, s) is byte array of the HTTP message signature output used in . To verify using this algorithm, the verifier applies the ECDSA algorithm defined in using the public key portion of the verification key material and the signature base re-created as described in . The hash function SHA-256 is applied to the signature base to create the digest content to which the signature verification function is applied, (M). The verifier extracts the HTTP message signature to be verified (S) as described in . This value is a 64-octet array consisting of the encoded values of r and s concatenated in order. These are both encoded in big-endian unsigned integers, zero-padded to 32-octets each. The resulting signature value (r, s) is used as input to the signature verification function. The results of the verification function indicate if the signature presented is valid. Note that the output of ECDSA algorithms are non-deterministic, and therefore it is not correct to re-calculate a new signature on the signature base and compare the results to an existing signature. Instead, the verification algorithm defined here needs to be used. See . Use of this algorithm can be indicated at runtime using the ecdsa-p256-sha256 value for the alg signature parameter.

ECDSA using curve P-384 DSS and SHA-384 To sign using this algorithm, the signer applies the ECDSA algorithm defined in using curve P-384 with the signer's private signing key and the signature base (). The hash SHA-384 is applied to the signature base to create the digest content to which the digital signature is applied, (M). The signature algorithm returns two integer values, r and s. These are both encoded as big-endian unsigned integers, zero-padded to 48-octets each. These encoded values are concatenated into a single 96-octet array consisting of the encoded value of r followed by the encoded value of s. The resulting concatenation of (r, s) is byte array of the HTTP message signature output used in . To verify using this algorithm, the verifier applies the ECDSA algorithm defined in using the public key portion of the verification key material and the signature base re-created as described in . The hash function SHA-384 is applied to the signature base to create the digest content to which the signature verification function is applied, (M). The verifier extracts the HTTP message signature to be verified (S) as described in . This value is a 96-octet array consisting of the encoded values of r and s concatenated in order. These are both encoded in big-endian unsigned integers, zero-padded to 48-octets each. The resulting signature value (r, s) is used as input to the signature verification function. The results of the verification function indicate if the signature presented is valid. Note that the output of ECDSA algorithms are non-deterministic, and therefore it is not correct to re-calculate a new signature on the signature base and compare the results to an existing signature. Instead, the verification algorithm defined here needs to be used. See . Use of this algorithm can be indicated at runtime using the ecdsa-p384-sha384 value for the alg signature parameter.

EdDSA using curve edwards25519 To sign using this algorithm, the signer applies the Ed25519 algorithm defined in with the signer's private signing key and the signature base (). The signature base is taken as the input message (M) with no pre-hash function. The signature is a 64-octet concatenation of R and S as specified in , and this is taken as a byte array for the HTTP message signature output used in . To verify using this algorithm, the signer applies the Ed25519 algorithm defined in using the public key portion of the verification key material (A) and the signature base re-created as described in . The signature base is taken as the input message (M) with no pre-hash function. The signature to be verified is processed as the 64-octet concatenation of R and S as specified in . The results of the verification function indicate if the signature presented is valid. Use of this algorithm can be indicated at runtime using the ed25519 value for the alg signature parameter.

JSON Web Signature (JWS) algorithms If the signing algorithm is a JOSE signing algorithm from the JSON Web Signature and Encryption Algorithms Registry established by , the JWS algorithm definition determines the signature and hashing algorithms to apply for both signing and verification. For both signing and verification, the HTTP messages signature base () is used as the entire "JWS Signing Input". The JOSE Header defined in is not used, and the signature base is not first encoded in Base64 before applying the algorithm. The output of the JWS signature is taken as a byte array prior to the Base64url encoding used in JOSE. The JWS algorithm MUST NOT be none and MUST NOT be any algorithm with a JOSE Implementation Requirement of Prohibited. JWA algorithm values from the JSON Web Signature and Encryption Algorithms Registry are not included as signature parameters. Typically, the JWS algorithm can be signaled using JSON Web Keys or other mechanisms common to JOSE implementations. In fact, JWA algorithm values are not registered in the HTTP Signature Algorithms registry, and so the explicit alg signature parameter is not used at all when using JOSE signing algorithms.

Including a Message Signature in a Message HTTP message signatures can be included within an HTTP message via the Signature-Input and Signature fields, both defined within this specification. The Signature-Input field identifies the covered components and parameters that describe how the signature was generated, while the Signature field contains the signature value. Each field MAY contain multiple labeled values. An HTTP message signature is identified by a label within an HTTP message. This label MUST be unique within a given HTTP message and MUST be used in both the Signature-Input and Signature fields. The label is chosen by the signer, except where a specific label is dictated by protocol negotiations such as described in . An HTTP message signature MUST use both Signature-Input and Signature fields and each field MUST contain the same labels. The presence of a label in one field but not in the other is an error.

The Signature-Input HTTP Field The Signature-Input field is a Dictionary structured field (defined in ) containing the metadata for one or more message signatures generated from components within the HTTP message. Each member describes a single message signature. The member's key is the label that uniquely identifies the message signature within the HTTP message. The member's value is the serialization of the covered components Inner List plus all signature metadata parameters identified by the label. To facilitate signature validation, the Signature-Input field value MUST contain the same serialized value used in generating the signature base's @signature-params value defined in . Note that in a structured field value, list order and parameter order have to be preserved. The signer MAY include the Signature-Input field as a trailer to facilitate signing a message after its content has been processed by the signer. However, since intermediaries are allowed to drop trailers as per , it is RECOMMENDED that the Signature-Input field be included only as a header to avoid signatures being inadvertently stripped from a message. Multiple Signature-Input fields MAY be included in a single HTTP message. The signature labels MUST be unique across all field values.

The Signature HTTP Field The Signature field is a Dictionary structured field defined in containing one or more message signatures generated from the signature context of the target message. The member's key is the label that uniquely identifies the message signature within the HTTP message. The member's value is a Byte Sequence containing the signature value for the message signature identified by the label. The signer MAY include the Signature field as a trailer to facilitate signing a message after its content has been processed by the signer. However, since intermediaries are allowed to drop trailers as per , it is RECOMMENDED that the Signature field be included only as a header to avoid signatures being inadvertently stripped from a message. Multiple Signature fields MAY be included in a single HTTP message. The signature labels MUST be unique across all field values.

Multiple Signatures Multiple distinct signatures MAY be included in a single message. Each distinct signature MUST have a unique label. These multiple signatures could be added all by the same signer or could come from several different signers. For example, a signer may include multiple signatures signing the same message components with different keys or algorithms to support verifiers with different capabilities, or a reverse proxy may include information about the client in fields when forwarding the request to a service host, including a signature over the client's original signature values. The following non-normative example starts with a signed request from the client. A reverse proxy takes this request and validates the client's signature. The proxy then alters the message before forwarding it on to the origin server, changing the target host and adding the Forwarded header field defined in . The proxy is in a position to validate the incoming client's signature and make its own statement to the origin server about the nature of the request that it is forwarding by adding its own signature over the new message before passing it along to the origin server. The proxy also includes all the elements from the original message that are relevant to the origin server's processing. In many cases, the proxy will want to cover all the same components that were covered by the client's signature, which is the case in this example. Note that in this example, the proxy is signing over the new authority value, which it has changed. The proxy also adds the Forwarded header to its own signature value. The proxy identifies its own key and algorithm and, in this example, includes an expiration for the signature to indicate to downstream systems that the proxy will not vouch for this signed message past this short time window. This results in a signature base of: And a signature output value of: These values are added to the HTTP request message by the proxy. The original signature is included under the identifier sig1, and the reverse proxy's signature is included under the label proxy_sig. The proxy uses the key test-key-rsa to create its signature using the rsa-v1_5-sha256 signature algorithm, while the client's original signature was made using the key id of test-key-rsa-pss and an RSA PSS signature algorithm. While the proxy could additionally include the client's Signature value and Signature-Input fields from the original message in the new signature's covered components, this practice is NOT RECOMMENDED due to known weaknesses in signing signature values as discussed in . The proxy is in a position to validate the client's signature, the changes the proxy makes to the message will invalidate the existing signature when the message is seen by the origin server. In this example, it is possible for the origin server to have additional information in its signature context to account for the change in authority, though this practice requires additional configuration and extra care as discussed in . In other applications, the origin server will not be able to verify the original signature itself but will still want to verify that the proxy has done the appropriate validation of the client's signature. An application that needs to signal successful processing or receipt of a signature would need to carefully specify alternative mechanisms for sending such a signal securely.

Requesting Signatures While a signer is free to attach a signature to a request or response without prompting, it is often desirable for a potential verifier to signal that it expects a signature from a potential signer using the Accept-Signature field. When the Accept-Signature field is sent in an HTTP request message, the field indicates that the client desires the server to sign the response using the identified parameters, and the target message is the response to this request. All responses from resources that support such signature negotiation SHOULD either be uncacheable or contain a Vary header field that lists Accept-Signature, in order to prevent a cache from returning a response with a signature intended for a different request. When the Accept-Signature field is used in an HTTP response message, the field indicates that the server desires the client to sign its next request to the server with the identified parameters, and the target message is the client's next request. The client can choose to also continue signing future requests to the same server in the same way. The target message of an Accept-Signature field MUST include all labeled signatures indicated in the Accept-Header signature, each covering the same identified components of the Accept-Signature field. The sender of an Accept-Signature field MUST include only identifiers that are appropriate for the type of the target message. For example, if the target message is a request, the covered components can not include the @status component identifier.

The Accept-Signature Field The Accept-Signature field is a Dictionary structured field (defined in ) containing the metadata for one or more requested message signatures to be generated from message components of the target HTTP message. Each member describes a single message signature. The member's key is the label that uniquely identifies the requested message signature within the context of the target HTTP message. The member's value is the serialization of the desired covered components of the target message, including any allowed component metadata parameters, using the serialization process defined in . The list of component identifiers indicates the exact set of component identifiers to be included in the requested signature, including all applicable component parameters. The signature request MAY include signature metadata parameters that indicate desired behavior for the signer. The following behavior is defined by this specification:

created: The signer is requested to generate and include a creation time. This parameter has no associated value when sent as a signature request.
expires: The signer is requested to generate and include an expiration time. This parameter has no associated value when sent as a signature request.
nonce: The signer is requested to include the value of this parameter as the signature nonce in the target signature.
alg: The signer is requested to use the indicated signature algorithm to create the target signature.
keyid: The signer is requested to use the indicated key material to create the target signature.
tag: The signer is requested to include the value of this parameter as the signature tag in the target signature.

Processing an Accept-Signature The receiver of an Accept-Signature field fulfills that header as follows:

Parse the field value as a Dictionary
For each member of the dictionary:
1. The key is taken as the label of the output signature as specified in .
2. Parse the value of the member to obtain the set of covered component identifiers.
3. Determine that the covered components are applicable to the target message. If not, the process fails and returns an error.
4. Process the requested parameters, such as the signing algorithm and key material. If any requested parameters cannot be fulfilled, or if the requested parameters conflict with those deemed appropriate to the target message, the process fails and returns an error.
5. Select and generate any additional parameters necessary for completing the signature.
6. Create the HTTP message signature over the target message.
7. Create the Signature-Input and Signature values and associate them with the label.
Optionally create any additional Signature-Input and Signature values, with unique labels not found in the Accept-Signature field.
Combine all labeled Signature-Input and Signature values and attach both fields to the target message.

By this process, a signature applied to a target message MUST have the same label, MUST include the same set of covered component, MUST process all requested parameters, and MAY have additional parameters. The receiver of an Accept-Signature field MAY ignore any signature request that does not fit application parameters. The target message MAY include additional signatures not specified by the Accept-Signature field. For example, to cover additional message components, the signer can create a second signature that includes the additional components as well as the signature output of the requested signature.

IANA Considerations IANA is asked to update one registry and create four new registries, according to the following sections.

HTTP Field Name Registration IANA is asked to update the "Hypertext Transfer Protocol (HTTP) Field Name Registry" registry, registering the following entries according to the table below:

Field Name	Status	Reference
Signature-Input	permanent	of RFC nnnn
Signature	permanent	of RFC nnnn
Accept-Signature	permanent	of RFC nnnn

HTTP Signature Algorithms Registry This document defines HTTP Signature Algorithms, for which IANA is asked to create and maintain a new registry titled "HTTP Signature Algorithms". Initial values for this registry are given in . Future assignments and modifications to existing assignment are to be made through the Expert Review registration policy . The Designated Expert (DE) is expected to ensure that the algorithms referenced by a registered algorithm identifier are fully defined with all parameters (such as salt, hash, required key length, etc) are fixed by the defining text. The DE is expected to ensure that the algorithm definition fully specifies the HTTP_SIGN and HTTP_VERIFY primitive functions, including how all defined inputs and outputs map to the underlying cryptographic algorithm. The DE is expected to reject any registrations that are aliases of existing registrations. The DE is expected to ensure all registrations follow the template presented in , including that the length of the name is not excessive while still being unique and recognizable. When setting a registered item's status to "Deprecated", the DE should ensure that a reason for the deprecation is documented, along with instructions for moving away from the deprecated functionality. This specification creates algorithm identifiers by including major parameters in the identifier string. However, algorithm identifiers in this registry are to be interpreted as whole string values and not as a combination of parts. That is to say, it is expected that implementors understand rsa-pss-sha512 as referring to one specific algorithm with its hash, mask, and salt values set as defined in the defining text that establishes this identifier. Implementors do not parse out the rsa, pss, and sha512 portions of the identifier to determine parameters of the signing algorithm from the string, and the registry of one combination of parameters does not imply the registration of other combinations.

Registration Template

Algorithm Name:: An identifier for the HTTP Signature Algorithm. The name MUST be an ASCII string that conforms to the sf-string ABNF rule in and SHOULD NOT exceed 20 characters in length. The identifier MUST be unique within the context of the registry.
Description:: A brief description of the algorithm used to sign the signature base.
Status:: A brief text description of the status of the algorithm. The description MUST begin with one of "Active" or "Deprecated", and MAY provide further context or explanation as to the reason for the status. A value of "Deprecated" indicates that the signature algorithm is no longer recommended for use and might be insecure or unsafe in practice.
Specification document(s):: Reference to the document(s) that specify the algorithm, preferably including a URI that can be used to retrieve a copy of the document(s). An indication of the relevant sections may also be included but is not required.

Initial Contents Initial contents of the HTTP Signature Algorithms Registry.

Algorithm Name	Description	Status	Specification document(s)
`rsa-pss-sha512`	RSASSA-PSS using SHA-512	Active	of RFC nnnn
`rsa-v1_5-sha256`	RSASSA-PKCS1-v1_5 using SHA-256	Active	of RFC nnnn
`hmac-sha256`	HMAC using SHA-256	Active	of RFC nnnn
`ecdsa-p256-sha256`	ECDSA using curve P-256 DSS and SHA-256	Active	of RFC nnnn
`ecdsa-p384-sha384`	ECDSA using curve P-384 DSS and SHA-384	Active	of RFC nnnn
`ed25519`	Edwards Curve DSA using curve edwards25519	Active	of RFC nnnn

HTTP Signature Metadata Parameters Registry This document defines the signature parameters structure in , which may have parameters containing metadata about a message signature. IANA is asked to create and maintain a new registry titled "HTTP Signature Metadata Parameters" to record and maintain the set of parameters defined for use with member values in the signature parameters structure. Initial values for this registry are given in . Future assignments and modifications to existing assignments are to be made through the Expert Review registration policy . The DE is expected to ensure that the name follows the template presented in , including that the length of the name is not excessive while still being unique and recognizable for its defined function. The DE is expected to ensure that the defined functionality is clear and does not conflict with other registered parameters. The DE is expected to ensure that the definition of the metadata parameter includes its behavior when used as part of the normal signature process as well as when used in an Accept-Signature field.

Registration Template

Name:: An identifier for the HTTP signature metadata parameter. The name MUST be an ASCII string that conforms to the key ABNF rule defined in and SHOULD NOT exceed 20 characters in length. The identifier MUST be unique within the context of the registry.
Description:: A brief description of the metadata parameter and what it represents.
Specification document(s):: Reference to the document(s) that specify the parameter, preferably including a URI that can be used to retrieve a copy of the document(s). An indication of the relevant sections may also be included but is not required.

Initial Contents The table below contains the initial contents of the HTTP Signature Metadata Parameters Registry. Each row in the table represents a distinct entry in the registry. Initial contents of the HTTP Signature Metadata Parameters Registry.

Name	Description	Specification document(s)
`alg`	Explicitly declared signature algorithm	of RFC nnnn
`created`	Timestamp of signature creation	of RFC nnnn
`expires`	Timestamp of proposed signature expiration	of RFC nnnn
`keyid`	Key identifier for the signing and verification keys used to create this signature	of RFC nnnn
`nonce`	A single-use nonce value	of RFC nnnn
`tag`	An application-specific tag for a signature	of RFC nnnn

HTTP Signature Derived Component Names Registry This document defines a method for canonicalizing HTTP message components, including components that can be derived from the context of the target message outside of the HTTP fields. These derived components are identified by a unique string, known as the component name. Component names for derived components always start with the "@" (at) symbol to distinguish them from HTTP field names. IANA is asked to create and maintain a new registry typed "HTTP Signature Derived Component Names" to record and maintain the set of non-field component names and the methods to produce their associated component values. Initial values for this registry are given in . Future assignments and modifications to existing assignments are to be made through the Expert Review registration policy . The DE is expected to ensure that the name follows the template presented in , including that the length of the name is not excessive while still being unique and recognizable for its defined function. The DE is expected to ensure that the component value represented by the registration request can be deterministically derived from the target HTTP message. The DE is expected to ensure that any parameters defined for the registration request are clearly documented, along with their effects on the component value. The DE should also ensure that the registration request is not sufficiently distinct from existing derived component definitions to warrant its registration. When setting a registered item's status to "Deprecated", the DE should ensure that a reason for the deprecation is documented, along with instructions for moving away from the deprecated functionality.

Registration Template

Name:: A name for the HTTP derived component. The name MUST begin with the "@" character followed by an ASCII string consisting only of lower-case characters ("a" - "z"), digits ("0" - "9"), and hyphens ("-"), and SHOULD NOT exceed 20 characters in length. The name MUST be unique within the context of the registry.
Description:: A description of the derived component.
Status:: A brief text description of the status of the algorithm. The description MUST begin with one of "Active" or "Deprecated", and MAY provide further context or explanation as to the reason for the status. A value of "Deprecated" indicates that the derived component name is no longer recommended for use.
Target:: The valid message targets for the derived parameter. MUST be one of the values "Request", "Response", or "Request, Response". The semantics of these are defined in .
Specification document(s):: Reference to the document(s) that specify the derived component, preferably including a URI that can be used to retrieve a copy of the document(s). An indication of the relevant sections may also be included but is not required.

Initial Contents The table below contains the initial contents of the HTTP Signature Derived Component Names Registry. Initial contents of the HTTP Signature Derived Component Names Registry.

Name	Description	Status	Target	Specification document(s)
`@signature-params`	Reserved for signature parameters line in signature base	Active	Request, Response	of RFC nnnn
`@method`	The HTTP request method	Active	Request	of RFC nnnn
`@authority`	The HTTP authority, or target host	Active	Request	of RFC nnnn
`@scheme`	The URI scheme of the request URI	Active	Request	of RFC nnnn
`@target-uri`	The full target URI of the request	Active	Request	of RFC nnnn
`@request-target`	The request target of the request	Active	Request	of RFC nnnn
`@path`	The full path of the request URI	Active	Request	of RFC nnnn
`@query`	The full query of the request URI	Active	Request	of RFC nnnn
`@query-param`	A single named query parameter	Active	Request	of RFC nnnn
`@status`	The status code of the response	Active	Response	of RFC nnnn

HTTP Signature Component Parameters Registry This document defines several kinds of component identifiers, some of which can be parameterized in specific circumstances to provide unique modified behavior. IANA is asked to create and maintain a new registry typed "HTTP Signature Component Parameters" to record and maintain the set of parameters names, the component identifiers they are associated with, and the modifications these parameters make to the component value. Definitions of parameters MUST define the targets to which they apply (such as specific field types, derived components, or contexts). Initial values for this registry are given in . Future assignments and modifications to existing assignments are to be made through the Expert Review registration policy . The DE is expected to ensure that the name follows the template presented in , including that the length of the name is not excessive while still being unique and recognizable for its defined function. The DE is expected to ensure that the definition of the field sufficiently defines any interactions incompatibilities with other existing parameters known at the time of the registration request. If the parameter changes the component value, the DE is expected to ensure that the component value defined by the component identifier with the parameter applied can be deterministically derived from the target HTTP message.

Registration Template

Name:: A name for the parameter. The name MUST be an ASCII string that conforms to the key ABNF rule defined in and SHOULD NOT exceed 20 characters in length. The name MUST be unique within the context of the registry.
Description:: A description of the parameter's function.
Specification document(s):: Reference to the document(s) that specify the derived component, preferably including a URI that can be used to retrieve a copy of the document(s). An indication of the relevant sections may also be included but is not required.

Initial Contents The table below contains the initial contents of the HTTP Signature Derived Component Names Registry. Initial contents of the HTTP Signature Component Parameters Registry.

Name	Description	Specification document(s)
`sf`	Strict structured field serialization	of RFC nnnn
`key`	Single key value of dictionary structured fields	of RFC nnnn
`bs`	Byte Sequence wrapping indicator	of RFC nnnn
`tr`	Trailer	of RFC nnnn
`req`	Related request indicator	of RFC nnnn
`name`	Single named query parameter	of RFC nnnn

Security Considerations In order for an HTTP message to be considered covered by a signature, all of the following conditions have to be true:

a signature is expected or allowed on the message by the verifier
the signature exists on the message
the signature is verified against the identified key material and algorithm
the key material and algorithm are appropriate for the context of the message
the signature is within expected time boundaries
the signature covers the expected content, including any critical components
the list of covered components is applicable to the context of the message

In addition to the application requirement definitions listed in , the following security considerations provide discussion and context to the requirements of creating and verifying signatures on HTTP messages.

General Considerations

Skipping Signature Verification HTTP Message Signatures only provide security if the signature is verified by the verifier. Since the message to which the signature is attached remains a valid HTTP message without the signature fields, it is possible for a verifier to ignore the output of the verification function and still process the message. Common reasons for this could be relaxed requirements in a development environment or a temporary suspension of enforcing verification during debugging an overall system. Such temporary suspensions are difficult to detect under positive-example testing since a good signature will always trigger a valid response whether or not it has been checked. To detect this, verifiers should be tested using both valid and invalid signatures, ensuring that the invalid signature fails as expected.

Use of TLS The use of HTTP Message Signatures does not negate the need for TLS or its equivalent to protect information in transit. Message signatures provide message integrity over the covered message components but do not provide any confidentiality for the communication between parties. TLS provides such confidentiality between the TLS endpoints. As part of this, TLS also protects the signature data itself from being captured by an attacker, which is an important step in preventing signature replay. When TLS is used, it needs to be deployed according to the recommendations in .

Message Processing and Selection

Insufficient Coverage Any portions of the message not covered by the signature are susceptible to modification by an attacker without affecting the signature. An attacker can take advantage of this by introducing or modifying a header field or other message component that will change the processing of the message but will not be covered by the signature. Such an altered message would still pass signature verification, but when the verifier processes the message as a whole, the unsigned content injected by the attacker would subvert the trust conveyed by the valid signature and change the outcome of processing the message. To combat this, an application of this specification should require as much of the message as possible to be signed, within the limits of the application and deployment. The verifier should only trust message components that have been signed. Verifiers could also strip out any sensitive unsigned portions of the message before processing of the message continues.

Signature Replay Since HTTP Message Signatures allows sub-portions of the HTTP message to be signed, it is possible for two different HTTP messages to validate against the same signature. The most extreme form of this would be a signature over no message components. If such a signature were intercepted, it could be replayed at will by an attacker, attached to any HTTP message. Even with sufficient component coverage, a given signature could be applied to two similar HTTP messages, allowing a message to be replayed by an attacker with the signature intact. To counteract these kinds of attacks, it's first important for the signer to cover sufficient portions of the message to differentiate it from other messages. In addition, the signature can use the nonce signature parameter to provide a per-message unique value to allow the verifier to detect replay of the signature itself if a nonce value is repeated. Furthermore, the signer can provide a timestamp for when the signature was created and a time at which the signer considers the signature to be expired, limiting the utility of a captured signature value. If a verifier wants to trigger a new signature from a signer, it can send the Accept-Signature header field with a new nonce parameter. An attacker that is simply replaying a signature would not be able to generate a new signature with the chosen nonce value.

Choosing Message Components Applications of HTTP Message Signatures need to decide which message components will be covered by the signature. Depending on the application, some components could be expected to be changed by intermediaries prior to the signature's verification. If these components are covered, such changes would, by design, break the signature. However, the HTTP Message Signature standard allows for flexibility in determining which components are signed precisely so that a given application can choose the appropriate portions of the message that need to be signed, avoiding problematic components. For example, a web application framework that relies on rewriting query parameters might avoid use of the @query derived component in favor of sub-indexing the query value using @query-param derived components instead. Some components are expected to be changed by intermediaries and ought not to be signed under most circumstance. The Via and Forwarded header fields, for example, are expected to be manipulated by proxies and other middle-boxes, including replacing or entirely dropping existing values. These fields should not be covered by the signature except in very limited and tightly-coupled scenarios. Additional considerations for choosing signature aspects are discussed in .

Choosing Signature Parameters and Derived Components over HTTP Fields Some HTTP fields have values and interpretations that are similar to HTTP signature parameters or derived components. In most cases, it is more desirable to sign the non-field alternative. In particular, the following fields should usually not be included in the signature unless the application specifically requires it:

"date": The "date" field value represents the timestamp of the HTTP message. However, the creation time of the signature itself is encoded in the created signature parameter. These two values can be different, depending on how the signature and the HTTP message are created and serialized. Applications processing signatures for valid time windows should use the created signature parameter for such calculations. An application could also put limits on how much skew there is between the "date" field and the created signature parameter, in order to limit the application of a generated signature to different HTTP messages. See also and .
"host": The "host" header field is specific to HTTP/1.1, and its functionality is subsumed by the "@authority" derived component, defined in . In order to preserve the value across different HTTP versions, applications should always use the "@authority" derived component. See also .

Signature Labels HTTP Message Signature values are identified in the Signature and Signature-Input field values by unique labels. These labels are chosen only when attaching the signature values to the message and are not accounted for in the signing process. An intermediary is allowed to re-label an existing signature when processing the message. Therefore, applications should not rely on specific labels being present, and applications should not put semantic meaning on the labels themselves. Instead, additional signature parameters can be used to convey whatever additional meaning is required to be attached to and covered by the signature. In particular, the tag parameter can be used to define an application-specific value as described in .

Multiple Signature Confusion Since multiple signatures can be applied to one message, it is possible for an attacker to attach their own signature to a captured message without modifying existing signatures. This new signature could be completely valid based on the attacker's key, or it could be an invalid signature for any number of reasons. Each of these situations need to be accounted for. A verifier processing a set of valid signatures needs to account for all of the signers, identified by the signing keys. Only signatures from expected signers should be accepted, regardless of the cryptographic validity of the signature itself. A verifier processing a set of signatures on a message also needs to determine what to do when one or more of the signatures are not valid. If a message is accepted when at least one signature is valid, then a verifier could drop all invalid signatures from the request before processing the message further. Alternatively, if the verifier rejects a message for a single invalid signature, an attacker could use this to deny service to otherwise valid messages by injecting invalid signatures alongside the valid ones.

Collision of Application-Specific Signature Tag Multiple applications and protocols could apply HTTP signatures on the same message simultaneously. In fact, this is a desired feature in many circumstances as described in . A naive verifier could become confused in processing multiple signatures, either accepting or rejecting a message based on an unrelated or irrelevant signature. In order to help an application select which signatures apply to its own processing, the application can declare a specific value for the tag signature parameter as defined in . For example, a signature targeting an application gateway could require tag="app-gateway" as part of the signature parameters for that application. The use of the tag parameter does not prevent an attacker from also using the same value as a target application, since the parameter's value is public and otherwise unrestricted. As a consequence, a verifier should only use value of the tag parameter to limit which signatures to check. Each signature still needs to be examined by the verifier to ensure that sufficient coverage is provided, as discussed in .

Message Content On its own, this specification does not provide coverage for the content of an HTTP message under the signature, either in request or response. However, defines a set of fields that allow a cryptographic digest of the content to be represented in a field. Once this field is created, it can be included just like any other field as defined in . For example, in the following response message: The digest of the content can be added to the Content-Digest field as follows: This field can be included in a signature base just like any other field along with the basic signature parameters: From here, the signing process proceeds as usual. Upon verification, it is important that the verifier validate not only the signature but also the value of the Content-Digest field itself against the actual received content. Unless the verifier performs this step, it would be possible for an attacker to substitute the message content but leave the Content-Digest field value untouched to pass the signature. Since only the field value is covered by the signature directly, checking only the signature is not sufficient protection against such a substitution attack. As discussed in , the value of the Content-Digest field is dependent on the content encoding of the message. If an intermediary changes the content encoding, the resulting Content-Digest value would change, which would in turn invalidate the signature. Any intermediary performing such an action would need to apply a new signature with the updated Content-Digest field value, similar to the reverse proxy use case discussed in . Applications that make use of the request-response parameter also need to be aware of the limitations in this functionality. Specifically, if a client does not include something like a Content-Digest header field in the request, the server is unable to include a signature that covers the request's content.

Cryptographic Considerations

Cryptography and Signature Collision The HTTP Message Signatures specification does not define any of its own cryptographic primitives, and instead relies on other specifications to define such elements. If the signature algorithm or key used to process the signature base is vulnerable to any attacks, the resulting signature will also be susceptible to these same attacks. A common attack against signature systems is to force a signature collision, where the same signature value successfully verifies against multiple different inputs. Since this specification relies on reconstruction of the signature base from an HTTP message, and the list of components signed is fixed in the signature, it is difficult but not impossible for an attacker to effect such a collision. An attacker would need to manipulate the HTTP message and its covered message components in order to make the collision effective. To counter this, only vetted keys and signature algorithms should be used to sign HTTP messages. The HTTP Message Signatures Algorithm Registry is one source of trusted signature algorithms for applications to apply to their messages. While it is possible for an attacker to substitute the signature parameters value or the signature value separately, the signature base generation algorithm always covers the signature parameters as the final value in the signature base using a deterministic serialization method. This step strongly binds the signature base with the signature value in a way that makes it much more difficult for an attacker to perform a partial substitution on the signature bases.

Key Theft A foundational assumption of signature-based cryptographic systems is that the signing key is not compromised by an attacker. If the keys used to sign the message are exfiltrated or stolen, the attacker will be able to generate their own signatures using those keys. As a consequence, signers have to protect any signing key material from exfiltration, capture, and use by an attacker. To combat this, signers can rotate keys over time to limit the amount of time stolen keys are useful. Signers can also use key escrow and storage systems to limit the attack surface against keys. Furthermore, the use of asymmetric signing algorithms exposes key material less than the use of symmetric signing algorithms.

Symmetric Cryptography The HTTP Message Signatures specification allows for both asymmetric and symmetric cryptography to be applied to HTTP messages. By its nature, symmetric cryptographic methods require the same key material to be known by both the signer and verifier. This effectively means that a verifier is capable of generating a valid signature, since they have access to the same key material. An attacker that is able to compromise a verifier would be able to then impersonate a signer. Where possible, asymmetric methods or secure key agreement mechanisms should be used in order to avoid this type of attack. When symmetric methods are used, distribution of the key material needs to be protected by the overall system. One technique for this is the use of separate cryptographic modules that separate the verification process (and therefore the key material) from other code, minimizing the vulnerable attack surface. Another technique is the use of key derivation functions that allow the signer and verifier to agree on unique keys for each message without having to share the key values directly. Additionally, if symmetric algorithms are allowed within a system, special care must be taken to avoid key downgrade attacks.

Key Specification Mix-Up The existence of a valid signature on an HTTP message is not sufficient to prove that the message has been signed by the appropriate party. It is up to the verifier to ensure that a given key and algorithm are appropriate for the message in question. If the verifier does not perform such a step, an attacker could substitute their own signature using their own key on a message and force a verifier to accept and process it. To combat this, the verifier needs to ensure that not only does the signature validate for a message, but that the key and algorithm used are appropriate.

Non-deterministic Signature Primitives Some cryptographic primitives such as RSA PSS and ECDSA have non-deterministic outputs, which include some amount of entropy within the algorithm. For such algorithms, multiple signatures generated in succession will not match. A lazy implementation of a verifier could ignore this distinction and simply check for the same value being created by re-signing the signature base. Such an implementation would work for deterministic algorithms such as HMAC and EdDSA but fail to verify valid signatures made using non-deterministic algorithms. It is therefore important that a verifier always use the correctly-defined verification function for the algorithm in question and not do a simple comparison.

Key and Algorithm Specification Downgrades Applications of this specification need to protect against key specification downgrade attacks. For example, the same RSA key can be used for both RSA-PSS and RSA v1.5 signatures. If an application expects a key to only be used with RSA-PSS, it needs to reject signatures for that key using the weaker RSA 1.5 specification. Another example of a downgrade attack occurs when an asymmetric algorithm is expected, such as RSA-PSS, but an attacker substitutes a signature using symmetric algorithm, such as HMAC. A naive verifier implementation could use the value of the public RSA key as the input to the HMAC verification function. Since the public key is known to the attacker, this would allow the attacker to create a valid HMAC signature against this known key. To prevent this, the verifier needs to ensure that both the key material and the algorithm are appropriate for the usage in question. Additionally, while this specification does allow runtime specification of the algorithm using the alg signature parameter, applications are encouraged to use other mechanisms such as static configuration or higher protocol-level algorithm specification instead, preventing an attacker from substituting the algorithm specified.

Signing Signature Values When applying the request-response parameter or multiple signatures to a message, it is possible to sign the value of an existing Signature field, thereby covering the bytes of the existing signature output in the new signature's value. While it would seem that this practice would transitively cover the components under the original signature in a verifiable fashion, the attacks described in can be used to impersonate a signature output value on an unrelated message. In this example, Alice intends to send a signed request to Bob, and Bob wants to provide a signed response to Alice that includes a cryptographic proof that Bob is responding to Alice's incoming message. Mallory wants to intercept this traffic and replace Alice's message with her own, without Alice being aware that the interception has taken place.

Alice creates a message, Req_A and applies a signature Sig_A using her private key Key_A_Sign.
Alice believes she is sending Req_A to Bob.
Mallory intercepts Req_A and reads the value Sig_A from this message.
Mallory generates a different message Req_M to send to Bob instead.
Mallory crafts a signing key Key_M_Sign such that she can create a valid signature Sig_M over her request Req_M using this key, but the byte value of Sig_M exactly equals that of Sig_A.
Mallory sends Req_M with Sig_M to Bob.
Bob validates Sig_M against Mallory's verification key, Key_M_Verify. At no time does Bob think that he's responding to Alice.
Bob responds with response message Res_B to Req_M and creates signature Sig_B over this message using his key, Key_B_Sign. Bob includes the value of Sig_M under Sig_B's covered components, but nothing elese from the request message.
Mallory receives the response Res_B from Bob, including the signature Sig_B value. Mallory replays this response to Alice.
Alice reads Res_B from Mallory and verifies Sig_B using Bob's verification key, Key_B_Verify. Alice includes the bytes of her original signature Sig_A in the signature base, and the signature verifies.
Alice is led to believe that Bob has responded to her message, and believes she has cryptographic proof of this happening, but in fact Bob responded to Mallory's malicious request and Alice is none the wiser.

To mitigate this, Bob can sign more portions of the request message than just the Signature field, in order to more fully differentiate Alice's message from Mallory's. Applications using this feature, particularly for non-repudiation purposes, can stipulate that any components required in the original signature also be covered separately in the second signature. For signed messages, requiring coverage of the corresponding Signature-Input field of the first signature ensures that unique items such as nonces and timestamps are also covered sufficiently by the second signature.

Matching Covered Components to Message

Modification of Required Message Parameters An attacker could effectively deny a service by modifying an otherwise benign signature parameter or signed message component. While rejecting a modified message is the desired behavior, consistently failing signatures could lead to the verifier turning off signature checking in order to make systems work again (see ), or to the application minimizing the signed component requirements. If such failures are common within an application, the signer and verifier should compare their generated signature bases with each other to determine which part of the message is being modified. If an expected modification is found, the signer and verifier can agree on an alternative set of requirements that will pass. However, the signer and verifier should not remove the requirement to sign the modified component when it is suspected an attacker is modifying the component.

Mismatch of Signature Parameters from Message The verifier needs to make sure that the signed message components match those in the message itself. For example, the @method derived component requires that the value within the signature base be the same as the HTTP method used when presenting this message. This specification encourages this by requiring the verifier to derive the signature base from the message, but lazy caching or conveyance of a raw signature base to a processing subsystem could lead to downstream verifiers accepting a message that does not match the presented signature. To counter this, the component that generates the signature base needs to be trusted by both the signer and verifier within a system.

Message Component Source and Context The signature context for deriving message component values includes the target HTTP Message itself, any associated messages (such as the request that triggered a response), and additional information that the signer or verifier has access to. Both signers and verifiers need to carefully consider the source of all information when creating component values for the signature base and take care not to take information from untrusted sources. Otherwise, an attacker could leverage such a loosely-defined message context to inject their own values into the signature base string, overriding or corrupting the intended values. For example, in most situations, the target URI of the message is defined in . However, let's say that there is an application that requires signing of the @authority of the incoming request, but the application doing the processing is behind a reverse proxy. Such an application would expect a change in the @authority value, and it could be configured to know the external target URI as seen by the client on the other side of the proxy. This application would use this configured value as its target URI for the purposes of deriving message component values such as @authority instead of using the target URI of the incoming message. This approach is not without problems, as a misconfigured system could accept signed requests intended for different components in the system. For this scenario, an intermediary could instead add its own signature to be verified by the application directly, as demonstrated in . This alternative approach requires a more active intermediary but relies less on the target application knowing external configuration values. For another example, defines a method for signing response messages but including portions of the request message that triggered the response. In this case, the context for component value calculation is the combination of the response and request message, not just the single message to which the signature is applied. For this feature, the req flag allows both signer to explicitly signal which part of the context is being sourced for a component identifier's value. Implementations need to ensure that only the intended message is being referred to for each component, otherwise an attacker could attempt to subvert a signature by manipulating one side or the other.

Multiple Message Component Contexts It is possible that the context for deriving message component values could be distinct for each signature present within a single message. This is particularly the case when proxies mutate messages and include signatures over the mutated values, in addition to any existing signatures. For example, a reverse proxy can replace a public hostname in a request to a service with the hostname for the individual service host that it is forwarding the request on to. If both the client and the reverse proxy add signatures covering @authority, the service host will see two signatures on the request, each signing different values for the @authority message component, reflecting the change to that component as the message made its way from the client to the service host. In such a case, it's common for the internal service to verify only one of the signatures or to use externally-configured information, as discussed in . However, a verifier processing both signatures has to use a different message component context for each signature, since the component value for the @authority component will be different for each signature. Verifiers like this need to be aware of both the reverse proxy's context for incoming messages as well as the target service's context for the message coming from the reverse proxy. The verifier needs to take particular care to apply the correct context to the correct signature, otherwise an attacker could use knowledge of this complex setup to confuse the inputs to the verifier. Such verifiers also need to ensure that any differences in message component contexts between signatures are expected and permitted. For example, in the above scenario, the reverse proxy could include the original hostname in a Forwarded header field, and sign @authority, forwarded, and the client's entry in the signature field. The verifier can use the hostname from the Forwarded header field to confirm that the hostname was transformed as expected.

HTTP Processing

Confusing HTTP Field Names for Derived Component Names The definition of HTTP field names does not allow for the use of the @ character anywhere in the name. As such, since all derived component names start with the @ character, these namespaces should be completely separate. However, some HTTP implementations are not sufficiently strict about the characters accepted in HTTP field names. In such implementations, a sender (or attacker) could inject a header field starting with an @ character and have it passed through to the application code. These invalid header fields could be used to override a portion of the derived message content and substitute an arbitrary value, providing a potential place for an attacker to mount a signature collision attack or other functional substitution attack (such as using the signature from a GET request on a crafted POST request). To combat this, when selecting values for a message component, if the component name starts with the @ character, it needs to be processed as a derived component and never taken as a fields. Only if the component name does not start with the @ character can it be taken from the fields of the message. The algorithm discussed in provides a safe order of operations.

Semantically Equivalent Field Values The signature base generation algorithm uses the value of an HTTP field as its component value. In the common case, this amounts to taking the actual bytes of the field value as the component value for both the signer and verifier. However, some field values allow for transformation of the values in semantically equivalent ways that alter the bytes used in the value itself. For example, a field definition can declare some or all of its value to be case-insensitive, or to have special handling of internal whitespace characters. Other fields have expected transformations from intermediaries, such as the removal of comments in the Via header field. In such cases, a verifier could be tripped up by using the equivalent transformed field value, which would differ from the byte value used by the signer. The verifier would have a difficult time finding this class of errors since the value of the field is still acceptable for the application, but the actual bytes required by the signature base would not match. When processing such fields, the signer and verifier have to agree how to handle such transformations, if at all. One option is to not sign problematic fields, but care must be taken to ensure that there is still sufficient signature coverage for the application. Another option is to define an application-specific canonicalization value for the field before it is added to the HTTP message, such as to always remove internal comments before signing, or to always transform values to lowercase. Since these transformations are applied prior to the field being used as input to the signature base generation algorithm, the signature base will still simply contain the byte value of the field as it appears within the message. If the transformations were to be applied after the value is extracted from the message but before it is added to the signature base, different attack surfaces such as value substitution attacks could be launched against the application. All application-specific additional rules are outside the scope of this specification, and by their very nature these transformations would harm interoperability of the implementation outside of this specific application. It is recommended that applications avoid the use of such additional rules wherever possible.

Parsing Structured Field Values Several parts of this specification rely on the parsing of structured field values . In particular, normalization of HTTP structured field values, referencing members of a dictionary structured field, and processing the @signature-input value when verifying a signature. While structured field values are designed to be relatively simple to parse, a naive or broken implementation of such a parser could lead to subtle attack surfaces being exposed in the implementation. For example, if a buggy parser of the @signature-input value does not enforce proper closing of quotes around string values within the list of component identifiers, an attacker could take advantage of this and inject additional content into the signature base through manipulating the Signature-Input field value on a message. To counteract this, implementations should use fully compliant and trusted parsers for all structured field processing, both on the signer and verifier side.

HTTP Versions and Component Ambiguity Some message components are expressed in different ways across HTTP versions. For example, the authority of the request target is sent using the Host header field in HTTP/1.1 but with the :authority pseudo-header in HTTP/2. If a signer sends an HTTP/1.1 message and signs the Host field, but the message is translated to HTTP/2 before it reaches the verifier, the signature will not validate as the Host header field could be dropped. It is for this reason that HTTP Message Signatures defines a set of derived components that define a single way to get value in question, such as the @authority derived component () in lieu of the Host header field. Applications should therefore prefer derived components for such options where possible.

Canonicalization Attacks Any ambiguity in the generation of the signature base could provide an attacker with leverage to substitute or break a signature on a message. Some message component values, particularly HTTP field values, are potentially susceptible to broken implementations that could lead to unexpected and insecure behavior. Naive implementations of this specification might implement HTTP field processing by taking the single value of a field and using it as the direct component value without processing it appropriately. For example, if the handling of obs-fold field values does not remove the internal line folding and whitespace, additional newlines could be introduced into the signature base by the signer, providing a potential place for an attacker to mount a signature collision attack. Alternatively, if header fields that appear multiple times are not joined into a single string value, as is required by this specification, similar attacks can be mounted as a signed component value would show up in the signature base more than once and could be substituted or otherwise attacked in this way. To counter this, the entire field value processing algorithm needs to be implemented by all implementations of signers and verifiers.

Non-List Field Values When an HTTP field occurs multiple times in a single message, these values need to be combined into a single one-line string value to be included in the HTTP signature base, as described in . Not all HTTP fields can be combined into a single value in this way and still be a valid value for the field. For the purposes of generating the signature base, the message component value is never meant to be read back out of the signature base string or used in the application. Therefore it is considered best practice to treat the signature base generation algorithm separately from processing the field values by the application, particularly for fields that are known to have this property. If the field values that are being signed do not validate, the signed message should also be rejected. If an HTTP field allows for unquoted commas within its values, combining multiple field values can lead to a situation where two semantically different messages produce the same line in a signature base. For example, take the following hypothetical header field with an internal comma in its syntax, here used to define two separate lists of values: For this header field, sending all of these values as a single field value results in a single list of values: Both of these messages would create the following line in the signature base: Since two semantically distinct inputs can create the same output in the signature base, special care has to be taken when handling such values. Specifically, the Set-Cookie field defines an internal syntax that does not conform to the List syntax in . In particular some portions allow unquoted commas, and the field is typically sent as multiple separate field lines with distinct values when sending multiple cookies. When multiple Set-Cookie fields are sent in the same message, it is not generally possible to combine these into a single line and be able to parse and use the results, as discussed in . Therefore, all the cookies need to be processed from their separate header values, without being combined, while the signature base needs to be processed from the special combined value generated solely for this purpose. If the cookie value is invalid, the signed message ought to be rejected as this is a possible padding attack as described in . To deal with this, an application can choose to limit signing of problematic fields like Set-Cookie, such as including the field in a signature only when a single field value is present and the results would be unambiguous. Similar caution needs to be taken with all fields that could have non-deterministic mappings into the signature base. Signers can also make use of the bs parameter to armor such fields, as described in .

Padding Attacks with Multiple Field Values Since HTTP field values need to be combined in a single string value to be included in the HTTP signature base, as described in , it is possible for an attacker to inject an additional value for a given field and add this to the signature base of the verifier. In most circumstances, this causes the signature validation to fail as expected, since the new signature base value will not match the one used by the signer to create the signature. However, it is theoretically possible for the attacker to inject both a garbage value to a field and a desired value to another field in order to force a particular input. This is a variation of the collision attack described in , where the attacker accomplishes their change in the message by adding to existing field values. To counter this, an application needs to validate the content of the fields covered in the signature in addition to ensuring that the signature itself validates. With such protections, the attacker's padding attack would be rejected by the field value processor, even in the case where the attacker could force a signature collision.

Ambiguous Handling of Query Elements The HTML form parameters format defined in the "application/x-www-form-urlencoded" section of , is widely deployed and supported by many application frameworks. For convenience, some of these frameworks in particular combine query parameters that are found in the HTTP query and those found in the message content, particularly for POST message with a Content-Type value of "application/x-www-form-urlencoded". The @query-param derived component identifier defined in draws its values only from the query section of the target URI of the request. As such, it would be possible for an attacker to shadow or replace query parameters in a request by overriding the signed query parameter with an unsigned form parameter, or vice versa. To counter this, an application needs to make sure that values used for the signature base and the application are drawn from a consistent context, in this case the query component of the target URI. Additionally, when the HTTP request has content, an application should sign the message content as well, as discussed in .

Privacy Considerations

Identification through Keys If a signer uses the same key with multiple verifiers, or uses the same key over time with a single verifier, the ongoing use of that key can be used to track the signer throughout the set of verifiers that messages are sent to. Since cryptographic keys are meant to be functionally unique, the use of the same key over time is a strong indicator that it is the same party signing multiple messages. In many applications, this is a desirable trait, and it allows HTTP Message Signatures to be used as part of authenticating the signer to the verifier. However, it could be unintentional tracking that a signer might not be aware of. To counter this kind of tracking, a signer can use a different key for each verifier that it is in communication with. Sometimes, a signer could also rotate their key when sending messages to a given verifier. These approaches do not negate the need for other anti-tracking techniques to be applied as necessary.

Signatures do not provide confidentiality HTTP Message Signatures do not provide confidentiality of any of the information protected by the signature. The content of the HTTP message, including the value of all fields and the value of the signature itself, is presented in plaintext to any party with access to the message. To provide confidentiality at the transport level, TLS or its equivalent can be used as discussed in .

Oracles It is important to balance the need for providing useful feedback to developers on error conditions without providing additional information to an attacker. For example, a naive but helpful server implementation might try to indicate the required key identifier needed for requesting a resource. If someone knows who controls that key, a correlation can be made between the resource's existence and the party identified by the key. Access to such information could be used by an attacker as a means to target the legitimate owner of the resource for further attacks.

Required Content A core design tenet of this specification is that all message components covered by the signature need to be available to the verifier in order to recreate the signature base and verify the signature. As a consequence, if an application of this specification requires that a particular field be signed, the verifier will need access to the value of that field. For example, in some complex systems with intermediary processors this could cause the surprising behavior of an intermediary not being able to remove privacy-sensitive information from a message before forwarding it on for processing, for fear of breaking the signature. A possible mitigation for this specific situation would be for the intermediary to verify the signature itself, then modifying the message to remove the privacy-sensitive information. The intermediary can add its own signature at this point to signal to the next destination that the incoming signature was validated, as is shown in the example in .

References Normative References ASCII format for network interchange HMAC: Keyed-Hashing for Message Authentication This document describes HMAC, a mechanism for message authentication using cryptographic hash functions. HMAC can be used with any iterative cryptographic hash function, e.g., MD5, SHA-1, in combination with a secret shared key. The cryptographic strength of HMAC depends on the properties of the underlying hash function. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF) Federal Information Processing Standard, FIPS JSON Web Key (JWK) A JSON Web Key (JWK) is a JavaScript Object Notation (JSON) data structure that represents a cryptographic key. This specification also defines a JWK Set JSON data structure that represents a set of JWKs. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and IANA registries established by that specification. JSON Web Algorithms (JWA) This specification registers cryptographic algorithms and identifiers to be used with the JSON Web Signature (JWS), JSON Web Encryption (JWE), and JSON Web Key (JWK) specifications. It defines several IANA registries for these identifiers. PKCS #1: RSA Cryptography Specifications Version 2.2 This document provides recommendations for the implementation of public-key cryptography based on the RSA algorithm, covering cryptographic primitives, encryption schemes, signature schemes with appendix, and ASN.1 syntax for representing keys and for identifying the schemes. This document represents a republication of PKCS #1 v2.2 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series. By publishing this RFC, change control is transferred to the IETF. This document also obsoletes RFC 3447. Edwards-Curve Digital Signature Algorithm (EdDSA) This document describes elliptic curve signature scheme Edwards-curve Digital Signature Algorithm (EdDSA). The algorithm is instantiated with recommended parameters for the edwards25519 and edwards448 curves. An example implementation and test vectors are provided. Structured Field Values for HTTP This document describes a set of data types and associated algorithms that are intended to make it easier and safer to define and handle HTTP header and trailer fields, known as "Structured Fields", "Structured Headers", or "Structured Trailers". It is intended for use by specifications of new HTTP fields that wish to use a common syntax that is more restrictive than traditional HTTP field values. Digital Signature Standard (DSS) The Open Group Base Specifications Issue 7, 2018 edition HTTP Semantics The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230. HTTP/1.1 The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document specifies the HTTP/1.1 message syntax, message parsing, connection management, and related security concerns. This document obsoletes portions of RFC 7230. Uniform Resource Identifier (URI): Generic Syntax A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] URL (Living Standard) Augmented BNF for Syntax Specifications: ABNF Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Forwarded HTTP Extension This document defines an HTTP extension header field that allows proxy components to disclose information lost in the proxying process, for example, the originating IP address of a request or IP address of the proxy on the user-agent-facing interface. In a path of proxying components, this makes it possible to arrange it so that each subsequent component will have access to, for example, all IP addresses used in the chain of proxied HTTP requests. This document also specifies guidelines for a proxy administrator to anonymize the origin of a request. JSON Web Signature (JWS) JSON Web Signature (JWS) represents content secured with digital signatures or Message Authentication Codes (MACs) using JSON-based data structures. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and an IANA registry defined by that specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) specification. Problem Details for HTTP APIs This document defines a "problem detail" as a way to carry machine- readable details of errors in a HTTP response to avoid the need to define new error response formats for HTTP APIs. Handling Long Lines in Content of Internet-Drafts and RFCs This document defines two strategies for handling long lines in width-bounded text content. One strategy, called the "single backslash" strategy, is based on the historical use of a single backslash ('\') character to indicate where line-folding has occurred, with the continuation occurring with the first character that is not a space character (' ') on the next line. The second strategy, called the "double backslash" strategy, extends the first strategy by adding a second backslash character to identify where the continuation begins and is thereby able to handle cases not supported by the first strategy. Both strategies use a self-describing header enabling automated reconstitution of the original content. Deprecating TLS 1.0 and TLS 1.1 This document formally deprecates Transport Layer Security (TLS) versions 1.0 (RFC 2246) and 1.1 (RFC 4346). Accordingly, those documents have been moved to Historic status. These versions lack support for current and recommended cryptographic algorithms and mechanisms, and various government and industry profiles of applications using TLS now mandate avoiding these old TLS versions. TLS version 1.2 became the recommended version for IETF protocols in 2008 (subsequently being obsoleted by TLS version 1.3 in 2018), providing sufficient time to transition away from older versions. Removing support for older versions from implementations reduces the attack surface, reduces opportunity for misconfiguration, and streamlines library and product maintenance. This document also deprecates Datagram TLS (DTLS) version 1.0 (RFC 4347) but not DTLS version 1.2, and there is no DTLS version 1.1. This document updates many RFCs that normatively refer to TLS version 1.0 or TLS version 1.1, as described herein. This document also updates the best practices for TLS usage in RFC 7525; hence, it is part of BCP 195. Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are used to protect data exchanged over a wide range of application protocols and can also form the basis for secure transport protocols. Over the years, the industry has witnessed several serious attacks on TLS and DTLS, including attacks on the most commonly used cipher suites and their modes of operation. This document provides the latest recommendations for ensuring the security of deployed services that use TLS and DTLS. These recommendations are applicable to the majority of use cases. RFC 7525, an earlier version of the TLS recommendations, was published when the industry was transitioning to TLS 1.2. Years later, this transition is largely complete, and TLS 1.3 is widely available. This document updates the guidance given the new environment and obsoletes RFC 7525. In addition, this document updates RFCs 5288 and 6066 in view of recent attacks. Client-Cert HTTP Header Field Ping Identity Akamai This document describes HTTP extension header fields that allow a TLS terminating reverse proxy to convey the client certificate information of a mutually authenticated TLS connection to the origin server in a common and predictable manner. Digest Fields Team Digitale, Italian Government Cloudflare This document defines HTTP fields that support integrity digests. The Content-Digest field can be used for the integrity of HTTP message content. The Repr-Digest field can be used for the integrity of HTTP representations. Want-Content-Digest and Want-Repr-Digest can be used to indicate a sender's interest and preferences for receiving the respective Integrity fields. This document obsoletes RFC 3230 and the Digest and Want-Digest HTTP fields. HTTP State Management Mechanism This document defines the HTTP Cookie and Set-Cookie header fields. These header fields can be used by HTTP servers to store state (called cookies) at HTTP user agents, letting the servers maintain a stateful session over the mostly stateless HTTP protocol. Although cookies have many historical infelicities that degrade their security and privacy, the Cookie and Set-Cookie header fields are widely used on the Internet. This document obsoletes RFC 2965. [STANDARDS-TRACK] Signing HTTP Messages Oracle Digital Bazaar When communicating over the Internet using the HTTP protocol, it can be desirable for a server or client to authenticate the sender of a particular message. It can also be desirable to ensure that the message was not tampered with during transit. This document describes a way for servers and clients to simultaneously add authentication and message integrity to HTTP messages by using a digital signature. A Method for Signing HTTP Requests for OAuth Ping Identity ARM Limited This document a method for offering data origin authentication and integrity protection of HTTP requests. To convey the relevant data items in the request a JSON-based encapsulation is used and the JSON Web Signature (JWS) technique is re-used. JWS offers integrity protection using symmetric as well as asymmetric cryptography. Authenticating Requests (AWS Signature Version 4) n.d. Seems Legit: Automated Analysis of Subtle Attacks on Protocols that Use Signatures University of Oxford CISPA Helmholtz Center for Information Security Independent Scholar Department of Computer Science, ETH Zurich The Transport Layer Security (TLS) Protocol Version 1.3 This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. Guidelines for Writing an IANA Considerations Section in RFCs Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226.

Detecting HTTP Message Signatures There have been many attempts to create signed HTTP messages in the past, including other non-standardized definitions of the Signature field, which is used within this specification. It is recommended that developers wishing to support both this specification and other historical drafts do so carefully and deliberately, as incompatibilities between this specification and various versions of other drafts could lead to unexpected problems. It is recommended that implementers first detect and validate the Signature-Input field defined in this specification to detect that this standard is in use and not an alternative. If the Signature-Input field is present, all Signature fields can be parsed and interpreted in the context of this specification.

Examples The following non-normative examples are provided as a means of testing implementations of HTTP Message Signatures. The signed messages given can be used to create the signature base with the stated parameters, creating signatures using the stated algorithms and keys. The private keys given can be used to generate signatures, though since several of the demonstrated algorithms are nondeterministic, the results of a signature are expected to be different from the exact bytes of the examples. The public keys given can be used to validate all signed examples.

Example Keys This section provides cryptographic keys that are referenced in example signatures throughout this document. These keys MUST NOT be used for any purpose other than testing. The key identifiers for each key are used throughout the examples in this specification. It is assumed for these examples that the signer and verifier can unambiguously dereference all key identifiers used here, and that the keys and algorithms used are appropriate for the context in which the signature is presented. The components for each private key in PEM format can be displayed by executing the following OpenSSL command: This command was tested with all the example keys on OpenSSL version 1.1.1m. Note that some systems cannot produce or use all of these keys directly, and may require additional processing. All keys are also made available in JWK format.

Example Key RSA test The following key is a 2048-bit RSA public and private key pair, referred to in this document as test-key-rsa. This key is encoded in PEM Format, with no encryption. The same public and private keypair in JWK format:

Example RSA PSS Key The following key is a 2048-bit RSA public and private key pair, referred to in this document as test-key-rsa-pss. This key is PCKS#8 encoded in PEM format, with no encryption. The same public and private keypair in JWK format:

Example ECC P-256 Test Key The following key is a public and private elliptical curve key pair over the curve P-256, referred to in this document as `test-key-ecc-p256. This key is encoded in PEM format, with no encryption. The same public and private keypair in JWK format:

Example Ed25519 Test Key The following key is an elliptical curve key over the Edwards curve ed25519, referred to in this document as test-key-ed25519. This key is PCKS#8 encoded in PEM format, with no encryption. The same public and private keypair in JWK format:

Example Shared Secret The following shared secret is 64 randomly-generated bytes encoded in Base64, referred to in this document as test-shared-secret.

Test Cases This section provides non-normative examples that may be used as test cases to validate implementation correctness. These examples are based on the following HTTP messages: For requests, this test-request message is used: For responses, this test-response message is used:

Minimal Signature Using rsa-pss-sha512 This example presents a minimal signature using the rsa-pss-sha512 algorithm over test-request, covering none of the components of the HTTP message, but providing a timestamped signature proof of possession of the key with a signer-provided nonce. The corresponding signature base is: This results in the following Signature-Input and Signature header fields being added to the message under the signature label sig-b21: Note that since the covered components list is empty, this signature could be applied by an attacker to an unrelated HTTP message. In this example, the nonce parameter is included to prevent the same signature from being replayed more than once, but if an attacker intercepts the signature and prevents its delivery to the verifier, the attacker could apply this signature to another message. Therefore, use of an empty covered components set is discouraged. See for more discussion. Note that the RSA PSS algorithm in use here is non-deterministic, meaning a different signature value will be created every time the algorithm is run. The signature value provided here can be validated against the given keys, but newly-generated signature values are not expected to match the example. See .

Selective Covered Components using rsa-pss-sha512 This example covers additional components (the authority, the Content-Digest header field, and a single named query parameter) in test-request using the rsa-pss-sha512 algorithm. This example also adds a tag parameter with the application-specific value of header-example. The corresponding signature base is: This results in the following Signature-Input and Signature header fields being added to the message under the label sig-b22: Note that the RSA PSS algorithm in use here is non-deterministic, meaning a different signature value will be created every time the algorithm is run. The signature value provided here can be validated against the given keys, but newly-generated signature values are not expected to match the example. See .

Full Coverage using rsa-pss-sha512 This example covers all applicable message components in test-request (including the content type and length) plus many derived components, again using the rsa-pss-sha512 algorithm. Note that the Host header field is not covered because the @authority derived component is included instead. The corresponding signature base is: This results in the following Signature-Input and Signature header fields being added to the message under the label sig-b23: Note in this example that the value of the Date header and the value of the created signature parameter need not be the same. This is due to the fact that the Date header is added when creating the HTTP Message and the created parameter is populated when creating the signature over that message, and these two times could vary. If the Date header is covered by the signature, it is up to the verifier to determine whether its value has to match that of the created parameter or not. See for more discussion. Note that the RSA PSS algorithm in use here is non-deterministic, meaning a different signature value will be created every time the algorithm is run. The signature value provided here can be validated against the given keys, but newly-generated signature values are not expected to match the example. See .

Signing a Response using ecdsa-p256-sha256 This example covers portions of the test-response response message using the ecdsa-p256-sha256 algorithm and the key test-key-ecc-p256. The corresponding signature base is: This results in the following Signature-Input and Signature header fields being added to the message under the label sig-b24: Note that the ECDSA algorithm in use here is non-deterministic, meaning a different signature value will be created every time the algorithm is run. The signature value provided here can be validated against the given keys, but newly-generated signature values are not expected to match the example. See .

Signing a Request using hmac-sha256 This example covers portions of the test-request using the hmac-sha256 algorithm and the secret test-shared-secret. The corresponding signature base is: This results in the following Signature-Input and Signature header fields being added to the message under the label sig-b25: Before using symmetric signatures in practice, see the discussion of the security tradeoffs in .

Signing a Request using ed25519 This example covers portions of the test-request using the ed25519 algorithm and the key test-key-ed25519. The corresponding signature base is: This results in the following Signature-Input and Signature header fields being added to the message under the label sig-b26:

TLS-Terminating Proxies In this example, there is a TLS-terminating reverse proxy sitting in front of the resource. The client does not sign the request but instead uses mutual TLS to make its call. The terminating proxy validates the TLS stream and injects a Client-Cert header according to , and then applies a signature to this field. By signing this header field, a reverse proxy can not only attest to its own validation of the initial request's TLS parameters but also authenticate itself to the backend system independently of the client's actions. The client makes the following request to the TLS terminating proxy using mutual TLS: The proxy processes the TLS connection and extracts the client's TLS certificate to a Client-Cert header field and passes it along to the internal service hosted at service.internal.example. This results in the following unsigned request: Without a signature, the internal service would need to trust that the incoming connection has the right information. By signing the Client-Cert header and other portions of the internal request, the internal service can be assured that the correct party, the trusted proxy, has processed the request and presented it to the correct service. The proxy's signature base consists of the following: This results in the following signature: Which results in the following signed request sent from the proxy to the internal service with the proxy's signature under the label ttrp: The internal service can validate the proxy's signature and therefore be able to trust that the client's certificate has been appropriately processed.

HTTP Message Transformations The HTTP protocol allows intermediaries and applications to transform an HTTP message without affecting the semantics of the message itself. HTTP message signatures are designed to be robust against many of these transformations in different circumstances. For example, the following HTTP request message has been signed using the ed25519 algorithm and the key test-key-ed25519. The signature base string for this message is: The following message has been altered by adding the Accept-Language header as well as adding a query parameter. However, since neither the Accept-Language header nor the query are covered by the signature, the same signature is still valid: The following message has been altered by removing the Date header, adding a Referer header, and collapsing the Accept header into a single line. The Date and Referrer headers are not covered by the signature, and the collapsing of the Accept header is an allowed transformation that is already accounted for by the canonicalization algorithm for HTTP field values. The same signature is still valid: The following message has been altered by re-ordering the field values of the original message, but not re-ordering the individual Accept headers. The same signature is still valid: The following message has been altered by changing the method to POST and the authority to "example.com" (inside the Host header). Since both the method and authority are covered by the signature, the same signature is NOT still valid: The following message has been altered by changing the order of the two instances of the Accept header. Since the order of fields with the same name is semantically significant in HTTP, this changes the value used in the signature base, and the same signature is NOT still valid:

Acknowledgements This specification was initially based on the internet draft. The editors would like to thank the authors of that draft, Mark Cavage and Manu Sporny, for their work on that draft and their continuing contributions. The specification also includes contributions from the internet draft and other similar efforts. The editors would also like to thank the following individuals for feedback, insight, and implementation of this draft and its predecessors (in alphabetical order): Mark Adamcin, Mark Allen, Paul Annesley, , , Sarven Capadisli, Liam Dennehy, Stephen Farrell, Phillip Hallam-Baker, Tyler Ham, Eric Holmes, Andrey Kislyuk, Adam Knight, Dave Lehn, Dave Longley, Ilari Liusvaara, James H. Manger, Kathleen Moriarty, Mark Nottingham, Yoav Nir, Adrian Palmer, Lucas Pardue, Roberto Polli, Julian Reschke, Michael Richardson, Wojciech Rygielski, Rich Salz, Adam Scarr, Cory J. Slep, Dirk Stein, Henry Story, Lukasz Szewc, Chris Webber, and Jeffrey Yasskin.

Document History RFC EDITOR: please remove this section before publication

draft-ietf-httpbis-message-signatures
- -17
  - Change encoding
  - Remove sign-the-signature examples and add explanations of why not to do that.
  - Query parameter values must be re-encoded for safety.
  - Query parameters now carry a warning of limitations.
  - Address field value encodings.
  - Discuss history and positioning of this specification.
  - Import obs-fold, reference US-ASCII, enforce ASCII-ness of Signature Base
- -16
  - Editorial cleanup from AD review.
  - Clarified dependency on structured field serialization rules.
  - Define use of all parameters in Accept-Signature.
  - Update example signature calculations.
  - Clarify how combined fields are handled.
  - Add more detailed instructions for IANA DE's.
  - Fix some references and anchors.
- -15
  - Editorial cleanup.
  - Defined "signature context".
- -14
  - Target raw non-decoded values for "@query" and "@path".
  - Add method for signing trailers.
  - Call out potential issues of list-based field values.
  - Update IANA registry for header fields.
  - Call out potential issues with Content-Digest in example.
  - Add JWK formats for all keys.
- -13
  - Renamed "context" parameter to "tag".
  - Added discussion on messages with multiple known contexts.
- -12
  - Added "context" parameter.
  - Added set of safe transformation examples.
  - Added ECDSA over P-384.
  - Expanded definiton of message component source context.
  - Sorted security considerations into categories.
- -11
  - Added ABNF references, coalesced ABNF rules.
  - Editorial and formatting fixes.
  - Update examples.
  - Added Byte Sequence field value wrapping.
- -10
  - Removed "related response" and "@request-response" in favor of generic "req" parameter.
  - Editorial fixes to comply with HTTP extension style guidelines.
  - Add security consideration on message content.
- -09
  - Explained key formats better.
  - Removed "host" and "date" from most examples.
  - Fixed query component generation.
  - Renamed "signature input" and "signature input string" to "signature base".
  - Added consideration for semantically equivalent field values.
- -08
  - Editorial fixes.
  - Changed "specialty component" to "derived component".
  - Expanded signature input generation and ABNF rules.
  - Added Ed25519 algorithm.
  - Clarified encoding of ECDSA signature.
  - Clarified use of non-deterministic algorithms.
- -07
  - Added security and privacy considerations.
  - Added pointers to algorithm values from definition sections.
  - Expanded IANA registry sections.
  - Clarified that the signing and verification algorithms take application requirements as inputs.
  - Defined "signature targets" of request, response, and related-response for specialty components.
- -06
  - Updated language for message components, including identifiers and values.
  - Clarified that Signature-Input and Signature are fields which can be used as headers or trailers.
  - Add "Accept-Signature" field and semantics for signature negotiation.
  - Define new specialty content identifiers, re-defined request-target identifier.
  - Added request-response binding.
- -05
  - Remove list prefixes.
  - Clarify signature algorithm parameters.
  - Update and fix examples.
  - Add examples for ECC and HMAC.
- -04
  - Moved signature component definitions up to intro.
  - Created formal function definitions for algorithms to fulfill.
  - Updated all examples.
  - Added nonce parameter field.
- -03
  - Clarified signing and verification processes.
  - Updated algorithm and key selection method.
  - Clearly defined core algorithm set.
  - Defined JOSE signature mapping process.
  - Removed legacy signature methods.
  - Define signature parameters separately from "signature" object model.
  - Define serialization values for signature-input header based on signature input.
- -02
  - Removed editorial comments on document sources.
  - Removed in-document issues list in favor of tracked issues.
  - Replaced unstructured Signature header with Signature-Input and Signature Dictionary Structured Header Fields.
  - Defined content identifiers for individual Dictionary members, e.g., "x-dictionary-field";key=member-name.
  - Defined content identifiers for first N members of a List, e.g., "x-list-field":prefix=4.
  - Fixed up examples.
  - Updated introduction now that it's adopted.
  - Defined specialty content identifiers and a means to extend them.
  - Required signature parameters to be included in signature.
  - Added guidance on backwards compatibility, detection, and use of signature methods.
- -01
  - Strengthened requirement for content identifiers for header fields to be lower-case (changed from SHOULD to MUST).
  - Added real example values for Creation Time and Expiration Time.
  - Minor editorial corrections and readability improvements.
- -00
  - Initialized from draft-richanna-http-message-signatures-00, following adoption by the working group.
draft-richanna-http-message-signatures
- -00
  - Converted to xml2rfc v3 and reformatted to comply with RFC style guides.
  - Removed Signature auth-scheme definition and related content.
  - Removed conflicting normative requirements for use of algorithm parameter. Now MUST NOT be relied upon.
  - Removed Extensions appendix.
  - Rewrote abstract and introduction to explain context and need, and challenges inherent in signing HTTP messages.
  - Rewrote and heavily expanded algorithm definition, retaining normative requirements.
  - Added definitions for key terms, referenced RFC 7230 for HTTP terms.
  - Added examples for canonicalization and signature generation steps.
  - Rewrote Signature header definition, retaining normative requirements.
  - Added default values for algorithm and expires parameters.
  - Rewrote HTTP Signature Algorithms registry definition. Added change control policy and registry template. Removed suggested URI.
  - Added IANA HTTP Signature Parameter registry.
  - Added additional normative and informative references.
  - Added Topics for Working Group Discussion section, to be removed prior to publication as an RFC.