CBOR Object Signing and Encryption (COSE): Headers for carrying and referencing X.509 certificates

Introduction In the process of writing discussions where held on the question of X.509 certificates and if there was a needed to provide for them. At the time there were no use cases presented that appeared to have a sufficient need for these attributes. Since that time a number of cases where X.509 certificate support is necessary have been defined. This document provides a set of attributes that will allow applications to transport and refer to X.509 certificates in a consistent manner. Some of the constrained device situations are being used where an X.509 PKI is already installed. One of these situations is the 6tish environment for enrollment of devices where the certificates are installed at the factory. The draft was also written with the idea that long term certificates could be used to provide for authentication of devices and uses them to establish session keys. A final scenario is the use of COSE as a messaging application where long term existence of keys can be used along with a central authentication authority. The use of certificates in this scenario allows for key management to be used which is well understood. Example COSE messages for the various headers defined below can be found at https://github.com/cose-wg/Examples. THIS IS NOT YET DONE BUT SHOULD BE COMING NOT LONG AFTER THE F2F MEETING.

Requirements Terminology 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.

Open Questions

Should we define an extended key usage?
Revocation info?

X.509 COSE Headers The use of X.509 certificates allows for an existing trust infrastructure to be used with COSE. This includes the full suite of enrollment protocols, trust anchors, trust chaining and revocation checking that have been defined over time by the IETF and other organizations. The key structures that have been defined in COSE currently do not support all of these properties although some may be found in COSE Web Tokens (CWT) . It is not necessarily expected that constrained devices will fully support the evaluation and processing of X.509 certificates, it is perfectly reasonable for a certificate to be assigned to a device which it can then provide to a relying party along with a signature or encrypted message, the relying party not being a constrained device. Certificates obtained from any of these methods MUST still be validated. This validation can be done via the PKIX rules in or by using a different trust structure, such as a trusted certificate distributer for self-signed certificates. The PKIX validation includes matching against the trust anchors configured for the application. These rules apply to certificates of a chain length of one as well as longer chains. If the application cannot establish a trust in the certificate, then it cannot be used. The header attributes defined in this document are:

x5bag:

This header attributes contains a bag of X.509 certificates. The set of certificates in this header are unordered and may contain self-signed certificates. The certificate bag can contain certificates which are completely extraneous to the message. (An example of this would be to carry a certificate with a key agreement key usage in a signed message.) As the certificates are unordered, the party evaluating the signature will need to do the necessary path building. Certificates needed for any particular chain to be built may be absent from the bag. As this header element does not provide any trust, the header attribute can be in either a protected or unprotected header attribute. This header attribute allows for a single or a bag of X.509 certificates to be carried in the message.

If a single certificate is conveyed, it is placed in a CBOR bstr.
If multiple certificates are conveyed, a CBOR array of bstrs is used. Each certificate being in its own bstr.

x5chain:

This header attribute contains an ordered array of X.509 certificates. The certificates are to be ordered starting with the certificate containing the end-entity key followed by the certificate which signed it and so on. There is no requirement for the entire chain to be present in the element if there is reason to believe that the relying party will already have it. This means that the relying party is still required to do path building, but that a candidate path is proposed in this attribute. As this header element does not provide any trust, the header attribute can be in either a protected or unprotected header attribute. This header attribute allows for a single or a chain of X.509 certificates to be carried in the message.

If a single certificate is conveyed, it is placed in a CBOR bstr.
If multiple certificates are conveyed, a CBOR array of bstrs is used. Each certificate being in it's own slot.

x5t:

This header attribute provides the ability to identify an X.509 certificate by a hash value. The attribute is an array of two elements. The first element is an algorithm identifier which is an integer or a string containing the hash algorithm identifier. The second element is a binary string containing the hash value. As this header element does not provide any trust, the header attribute can be in either a protected or unprotected header attribute. For interoperability, applications which use this header attribute MUST support the hash algorithm 'sha256', but can use other hash algorithms.

x5u:

This header attribute provides the ability to identify an X.509 certificate by a URI. The referenced resource can be any of the following media types:

application/pkix-cert
application/pkcs7-mime; smime-type="certs-only"

As this header attribute implies a trust relationship, the attribute MUST be in the protected attributes. The URI provided MUST provide integrity protection and server authentication. For example, an HTTP or CoAP GET request to retrieve a certificate MUST use TLS or DTLS . If the certificate does not chain to an existing trust anchor, the certificate MUST NOT be trusted unless the server is configured as trusted to provide new trust anchors. This will normally be the situation when self-signed certificates are used.

The header attributes are used in the following locations:

COSE_Signature and COSE_Sign0 objects, in these objects they identify the key that was used for generating signature.
COSE_recipient objects, in this location they may be used to identify the certificate for the recipient of the message.

X.509 COSE Headers

Name	Value	value type	description
x5bag	TBD4	COSE_X509	An unordered bag of X.509 certificates
x5chain	TBD3	COSE_X509	An ordered chain of X.509 certificates
x5t	TBD1	COSE_CertHash	Hash of an X.509 certificate
x5u	TBD2	uri	URI pointing to an X.509 certificate

Below is an equivalent CDDL description of the text above.

X.509 certificates and static-static ECDH The header attributes defined in the previous section are used to identify the recipient certificates for the ECDH key agreement algorithms. In this section we define the algorithm specific parameters that are used for identifying or transporting the senders key for static-static key agreement algorithms. These attributes are defined analogously to those in the previous section. There is no definition for the certificate bag as the same attribute would be used for both the sender and recipient certificates.

x5chain-sender:: This header attribute contains the chain of certificates starting with the sender's key exchange certificate. The structure is the same as 'x5bag'.
x5t-sender:: This header attribute contains the hash value for the sender's key exchange certificate. The structure is the same as 'x5t'.
x5u-sender:: This header attribute contains a URI for the sender's key exchange certificate. The structure and processing are the same as 'x5u'.

Static ECDH Algorithm Values

Name	Value	Type	Algorithm	Description
x5t-sender	TBD	COSE_CertHash	ECDH-SS+HKDF-256, ECDH-SS+HKDF-512, ECDH-SS+A128KW, ECDH-SS+AES192KW, ECDH-SS+AES256KW	Thumbprint for the senders X.509 certificate
x5u-sender	TBD	uri	ECDH-SS+HKDF-256, ECDH-SS+HKDF-512, ECDH-SS+A128KW, ECDH-SS+AES192KW, ECDH-SS+AES256KW	URI for the senders X.509 certificate
x5chain-sender	TBD	COSE_X509	ECDH-SS+HKDF-256, ECDH-SS+HKDF-512, ECDH-SS+A128KW, ECDH-SS+AES192KW, ECDH-SS+AES256KW	static key X.509 certificate chain

IANA Considerations

COSE Header Parameter Registry IANA is requested to register the new COSE Header items in in the "COSE Header Parameters" registry.

COSE Header Algorithm Parameter Registry IANA is requested to register the new COSE Header items in in the "COSE Header Algorithm Parameters" registry.

Security Considerations Establishing trust in a certificate is a vital part of processing. Trust cannot be assumed whenever a new self-signed certificate appears on the client, instead a well defined process is required. One common way for a new trust anchor to be added (or removed) from a device is by doing a new firmware upgrade. In constrained systems, there is a trade-off between the order of checking the signature and checking the certificate for validity. Validating certificates can require that network resources be accessed in order to get revocation information or retrieve certificates during path building. Doing the network access can consume resources dealing with power and network bandwidth. On the other hand, an oracle can potentially be built based on if the network resources are only accessed if the signature validation passes. In any event, both the signature and certificate validation MUST be checked before acting on any requests. As called out in the COSE algorithms document basic checking on the keys in a certificate needs to be performed prior to using them. These can include validating that points are on curves for elliptical curve algorithms and that sizes of keys are acceptable for RSA. The use of unvalidated keys can lead either to loss of security or excessive consumption of resources.