]> RISE AB

shahid.raza@ri.se

RISE AB

joel.hoglund@ri.se

Ericsson AB

goran.selander@ericsson.com

Ericsson AB

john.mattsson@ericsson.com

Nexus Group

martin.furuhed@nexusgroup.com

This document specifies a CBOR encoding of PKIX profiled X.509 Certificates. The resulting certificates are called “CBOR certificates”. The CBOR encoding supports a large subset of RFC 5280, while at the same time producing very small sizes for certificates compatible with RFC 7925. The CBOR encoding can be used to compress DER encoded X.509 certificated to encode natively signed certificated. When uses to compress DER encoded X.509 certificates, the CBOR encoding can in many cases compress RFC 7925 profiled certificates with over 50%. The document also specifies COSE headers for CBOR certificates as well as a TLS certificate type for CBOR certificates.

One of the challenges with deploying a Public Key Infrastructure (PKI) for the Internet of Things (IoT) is the size and encoding of X.509 public key certificates , since those are not optimized for constrained environments . More compact certificate representations are desirable. 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. 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 . RFC 7925 specifies a certificate profile for Internet of Things deployments which can be applied for lightweight certificate based authentication with e.g. TLS , DTLS , COSE , or EDHOC . This document specifies a CBOR encoding which can support large parts of based on . The encoding support all profiled X.509 certificates. Two variants are defined using the same CBOR encoding and differing only in what is being signed: CBOR compression of DER encoded X.509 certificates , which can be decompressed into the original DER encoded X.509 certificate. Natively signed CBOR certificates, which further optimizes the performance in constrained environments but is not backwards compatible with , see . This document specifies COSE headers for use of the CBOR certificates with COSE, see . The document also specifies a TLS certificate type for use of the CBOR certificates with TLS (with or without additional TLS certificate compression), see .

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 .

This section specifies the content and encoding for CBOR certificates, with the overall objective to produce a very compact representation supporting large parts of and everything in . In the CBOR encoding, static fields are elided, elliptic curve points are compressed, OID are replaced with short integers, time values are compressed, and redundant encoding is removed. Combining these different components reduces the certificate size significantly, which is not possible with general purpose compressions algorithms, see . The CBOR certificate can be either a CBOR compressed 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 CBOR certificate, in which case the signature is calculated directly on the CBOR encoded data (see ). In both cases the certificate content is adhering to the restrictions given by . When used as for compression of an existing X.509 certificate, the encoding only works on canonical encoded certificates. The encoding is known to work with DER but might work with other canonical encodings. The compression 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.

The X.509 fields and their CBOR encodings are listed below. CBOR certificates are defined in terms of DER encoded X.509 certificates: version. The ‘version’ field is known (fixed to v3) and is omitted in the CBOR encoding. serialNumber. The ‘serialNumber’ INTEGER value field is encoded as a CBOR byte string ‘certificateSerialNumber’. Any leading 0x00 byte (to indicate that the number is not negative) is omitted. signatureAlgorithm. The ‘signatureAlgorithm’ field is encoded as a CBOR int ‘issuerSignatureAlgorithm’ (see ) or a relativeOID byte string. Algorithms with parameters are not supported except RSA algorithms that use parameters = NULL. signature. The ‘signature’ field is always the same as the ‘signatureAlgorithm’ field and always omitted from the CBOR encoding. issuer. In the general case, the sequence of ‘RelativeDistinguishedName’ is encoded as CBOR array of CBOR arrays of Attributes, where each Attribute type and value is encoded as a (CBOR int, CBOR text string) pair. Each AttributeType is encoded as a CBOR int (see ), where the sign is used to represent the character string type; positive for printableString, negative for utf8String. The string types teletexString, universalString, and bmpString are not supported. If exactly one ‘RelativeDistinguishedName’ is present, the outer array is omitted, and issuer is encoded as a single CBOR array. If a RelativeDistinguishedName contains a single Attribute containing an utf8String encoded ‘common name’, the int is omitted and the Attribute is encoded as a single CBOR text string. If the utf8String encoded ‘common name’ contains an EUI-64 mapped from a 48-bit MAC address (i.e. of the form “hh-hh-hh-FF-FE-hh-hh-hh) it is encoded as a CBOR byte string of length 6. Other EUI-64 is encoded as a CBOR byte string of length 8. validity. The ‘notBefore’ and ‘notAfter’ fields are ASCII string of the form “yymmddHHMMSSZ” for UTCTime and “yyyymmddHHMMSSZ” for GeneralizedTime. They are encoded as unsigned integers using the following invertible encoding (Horner’s method with different bases). n = SS + 61 * (MM + 60 * (HH + 24 * (dd + 32 * (mm + 13 * (yy)yy)))) They are encoded as a byte string, which is interpreted as an unsigned integer n in network byte order. UTCTime and GeneralizedTime are encoded as a byte strings of length 4 and 5 respectively. Decoding can be done by a succession of modulo and subtraction operations. I.e. SS = n mod 61, MM = ((n - SS) / 61) mod 60, etc. subject. The ‘subject’ is encoded exactly like issuer. subjectPublicKeyInfo. The ‘algorithm’ field is encoded as the CBOR int ‘subjectPublicKeyAlgorithm’ (see ) or a relativeOID byte string. Algorithms with parameters are not supported except id-ecPublicKey with named curves and the RSA algorithms that use parameters = NULL. For id-ecPublicKey the namedCurve parameter is encoded in the CBOR int. 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. Uncompressed public keys of type id-ecPublicKey are point compressed as defined in Section 2.3.3 of . If a DER encoded certificate with a point compressed public key of type id-ecPublicKey is comreseed, the octets 0xfe and 0xfd are used instead of 0x02 and 0x03 in the CBOR encoding to represent a even and off y-coordinate respectively. extensions. The ‘extensions’ field is encoded as a CBOR array where each extension is encoded as either a registered extension (an CBOR int followed by an optional CBOR item of any type) or a raw extension (a relative OID byte string, a bool, and a the DER encoved value of ‘extnValue’). If the array contains exactly one int, the array is omitted. Extensions are encoded as specified in . The extensions mandated to be supported by are given special treatment. signatureValue. 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. ECDSA signatures are given special treatment. For ECDSA signatures the SEQUENCE and INTEGER type and length fields are omitted and the two INTEGER value fields are padded to the fixed length L = ceil( log2(n) / 8 ), where n is the size of the largest prime-order subgroup. For secp256r1, secp384r1, and secp521r1, L is 32, 48, and 66 respectively. For natively signed CBOR certificates the signatureValue is calculated over the CBOR sequence TBSCertificate. In addition to the above fields present in X.509, the CBOR encoding introduces an additional field: cborCertificateType. A CBOR int used to indicate the type of CBOR certificate. Currently, type can be a natively signed CBOR certificate (cborCertificateType = 0) or a CBOR compressed X.509 certificates (cborCertificateType = 1), see . The following Concise Data Definition Language (CDDL) defines CBORCertificate and TBSCertificate, which are encoded as CBOR Sequences . The member names therefore only have documentary value.

EDITOR’S NOTE: The current specification encodes many common extensions with a DER encoded byte string. It should be discussed if more or all commonly active extensions should be natively encoded with CBOR. Would an specific CBOR encoding have to be specified for each extension or can a general CBOR encoding that apply to all remaining extensions be specified? This section details the encoding of the ‘extensions’ field. The ‘extensions’ field is encoded as a CBOR array where each extension is encoded as either a registered extension (an CBOR int followed by an optional CBOR item of any type) or a raw extension (a relative OID byte string, a bool, and a the DER encoved value of ‘extnValue’). For registered extensions each ‘extnID’ field is encoded as a CBOR int (see ), where the sign is used to encode if the extension ‘critical’ field. Critical extensions are encoded with a positive sign and non-critical extensions are encoded with a negative sign. If the array contains exactly one int, the array is omitted. The ‘extnValue’ OCTET STREAM value field is encoded as the CBOR byte string ‘extensionValue’ except for the extensions specified below. The extensions mandated to be supported by are given special treatment. Below the boolean values (cA, digitalSignature, keyAgreement, etc.) are set to 0 or 1 according to their value in the DER encoding.: basicConstraints. A basic constrained with ‘cA’ = false is encoded as extensionType = 1, a basic constrained with ‘cA’ = true without ‘pathLenConstraint’ is encoded as extensionType = 2, and a basic constrained with ‘cA’ = true with ‘pathLenConstraint’ is encoded as extensionType = 3 followed by and int extensionValue encoding the value of ‘pathLenConstraint’. keyUsage. The extensionType is encoded as below. If none of the bits except digitalSignature, keyAgreement, and keyCertSign are set, the extensionValue is omitted. Otherwise the ‘KeyUsage’ BIT STRING is interpreted as an unsigned integer n in network byte order and encoded as a CBOR int.

extKeyUsage. extensionType is encoded as defined by and extensionValue is encoded as an array of ints or relativeOID where each ints or relativeOID encodes a key usage purpose (see for registered ints). If the array contains a single item, the array is omitted.

subjectAltName. extensionType is encoded as defined by and extensionValue is encoded as an [ * ( int, any ) ] array where each (int, any) 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. Consequently: A critical basicConstraints (cA = 1) without pathLenConstraint is encoded as the CBOR int -2. A non-critical keyUsage with only keyAgreement asserted is encoded as the CBOR int 6 (= 4 + 2). A non-critical extKeyUsage containing id-kp-codeSigning and id-kp-OCSPSigning is encoded as the CBOR int 12 followed by the CBOR array [ 3, 6 ]. A non-critical subjectAltName containing only the dNSName example.com is encoded as the CBOR int 13 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 [ -2, 6, 12, [ 3, 6 ], 13, “example.com” ].

For general purpose applications, the normative requirements of applies. This section describes the mandatory to implement algorithms and OIDs for constrained IoT application; the values of the OIDs including certificate fields and extensions, time format, attributes in distinguished names, etc. TODO: Write this section

CBOR certificates can be deployed with legacy X.509 certificates and CA infrastructure. In order to verify the signature, the CBOR certificate is used to recreate the original X.509 data structure to be able 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 CBOR 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 CBOR 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 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 CBOR 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.

The CBOR encoding of the sample certificate given in results in the numbers shown 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 will be small, mainly corresponding to making the ASN.1 encoding more compact. The zlib number was calculated with zlib-flate.

cert.compressed ]]>

The difference between CBOR compressed X.509 certificate and natively signed CBOR certificate is that the signature is calculated over the CBOR encoding of the CBOR sequence TBSCertficate rather than the DER encoded ASN.1 data. This removes entirely the need for ASN.1 DER and base64 encoding which reduces the processing in the authenticating devices and avoids known complexities with these encoding. Natively signed CBOR certificates can be applied in devices that are only required to authenticate to natively signed CBOR certificate compatible servers. This is not a major restriction for many IoT deployments, where the parties issuing and verifying certificates can be a restricted ecosystem which not necessarily involves public CAs. CBOR compressed X.509 certificates provides an intermediate step between profiled X.509 certificates and natively signed CBOR certificates: An implementation of CBOR compressed X.509 certificates contains both the CBOR encoding of the X.509 certificate and the signature operations sufficient for natively signed CBOR certificates. The natively signed approach based on DER encoded X.509 certificates described in this document has a lot of benefits. A CA can use existing ASN.1 machinery to create a DER encoded certificate, the DER encoded certificate can then be transformed to CBOR before signing.

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. 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.

For all items, the ‘Reference’ field points to this document.

IANA has created a new registry titled “CBOR Certificate Types” under the new heading “CBOR Certificate”. For values in the interval [-24, 23] the registration procedure is “IETF Review”. For all other values the registration procedure is “Expert Review”. The columns of the registry are Value, Description, and Reference, where Value is an integer, and the other columns are text strings. The initial contents of the registry are:

IANA has created a new registry titled “CBOR Attribute Type Registry” under the new heading “CBOR Certificate”. The columns of the registry are Value, X.509 Attribute Type, and Reference, where Value is an integer, and the other columns are text strings. Only positive values can be registered. For values in the interval [1, 23] the registration procedure is “IETF Review”. For all other values the registration procedure is “Expert Review”. The initial contents of the registry are:

IANA has created a new registry titled “CBOR Extension Type Registry” under the new heading “CBOR Certificate”. The columns of the registry are Value, X.509 Extension Type, and Reference, where Value is an integer, and the other columns are text strings. Only positive values can be registered. For values in the interval [1, 23] the registration procedure is “IETF Review”. For all other values the registration procedure is “Expert Review”. The initial contents of the registry are:

IANA has created a new registry titled “CBOR Extended Key Usage Registry” under the new heading “CBOR Certificate”. The columns of the registry are Value, Extended Key Usage Purpose, 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”. For all other values the registration procedure is “Expert Review”. The initial contents of the registry are:

IANA has created a new registry titled “CBOR Subject Alternative Name Registry” under the new heading “CBOR Certificate”. The columns of the registry are Value, Extended Key Usage Purpose, 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”. For all other values the registration procedure is “Expert Review”. The initial contents of the registry are:

IANA has created a new registry titled “CBOR Certificate Signature Algorithms” under the new heading “CBOR Certificate”. For values in the interval [-24, 23] the registration procedure is “IETF Review”. For all other values the registration procedure is “Expert Review”. The columns of the registry are Value, X.509 Algorithm, and Reference, where Value is an integer, and the other columns are text strings. The initial contents of the registry are: EDITOR’S NOTE: This is probably to many algorithms. All sha224, sha3, and maybe ecdsa-with-SHA1 can probably be removed.

IANA has created a new registry titled “CBOR Certificate Public Key Algorithms” under the new heading “CBOR Certificate”. For values in the interval [-24, 23] the registration procedure is “IETF Review”. For all other values the registration procedure is “Expert Review”. The columns of the registry are Value, X.509 Algorithm, and Reference, where Value is an integer, and the other columns are text strings. The initial contents of the registry are:

This document registers the following entries in the “COSE Header Parameters” registry under the “CBOR Object Signing and Encryption (COSE)” heading. The formatting and processing are the same as the corresponding x5bag, x5chain, x5t, and x5u defined in except that the certificates are CBOR encoded instead of DER encoded.

This document registers the following entry in the “TLS Certificate Types” registry under the “Transport Layer Security (TLS) Extensions” heading. 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 CBOR certificates, it could perhaps be presented in the TLS WG. The TLS WG might e.g. want a separate draft in the TLS WG.

&RFC2119; &RFC5280; &RFC7049; &RFC7925; &RFC8152; &RFC8174; &RFC8446; &RFC8610; &RFC8742; &I-D.ietf-tls-dtls13; &I-D.ietf-tls-certificate-compression; &RFC7228; &I-D.ietf-cose-x509; &I-D.ietf-lake-edhoc;

Example of profiled X.509 certificate parsed with OpenSSL.

The DER encoding of the above certificate is 314 bytes.

The CBOR certificate compression of the X.509 in CBOR diagnostic format is:

The CBOR encoding (CBOR sequence) of the CBOR certificate is 138 bytes.

The corresponding natively signed CBOR certificate in CBOR diagnostic format is identical except for type and signatureValue.

The CBOR encoding (CBOR sequence) of the CBOR certificate is 138 bytes.

The DER encoding of the tools.ietf.org certificate is 1647 bytes.

The CBOR certificate compression of the X.509 in CBOR diagnostic format is:

The CBOR encoding (CBOR sequence) of the CBOR certificate is 1374 bytes.

EDITOR’S NOTE: The ASN.1 below is not up to date with the rest of the specification. The below ASN.1 for RFC 7925 profile should be in draft-ietf-uta-tls13-iot-profile instead. If CBOR Certificates support a large subset of RFC 5280, we should probably not duplicate all the ASN.1 in that document. Should be discussed what kind and how much (if any) ASN.1 this document needs. If possible, one option would be to have ASN.1 for the restrictions compared to RFC 5280.

The authors want to thank Henk Birkholz, Carsten Bormann, Russ Housley, Ilari Liusvaara, Laurence Lundblade, Thomas Peterson, Michael Richardson, Jim Schaad, and Rene Struik for reviewing and commenting on intermediate versions of the draft.