CBOR Encoded X.509 Certificates (C509 Certificates)

CBOR Encoded X.509 Certificates (C509 Certificates) 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

NIO

lijun.liao@nio.io

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 common certificate profiles, and it 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 computed over the CBOR encoding instead of the DER encoding, thereby avoiding the use of ASN.1. Both types of certificates have the same semantics as X.509 while providing comparable size reduction. This document also specifies CBOR-encoded data structures for certification requests and certification request templates, new COSE headers, as well as a TLS certificate type and a file format for C509. This document updates RFC 6698 by extending the TLSA selectors registry 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 Resource Public Key Infrastructure (RPKI) where a single certificate can be very large. More compact certificate representations are, therefore, desirable in many use cases. X.509 certificates are defined using Abstract Syntax Notation One (ASN.1) and encoded using the Distinguished Encoding Rules (DER) . This document specifies an alternative encoding of X.509 certificates using the Concise Binary Object Representation (CBOR) , initially proposed in . The use of a more compact encoding reduces certificate size, which has known performance benefits in terms of decreased communication overhead, power consumption, latency, and storage requirements. The re-encoding of X.509 is called C509, and the resulting certificates are termed C509 certificates. C509 is not a general CBOR encoding for ASN.1 data structures. CBOR is a data format designed for small code size and small message size in systems with very limited memory, processor power, and instruction sets. CBOR builds on the JSON data model but extends it by, for example, encoding binary data directly without base64 conversion. In addition to the binary CBOR encoding, CBOR also has a diagnostic notation that is human-readable and editable, which simplifies development and debugging. The Concise Data Definition Language (CDDL) provides a way to express structures for protocol messages and APIs that use CBOR. also extends the diagnostic notation. For a complete specification and examples, see , , and . A tool for becoming familiar with CBOR, available at the time of publication, is the CBOR playground . The C509 encoding supports a large subset of and all certificates profiled for , IEEE 802.1AR (DevID) , CAB Baseline , , RPKI , Wi-SUN , and eUICC . C509 is designed for small code size and compact encoding of certificates in constrained environments including certificates profiled specifically for IoT deployments, but can be applied to certificate-based authentication in general, for example, using TLS , QUIC , DTLS , COSE and EDHOC . This document does not specify a certificate profile. At the time of publication, there are several C509 implementations targeting, for example, in-vehicle and vehicle-to-cloud communication, Uncrewed Aircraft Systems (UAS), and Global Navigation Satellite System (GNSS) deployments. When used to re-encode DER-encoded X.509 certificates, the CBOR encoding can reduce the size of -profiled certificates by more than 50%; see . C509 is designed to be extensible to additional X.509 features, for example, support for new algorithms, including new Post-Quantum (PQ) algorithms, which can be registered in the IANA registry as they are specified; see . This document defines two types of C509 using the same CBOR encoding and differing only in what is being signed:

An invertible CBOR re-encoding of DER-encoded X.509 certificates , which can be reversed to obtain the original DER-encoded X.509 certificate, which can be verified using legacy certificate software. Due to the widespread deployment of X.509, it is necessary to allow backward compatibility.
Natively signed C509 certificates, where the signature is calculated over the CBOR encoding instead of the DER encoding. This removes the need for ASN.1 and DER parsing and the associated complexity, but such certificates are not backward compatible with implementations requiring DER-encoded X.509. Natively signed C509 certificates can be used in devices that are only required to authenticate to servers compatible with natively signed C509 certificates. This is not a major restriction in many IoT deployments in which the parties that issue and verify certificates are part of a restricted ecosystem.

This document also specifies C509 Certification Requests; see . It further specifies COSE headers for use with C509 certificates in COSE; see ; a TLS certificate type for use with C509 certificates in TLS and QUIC, with or without additional TLS certificate compression; see ; and a C509 file format. By extending the TLSA selectors registry to include C509 certificates, 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 and of .

C509 Certificate This section specifies the content and encoding of C509 certificates, with the objective of producing a compact representation that supports a large part of and all of , , RPKI , GSMA eUICC , and CAB Baseline . In the CBOR encoding, static fields are elided, elliptic curve points and time values are compressed, OIDs are replaced with short integers or complemented with CBOR OID encoding , and redundant encoding is removed. Combining these techniques significantly reduces certificate size, which is not achievable with general-purpose compression algorithms; see . A 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 adheres to the restrictions given in . The re-encoding is known to work with DER-encoded certificates, but it might also 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 This section describes the X.509 fields and their CBOR encodings and uses them in the definition of C509 certificates; see . While many of encodings are supported, there are a few instances marked "not supported" for which no alternative is provided and, therefore, no C509 encoding can be generated. The following Concise Data Definition Language (CDDL) defines the CBOR array C509Certificate and the CBOR Sequence TBSCertificate. The member names therefore have documentary value only. Examples are given in the appendices; see, for example, .

CDDL for C509Certificate. The media type of C509Certificate is application/cose-c509-cert+cbor, see , with corresponding CoAP Content-Format defined in . The "magic number" TBD20 is defined using the reserved CBOR tag 55799 and the Content-Format TBD21, enveloped as described in . 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.

certificateSerialNumber The 'certificateSerialNumber' 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 'RDNAttribute' is encoded as a CBOR array consisting of RDNAttribute elements. RelativeDistinguishedName with more than one AttributeTypeAndValue is not supported. Each RDNAttribute is CBOR-encoded as (type, value), either as an (int, SpecialText) pair or as 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 restrictions. The value of the attributes serialNumber and countryName SHALL contain only characters from the 74-character ASCII subset permitted by PrintableString. Additionally, the value of the countryName attribute 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 of at least 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 attribute 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 null 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' field 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, it is followed by the DER-encoded extnValue encoded in the following way:
- if the extension is non-critical, the extnValue OCTET STRING value field is encoded as a CBOR byte string;
- if the extension is critical, the extnValue OCTET STRING value field is encoded as a CBOR byte string and further wrapped in a CBOR array consisting of only this element.

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 ECDSA signatures are encoded in the same way as in . 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, the same OID 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 can, for example, 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:

IPAddrBlocks (id-pe-ipAddrBlocks). The X.509 extension IPAddrBlocks is specified in . The ASN.1 BIT STRING value of IPAddress is converted to a byte sequence defined as: unusedBits || value where unusedBits is a single octet indicating the number of unused bits in the final octet of the BIT STRING, and value is the sequence of octets containing the BIT STRING value. This byte sequence preserves the exact information contained in the ASN.1 BIT STRING. For each IPAddressFamily, the representation is selected as follows:
- If inherit is present, null SHALL be used.
- Otherwise, if the byte sequence of any IPAddress (including addressPrefix, and the min and max fields of addressRange) exceeds 8 octets in length, the IPAddressChoice representation SHALL be used.
- Otherwise, the IntIPAddressChoice representation SHALL be used.
For IntIPAddressChoice, IntAddressPrefix and the min and max values of IntAddressRange SHALL be encoded as big-endian integers representing the following byte sequence: (unusedBits + 1) || value The first byte is encoded as (unusedBits + 1) instead of unusedBits in order to guarantee a non-zero value. With the exception of the first IPAddress, each subsequent IPAddress SHALL be encoded as a CBOR integer representing the difference from the previous IPAddress. As specified in , the IPAddressFamily element contains an Address Family Identifier (AFI) and, optionally, a Subsequent Address Family Identifier (SAFI). AFIs and SAFIs are defined in and , respectively. The limitations specified in apply here as well.

IPAddrBlocks v2 (id-pe-ipAddrBlocks-v2). The X.509 extension IPAddrBlocks v2 is specified in . The extension value is encoded exactly like in the extension "IPAddrBlocks".
OCSP No Check (id-pkix-ocsp-nocheck). 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.

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 otherName values with type-id id-on-hardwareModuleName, id-on-SmtpUTF8Mailbox and id-on-MACAddress have been given their own ints; such otherName values are encoded as follows:
- For id-on-hardwareModuleName, the value is a CBOR array [ hwType: ~oid, hwSerialNum: bytes ] as specified in .
- For id-on-SmtpUTF8Mailbox, the value is a CBOR text as specified in .
- For id-on-MACAddress, the value is a CBOR byte string containing 6 octets for EUI-48 and 8 octets for EUI-64 as specified in .

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

CRL Distribution Points (cRLDistributionPoints). If all DistributionPoint elements contain the distributionPoint with fullName choice of uniformResourceIdentifier, optional reasons, and optional cRLIssuer with one directoryName, the extension value can be CBOR-encoded. The 'reasons' BIT STRING is interpreted as an unsigned integer in network byte order and encoded as a CBOR uint. If the CRLDistributionPoints consists of only one DistributionPointName, which in turn has only the fullName field of type CBOR text, it is encoded as CBOR text; otherwise it is encoded as a CBOR array.

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, authorityCertIssuer, and authorityCertSerialNumber, 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 values are encoded as UTF8String, the extension value can be CBOR-encoded. OIDs registered in are encoded as an int. The policyQualifierId is encoded as a 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 are therefore omitted. For IPv4 addresses, the iPAddress field MUST contain five octets, and for IPv6 addresses, the field MUST contain 17 octets. In both cases the last octet indicates the number of bits in the prefix. 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 Identifiers (id-pe-autonomousSysIds). The X.509 extension AS Identifiers is specified in . If 'rdi' is not present, the extension value can be CBOR-encoded. Each ASId is encoded as a CBOR uint. With the exception of the first ASId, each subsequent ASId is encoded as the difference from the previous ASId.

AS Identifiers v2 (id-pe-autonomousSysIds-v2). The X.509 extension AS Identifiers v2 is specified in . The extension value is encoded exactly like in the extension "AS Identifiers".

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" ].

C509 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 content thus includes the CBOR-encoded 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+cbor, 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, analogously to x5u in , provides the ability to identify a C509 certificate by a URI . It contains a CBOR text string (media type application/cbor and CoAP Content-Format 60). The referenced resource can be any of the following media types:

application/cose-c509-cert+cbor ()
application/cose-c509+cbor ()
application/cose-c509+cbor; usage=chain ()

When the application/cose-c509+cbor media type is used, the data is a CBOR sequence of single-entry COSE_C509 structures (encoding "bytes"). If the parameter "usage" is set to "chain", this sequence indicates a certificate chain. 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. C509 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 C509 certificate

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

C509 COSE Header Algorithm Parameters This section defines the COSE header parameters used for identifying or transporting the sender's key for static-static key agreement algorithms corresponding to , see .

c5c-sender contains the chain of certificates starting with the sender's key exchange certificate. The structure is the same as 'c5c'.
c5t-sender contains the hash value for the sender's key exchange certificate. The structure is the same as 'c5t'.
c5u-sender, analogously to x5u-sender in , contains a URI for the sender's key exchange certificate. The structure and processing are the same as 'c5u'.

Static ECDH Algorithm Values

Name	Algorithm	Label	Type	Description
c5c-sender	ECDH-SS+HKDF-256, ECDH-SS+HKDF-512, ECDH-SS+A128KW, ECDH-SS+A192KW, ECDH-SS+A256KW	-30 (suggested)	COSE_C509	An ordered chain of C509 certificates
c5t-sender	ECDH-SS+HKDF-256, ECDH-SS+HKDF-512, ECDH-SS+A128KW, ECDH-SS+A192KW, ECDH-SS+A256KW	-31 (suggested)	COSE_CertHash	Hash of a C509Certificate
c5u-sender	ECDH-SS+HKDF-256, ECDH-SS+HKDF-512, ECDH-SS+A128KW, ECDH-SS+A192KW, ECDH-SS+A256KW	-32 (suggested)	uri	URI pointing to a C509 certificate

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+cbor 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+cbor 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. Although 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, in which new encodings can be defined, for example, to optimize extensions for which no special CBOR encoding has previously been defined. Where both a specific and a generic CBOR encoding are supported, the specific CBOR encoding MUST be used. For example, when a specific CBOR encoding of an extension is defined in and the C509 Extensions Registry, that specific encoding MUST be used. In particular, when a specific otherName encoding is available, identified by a negative integer value in the C509 General Names Registry, it MUST be used. Native C509 certificates MUST use only specific CBOR-encoded fields. However, when decoding non-native C509 certificates, the decoder may need to support, for example, the (extensionID: ~oid, extensionValue: bytes / [bytes]) encoding of an extension for which an (extensionID: int, extensionValue: Defined) encoding exists. One reason is that the certificate might have been issued before the specific CBOR extension was registered.

C509 Name in TLS and DTLS In TLS and DTLS, the subject of a trusted authority may be sent to the peer to help it select the certificate chain, as in the CertificateAuthoritiesExtension in , in the certificate_authorities field of CertificateRequest in , or in the TrustedAuthorities in . For such usage in TLS and DTLS, the C509 name is wrapped in a distinguished name with exactly one RelativeDistinguishedName, which in turn contains exactly one AttributeTypeAndValue with the attribute C509Name. The attribute value is the raw byte string of the encoded C509 Name as specified in . The attribute for C509 Name has the following structure:

C509 Certification Request This section defines the format of a C509 Certification Request based on . It reuses the encodings of C509 certificates defined in . A Certification Request is commonly referred to as a Certificate Signing Request (CSR). The CDDL for the C509 Certification Request is shown in . The fields have the same encoding as the corresponding fields of the C509 Certificate, see .

CDDL for C509CertificationRequest. After verifying subjectSignatureValue, the Certification Authority (CA) MAY transform the C509CertificationRequest into a CertificationRequestInfo for compatibility with existing procedures and implementations. The media type of C509CertificationRequest is application/cose-c509-pkcs10+cbor, 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 .

Certification Request Types Two types of C509 Certification Requests are defined. Both use the same CBOR encoding and differ only in what is being signed; see . A C509 Certification Request is either an invertible CBOR re-encoding of a DER-encoded certification request or a natively signed request in which the signature is calculated over the CBOR encoding instead of the DER encoding.

c509CertificationRequestType = 2. This type indicates that the C509 Certification Request is natively signed, i.e., that subjectSignatureValue contains the signature over the CBOR Sequence TBSCertificationRequest; see . This encoding removes the need for ASN.1 and DER parsing and for re-encoding by the requesting party.
c509CertificationRequestType = 3. This type indicates that the C509 Certification Request is a CBOR re-encoded certification request, as defined in . This encoding is backward compatible with legacy RFC 2986 certification requests and reduces transport overhead.

The type of certificate issued in response to the request is determined by the application. For C509, the default is c509CertificateType = c509CertificationRequestType. An implementation MAY only support certain values of c509CertificationRequestType.

Subject Signature Algorithm subjectSignatureAlgorithm can be a signature algorithm or a non-signature proof-of-possession algorithm, for example, as defined in . In the case of , the signature is replaced by a MAC and requires a public Diffie-Hellman key of the verifier to be 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 proof-of-possession algorithms with SHA-2 and HMAC-SHA2 (see values 14-16 in ) require 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 certification request, see .

Certification Request Attributes The 'attributes' field specifies the attributes contained in a certification request. The 'attributes' field with no GeneralAttribute SHALL be encoded as an empty CBOR array. The remainder of this section specifies CBOR-encoded attributes for Certification Requests.

Extension Request The X.509 attribute "Extension Request" is defined in . The 'attributeValue' field has type Extensions as in . An empty CBOR array indicates no extensions.

Challenge Password The X.509 attribute "Challenge Password" is defined in . The 'attributeValue' field has type ChallengePassword. A UTF8 String is encoded as CBOR text, and a Printable String is tagged with number 121 (alternative 0 as defined in ). All other string types are not supported. For certification request type 2, only UTF8 String is allowed.

Private Key Possession Statement The X.509 attribute "Statement of Possession of a Private Key" is defined in . The 'attributeValue' field has type PrivateKeyPossessionStatement.

Certification 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 Certification Request. Alternatively to the unstructured mechanism specified in , describes an approach using a Certification Request Template in response to a GET /csrattrs request by the EST client. The EST server thus returns a Certification 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. The approach of is also followed for C509. The C509CertificationRequestTemplate is based on TBSCertificationRequest of the C509CertificationRequest, see , but excludes the subjectSignatureValue field from the template since that needs no further specification. The C509 Certification Request Template is shown in .

CDDL for C509CertificationRequestTemplate. Except as specified in this section, the fields have the same encoding as the corresponding fields of the TBSCertificationRequest, 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 Certification Request Templates can be defined (see ), distinguished by the c509CertificationRequestTemplateType integer. Each type may have its own CDDL structure. The presence of a Defined (non-undefined) value in a C509CertificationRequestTemplate indicates that the server expects the client to use that value in the certification request. If multiple AlgorithmIdentifier or c509CertificationRequestType values are present, the server expects the client to select one of them for use in the Certification Request. The presence of an undefined value indicates that the client is expected to provide an appropriate value for that field. For example, if the 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. For RDNAttributeTemplate, the minOccurs and maxOccurs fields specify the minimal and maximal occurrences of attributes of the given attributeType; maximal shall not be less than minimal, and maximal shall be positive. Negative attributeType is not allowed. For ExtensionTemplate, the field "optional" specifies whether an extension of the given extensionID is optional. Negative extensionID is not allowed. The media type of C509CertificationRequestTemplate is application/cose-c509-crtemplate+cbor, 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 Certification Request format defined in follows the PKCS#10 format to enable a direct mapping to the certification request information, see . The CA can make use of a Certification Request Template defined in , for simplified configuration. When a certification 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.

Operational Considerations

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 Certification Request, the CA transforms it into a DER-encoded 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 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 (ECDSA) and cabforum.org (RSA)) 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. In the examples using FN-DSA and ML-DSA certificate chains, the largest portion of the certificate size consists of the public keys and signatures, which are essentially random. As a result, both Brotli and C509 achieve only very limited size reduction. However, C509 still performs slightly better.

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

Comparing Sizes of Certificate Chains with TLS 1.3. 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. The same security procedures applies as for X.509. For example the same certificate path validation as defined in MUST be performed on C509 certificates before they can be considered trusted. 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 which may violate identity protection 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 apply, see . Security considerations of the COSE protected and unprotected headers are discussed in . The specification makes no recommendations on the use of algorithms, paddings, or other security constructs applied in the encoding of a certificate. In particular, an IANA registration does not imply a recommendation. For example, some deprecated algorithms are assigned code points only for backward compatibility to enable CBOR encoding of existing certificates.

IANA Considerations This document creates several new registries in the new registry group "CBOR Encoded X.509 (C509)". For all items, the 'Reference' field points to this document. Editor's note: Add informative reference to the newly created IANA registries and updated existing registries.

Designated Expert Guidance Reviewers are encouraged to get sufficient information for registration requests to ensure that the usage is not going to duplicate one that is already registered and that the point is likely to be used in deployments. Experts should take into account the expected usage of entries when approving point assignment. 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 an "IETF Review" basis per with "Expert Review" additionally required per . 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 registry group "CBOR Encoded X.509 (C509)". 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 Certification Request Types Registry IANA has created a new registry titled "C509 Certification Request Types" under the new registry group "CBOR Encoded X.509 (C509)". 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 Certification Request Types

C509 Private Key Types Registry IANA has created a new registry titled "C509 Private Key Types" in the new registry group "CBOR Encoded X.509 (C509)". The fields of the registry are Value, Comments, subjectPrivateKey, and Reference, where Value is an integer, and the other columns are text strings. The subjectPrivateKey describes the encoding of the subject private key, see . 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 Certification Request Templates Types Registry IANA has created a new registry titled "C509 Certification Request Templates Types" under the new registry group "CBOR Encoded X.509 (C509)". 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 with Expert Review". For all other values the registration procedure is "Expert Review". The initial contents of the registry are:

C509 Certification Request Templates Types

C509 RDN Attributes Registry IANA has created a new registry titled "C509 RDN Attributes" in the new registry group "CBOR Encoded X.509 (C509)". 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 RDN Attributes specified only for CBOR encoded certificates where no OID is defined, the OID and DER fields are marked "N/A". If there is an OID defined, the OID is given in dotted decimal representation, and the DER column contains the hex string of the DER-encoded OID . If it is not expected to be understood from the other information (e.g. the OID), then the Comments field must contain a reference to where the RDN Attribute is described. 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". The initial contents of the registry are:

C509 RDN Attributes

C509 CR Attributes Registry IANA has created a new registry titled "C509 CR Attributes" under the registry group "CBOR Encoded X.509 (C509)". The fields of the registry are Value, Name, Identifiers, OID, DER, Comments, attributeValue, and Reference, where Value is an integer, and the other columns are text strings. Name and Identifiers are informal descriptions. The fields Name, OID, and DER are mandatory. For CR Attributes specified only for CBOR encoded certificates where no OID is defined, the OID and DER fields are marked "N/A". If OID is present, the OID is given in dotted decimal representation, and the DER column contains the hex string of the DER-encoded OID . If it is not expected to be understood from the other information (e.g. the OID), then the Comments field must contain a reference to where the CR Attribute is described. 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 CRAttributes

C509 Extensions Registry IANA has created a new registry titled "C509 Extensions" under the new registry group "CBOR Encoded X.509 (C509)". 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, and extensionValue are mandatory. For all other values the registration procedure is "Expert Review". For Extensions specified only for CBOR encoded certificates where no OID is defined, the OID and DER fields are marked "N/A". If it is not expected to be understood from the other information (e.g. the OID), then the Comments field must contain a reference to where the Extension is described. For values in the interval [1, 23] the registration procedure is "IETF Review with Expert Review". Values 32768 are reserved for Private Use. The initial contents of the registry are:

C509 Extensions

C509 Certificate Policies Registry IANA has created a new registry titled "C509 Certificate Policies" under the registry group "CBOR Encoded X.509 (C509)". 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 all other values the registration procedure is "Expert Review". For Certificate Policies specified only for CBOR encoded certificates where no OID is defined, the OID and DER fields are marked "N/A". If it is not expected to be understood from the other information (e.g. the OID), then the Comments field must contain a reference to where the Certificate Policy is described. For values in the interval [-24, 23] the registration procedure is "IETF Review with Expert Review". Values 32768 are reserved for Private Use. The initial contents of the registry are:

C509 Certificate Policies

C509 Policies Qualifiers Registry IANA has created a new registry titled "C509 Policies Qualifiers" under the registry group "CBOR Encoded X.509 (C509)". 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. If it is not expected to be understood from the other information (e.g. the OID), then the Comments field must contain a reference to where the Policy Qualifier is described. 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" under the registry group "CBOR Encoded X.509 (C509)". 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. If it is not expected to be understood from the other information (e.g. the OID), then the Comments field must contain a reference to where the Information Access is described. 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" under the registry group "CBOR Encoded X.509 (C509)". 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 Extended Key Usage specified only for CBOR encoded certificates where no OID is defined, the OID and DER fields are marked "N/A". If it is not expected to be understood from the other information (e.g. the OID), then the Comments field must contain a reference to where the Extended Key Usage is described. 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" under the registry group "CBOR Encoded X.509 (C509)". The fields of the registry are Value, Name, Comments, GeneralNameValue, and Reference, where Value is an integer, and the other columns are text strings. The fields Name and GeneralNameValue are mandatory. If it is not expected to be understood from the other information (e.g. the OID), then the Comments field must contain a reference to where the General Name is described. 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 registry group "CBOR Encoded X.509 (C509)". 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. Alignment with the value of public key algorithm must be considered, see instruction in . If it is not expected to be understood from the other information (e.g. the OID), then the Comments field must contain a reference to where the Signature Algorithm is described. 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 registry group "CBOR Encoded X.509 (C509)". 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. If the public key can only be used with one signature algorithm and the OID of the public key algorithm is the same as the signature algorithm, then the value must be chosen equal to the value of signature algorithm, see . If it is not expected to be understood from the other information (e.g. the OID), then the Comments field must contain a reference to where the Public Key Algorithm is described. 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

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

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

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

Media Type application/cose-c509-cert+cbor When the application/cose-c509-cert+cbor media type is used, the data is a C509Certificate structure. Type name: application Subtype name: cose-c509-cert+cbor 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 use C509 as a certificate type. Fragment identifier considerations: N/A Additional information:

Deprecated alias names for this type: N/A
Magic number(s): TBD20
File extension(s): .c509
Macintosh file type code(s): N/A