]> 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/compression of RFC 7925 profiled certificates. By using the fact that the certificates are profiled, the CBOR certificate compression algorithms can in many cases compress RFC 7925 profiled certificates with over 50%. This document also specifies COSE headers for CBOR encoded certificates as well as the use of the CBOR certificate compression algorithm with TLS Certificate Compression in TLS 1.3 and DTLS 1.3.

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 X.509 certificates, keeping X.509 compatibility 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 the CBOR encoding/compression of RFC 7925 profiled X.509 certificates based on . Two variants are defined using exactly the same CBOR encoding and differing only in what is being signed: The CBOR compressed X.509 certificate, which can be decompressed into a certificate that can be verified by code compatible with RFC 7925. The “native” CBOR encoded certificate, which further optimizes the performance in constrained environments but is not backwards compatible with RFC 7925, see . Other work has looked at reducing the size of X.509 certificates. The purpose of this document is to stimulate a discussion on CBOR based certificates: what field values (in particular for ‘issuer’/’subject’) are relevant for constrained IoT applications, what is the maximum compression that can be expected with CBOR, and what is the right trade-off between compactness and generality. This document specifies COSE headers for use of the CBOR certificate encoding with COSE. The document also specifies the CBOR certificate compression algorithm for use as TLS Certificate Compression with TLS 1.3 and DTLS 1.3.

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 .

This section specifies the content and encoding for CBOR certificates, with the overall objective to produce a very compact representation of the certificate profile defined in . 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 native 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 . The corresponding ASN.1 schema is given in . The encoding and compression has several components including: ASN.1 DER and base64 encoding are replaced with CBOR encoding, static fields are elided, and elliptic curve points are compressed. The X.509 fields and their CBOR encodings are listed below. Combining these different components reduces the certificate size significantly, which is not possible with general purpose compressions algorithms, see . CBOR certificates are defined in terms of RFC 7925 profiled X.509 certificates: version. The ‘version’ field is known (fixed to v3), and is omitted in the CBOR encoding. serialNumber. The ‘serialNumber’ field is encoded as a CBOR byte string. This allows encoding of all lengths with minimal overhead. 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 Distinguished Name is encoded as CBOR map, but if only CN is present the value can be encoded as a single text value. validity. The ‘notBefore’ and ‘notAfter’ UTCTime fields are ASCII string of the form “yymmddHHMMSSZ”. They are encoded as the unsigned integers using the following invertible encoding (Horner’s method with different bases). The resulting integer n always fit in a 32 bit usigned integer. n = SS + 60 * (MM + 60 * (HH + 24 * (dd + 32 * (mm + 13 * yy)))) Decoding can be done by a succession of modulo and substraction operations. I.e. SS = n mod 60, MM = ((n - SS) / 60) mod 60, etc. subject. The ‘subject’ field is restricted to specifying the value of the common name. By RFC 7925 an IoT subject is identified by either an EUI-64 for clients, or by a FQDN for servers. An EUI-64 mapped from a 48-bit MAC address is encoded as a CBOR byte string of length 6. Other EUI-64 is encoded as a CBOR byte string of length 8. A FQDN is encoded as a CBOR text string. subjectPublicKeyInfo. If the ‘algorithm’ field is the default (id-ecPublicKey and prime256v1), it is omitted in the CBOR encoding, otherwise it is included in the subjectPublicKeyInfo_algorithm field encoded as an int, (see ). The ‘subjectPublicKey’ is encoded as a CBOR byte string. Public keys of type id-ecPublicKey are point compressed as defined in Section 2.3.3 of . extensions. The ‘extensions’ field is encoded as a CBOR array where each extension is represented with an int. This is the most compact representation of the allowed extensions. The extensions mandated to be supported by RFC 7925 is encodeded as specified below, where critical extensions are encoded with a negative sign. TODO: need to make things mod 3 instead. I.e. non-critical keyUsage keyAgreement is encoded as 5, critical basicConstraints cA is encodes as -3, and non-criticical extKeyUsage id-kp-codeSigning + id-kp-OCSPSigning is encoded as 22. If subjectAltName is present, the value is placed at the end of the array encoded as a byte or text string following the encoding rules for the subject field. If the array contains a single int, extensions is encoded as the int instead of an array.

signatureAlgorithm. If the ‘signatureAlgorithm’ field is the default (ecdsa-with-SHA256) it is omitted in the CBOR encoding, otherwise it is included in the signatureAlgorithm field encoded as an CBOR int (see ). signatureValue. Since the signature algorithm and resulting signature length are known, padding and extra length fields which are present in the ASN.1 encoding are omitted and the ‘signatureValue’ field is encoded as a CBOR byte string. For native CBOR certificates the signatureValue is calculated over the certificate CBOR sequence excluding the signatureValue. In addition to the above fields present in X.509, the CBOR ecoding introduces an additional field type. A CBOR int used to indicate the type of CBOR certificate. Currently type can be a native CBOR certificate (type = 0) or a CBOR compressed X.509 certificates (type = 1), see . The following Concise Data Definition Language (CDDL) defines a group, the elements of which are to be used in an unadorned CBOR Sequence . The member names therefore only have documentary value.

bytes } / text, validity_notBefore: uint, validity_notAfter: uint, subject : text / bytes subjectPublicKey : bytes extensions : [ *4 int, ? text / bytes ] / int, signatureValue : bytes, ? ( signatureAlgorithm : int, subjectPublicKeyInfo_algorithm : int ) ) ]]>

The signatureValue for native CBOR certificates is calculated over the CBOR sequence:

bytes } / text, validity_notBefore: uint, validity_notAfter: uint, subject : text / bytes subjectPublicKey : bytes extensions : [ *4 int, ? text / bytes ] / int, ? ( signatureAlgorithm : int, subjectPublicKeyInfo_algorithm : int ) ) ]]>

TODO - Specify exactly how issuer is encoded into a map / text and back again. This is a compromise between compactness and complete generality.

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 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 will be small, mainly corresponding to making the ASN.1 endcoding more compact. The zlib number was calculated with zlib-flate.

cert.compressed ]]>

The difference between CBOR compressed X.509 certificate and native CBOR certificate is that the signature is calculated over the CBOR encoding 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 encodings. Native CBOR certificates can be applied in devices that are only required to authenticate to native 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 RFC 7925 profiled X.509 certificates and native 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 native CBOR certificates.

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”. 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 Certificate Signature Algorithms” under the new heading “CBOR Certificate”. 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:

IANA has created a new registry titled “CBOR Certificate Public Key Algorithms” under the new heading “CBOR Certificate”. 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 x5chain and x5u defined in except that the certificates are CBOR encoded instead of DER encoded.

This document registers the following entry in the “Certificate Compression Algorithm IDs” registry under the “Transport Layer Security (TLS) Extensions” heading.

&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 RFC 7925 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 136 bytes.

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

The CBOR encoding (CBOR sequence) of the CBOR certificate is 136 bytes.

TODO - This ASN.1 profile should probably be in a document that updates RFC 7925.