]> Ericsson AB

john.mattsson@ericsson.com

Ericsson AB

goran.selander@ericsson.com

RISE AB

shahid.raza@ri.se

RISE AB

joel.hoglund@ri.se

Nexus Group

martin.furuhed@nexusgroup.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 and all certificates compatible with the RFC 7925, IEEE 802.1AR (DevID), CNSA, RPKI, GSMA eUICC, and CA/Browser Forum Baseline Requirements profiles. 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 with over 50%. The CBOR encoded structure can alternatively be signed directly ("natively signed"), which does not require re-encoding for the signature to be verified. The document also specifies C509 COSE headers, a C509 TLS certificate type, and a C509 file format. 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. 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), byte strings (bstr), and text strings (tstr), 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 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 based on , which can support large parts of RFC 5280. The encoding supports all RFC 7925, IEEE 802.1AR, CAB Baseline , , RPKI , eUICC profiled X.509 certificates. The resulting certificates are called C509 Certificates. 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 1. 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 specifies COSE headers for use of the C509 certificates with COSE, see . The document also specifies a TLS certificate type for use of the C509 certificates with TLS and QUIC (with or without additional TLS certificate compression), see .

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, 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 order of elements in arrays are always encoded in the same order as the elements or the corresponding SEQUENCE or SET in the DER encoding.

Message Fields The X.509 fields and their CBOR encodings are listed below, and used in the definition of C509 certificates, see . C509 certificates are defined in terms of DER encoded X.509 certificates:

• version. The 'version' field is encoded in the 'c509CertificateType' CBOR int. The field 'c509CertificateType' also indicates the type of the C509 certificate. Currently, the type can be a natively signed C509 certificate following X.509 v3 (c509CertificateType = 0) or a CBOR re-encoded X.509 v3 DER certificate (c509CertificateType = 1), see .
• 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 is always the same as the 'signatureAlgorithm' field and therefore omitted from the CBOR encoding.
• issuer. In the general case, the sequence of 'RelativeDistinguishedName' is encoded as a CBOR array of CBOR arrays of Attributes. Typically, each RelativeDistinguishedName only contains a single attribute and the sequence is then encoded as a CBOR array of Attributes. Each Attribute is encoded as a (CBOR int, CBOR text string) pair or as a (unwrapped CBOR OID, CBOR bytes) pair. The absolute value of the CBOR int (see ) encodes the attribute type and the sign is used to represent the character string type; positive for Utf8String, negative for PrintableString. The Attribute Email Address is always an IA5String. In natively signed C509 certificates all text strings are UTF-8 encoded and all attributeType 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. The string types teletexString, universalString, and bmpString are not supported. If Name contains a single Attribute containing an utf8String encoded 'common name' it is encoded as follows:
  • If the text string contains only the symbols '0'–'9' or 'a'–'f', it is encoded as a CBOR byte string, prefixed with an initial byte set to '00'.
  • If the text string contains an EUI-64 of the form "HH-HH-HH-HH-HH-HH-HH-HH" where 'H' is one of the symbols '0'–'9' or 'A'–'F' it is encoded as a CBOR byte string prefixed with an initial byte set to '01', for a total length of 9. An EUI-64 mapped from a 48-bit MAC address (i.e., of the form "HH-HH-HH-FF-FE-HH-HH-HH) is encoded as a CBOR byte string prefixed with an initial byte set to '01', for a total length of 7.
  • Otherwise it is encoded as a CBOR text string.
• 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 CBOR null.
• subject. The 'subject' is encoded exactly like issuer.
• 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. 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 as a CBOR array where each extension is encoded as either a CBOR int (see ) followed by an optional CBOR item of any type or an unwrapped CBOR OID tag followed by a CBOR bool encoding 'critical' and the DER encoded value of the 'extnValue' encoded as a CBOR byte string. If the array contains exactly two ints and the absolute value of the first int is 2 (corresponding to keyUsage), the 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. Extensions are encoded as specified in . The extensions mandated to be supported by and are given special treatment. An omitted 'extensions' field is encoded as an empty CBOR array.
• signatureAlgorithm. The 'signatureAlgorithm' field 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.
• signatureValue. In general, the 'signatureValue' BIT STRING value field is encoded as the CBOR byte string issuerSignatureValue. This specification assumes 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

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).

CDDL for C509Certificate.

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 is omitted and subjectPublicKey consist 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 a uncompressed public key of type id-ecPublicKey is CBOR encoded with point compression, the octets 0xfe and 0xfd are used instead of 0x02 and 0x03 in the CBOR encoding to represent even and odd y-coordinate, respectively.

Encoding of issuerSignatureValue For ECDSA signatures, the SEQUENCE and INTEGER type and length fields as well as the any leading 0x00 byte (to indicate that the number is not negative) are omitted. If the two INTEGER value fields have different lengths, the shorter INTEGER value field is padded with zeroes so that the two fields have the same length. The resulting byte string is encoded as a CBOR byte string.

Encoding of Extensions This section details the encoding of the 'extensions' field. The 'extensions' field is encoded as a CBOR array where each extensionID is encoded either as a CBOR int or as an unwrapped CBOR OID tag. If 'extensionID' is encoded an int (see ), the sign is used to encode if the extension is critical and the 'critical' field is omitted. Critical extensions are encoded with a negative sign and non-critical extensions are encoded with a positive sign. The 'extnValue' OCTET STRING value field is encoded as the CBOR byte string 'extensionValue' except for the extensions specified below. For some extensions, only commonly used parts are supported by the CBOR encoding. If unsupported parts are used, the CBOR encoding cannot be used. CBOR encoding of the following extension values are fully supported:

• Subject Key Identifier (subjectKeyIdentifier). 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 tag 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 identify the pair ( hwType, hwSerialEntries ) 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 certSerialNumberm 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 contains general names registered in the extension value can be CBOR encoded. Note that requires that minimum MUST be zero, and maximum MUST be absent.

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

• AS Resources (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 (autonomousSysIds-v2). Encoded exactly like autonomousSysIds.
• IP Resources (id-pe-ipAddrBlocks). If rdi and SAFI is 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 is set to ones. With the exception of the first Address, if the byte string has the same length as the previous Address, the Address is encoded as an uint with the the difference to the previous Address. It should be noted that using address differences for compactness prevents encoding an address range larger than 2^64 - 1 corresponding to the cbor integer max value.

• IP Resources v2 (id-pe-ipAddrBlocks-v2). Encoded exactly like id-pe-ipAddrBlocks.
• Signed Certificate Timestamp. If all the SCTs are version 1, and there are no SCT extensions, the extension value can be CBOR encoded. LogIDs are encoded as CBOR byte strings, the timestamp is encoded as a CBOR int (milliseconds since validityNotBefore), and the signature is encoded with an (AlgorithmIdentifier, any) pair in the same way as issuerSignatureAlgorithm and issuerSignatureValue.

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 and keyAgreement asserted is encoded as the two CBOR ints 2, 17 (2^0 + 2^4 = 17).
• 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, 6 ].
• 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, 17, 8, [ 3, 6 ], 3, "example.com" ].

COSE Header Parameters The formatting and processing for c5b, c5c, and c5t, and c5u, defined in are similar to x5bag, x5chain, x5t, x5u defined in except that the certificates are C509 instead of DER encoded X.509 and uses a COSE_C509 structure instead of COSE_X509. c5u provides an alternative way to identify an untrusted certificate bag/chain by reference with a URI. The content is a COSE_C509 item served with the application/cose-c509 content format. The COSE_C509 structure used in c5b, c5c, and c5u is defined as: 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 some out-of-band confirmation. COSE Header Parameters

Name	Label	Value Type	Description
c5b	TBD1	COSE_C509	An unordered bag of C509 certificates
c5c	TBD2	COSE_C509	An ordered chain of C509 certificates
c5t	TBD3	COSE_CertHash	Hash of a C509Certificate
c5u	TBD4	uri	URI pointing to a COSE_C509 containing a 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.

C509 Certificate Signing Request The section defines the format of a C509 Certificate Signing Request (CSR), also known as a C509 Certificate Request, based on and compatible with RFC 2986 reusing the formatting for C509 certificates defined in . There are currently two c509CertificateRequestType values defined, c509CertificateRequestType = 0 requests a c509CertificateType = 0 and c509CertificateRequestType = 1 requests a c509CertificateType = 1. subjectSignatureAlgorithm can be a signature algorithm or a non-signature proof-of-possession algorithm, e.g. as defined in . In the latter case, the signature is replaced by a MAC and requires a public Diffie-Hellman key of the verifier distributed out-of-band. Both kinds are listed in the C509 Signature Algorithms Registry, see . Certificate request attributes, i.e. attributes for use with certificate requests providing additional information about the subject of the certificate, are defined in . The attribute extensionRequest is supported with a dedicated element. Other certificate request attributes are included using the same Extensions structure as in extensionsRequest, both extensions and attributes are listed in the C509 Extensions Registry, see . The only other certificate request attribute specified in this document is challengePassword which is defined for utf8String values and encoded as CBOR text string, except if the text string contains only the symbols '0'–'9' or 'a'–'f', in which case it is encoded as a CBOR byte string.

CDDL for C509CertificateRequest.

After verifying the subjectSignatureValue, the CA MAY transform the C509CertificateRequest into a CertificationRequestInfo for compatibility with existing procedures and code.

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 Signing Request (CSR)) format defined in Section 4 follows the PKCS#10 format to enable a direct mapping to the certification request information, see Section 4.1 of . 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 of the public key, before normal certificate generation can take place. In the reverse direction, in case c509CertificateType = 1 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 = 0 was requested, the C509 processing needs to be performed before signing the certificate, in which case a tighter integration with 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 use that with existing processes and code to produce an RFC 5280 DER encoded X.509 certificate. The DER encoded X509 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 the handshake is 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 a constrained wireless links. The conversion to X.509 within an 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 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. For the example HTTPS certificate chains (www.ietf.org and tools.ietf.org) both C509 and Brotli perform well complementing each other. C509 use dedicated information to compress individual certificates, while Brotli can compress duplicate information in the entire chain. Note that C509 certificates of type 0 and 1 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). X509 and C509 are Certificate messages. X509 + Brotli and C509 + Brotli are CompressedCertificate messages.

Security Considerations The CBOR profiling 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 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 draft does not reveal any additional information compared to X.509. Because of 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.

IANA Considerations This document creates several new registries under the new heading "C509 Certificate". 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 one 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 encodings, other values in the interval [-256, 255] have a 2 byte encodings, and the remaining values in the interval [-65536, 65535] have 3 byte encodings.

C509 Certificate Types Registry IANA has created a new registry titled "C509 Certificate Types" under the new heading "C509 Certificate". The columns of the registry are Value, Description, and Reference, where Value is an integer, and the other columns are text strings. 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 Types

C509 Attributes Registry IANA has created a new registry titled "C509 Attributes" under the new heading "CBOR Encoded X509 Certificates (C509 Certificates)". The columns of the registry are Value, Name, Identifiers, OID, DER, Comments, and Reference, where Value is an non-negative integer, and the other columns are text strings. For values in the interval [0, 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 Attributes

C509 Extensions Registry IANA has created a new registry titled "C509 Extensions Registry" under the new heading "CBOR Encoded X509 Certificates (C509 Certificates)". The columns of the registry are Value, Name, Identifiers, OID, DER, Comments, extensionValue, and Reference, where Value is an positive integer, and the other columns are text strings. The registry also contains CSR attributes for use in Certificate Requests, see . For values in the interval [1, 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 Extensions and CSR Attributes

C509 Certificate Policies Registry IANA has created a new registry titled "C509 Certificate Policies Registry" under the new heading "CBOR Encoded X509 Certificates (C509 Certificates)". The columns of the registry are Value, Name, Identifiers, OID, DER, Comments, and Reference, where Value is an integer, and the other columns are text strings. 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 Policies

C509 Policies Qualifiers Registry IANA has created a new registry titled "C509 Policies Qualifiers Registry" under the new heading "CBOR Encoded X509 Certificates (C509 Certificates)". The columns of the registry are Value, Name, Identifiers, OID, DER, Comments, and Reference, where Value is an integer, and the other columns are text strings. 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 Policies Qualifiers

C509 Information Access Registry IANA has created a new registry titled "C509 Information Access Registry" under the new heading "CBOR Encoded X509 Certificates (C509 Certificates)". The columns of the registry are Value, Name, Identifiers, OID, DER, Comments, and Reference, where Value is an integer, and the other columns are text strings. 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 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 X509 Certificates (C509 Certificates)". The columns of the registry are Value, Name, Identifiers, OID, DER, Comments, and Reference, where Value is an integer, and the other columns are text strings. 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 Extended Key Usages

C509 General Names Registry IANA has created a new registry titled "C509 General Names Registry" under the new heading "CBOR Encoded X509 Certificates (C509 Certificates)". The columns of the registry are Value, General Name, and Reference, where Value is an integer, and the other columns are text strings. 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 General Names

C509 Signature Algorithms Registry IANA has created a new registry titled "C509 Signature Algorithms" under the new heading "CBOR Encoded X509 Certificates (C509 Certificates)". The registry includes both signature algorithms and non-signature proof-of-possession algorithms. The columns 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. 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 Signature Algorithms

C509 Public Key Algorithms Registry IANA has created a new registry titled "C509 Public Key Algorithms" under the new heading "CBOR Encoded X509 Certificates (C509 Certificates)". The columns 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. 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". T 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 specify 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/cose-c509 When the application/cose-c509 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. IANA has registered the following media type : Type name: application Subtype name: cose-c509 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): 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: IESG

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 as X509, but uses a different value and instead of DER-encoded X.509 certificate, opaque cert_data<1..2^24-1> contains a the CBOR sequence ~C509Certificate (an unwrapped C509Certificate). EDITOR'S NOTE: The TLS registrations should be discussed and approved by the TLS WG at a later stage. When COSE WG has adopted work on C509 certificates, it could perhaps be presented in the TLS WG. The TLS WG might e.g. want a separate draft in the TLS WG.

CBOR Tags Registry This document registers the following entries in the "CBOR Tags" registry under the "Concise Binary Object Representation (CBOR) Tags" heading.

References Normative References 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. 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 procedures for the specification and registration of media types for use in HTTP, MIME, and other Internet protocols. This memo documents an Internet Best Current Practice. 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. 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. 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. Informative 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. 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. 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. Ericsson AB Ericsson AB Ericsson AB 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 OSCORE security context. By reusing COSE for cryptography, CBOR for encoding, and CoAP for transport, the additional code size can be kept very low. Linaro This document is a companion to RFC 7925 and defines TLS/DTLS 1.3 profiles for Internet of Things devices. It also updates RFC 7925 with regards to the X.509 certificate profile. Mozilla Cisco Google 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.

Example C509 Certificates

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 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 139 bytes. The point compressed public key is represented as described in .

Example: Natively Signed C509 Certificate The corresponding natively signed C509 certificate in CBOR diagnostic format is identical, except for c509CertificateType, encoding of point compression (see ), and signatureValue. The size of the CBOR encoding (CBOR sequence) is 139 bytes.

C509 for Diffie-Hellman keys The two previous examples illustrate the common key usage 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.

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.

Example IEEE 802.1AR profiled X.509 Certificate EDITOR'S NOTE: Need good example, or remove section One example of an IEEE 802.1AR profiled X.509 certificate (IDevID) is provided in . The DER encoding is given in the second example in Appendix A.2, and the plain text is in the second example of Appendix C.2. The X509v3 Subject Alternative Name is of type OtherName (see ), with type-id = id-on-hardwareModuleName (see ) consisting of hardware type OID and hardware serial number of the device, in that example being 1.3.6.1.4.1.6715.1.10 and 01020304, respectively.

Example C509 Certificate Encoding

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, Carsten Bormann, Russ Housley, Olle Johansson, Benjamin Kaduk, Ilari Liusvaara, Laurence Lundblade, Francesca Palombini, Thomas Peterson, Michael Richardson, Stefan Santesson, Jim Schaad, Brian Sipos, Fraser Tweedale, and Rene Struik for reviewing and commenting on intermediate versions of the draft and helping with GitHub.