]> Ericsson AB

john.mattsson@ericsson.com

Ericsson AB

goran.selander@ericsson.com

University of Glasgow

shahid.raza@glasgow.ac.uk

RISE AB

joel.hoglund@ri.se

IN Groupe

martin.furuhed@ingroupe.com

This document specifies a CBOR encoding of X.509 certificates. The resulting certificates are called C509 certificates. The CBOR encoding supports a large subset of RFC 5280, common certificate profiles and is extensible. Two types of C509 certificates are defined. One type is an invertible CBOR re-encoding of DER encoded X.509 certificates with the signature field copied from the DER encoding. The other type is identical except that the signature is over the CBOR encoding instead of the DER encoding, avoiding the use of ASN.1. Both types of certificates have the same semantics as X.509 and the same reduced size compared to X.509. The document also specifies CBOR encoded data structures for certificate (signing) requests and certificate request templates, new COSE headers, as well as a TLS certificate type and a file format for C509. This document updates RFC 6698; the TLSA selectors registry is extended to include C509 certificates. About This Document Status information for this document may be found at . Discussion of this document takes place on the CBOR Object Signing and Encryption Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction One of the challenges with deploying a Public Key Infrastructure (PKI) for the Internet of Things (IoT) is the size and parsing of X.509 public key certificates , since those are not optimized for constrained environments . Large certificate chains are also problematic in non-constrained protocols such as EAP-TLS where authenticators typically drop an EAP session after only 40–50 round-trips, QUIC where the latency increases significantly unless the server sends less than three times as many bytes as received prior to validating the client address, and RPKI where a single certificate can be very large. More compact certificate representations are therefore desirable in many use cases. Due to the current PKI usage of DER encoded X.509 certificates, keeping compatibility with DER encoded X.509 is necessary at least for a transition period. However, the use of a more compact encoding with the Concise Binary Object Representation (CBOR) reduces the certificate size significantly, which has known performance benefits in terms of decreased communication overhead, power consumption, latency, storage, etc. The use of CBOR also reduces code complexity, code size, memory usage, and CPU usage. CBOR is a data format designed for small code size and small message size. CBOR builds on the JSON data model but extends it by, e.g., encoding binary data directly without base64 conversion. In addition to the binary CBOR encoding, CBOR also has a diagnostic notation that is readable and editable by humans. The Concise Data Definition Language (CDDL) provides a way to express structures for protocol messages and APIs that use CBOR. RFC 8610 also extends the diagnostic notation. CBOR data items are encoded to or decoded from byte strings using a type-length-value encoding scheme, where the three highest order bits of the initial byte contain information about the major type. CBOR supports several different types of data items, in addition to integers (int, uint), simple values (e.g. null, undefined), byte strings (bytes), and text strings (text), CBOR also supports arrays [] of data items, maps {} of pairs of data items, and sequences of data items. For a complete specification and examples, see , , and . We recommend implementors to get used to CBOR by using the CBOR playground . CAB Baseline Requirements , RFC 7925 , IEEE 802.1AR , and CNSA 1.0 specify certificate profiles which can be applied to certificate based authentication with, e.g., TLS , QUIC , DTLS , COSE , EDHOC , or Compact TLS 1.3 . RFC 7925 , RFC7925bis , and IEEE 802.1AR specifically target Internet of Things deployments. This document specifies a CBOR encoding of X.509 certificates based on . The resulting certificates are called C509 certificates. The CBOR encoding supports a large subset of RFC 5280 and all certificates compatible with the RFC 7925, IEEE 802.1AR (DevID), CAB Baseline , , RPKI , eUICC profiled X.509 certificates, and is designed to render a compact encoding of certificates used in constrained environments. C509 is deployed in, e.g., in-vehicle and vehicle-to-cloud communication, Unmanned Aircraft Systems (UAS), and Global Navigation Satellite System (GNSS). When used to re-encode DER encoded X.509 certificates, the CBOR encoding can in many cases reduce the size of RFC 7925 profiled certificates by over 50% while also significantly reducing memory and code size compared to ASN.1. C509 is not a general CBOR ecoding for Abstract Syntax Notation One (ASN.1) data structures. C509 is designed to be extensible to additional features of X.509, for example support for new algorithms, including new post-quantum algorithms, which can be registered in the IANA registry as they become specified, see . This document does not specify a certificate profile. Two variants are defined using the same CBOR encoding and differing only in what is being signed:

1. An invertible CBOR re-encoding of DER encoded X.509 certificates , which can be reversed to obtain the original DER encoded X.509 certificate.
2. Natively signed C509 certificates, where the signature is calculated over the CBOR encoding instead of over the DER encoding as in the first variant. This removes the need for ASN.1 and DER parsing and the associated complexity but they are not backwards compatible with implementations requiring DER encoded X.509.

Natively signed C509 certificates can be applied in devices that are only required to authenticate to natively signed C509 certificate compatible servers, which is not a major restriction for many IoT deployments where the parties issuing and verifying certificates can be a restricted ecosystem. This document also specifies C509 Certificate Requests, see ; COSE headers for use of the C509 certificates with COSE, see ; a TLS certificate type for use of the C509 certificates with TLS and QUIC (with or without additional TLS certificate compression), see ; and a C509 file format. The TLSA selectors registry is extended to include C509 certificates, thus this document updates .

Notational Conventions 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. This specification makes use of the terminology in , , , , and . When referring to CBOR, this specification always refers to Deterministically Encoded CBOR as specified in Sections 4.2.1 and 4.2.2 of .

C509 Certificate This section specifies the content and encoding for C509 certificates, with the overall objective to produce a very compact representation supporting large parts of , and everything in , , RPKI , GSMA eUICC , and CAB Baseline . In the CBOR encoding, static fields are elided, elliptic curve points and time values are compressed, OID are replaced with short integers or complemented with CBOR OID encoding , and redundant encoding is removed. Combining these different components reduces the certificate size significantly, which is not possible with general purpose compression algorithms, see . The C509 certificate can be either a CBOR re-encoding of a DER encoded X.509 certificate, in which case the signature is calculated on the DER encoded ASN.1 data in the X.509 certificate, or a natively signed C509 certificate, in which case the signature is calculated directly on the CBOR encoded data. In both cases the certificate content is adhering to the restrictions given by . The re-encoding is known to work with DER encoded certificates but might work with other canonical encodings. The re-encoding does not work for BER encoded certificates. In the encoding described below, the elements in arrays are always encoded in the same order as elements of the corresponding SEQUENCE or SET in the DER encoding.

Message Fields The X.509 fields and their CBOR encodings are described in this section, and used in the definition of C509 certificates, see . The following Concise Data Definition Language (CDDL) defines the CBOR array C509Certificate and the CBOR sequence TBSCertificate. The member names therefore only have documentary value. Applications not requiring a CBOR item MAY represent C509 certificates with the CBOR sequence ~C509Certificate (unwrapped C509Certificate). Examples are given in the appendices, e.g., .

CDDL for C509Certificate.

C509 certificates are defined in terms of DER encoded X.509 certificates as detailed in the following subsections.

version The 'version' field is encoded in the 'c509CertificateType' CBOR int. The field 'c509CertificateType' also indicates the type of the C509 certificate. Two types are defined in this document: natively signed C509 certificates, following X.509 v3 (c509CertificateType = 2); and CBOR re-encoded X.509 v3 DER certificate (c509CertificateType = 3), see . The number of elements in TBSCertificate is fixed and determined by the type. Additional types may be added in the future.

serialNumber The 'serialNumber' INTEGER value field is encoded as the unwrapped CBOR unsigned bignum (~biguint) 'certificateSerialNumber'. Any leading 0x00 byte (to indicate that the number is not negative) is therefore omitted.

signature The 'signature' field, containing the signature algorithm including parameters, is encoded as a CBOR int (see ) or as an array with an unwrapped CBOR OID tag optionally followed by the parameters encoded as a CBOR byte string.

issuer In the general case, the sequence of 'Attribute' is encoded as a CBOR array consisting of Attribute elements. RelativeDistinguishedName with more than one AttributeTypeAndValue is not supported. Each Attribute is CBOR encoded as (type, value) either as a (int, SpecialText) pair, or a (~oid, bytes) tuple. In the former case, the absolute value of the int encodes the attribute type (see ) and the sign is used to represent the character string type in the X.509 certificate; positive for utf8String, negative for printableString. Attribute values which are always of type IA5String are unambiguously represented using a non-negative int. Examples include emailAddress and domainComponent (see ). In CBOR, all text strings are UTF-8 encoded and in natively signed C509 certificates all CBOR ints SHALL be non-negative. Text strings SHALL still adhere to any X.509 restrictions, i.e., serialNumber SHALL only contain the 74-character subset of ASCII allowed by printableString and countryName SHALL have length 2. CBOR encoding is allowed for IA5String (if this is the only allowed type, e.g., emailAddress), printableString and utf8String, whereas the string types teletexString, universalString, and bmpString are not supported. The text strings are further optimized as follows:

• If the text string has an even length 2 and contains only the symbols '0'–'9' or 'a'–'f', it is encoded as a CBOR byte string.
• If the text string contains an EUI-64 of the form "HH-HH-HH-HH-HH-HH-HH-HH" where each 'H' is one of the symbols '0'-'9' or 'A'-'F', it is encoded as a CBOR tagged MAC address using the CBOR tag 48, see . If of the form "HH-HH-HH-FF-FE-HH-HH-HH", it is encoded as a 48-bit MAC address, otherwise as a 64-bit MAC address. See example in .
• Otherwise it is encoded as a CBOR text string.

The final encoding of the extension value may therefore be text, bytes, or tag, i.e., SpecialText. If Name contains a single 'common name' attribute with attributeType = +1, it is for compactness encoded as just the SpecialText containing the single attribute value. In natively signed C509 certificates, bytes and tag 48 do not correspond to any predefined text string encoding and may also be used for other attribute types. If the 'issuer' field is identical to the 'subject' field, e.g., in case of self-signed certificates, then the 'issuer' field MUST be encoded as the CBOR simple value null (0xf6).

validity The 'notBefore' and 'notAfter' fields are encoded as unwrapped CBOR epoch-based date/time (~time) where the tag content is an unsigned integer. In POSIX time, leap seconds are ignored, with a leap second having the same POSIX time as the second before it. Compression of X.509 certificates with the time 23:59:60 UTC is therefore not supported. Note that RFC 5280 mandates encoding of dates through the year 2049 as UTCTime, and later dates as GeneralizedTime. The value "99991231235959Z" (no expiration date) is encoded as the CBOR simple value null.

subject The 'subject' field is encoded exactly like issuer, except that the CBOR simple value is not a valid value.

subjectPublicKeyInfo The 'AlgorithmIdentifier' field including parameters is encoded as the CBOR int 'subjectPublicKeyAlgorithm' (see ) or as an array with an unwrapped CBOR OID tag optionally followed by the parameters encoded as a CBOR byte string. In general, the 'subjectPublicKey' BIT STRING value field is encoded as a CBOR byte string, but may be encoded as a CBOR item of any type except undefined (see ). This specification assumes the BIT STRING has zero unused bits, and the unused bits byte is omitted. For rsaEncryption and id-ecPublicKey, the encoding of subjectPublicKey is further optimized as described in .

issuerUniqueID Not supported.

subjectUniqueID Not supported.

extensions The 'extensions' field is encoded either as a CBOR array or as a CBOR int. An omitted 'extensions' field is encoded as an empty CBOR array. Each 'extensionID' in the CBOR array is encoded either as a CBOR int (see ) or as an unwrapped CBOR OID tag .

• If 'extensionID' is encoded as a CBOR int, it is followed by a CBOR item of any type except undefined (see ), and the sign of the int is used to encode if the extension is critical: Critical extensions are encoded with a negative sign and non-critical extensions are encoded with a positive sign. If the CBOR array contains exactly two ints and the absolute value of the first int is 2 (corresponding to keyUsage, see ), the CBOR array is omitted and the extensions is encoded as a single CBOR int with the absolute value of the second int and the sign of the first int.
• If extensionID is encoded as an unwrapped CBOR OID tag, then it is followed by an optional CBOR simple value true (0xf5) 'critical', and the DER-encoded value of the extnValue. The presence of the CBOR true value in the array indicates that the extension is critical; its absence means that the extension is non-critical (see ). The extnValue OCTET STRING value field is encoded as the CBOR byte string 'extensionValue'.

The processing of critical and non-critical extensions is specified in . The currently defined extension values for which there is CBOR int encoded 'extensionID' are specified in . The extensions mandated to be supported by and are given special treatment. More details about extensions in .

signatureAlgorithm The 'signatureAlgorithm' field is always the same as the 'signature' field and therefore omitted from the CBOR encoding.

signatureValue In general, the 'signatureValue' BIT STRING value field is encoded as the CBOR byte string issuerSignatureValue. This specification assumes that the BIT STRING has zero unused bits, and the unused bits byte is omitted. For natively signed C509 certificates, the signatureValue is calculated over the CBOR sequence TBSCertificate. For ECDSA, the encoding of issuerSignatureValue is further optimized as described in

Encoding of subjectPublicKey and issuerSignatureValue

Encoding of subjectPublicKey For RSA public keys (rsaEncryption), the SEQUENCE and INTEGER type and length fields are omitted, and the two INTEGER value fields (modulus, exponent) are encoded as an array of two unwrapped CBOR unsigned bignum (~biguint), i.e., [ modulus : ~biguint, exponent : ~biguint ]. If the exponent is 65537, the array and the exponent are omitted and subjectPublicKey consists of only the modulus encoded as an unwrapped CBOR unsigned bignum (~biguint). For elliptic curve public keys in Weierstraß form (id-ecPublicKey), keys may be point compressed as defined in Section 2.3.3 of . Native C509 certificates with Weierstraß form keys use the octets 0x02, 0x03, and 0x04 as defined in . If a DER encoded certificate with an uncompressed public key of type id-ecPublicKey is CBOR encoded with point compression, then the octet 0xfe is used instead of 0x02 to represent an even y-coordinate, and the octet 0xfd is used instead of 0x03 to represent an odd y-coordinate.

Encoding of issuerSignatureValue For ECDSA signatures, the SEQUENCE and INTEGER type and length fields as well as any leading 0x00 byte (to indicate that the number is not negative) are omitted. Each of the two INTEGER value fields are then padded with leading zeroes to the same fixed length, given by the number of bytes needed to represent the order n of the cyclic subgroup used with the algorithm. For example, for P-256, the number of bytes for each integer is 32. The resulting byte string is encoded as a CBOR byte string.

Encoding of Extensions The 'extensions' field is encoded as specified in with further details provided in this section. For some extensions, the CBOR int encoded extensionID is only supported for commonly used values of the extension. In case of extension values for which the CBOR int encoded extensionID is not supported, the extension MUST be encoded using the unwrapped CBOR OID tag encoded extensionID. A note on extensionID naming: in existing OID databases, most IDs can be found in versions with and without an 'id-pe' or 'id-ce' prefix. We have excluded the prefix for the commonly used extensions defined in and included them for extensions defined elsewhere. CBOR encoding of the following extension values is fully supported:

• Subject Key Identifier (subjectKeyIdentifier). In natively signed certificates, KeyIdentifier SHOULD be composed of the leftmost 160-bits of the SHA-256 hash of the CBOR encoded subjectPublicKey. Other methods of generating unique numbers can be used. The extensionValue is encoded as follows:

• Key Usage (keyUsage). The 'KeyUsage' BIT STRING is interpreted as an unsigned integer in network byte order and encoded as a CBOR int. See for special encoding in case keyUsage is the only extension present.

• Policy Mappings (policyMappings). extensionValue is encoded as follows:

• Basic Constraints (basicConstraints). If 'cA' = false then extensionValue = -2, if 'cA' = true and 'pathLenConstraint' is not present then extensionValue = -1, and if 'cA' = true and 'pathLenConstraint' is present then extensionValue = pathLenConstraint.

• Policy Constraints (policyConstraints). extensionValue is encoded as follows:

• Extended Key Usage (extKeyUsage). extensionValue is encoded as an array of CBOR ints (see ), or unwrapped CBOR OID tags , where each int or OID encodes a key usage purpose. If the array contains a single KeyPurposeId, the array is omitted.

• Inhibit anyPolicy (inhibitAnyPolicy). extensionValue is encoded as follows:

CBOR encoding of the following extension values are partly supported:

• Subject Alternative Name (subjectAltName). If the subject alternative name only contains general names registered in the extension value can be CBOR encoded. extensionValue is encoded as an array of (int, any) pairs where each pair encodes a general name (see ). If subjectAltName contains exactly one dNSName, the array and the int are omitted and extensionValue is the dNSName encoded as a CBOR text string. In addition to the general names defined in , the hardwareModuleName type of otherName has been given its own int due to its mandatory use in IEEE 802.1AR. When 'otherName + hardwareModuleName' is used, then [ ~oid, bytes ] is used to contain the pair ( hwType, hwSerialNum ) directly as specified in . Only the general names in are supported.

• Issuer Alternative Name (issuerAltName). extensionValue is encoded exactly like subjectAltName.

• CRL Distribution Points (cRLDistributionPoints). If the CRL Distribution Points is a sequence of DistributionPointName, where each DistributionPointName only contains uniformResourceIdentifiers, the extension value can be CBOR encoded. extensionValue is encoded as follows:

• Freshest CRL (freshestCRL). extensionValue is encoded exactly like cRLDistributionPoints.

• Authority Information Access (authorityInfoAccess). If all the GeneralNames in authorityInfoAccess are of type uniformResourceIdentifier, the extension value can be CBOR encoded. Each accessMethod is encoded as a CBOR int (see ) or an unwrapped CBOR OID tag . The uniformResourceIdentifiers are encoded as CBOR text strings.

• Subject Information Access (subjectInfoAccess). Encoded exactly like authorityInfoAccess.

• Authority Key Identifier (authorityKeyIdentifier). If the authority key identifier contains all of keyIdentifier, certIssuer, and certSerialNumber or if only keyIdentifier is present the extension value can be CBOR encoded. If all three are present a CBOR array is used, if only keyIdentifier is present, the array is omitted:

• Certificate Policies (certificatePolicies). If noticeRef is not used and any explicitText are encoded as UTF8String, the extension value can be CBOR encoded. OIDs registered in are encoded as an int. The policyQualifierId is encoded as an CBOR int (see ) or an unwrapped CBOR OID tag .

• Name Constraints (nameConstraints). If the name constraints only contain general names registered in the extension value can be CBOR encoded. C509 uses the same additions and restrictions as defined in . Note that the minimum and maximum fields are not used and therefore omitted. For IPv4 addresses, the iPAddress field MUST contain five octets and for IPv6 addresses, the field MUST contain 17 octets, where the last octet indicates the number of bits in the netmask. As an example, the address block 192.0.2.0/24 is encoded as C0 00 02 00 18 instead of C0 00 02 00 FF FF FF 00 as in the DER encoding.

• Subject Directory Attributes (subjectDirectoryAttributes). Encoded as attributes in issuer and subject with the difference that there can be more than one attributeValue.

• AS Resources (id-pe-autonomousSysIds). If rdi is not present, the extension value can be CBOR encoded. Each ASId is encoded as an uint. With the exception of the first ASId, the ASid is encoded as the difference to the previous ASid.

• AS Resources v2 (id-pe-autonomousSysIds-v2). Encoded exactly like autonomousSysIds.
• IP Resources (id-pe-ipAddrBlocks). If rdi and SAFI are not present, the extension value can be CBOR encoded. Each AddressPrefix is encoded as a CBOR bytes string (without the unused bits octet) followed by the number of unused bits encoded as a CBOR uint. Each AddressRange is encoded as an array of two CBOR byte strings. The unused bits for min and max are omitted, but the unused bits in max IPAddress are set to one. With the exception of the first Address, if the byte string has the same length as the previous Address, the Address is encoded as a uint with the difference to the previous Address. It should be noted that using address differences for compactness prevents encoding an address range larger than 2⁶⁴ - 1 corresponding to the CBOR integer max value.

• IP Resources v2 (id-pe-ipAddrBlocks-v2). Encoded exactly like id-pe-ipAddrBlocks.
• Signed Certificate Timestamp (Certificate Transparency). If all the SCTs are version v1 , and there are no SCT extensions, the extension value can be CBOR encoded. Other versions of SCT are out of scope for this document. LogIDs are encoded as CBOR byte strings, the timestamp is encoded as a CBOR uint (milliseconds since validityNotBefore), and the signature is encoded with an (AlgorithmIdentifier, any) pair in the same way as issuerSignatureAlgorithm and issuerSignatureValue.

• OCSP No Check (id-pkix-ocsp-nocheck). If the extension value is NULL, it can be CBOR encoded. The CBOR encoded extensionValue is the value null.
• Precertificate Signing Certificate. The CBOR encoded extensionValue is the value null.
• TLS Features (id-pe-tlsfeature). The extensionValue is encoded as an array of integers, where each integer represents a TLS extension.

Example Encoding of Extensions The examples below use values from , , and :

• A critical basicConstraints ('cA' = true) without pathLenConstraint is encoded as the two CBOR ints -4, -1.
• A non-critical keyUsage with digitalSignature (0), nonRepudiation (1), keyEncipherment (2) and keyAgreement (4) asserted is encoded as the two CBOR ints 2, 23 (2⁰ + 2¹ + 2² + 2⁴ = 23).
• A non-critical extKeyUsage containing id-kp-codeSigning and id-kp-OCSPSigning is encoded as the CBOR int 8 followed by the CBOR array [ 3, 9 ].
• A non-critical subjectAltName containing only the dNSName example.com is encoded as the CBOR int 3 followed by the CBOR text string "example.com".

Thus, the extension field of a certificate containing all of the above extensions in the given order would be encoded as the CBOR array [ -4, -1, 2, 23, 8, [ 3, 9 ], 3, "example.com" ].

COSE Header Parameters The formatting and processing for c5b, c5c, c5t, and c5u, defined in below, are similar to x5bag, x5chain, x5t, x5u defined in except that the certificates are C509 instead of DER encoded X.509 and use a COSE_C509 structure instead of COSE_X509. The COSE_C509 structure used in c5b, c5c, and c5u is defined as: C509CertData thus includes the unwrapped CBOR sequence, ~C509Certificate. The byte string encoding includes the length of each certificate which simplifies parsing. See for an example. The COSE_C509 item has media type application/cose-c509-cert, see . Different CoAP Content-Formats are defined depending on "usage" = "chain" or not, see . Stored file formats are defined for the cases with/without ("usage" = "chain") with "magic numbers" TBD8/TBD6 using the reserved CBOR tag 55799 and the corresponding Content-Formats TBD15/TBD3, enveloped as described in . The value type of c5t is the COSE_CertHash structure defined in , which contains the hash value of the C509 certificate calculated over ~C509Certificate. Thus C509CertData contains all data necessary to calculate the thumbprint c5t. c5u provides an alternative way to identify an untrusted certificate chain by reference with a URI , encoded as a CBOR text string (media type application/cbor and CoAP Content-Format 60). The referenced resource is a COSE_C509 item served with the application/cose-c509-cert media type ("usage" = "chain"), as described above. As the contents of c5b, c5c, c5t, and c5u are untrusted input, the header parameters can be in either the protected or unprotected header bucket. The trust mechanism MUST process any certificates in the c5b, c5c, and c5u parameters as untrusted input. The presence of a self-signed certificate in the parameter MUST NOT cause the update of the set of trust anchors without appropriate authorization. COSE Header Parameters

Name	Label	Value Type	Description
c5b	24	COSE_C509	An unordered bag of C509 certificates
c5c	25	COSE_C509	An ordered chain of C509 certificates
c5t	22	COSE_CertHash	Hash of a ~C509Certificate
c5u	23	uri	URI pointing to a COSE_C509 containing an ordered chain of certificates

Note that certificates can also be identified with a 'kid' header parameter by storing 'kid' and the associated bag or chain in a dictionary.

Private Key Structures Certificate management also makes use of data structures including private keys, see, e.g., . This section defines the following CBOR encoded structures: The field 'C509PrivateKeyType' indicates the type of the C509 private key. Different types of C509 Private Key Structures can be defined, see . Currently, two types are defined. When C509PrivateKeyType = 0, the subjectPrivateKey is the CBOR byte string encoding of the PrivateKey OCTET STRING value field defined in . When C509PrivateKeyType = 1, the subjectPrivateKey is a COSE_KEY structure containing a private key as defined in . Note that COSE_KEY might not be possible to use with all algorithms that have a C509 AlgorithmIdentifier defined. The C509PrivateKey item is served with the application/cose-c509-privkey media type, see , with corresponding CoAP Content-Format defined in . A stored file format is defined with "magic number" TBD12 using the reserved CBOR tag 55799 and the Content-Format TBD10, enveloped as described in . The C509PEM item is served with the application/cose-c509-pem media type, see , with corresponding CoAP Content-Format defined in . A stored file format is defined with "magic number" TBD13 using the reserved CBOR tag 55799 and the Content-Format TBD11, enveloped as described in .

Deterministic Encoding In some use cases it is desirable to be able to specify a unique C509 representation of a given X.509 certificate. While this specification requires the use of Deterministically Encoded CBOR (see ), it is still possible to represent certain X.509 certificate fields in different ways. This is a consequence of the extensibility of the C509 format where new encodings can be defined, for example to optimize extensions for which no special CBOR encoding have been previously defined. Where there is support for a specific and a generic CBOR encoding, the specific CBOR encoding MUST be used. For example, when there is support for specific CBOR encoding of an extension, as specified in and the C509 Extensions Registry, it MUST be used. In particular, when there is support for a specific otherName encoding (negative integer value in C509 General Names Registry) it MUST be used. Native C509 certificates MUST only use specific CBOR encoded fields. However, when decoding a non-native C509 certificates, the decoder may need to support, for example, (extensionID:~oid, ? critical: true, extensionValue:bytes)-encoding of an extension for which there is an (extensionID:int, extensionValue:Defined)-encoding. One reason is that the certificate was issued before the specific CBOR extension was registered.

C509 Certificate (Signing) Request This section defines the format of a C509 Certificate Request, also known as a C509 Certificate Signing Request (CSR), based on and compatible with RFC 2986 , and reusing the formatting of C509 certificates defined in . The CDDL for the C509 Certificate Request is shown in . The fields have the same encoding as the corresponding fields of the C509 Certificate, see .

CDDL for C509CertificateRequest.

After verifying the subjectSignatureValue, the CA MAY transform the C509CertificateRequest into a CertificationRequestInfo for compatibility with existing procedures and code. The media type of C509CertificateRequest is application/cose-c509-pkcs10, see , with corresponding CoAP Content-Format defined in . The "magic number" TBD9 is defined using the reserved CBOR tag 55799 and the Content-Format TBD4, enveloped as described in .

Certificate Request Types Different types of C509 Certificate Requests are defined, see , all using the same CBOR encoding and differing only in what is being signed and what type of C509 certificate is being requested:

• The C509 Certificate Request can either be an invertible CBOR re-encoding of a DER encoded RFC 2986 certification request, or it can be natively signed where the signature is calculated over the CBOR encoding instead of the DER encoding.
• The requested C509 certificate in the C509 Certificate Request can either be of type 2 or of type 3, see .

Combining these options enables the four instances of c509CertificateRequestType defined in and illustrated in .

C509 Certificate Request Types 0, 1, 2 and 3.

An implementation MAY only support certain values of c509CertificateRequestType. The most common values are expected to be:

• c509CertificateRequestType = 0. This type indicates that the C509 Certificate Request is natively signed, and that the requested certificate format has c509CertificateType = 2. This encoding removes the need for ASN.1 and DER parsing and re-encoding in the requesting party.
• c509CertificateRequestType = 3. This type indicates that the C509 Certificate Request is CBOR re-encoded RFC 2986 certification requests, and that the requested certificate format has c509CertificateType = 3. This encoding is backwards compatible with legacy RFC 2986 certification requests and X.509 certificates, but enables a reduced transport overhead.

Subject Signature Algorithm subjectSignatureAlgorithm can be a signature algorithm or a non-signature proof-of-possession algorithm, e.g., as defined in . In the case of , the signature is replaced by a MAC and requires a public Diffie-Hellman key of the verifier distributed out-of-band. Both signature algorithms and non-signature proof-of-possession algorithms are listed in the C509 Signature Algorithms Registry, see . The non-signature roof-of-possession algorithms with SHA-2 and HMAC-SHA2 (see values 14-16 in ) requires a signature value with syntax DhSigStatic defined as follows: Note that a key agreement key pair may be used with a signature algorithm in a certificate request, see .

Certificate Request Attributes specifies two attribute types that may be included in the certificate request: extension request and challenge password.

Extensions Request The extensionRequest field is used to carry information about certificate extensions the entity requesting certification wishes to be included in the certificate, encoded as Extensions in . An empty CBOR array indicates no extensions.

Challenge Password Other certificate request attributes are included using the Extensions structure and the extensionRequest field. The only other certificate request attribute specified in this document is challengePassword, listed in the C509 Extensions Registry, see . The extensionValue is encoded as follows: In natively signed requests (types 0 and 2), a positive extensionID is used. In CBOR re-encoding of a DER encoded request (types 1 and 3), the sign of extensionID of challengePassword indicates the string type in the DER encoded challengePassword (instead of the criticalness in extensions): positive for utf8String and negative for printableString. The same text string encoding optimizations applies as in .

Certificate Request Template Enrollment over Secure Transport (EST, ) defines, and clarifies, how an EST server can specify what it expects the EST client to include in a subsequent Certificate Signing Request (CSR). Alternatively to the unstructured mechanism specified in , describes an approach using a Certificate Request Template in response to a GET /csrattrs request by the EST client. The EST server thus returns an Certificate Request-like object with various fields filled out, and other fields waiting to be filled in and a signature to be added by the EST client. For C509 we follow the approach of . The C509CertificateRequestTemplate is based on TBSCertificateRequest of the C509CertificateRequest, see , but excludes the subjectSignatureValue field from the template since that needs no further specification. The C509 Certificate Request Template is shown in .

CDDL for C509CertificateRequestTemplate.

Except as specified in this section, the fields have the same encoding as the corresponding fields of the TBSCertificateRequest, see . The specification of the template makes use of the CBOR simple value undefined (0xf7) to indicate fields to fill in. Consistent with this rule, note that the subjectPublicKey field always has the value undefined in the template. Different types of Certificate Request Templates can be defined (see ), distinguished by the c509CertificateRequestTemplateType integer. Each type may have its own CDDL structure. The presence of a Defined (non-undefined) value in a C509CertificateRequestTemplate indicates that the EST server expects the EST client to use that value in the certificate request. If multiple AlgorithmIdentifier or c509CertificateRequestType values are present, the EST server expects the EST client to select one of them for use in the Certificate Request. The presence of an undefined value indicates that the EST client is expected to provide an appropriate value for that field. For example, if the EST server includes a subjectAltName with a GeneralNameType iPAddress and a GeneralNameValue empty byte string, this means that the client SHOULD fill in a corresponding GeneralNameValue. The media type of C509CertificateRequestTemplate is application/cose-c509-crtemplate, see , with corresponding CoAP Content-Format defined in . The "magic number" TBD18 is defined using the reserved CBOR tag 55799 and the Content-Format TBD19, enveloped as described in .

C509 Processing and Certificate Issuance It is straightforward to integrate the C509 format into legacy X.509 processing during certificate issuance. C509 processing can be performed as an isolated function of the CA, or as a separate function trusted by the CA. The Certificate Request format defined in follows the PKCS#10 format to enable a direct mapping to the certification request information, see Section 4.1 of . The CA can make use of a Certificate Request Template defined in , for simplified configuration. When a certificate request is received, the CA, or function trusted by the CA, needs to perform some limited C509 processing and verify the proof-of-possession corresponding to the public key, before normal certificate generation can take place. In the reverse direction, in case c509CertificateType = 3 was requested, a separate C509 processing function can perform the conversion from a generated X.509 certificate to C509 as a bump-in-the-wire. In case c509CertificateType = 2 was requested, the C509 processing needs to be performed before signing the certificate, in which case a tighter integration with the CA may be needed.

Legacy Considerations C509 certificates can be deployed with legacy X.509 certificates and CA infrastructure. An existing CA can continue to use its existing procedures and code for PKCS#10, and DER encoded X.509 and only implement C509 as a thin processing layer on top. When receiving a C509 CSR, the CA transforms it into a DER encoded RFC 2986 CertificationRequestInfo and uses that with existing processes and code to produce an RFC 5280 DER encoded X.509 certificate. The DER encoded X.509 is then transformed into a C509 certificate. At any later point, the C509 certificate can be used to recreate the original X.509 data structure needed to verify the signature. For protocols like TLS/DTLS 1.2, where certificates are sent unencrypted, the actual encoding and compression can be done at different locations depending on the deployment setting. For example, the mapping between C509 certificate and standard X.509 certificate can take place in a 6LoWPAN border gateway, which allows the server side to stay unmodified. This case gives the advantage of the low overhead of a C509 certificate over constrained wireless links. The conversion to X.509 within a constrained IoT device will incur a computational overhead. However, measured in energy, this is likely to be negligible compared to the reduced communication overhead. For the setting with constrained server and server-only authentication, the server only needs to be provisioned with the C509 certificate and does not perform the conversion to X.509. This option is viable when client authentication can be asserted by other means. For protocols like IKEv2, TLS/DTLS 1.3, and EDHOC, where certificates are encrypted, the proposed encoding needs to be done fully end-to-end, through adding the encoding/decoding functionality to the server.

Expected Certificate Sizes The CBOR encoding of the sample certificate chains given in results in the numbers shown in Figures and . COSE_X509 is defined in and COSE_C509 is defined in . After RFC 7925 profiling, most duplicated information has been removed, and the remaining text strings are minimal in size. Therefore, the further size reduction reached with general compression mechanisms such as Brotli will be small, mainly corresponding to making the ASN.1 encoding more compact. CBOR encoding can however significantly compress RFC 7925 profiled certificates. In the examples with HTTPS certificate chains (www.ietf.org and tools.ietf.org) both C509 and Brotli perform well complementing each other. C509 uses dedicated information to compress individual certificates, while Brotli can compress duplicate information in the entire chain. Note that C509 certificates of type 2 and 3 have the same size. For Brotli, the Rust crate Brotli 3.3.0 was used with compression level 11 and window size 22.

Comparing Sizes of Certificate Chains in COSE. Number of bytes (length of certificate chain).

Comparing Sizes of Certificate Chains with TLS. Number of bytes (length of certificate chain). X.509 and C509 are Certificate messages. X.509 + Brotli and C509 + Brotli are CompressedCertificate messages.

Security Considerations The CBOR encoding of X.509 certificates does not change the security assumptions needed when deploying standard X.509 certificates but decreases the number of fields transmitted, which reduces the risk for implementation errors. The security considerations of apply. The use of natively signed C509 certificates removes the need for ASN.1 encoding, which is a rich source of security vulnerabilities. Conversion between the certificate formats can be made in constant time to reduce risk of information leakage through side channels. The mechanism in this document does not reveal any additional information compared to X.509. Because of the difference in size, it will be possible to detect that this profile is used. The gateway solution described in requires unencrypted certificates and is not recommended. Any issues with decoding or parsing a C509 certificate should be handled exactly as how such errors would be handled for the corresponding X.509 certificate. For example, a non-critical extension MAY be ignored if it is not recognized, see . As stated in , the contents of the COSE Header Parameters c5b, c5c, c5t, c5u is untrusted input that potentially may be verified using existing trust anchors or other trust establishment mechanism out of scope of this document. Similar security considerations as x5bag, x5chain, x5t and x5u applies, see . Security considerations of the COSE protected and unprotected headers is discussed in .

IANA Considerations This document creates several new registries under the new heading "CBOR Encoded X.509 (C509) Parameters". For all items, the 'Reference' field points to this document. The expert reviewers for the registries defined in this document are expected to ensure that the usage solves a valid use case that could not be solved better in a different way, that it is not going to duplicate an entry that is already registered, and that the registered point is likely to be used in deployments. They are furthermore expected to check the clarity of purpose and use of the requested code points. Experts should take into account the expected usage of entries when approving point assignment, and the length of the encoded value should be weighed against the number of code points left that encode to that size and how constrained the systems it will be used on are. Values in the interval [-24, 23] have a 1-byte encoding, other values in the interval [-256, 255] have a 2-byte encoding, and the remaining values in the interval [-65536, 65535] have a 3-byte encoding. All assignments according to "IETF Review with Expert Review" are made on a "IETF Review" basis per Section 4.8 of with "Expert Review" additionally required per Section 4.5 of . The procedure for early IANA allocation of "standards track code points" defined in also applies. When such a procedure is used, IANA will ask the designated expert(s) to approve the early allocation before registration. In addition, working group chairs are encouraged to consult the expert(s) early during the process outlined in Section 3.1 of .

C509 Certificate Types Registry IANA has created a new registry titled "C509 Certificate Types" under the new heading "CBOR Encoded X.509 (C509) Parameters". The fields of the registry are Value, Description, and Reference, where Value is an integer, and the other columns are text strings. It is mandatory to specify content in all columns. For values in the interval [-24, 23], the registration procedure is "IETF Review with Expert Review". For all other values, the registration procedure is "Expert Review". The initial contents of the registry are (see ):

C509 Certificate Types

C509 Certificate Request Types Registry IANA has created a new registry titled "C509 Certificate Request Types" under the new heading "CBOR Encoded X.509 (C509) Parameters". The fields of the registry are Value, Description, and Reference, where Value is an integer, and the other columns are text strings. All columns are mandatory. For values in the interval [-24, 23] the registration procedure is "IETF Review with Expert Review". For all other values the registration procedure is "Expert Review". The initial contents of the registry are:

C509 Certificate Request Types

C509 Private Key Types Registry IANA has created a new registry titled "C509 Private Key Types" under the new heading "CBOR Encoded X.509 (C509) Parameters". The fields of the registry are Value, Comments, and subjectPrivateKey, and Reference, where Value is an integer, and the other columns are text strings. All columns are mandatory. For values in the interval [-24, 23] the registration procedure is "IETF Review with Expert Review". For all other values the registration procedure is "Expert Review". The initial contents of the registry are:

C509 Private Key Types

C509 Certificate Request Templates Types Registry IANA has created a new registry titled "C509 Certificate Request Templates Types" under the new heading "CBOR Encoded X.509 (C509) Parameters". The columns of the registry are Value, Description, and Reference, where Value is an integer, and the other columns are text strings. All columns are mandatory. For values in the interval [-24, 23] the registration procedure is "IETF Review" and "Expert Review". For all other values the registration procedure is "Expert Review". The initial contents of the registry are:

C509 Certificate Request Templates Types

C509 Attributes Registry IANA has created a new registry titled "C509 Attributes" under the new heading "CBOR Encoded X.509 (C509) Parameters". The fields of the registry are Value, Name, Identifiers, OID, DER, Comments, and Reference, where Value is a non-negative integer, and the other columns are text strings. Name and Identifiers are informal descriptions. The fields Name, OID, and DER are mandatory. For values in the interval [0, 23] the registration procedure is "IETF Review with Expert Review". Values 32768 are reserved for Private Use. For all other values the registration procedure is "Expert Review". Name and Identifiers are informal descriptions. The OID is given in dotted decimal representation. The DER column contains the hex string of the DER-encoded OID . The initial contents of the registry are:

C509 Attributes

C509 Extensions Registry IANA has created a new registry titled "C509 Extensions Registry" under the new heading "CBOR Encoded X.509 (C509) Parameters". The fields of the registry are Value, Name, Identifiers, OID, DER, Comments, extensionValue, and Reference, where Value is a positive integer, and the other columns are text strings. The fields Name, OID, DER, abd extensionValue are mandatory. The registry also contains certificate request attributes for use in Certificate Requests, see . For values in the interval [1, 23] the registration procedure is "IETF Review with Expert Review". Values 32768 are reserved for Private Use. For all other values the registration procedure is "Expert Review". The initial contents of the registry are:

C509 Extensions and Certificate Request Attributes

C509 Certificate Policies Registry IANA has created a new registry titled "C509 Certificate Policies Registry" under the new heading "CBOR Encoded X.509 (C509) Parameters". The fields of the registry are Value, Name, Identifiers, OID, DER, Comments, and Reference, where Value is an integer, and the other columns are text strings. The fields Name, OID, and DER are mandatory. For values in the interval [-24, 23] the registration procedure is "IETF Review with Expert Review". Values 32768 are reserved for Private Use. For all other values the registration procedure is "Expert Review". The initial contents of the registry are:

C509 Certificate Policies

C509 Policies Qualifiers Registry IANA has created a new registry titled "C509 Policies Qualifiers Registry" under the new heading "CBOR Encoded X.509 (C509) Parameters". The fields of the registry are Value, Name, Identifiers, OID, DER, Comments, and Reference, where Value is an integer, and the other columns are text strings. The fields Name, OID, and DER are mandatory. For values in the interval [-24, 23] the registration procedure is "IETF Review with Expert Review". Values 32768 are reserved for Private Use. For all other values the registration procedure is "Expert Review". The initial contents of the registry are:

C509 Policies Qualifiers

C509 Information Access Registry IANA has created a new registry titled "C509 Information Access Registry" under the new heading "CBOR Encoded X.509 (C509) Parameters". The fields of the registry are Value, Name, Identifiers, OID, DER, Comments, and Reference, where Value is an integer, and the other columns are text strings. The fields Name, OID, and DER are mandatory. For values in the interval [-24, 23] the registration procedure is "IETF Review with Expert Review". For all other values the registration procedure is "Expert Review". The initial contents of the registry are:

C509 Information Accesses

C509 Extended Key Usages Registry IANA has created a new registry titled "C509 Extended Key Usages Registry" under the new heading "CBOR Encoded X.509 (C509) Parameters". The fields of the registry are Value, Name, Identifiers, OID, DER, Comments, and Reference, where Value is an integer, and the other columns are text strings. The fields Name, OID, and DER are mandatory. For values in the interval [-24, 23] the registration procedure is "IETF Review with Expert Review". Values 32768 are reserved for Private Use. For all other values the registration procedure is "Expert Review". The initial contents of the registry are:

C509 Extended Key Usages

C509 General Names Registry IANA has created a new registry titled "C509 General Names Registry" under the new heading "CBOR Encoded X.509 (C509) Parameters". The fields of the registry are Value, General Name, and Reference, where Value is an integer, and the other columns are text strings. The fields Name and Value are mandatory. For values in the interval [-24, 23] the registration procedure is "IETF Review with Expert Review". For all other values the registration procedure is "Expert Review". The initial contents of the registry are:

C509 General Names

C509 Signature Algorithms Registry IANA has created a new registry titled "C509 Signature Algorithms" under the new heading "CBOR Encoded X.509 (C509) Parameters". The registry includes both signature algorithms and non-signature proof-of-possession algorithms. The fields of the registry are Value, Name, Identifiers, OID, Parameters, DER, Comments, and Reference, where Value is an integer, and the other columns are text strings. The fields Name, OID, Parameters, and DER are mandatory. For values in the interval [-24, 23] the registration procedure is "IETF Review with Expert Review". For all other values the registration procedure is "Expert Review". The initial contents of the registry are:

C509 Signature Algorithms

C509 Public Key Algorithms Registry IANA has created a new registry titled "C509 Public Key Algorithms" under the new heading "CBOR Encoded X.509 (C509) Parameters". The fields of the registry are Value, Name, Identifiers, OID, Parameters, DER, Comments, and Reference, where Value is an integer, and the other columns are text strings. The fields Name, OID, Parameters, and DER are mandatory. For values in the interval [-24, 23] the registration procedure is "IETF Review with Expert Review". For all other values the registration procedure is "Expert Review". The initial contents of the registry are:

C509 Public Key Algorithms

Suitability of different public key algorithms for use within IoT scenarios The public key algorithms registry specifies a number of algorithms, not all which are suitable for usage with constrained devices. RSA requires large keys and large signature sizes compared to elliptic curve cryptography (ECC), which together with resource-efficient implementations of named elliptic curves (Montgomery, Edwards and Weierstraß curves) make them suitable candidates for IoT public key usage. These curves are represented by ids 1–11 and 24–28 in .

COSE Header Parameters Registry IANA is requested to assign the entries in to the "COSE Header Parameters" registry under the "CBOR Object Signing and Encryption (COSE)" heading with this document as reference.

Media Type Application Registry IANA is requested to assign the following entries in to the "application" registry under the "Media Types" heading with this document as reference.

Media Type application/cose-c509-cert When the application/cose-c509-cert media type is used, the data is a COSE_C509 structure. If the parameter "usage" is set to "chain", this sequence indicates a certificate chain. Type name: application Subtype name: cose-c509-cert Required parameters: N/A Optional parameters: usage

• Can be absent to provide no further information about the intended meaning of the order in the CBOR sequence of certificates.
• Can be set to "chain" to indicate that the sequence of data items is to be interpreted as a certificate chain.

Encoding considerations: binary Security considerations: See the Security Considerations section of [[this document]]. Interoperability considerations: N/A Published specification: [[this document]] Applications that use this media type: Applications that employ COSE and use C509 as a certificate type. Fragment identifier considerations: N/A Additional information:

• Deprecated alias names for this type: N/A
• Magic number(s): TBD8, TBD6
• File extension(s): .c509
• Macintosh file type code(s): N/A

Person & email address to contact for further information: iesg@ietf.org Intended usage: COMMON Restrictions on usage: N/A Author: COSE WG Change controller: IETF

Media Type application/cose-c509-pkcs10 When the application/cose-c509-pkcs10 media type is used, the data is a C509CertificateRequest structure. Type name: application Subtype name: cose-c509-pkcs10 Required parameters: N/A Optional parameters: N/A Encoding considerations: binary Security considerations: See the Security Considerations section of [[this document]]. Interoperability considerations: N/A Published specification: [[this document]] Applications that use this media type: Applications that employ COSE and C509 Certificate Request. Fragment identifier considerations: N/A Additional information:

• Deprecated alias names for this type: N/A
• Magic number(s): TBD9
• File extension(s): .c509
• Macintosh file type code(s): N/A

Person & email address to contact for further information: iesg@ietf.org Intended usage: COMMON Restrictions on usage: N/A Author: COSE WG Change controller: IETF

Media Type application/cose-c509-crtemplate When the application/cose-c509-crtemplate media type is used, the data is a C509CertificateRequestTemplate structure. Type name: application Subtype name: cose-c509-crtemplate Required parameters: N/A Optional parameters: N/A Encoding considerations: binary Security considerations: See the Security Considerations section of [[this document]]. Interoperability considerations: N/A Published specification: [[this document]] Applications that use this media type: Applications that employ COSE and C509 Certificate Request. Fragment identifier considerations: N/A Additional information:

• Deprecated alias names for this type: N/A
• Magic number(s): TBD18
• File extension(s): .c509
• Macintosh file type code(s): N/A

Person & email address to contact for further information: iesg@ietf.org Intended usage: COMMON Restrictions on usage: N/A Author: COSE WG Change controller: IETF

Media Type application/cose-c509-privkey When the application/cose-c509-privkey media type is used, the data is a C509PrivateKey structure. Type name: application Subtype name: cose-c509-privkey Required parameters: N/A Optional parameters: usage Encoding considerations: binary Security considerations: See the Security Considerations section of [[this document]]. Interoperability considerations: N/A Published specification: [[this document]] Applications that use this media type: Applications that employ COSE and use C509 as a certificate type. Fragment identifier considerations: N/A Additional information:

• Deprecated alias names for this type: N/A
• Magic number(s): TBD12
• File extension(s): .c509
• Macintosh file type code(s): N/A

Person & email address to contact for further information: iesg@ietf.org Intended usage: COMMON Restrictions on usage: N/A Author: COSE WG Change controller: IETF

Media Type application/cose-c509-pem When the application/cose-c509-pem media type is used, the data is a C509PEM structure. Type name: application Subtype name: cose-c509-pem Required parameters: N/A Optional parameters: usage Encoding considerations: binary Security considerations: See the Security Considerations section of [[this document]]. Interoperability considerations: N/A Published specification: [[this document]] Applications that use this media type: Applications that employ COSE and use C509 as a certificate type. Fragment identifier considerations: N/A Additional information:

• Deprecated alias names for this type: N/A
• Magic number(s): TBD13
• File extension(s): .c509
• Macintosh file type code(s): N/A

Person & email address to contact for further information: iesg@ietf.org Intended usage: COMMON Restrictions on usage: N/A Author: COSE WG Change controller: IETF

Media Type application/cose-certhash When the application/cose-certhash media type is used, the data is a COSE_CertHash structure, see . Type name: application Subtype name: cose-certhash Required parameters: N/A Optional parameters: usage

• Can be absent to provide no further information about what the hash value is calculated over.
• Can be set to "c509" to indicate that the hash value is calculated over a C509 certificate, see .

Encoding considerations: binary Security considerations: See the Security Considerations section of . Interoperability considerations: N/A Published specification: [[this document]] Applications that use this media type: Applications that employ COSE and use X.509 or C509 as certificate type. Fragment identifier considerations: N/A Additional information:

• Deprecated alias names for this type: N/A
• Magic number(s): N/A
• File extension(s): N/A
• Macintosh file type code(s): N/A

Person & email address to contact for further information: iesg@ietf.org Intended usage: COMMON Restrictions on usage: N/A Author: COSE WG Change controller: IETF

CoAP Content-Formats Registry IANA is requested to add entries for "application/cose-c509-cert", "application/cose-c509-pkcs10", "application/cose-c509-crtemplate", "application/cose-c509-privkey" and "application/cose-c509-pem" to the "CoAP Content-Formats" registry under the registry group "Constrained RESTful Environments (CoRE) Parameters". A dedicated Content-Format ID is requested for the "application/cose-c509-cert" media type in the case when the parameter "usage" is set to "chain", see . IANA is requested to add entries for "application/cose-certhash" to the "CoAP Content-Formats" registry under the registry group "Constrained RESTful Environments (CoRE) Parameters". A dedicated Content-Format ID is requested in the case when the parameter "usage" is set to "c509", see . IANA is requested to add entries for "application/cbor" to the "CoAP Content-Formats" registry under the registry group "Constrained RESTful Environments (CoRE) Parameters", in the case when the encoding is a CBOR text string containing a URI, see .

CoAP Content-Format IDs

TLS Certificate Types Registry This document registers the following entry in the "TLS Certificate Types" registry under the "Transport Layer Security (TLS) Extensions" heading. The new certificate type can be used with additional TLS certificate compression . C509 is defined in the same way as X.509, but uses a different value and instead of the DER-encoded X.509 certificate, opaque cert_data<1..2^24-1> in TLS 1.3 and opaque ASN.1Cert<1..2^24-1> in TLS 1.2, contains the CBOR sequence ~C509Certificate (an unwrapped C509Certificate). Similar to COSE_C509, the TLS handshake contains the length of each certificate. The TLS extensions client_certificate_type and server_certificate_type are used to negotiate the use of C509.

TLSA Selectors Registry This document registers the following entries in the "TLSA Selectors" registry under the "DNS-Based Authentication of Named Entities (DANE) Parameters" heading. The TLSA selectors registry defined in originally only applied to PKIX certificates in DER encoding. This specification updates to accept the use of C509 certificates, which are essentially CBOR encoded PKIX certificates.

EDHOC Authentication Credential Types Registry This document registers the following entry in the "EDHOC Authentication Credential Types" registry under the "Ephemeral Diffie-Hellman Over COSE (EDHOC)" heading. This is used to identify supported authentication credential type, for example, during discovery of EDHOC resources, see .

References Normative References This memo represents a republication of PKCS #9 v2.0 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document, except for the security considerations section, is taken directly from that specification. This memo provides information for the Internet community. This memo represents a republication of PKCS #10 v1.7 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document, except for the security considerations section, is taken directly from the PKCS #9 v2.0 or the PKCS #10 v1.7 document. This memo provides information for the Internet community. 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] This document describes the use of the Cryptographic Message Syntax (CMS) to protect firmware packages, which provide object code for one or more hardware module components. CMS is specified in RFC 3852. A digital signature is used to protect the firmware package from undetected modification and to provide data origin authentication. Encryption is optionally used to protect the firmware package from disclosure, and compression is optionally used to reduce the size of the protected firmware package. A firmware package loading receipt can optionally be generated to acknowledge the successful loading of a firmware package. Similarly, a firmware package load error report can optionally be generated to convey the failure to load a firmware package. [STANDARDS-TRACK] This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This document defines the syntax for private-key information and a content type for it. Private-key information includes a private key for a specified public-key algorithm and a set of attributes. The Cryptographic Message Syntax (CMS), as defined in RFC 5652, can be used to digitally sign, digest, authenticate, or encrypt the asymmetric key format content type. This document obsoletes RFC 5208. [STANDARDS-TRACK] Encrypted communication on the Internet often uses Transport Layer Security (TLS), which depends on third parties to certify the keys used. This document improves on that situation by enabling the administrators of domain names to specify the keys used in that domain's TLS servers. This requires matching improvements in TLS client software, but no change in TLS server software. [STANDARDS-TRACK] This document describes an experimental protocol for publicly logging the existence of Transport Layer Security (TLS) certificates as they are issued or observed, in a manner that allows anyone to audit certificate authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs. Logs are network services that implement the protocol operations for submissions and queries that are defined in this document. This document profiles certificate enrollment for clients using Certificate Management over CMS (CMC) messages over a secure transport. This profile, called Enrollment over Secure Transport (EST), describes a simple, yet functional, certificate management protocol targeting Public Key Infrastructure (PKI) clients that need to acquire client certificates and associated Certification Authority (CA) certificates. It also supports client-generated public/private key pairs as well as key pairs generated by the CA. This memo describes the process for early allocation of code points by IANA from registries for which "Specification Required", "RFC Required", "IETF Review", or "Standards Action" policies apply. This process can be used to alleviate the problem where code point allocation is needed to facilitate desired or required implementation and deployment experience prior to publication of an RFC, which would normally trigger code point allocation. The procedures in this document are intended to apply only to IETF Stream documents. This document specifies a new certificate type and two TLS extensions for exchanging raw public keys in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS). The new certificate type allows raw public keys to be used for authentication. 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. The EST (Enrollment over Secure Transport) protocol defines the Well-Known URI (Uniform Resource Identifier) -- /.well-known/est -- along with a number of other path components that clients use for PKI (Public Key Infrastructure) services, namely certificate enrollment (e.g., /simpleenroll). This document defines a number of other PKI services as additional path components -- specifically, firmware and trust anchors as well as symmetric, asymmetric, and encrypted keys. This document also specifies the PAL (Package Availability List), which is an XML (Extensible Markup Language) file or JSON (JavaScript Object Notation) object that clients use to retrieve packages available and authorized for them. This document extends the EST server path components to provide these additional services. This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. This document describes the Concise Binary Object Representation (CBOR) Sequence format and associated media type "application/cbor-seq". A CBOR Sequence consists of any number of encoded CBOR data items, simply concatenated in sequence. Structured syntax suffixes for media types allow other media types to build on them and make it explicit that they are built on an existing media type as their foundation. This specification defines and registers "+cbor-seq" as a structured syntax suffix for CBOR Sequences. The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. The Concise Binary Object Representation (CBOR), defined in RFC 8949, is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. This document defines CBOR tags for object identifiers (OIDs) and is the reference document for the IANA registration of the CBOR tags so defined. This document defines a stored ("file") format for Concise Binary Object Representation (CBOR) data items that is friendly to common systems that recognize file types, such as the Unix file(1) command. The CBOR Object Signing and Encryption (COSE) message structure uses references to keys in general. For some algorithms, additional properties are defined that carry parameters relating to keys as needed. The COSE Key structure is used for transporting keys outside of COSE messages. This document extends the way that keys can be identified and transported by providing attributes that refer to or contain X.509 certificates. Some IETF protocols make use of Ethernet frame formats and IEEE 802 parameters. This document discusses several aspects of such parameters and their use in IETF protocols, specifies IANA considerations for assignment of points under the IANA Organizationally Unique Identifier (OUI), and provides some values for use in documentation. This document obsoletes RFC 7042. The lightweight authenticated key exchange protocol Ephemeral Diffie-Hellman Over COSE (EDHOC) can be run over the Constrained Application Protocol (CoAP) and used by two peers to establish a Security Context for the security protocol Object Security for Constrained RESTful Environments (OSCORE). This document details this use of the EDHOC protocol by specifying a number of additional and optional mechanisms, including an optimization approach for combining the execution of EDHOC with the first OSCORE transaction. This combination reduces the number of round trips required to set up an OSCORE Security Context and to complete an OSCORE transaction using that Security Context. n.d. 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. 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 This document defines a standard profile for X.509 certificates for the purpose of supporting validation of assertions of "right-of-use" of Internet Number Resources (INRs). The certificates issued under this profile are used to convey the issuer's authorization of the subject to be regarded as the current holder of a "right-of-use" of the INRs that are described in the certificate. This document contains the normative specification of Certificate and Certificate Revocation List (CRL) syntax in the Resource Public Key Infrastructure (RPKI). This document also specifies profiles for the format of certificate requests and specifies the Relying Party RPKI certificate path validation procedure. [STANDARDS-TRACK] This document describes two methods for producing an integrity check value from a Diffie-Hellman key pair and one method for producing an integrity check value from an Elliptic Curve key pair. This behavior is needed for such operations as creating the signature of a Public-Key Cryptography Standards (PKCS) #10 Certification Request. These algorithms are designed to provide a Proof-of-Possession of the private key and not to be a general purpose signing algorithm. This document obsoletes RFC 2875. The Internet Protocol Suite is increasingly used on small devices with severe constraints on power, memory, and processing resources, creating constrained-node networks. This document provides a number of basic terms that have been useful in the standardization work for constrained-node networks. This document describes and discusses the textual encodings of the Public-Key Infrastructure X.509 (PKIX), Public-Key Cryptography Standards (PKCS), and Cryptographic Message Syntax (CMS). The textual encodings are well-known, are implemented by several applications and libraries, and are widely deployed. This document articulates the de facto rules by which existing implementations operate and defines them so that future implementations can interoperate. A common design pattern in Internet of Things (IoT) deployments is the use of a constrained device that collects data via sensors or controls actuators for use in home automation, industrial control systems, smart cities, and other IoT deployments. This document defines a Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) 1.2 profile that offers communications security for this data exchange thereby preventing eavesdropping, tampering, and message forgery. The lack of communication security is a common vulnerability in IoT products that can easily be solved by using these well-researched and widely deployed Internet security protocols. This specification defines a lossless compressed data format that compresses data using a combination of the LZ77 algorithm and Huffman coding, with efficiency comparable to the best currently available general-purpose compression methods. 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. This document specifies a base profile for X.509 v3 Certificates and X.509 v2 Certificate Revocation Lists (CRLs) for use with the United States National Security Agency's Commercial National Security Algorithm (CNSA) Suite. The profile applies to the capabilities, configuration, and operation of all components of US National Security Systems that employ such X.509 certificates. US National Security Systems are described in NIST Special Publication 800-59. It is also appropriate for all other US Government systems that process high-value information. It is made publicly available for use by developers and operators of these and any other system deployments. In TLS handshakes, certificate chains often take up the majority of the bytes transmitted. This document describes how certificate chains can be compressed to reduce the amount of data transmitted and avoid some round trips. This document defines the core of the QUIC transport protocol. QUIC provides applications with flow-controlled streams for structured communication, low-latency connection establishment, and network path migration. QUIC includes security measures that ensure confidentiality, integrity, and availability in a range of deployment circumstances. Accompanying documents describe the integration of TLS for key negotiation, loss detection, and an exemplary congestion control algorithm. This document specifies version 1.3 of the Datagram Transport Layer Security (DTLS) protocol. DTLS 1.3 allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. The DTLS 1.3 protocol is based on the Transport Layer Security (TLS) 1.3 protocol and provides equivalent security guarantees with the exception of order protection / non-replayability. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document obsoletes RFC 6347. Enrollment over Secure Transport (EST) is used as a certificate provisioning protocol over HTTPS. Low-resource devices often use the lightweight Constrained Application Protocol (CoAP) for message exchanges. This document defines how to transport EST payloads over secure CoAP (EST-coaps), which allows constrained devices to use existing EST functionality for provisioning certificates. The Extensible Authentication Protocol (EAP), defined in RFC 3748, provides a standard mechanism for support of multiple authentication methods. This document specifies the use of EAP-TLS with TLS 1.3 while remaining backwards compatible with existing implementations of EAP-TLS. TLS 1.3 provides significantly improved security and privacy, and reduced latency when compared to earlier versions of TLS. EAP-TLS with TLS 1.3 (EAP-TLS 1.3) further improves security and privacy by always providing forward secrecy, never disclosing the peer identity, and by mandating use of revocation checking when compared to EAP-TLS with earlier versions of TLS. This document also provides guidance on authentication, authorization, and resumption for EAP-TLS in general (regardless of the underlying TLS version used). This document updates RFC 5216. The Extensible Authentication Protocol (EAP), defined in RFC 3748, provides a standard mechanism for support of multiple authentication methods. EAP-TLS and other TLS-based EAP methods are widely deployed and used for network access authentication. Large certificates and long certificate chains combined with authenticators that drop an EAP session after only 40 - 50 round trips is a major deployment problem. This document looks at this problem in detail and describes the potential solutions available. This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a very compact and lightweight authenticated Diffie-Hellman key exchange with ephemeral keys. EDHOC provides mutual authentication, forward secrecy, and identity protection. EDHOC is intended for usage in constrained scenarios, and a main use case is to establish an Object Security for Constrained RESTful Environments (OSCORE) security context. By reusing CBOR Object Signing and Encryption (COSE) for cryptography, Concise Binary Object Representation (CBOR) for encoding, and Constrained Application Protocol (CoAP) for transport, the additional code size can be kept very low. University of Applied Sciences Bonn-Rhein-Sieg Linaro Sandelman Software Works RFC 7925 offers guidance to developers on using TLS/DTLS 1.2 for Internet of Things (IoT) devices with resource constraints. This document is a companion to RFC 7925, defining TLS/DTLS 1.3 profiles for IoT devices. Additionally, it updates RFC 7925 with respect to the X.509 certificate profile and ciphersuite requirements. Windy Hill Systems, LLC Cisco Meta Platforms, Inc. This document specifies a "compact" version of TLS 1.3 and DTLS 1.3. It saves bandwidth by trimming obsolete material, tighter encoding, a template-based specialization technique, and alternative cryptographic techniques. cTLS is not directly interoperable with TLS 1.3 or DTLS 1.3 since the over-the-wire framing is different. A single server can, however, offer cTLS alongside TLS or DTLS. Sandelman Software Works Cisco Siemens The Industrial Lounge This document updates RFC7030, Enrollment over Secure Transport (EST), clarifying how the Certificate Signiing Request (CSR) Attributes Response can be used by an EST server to specify both CSR attribute Object IDs (OID) and also CSR attribute values, in particular X.509 extension values, that the server expects the client to include in subsequent CSR request. RFC9148 is derived from RFC7030, and it is also updated. RFC7030 (EST) is ambiguous in its specification of the CSR Attributes Response. This has resulted in implementation challenges and implementor confusion. As a result, there was not universal understanding of what was specified. This document clarifies the encoding rules. This document therefore also provides a new straightforward approach: using a template for CSR contents that may be partially filled in by the server. This also allows an EST server to specify a subject Distinguished Name (DN).

C509 Certificate Examples

Example: RFC 7925 profiled X.509 Certificate Example of profiled X.509 certificate parsed with OpenSSL. The DER encoding of the above certificate is 316 bytes.

Example: C509 Certificate Encoding This section shows the C509 encoding of the X.509 certificate in the previous section. The point compressed public key is represented as described in . shows the diagnostic notation of the unwrapped CBOR sequence, ~C509Certificate, see .

CBOR diagnostic notation of ~C509Certificate

shows the plain hex format of the unwrapped CBOR sequence, the size is 140 bytes.

CBOR plain hex format of ~C509Certificate.

Example: Natively Signed C509 Certificate This section shows the natively signed C509 certificate corresponding to that of the previous section, which is identical except for c509CertificateType, encoding of point compression (see ), and signatureValue. shows the diagnostic notation of the natively signed unwrapped CBOR sequence, ~C509Certificate.

CBOR diagnostic notation of ~C509Certificate

shows the plain hex format of the natively signed unwrapped CBOR sequence, the size is 140 bytes.

CBOR plain hex format of ~C509Certificate.

C509 for Diffie-Hellman keys The two previous examples illustrate keyUsage digitalSignature. A C509 certificate for a public Diffie-Hellman key would instead have key usage keyAgreement encoded according to (in this case of single extension encoded as integer 16 instead of 1 for digital signature) but otherwise identical in format. Note that Section 5.6.3.2 of allows a key agreement key pair to be used to sign a certificate request.

Example: Additional Keys for the Example Certificates Below are the issuer key pair and the subject private key belonging to the above example certificates. The private keys are encoded as in COSE . These issuer key pair can be used to sign or verify the example certificates and the subject private key allows the example certificates to be used in test vectors for other protocols like EDHOC.

Examples: C509Certificate and C509CertData This section examplifies other CBOR objects defined in this specification, based on the natively signed C509 certificate in . shows the encoding of the corresponding C509Certificate, i.e., the CBOR array wrapping of the CBOR sequence ~C509Certificate, see .

C509Certificate: The CBOR array wrapping of ~C509Certificate

Note that C509Certificate is identical to ~C509Certificate in except for the prefix 8B (which indicates that it is a CBOR array with 11 elements). shows the encoding of the corresponding C509CertData, i.e., the CBOR byte string wrapping of the CBOR sequence ~C509Certificate, see .

C509CertData: CBOR byte string wrapping of ~C509Certificate.

Note that C509CertData is identical to ~C509Certificate in except for the prefix 58 8C (which indicates that it is a CBOR byte string of 140 bytes).

Example: IEEE 802.1AR profiled X.509 Certificate An example of an IEEE 802.1AR profiled X.509 certificate (Secure Device Identifier, DevID) is provided in Appendix C.2 of . The certificate is shown below including details of the hardwareModuleName type of otherName in subjectAltName, see . The DER encoding of the certificate is 577 bytes:

Example: C509 Certificate Encoding The CBOR encoding (~C509Certificate) of the same X.509 certificate is shown below in CBOR diagnostic format. The size of the CBOR encoding (CBOR sequence) is 275 bytes:

Example: CAB Baseline ECDSA HTTPS X.509 Certificate The www.ietf.org HTTPS server replies with a certificate message with 2 certificates. The DER encoding of the first certificate is 1209 bytes.

Example: C509 Certificate Encoding The CBOR encoding (~C509Certificate) of the first X.509 certificate is shown below in CBOR diagnostic format. The size of the CBOR encoding (CBOR sequence) is 783 bytes.

Example: CAB Baseline RSA HTTPS X.509 Certificate The tools.ietf.org HTTPS server replies with a certificate message with 4 certificates. The DER encoding of the first certificate is 1647 bytes.

Example: C509 Certificate Encoding The CBOR encoding (~C509Certificate) of the first X.509 certificate is shown below in CBOR diagnostic format. The size of the CBOR encoding (CBOR sequence) is 1245 bytes.

Acknowledgments The authors want to thank Henk Birkholz, Corey Bonnell, Carsten Bormann, Russ Housley, Olle Johansson, Benjamin Kaduk, Ilari Liusvaara, Laurence Lundblade, Francesca Palombini, Thomas Peterson, Michael Richardson, Stefan Santesson, Jim Schaad, Brian Sipos, Rene Struik, Fraser Tweedale, and Paul Wouters for reviewing and commenting on intermediate versions of the draft and help with GitHub.

Contributors Special acknowledgement goes to Lijun Liao for his careful review and many detailed improvement proposals.