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Table of Contents

1. Introduction

In the process of writing [RFC8152] and [RFC9052], the CBOR Object Signing and Encryption (COSE) Working Group discussed X.509 certificates [RFC5280] and decided that no use cases were presented that showed a need to support certificates. Since that time, a number of cases have been defined in which X.509 certificate support is necessary, and by implication, applications will need a documented and consistent way to handle such certificates. This document defines a set of attributes that will allow applications to transport and refer to X.509 certificates in a consistent manner.¶

In some of these cases, a constrained device is being deployed in the context of an existing X.509 PKI: for example, [Constrained-BRSKI] describes a device enrollment solution that relies on the presence of a factory-installed certificate on the device. [EDHOC] was also written with the idea that long-term certificates could be used to provide for authentication of devices and establish session keys. Another possible scenario is the use of COSE as the basis for a secure messaging application. This scenario assumes the presence of long-term keys and a central authentication authority. Basing such an application on public key certificates allows it to make use of well-established key management disciplines.¶

2. X.509 COSE Header Parameters

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 Concise Binary Object Representation (CBOR) key structures [RFC8949] that have been defined in COSE currently do not support all of these properties, although some may be found in CBOR Web Tokens (CWTs) [RFC8392].¶

It is not necessarily expected that constrained devices themselves will evaluate and process X.509 certificates: it is perfectly reasonable for a constrained device to be provisioned with a certificate that it subsequently provides to a relying party -- along with a signature or encrypted message -- on the assumption that the relying party is not a constrained device and is capable of performing the required certificate evaluation and processing. It is also reasonable that a constrained device would have the hash of a certificate associated with a public key and be configured to use a public key for that thumbprint, but without performing the certificate evaluation or even having the entire certificate. In any case, there still needs to be an entity that is responsible for handling the possible certificate revocation.¶

Parties that intend to rely on the assertions made by a certificate obtained from any of these methods still need to validate it. This validation can be done according to the PKIX rules specified in [RFC5280] or by using a different trust structure, such as a trusted certificate distributor for self-signed certificates. The PKIX validation includes matching against the trust anchors configured for the application. These rules apply when the validation succeeds in a single step as well as when certificate chains need to be built. If the application cannot establish trust in the certificate, the public key contained in the certificate cannot be used for cryptographic operations.¶

The header parameters defined in this document are as follows:¶

x5bag:

This header parameter contains a bag of X.509 certificates. The set of certificates in this header parameter is unordered and may contain self-signed certificates. Note that there could be duplicate certificates. The certificate bag can contain certificates that are completely extraneous to the message. (An example of this would be where a signed message is being used to transport a certificate containing a key agreement key.) As the certificates are unordered, the party evaluating the signature will need to be capable of building the certificate path as necessary. That party will also have to take into account that the bag may not contain the full set of certificates needed to build any particular chain.¶

The trust mechanism MUST process any certificates in this parameter as untrusted input. The presence of a self-signed certificate in the parameter MUST NOT cause the update of the set of trust anchors without some out-of-band confirmation. As the contents of this header parameter are untrusted input, the header parameter can be in either the protected or unprotected header bucket. Sending the header parameter in the unprotected header bucket allows an intermediary to remove or add certificates.¶

The end-entity certificate MUST be integrity protected by COSE. This can, for example, be done by sending the header parameter in the protected header, sending an 'x5bag' in the unprotected header combined with an 'x5t' in the protected header, or including the end-entity certificate in the external_aad.¶

This header parameter allows for a single X.509 certificate 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 byte string.¶
• If multiple certificates are conveyed, a CBOR array of byte strings is used, with each certificate being in its own byte string.¶

x5chain:

This header parameter 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 that 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 already has, or can locate, the missing certificates. This means that the relying party is still required to do path building but that a candidate path is proposed in this header parameter.¶

The trust mechanism MUST process any certificates in this parameter as untrusted input. The presence of a self-signed certificate in the parameter MUST NOT cause the update of the set of trust anchors without some out-of-band confirmation. As the contents of this header parameter are untrusted input, the header parameter can be in either the protected or unprotected header bucket. Sending the header parameter in the unprotected header bucket allows an intermediary to remove or add certificates.¶

The end-entity certificate MUST be integrity protected by COSE. This can, for example, be done by sending the header parameter in the protected header, sending an 'x5chain' in the unprotected header combined with an 'x5t' in the protected header, or including the end-entity certificate in the external_aad.¶

This header parameter allows for a single X.509 certificate 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 byte string.¶
• If multiple certificates are conveyed, a CBOR array of byte strings is used, with each certificate being in its own byte string.¶

x5t:

This header parameter identifies the end-entity X.509 certificate by a hash value (a thumbprint). The 'x5t' header parameter is represented as an array of two elements. The first element is an algorithm identifier that is an integer or a string containing the hash algorithm identifier corresponding to the Value column (integer or text string) of the algorithm registered in the "COSE Algorithms" registry (see <https://www.iana.org/assignments/cose/>). The second element is a binary string containing the hash value computed over the DER-encoded certificate.¶

As this header parameter does not provide any trust, the header parameter can be in either a protected or unprotected header bucket.¶

The identification of the end-entity certificate MUST be integrity protected by COSE. This can be done by sending the header parameter in the protected header or including the end-entity certificate in the external_aad.¶

The 'x5t' header parameter can be used alone or together with the 'x5bag', 'x5chain', or 'x5u' header parameters to provide integrity protection of the end-entity certificate.¶

For interoperability, applications that use this header parameter MUST support the hash algorithm 'SHA-256' but can use other hash algorithms. This requirement allows for different implementations to be configured to use an interoperable algorithm, but does not preclude the use (by prior agreement) of other algorithms.¶

x5u:

This header parameter provides the ability to identify an X.509 certificate by a URI [RFC3986]. It contains a CBOR text string. The referenced resource can be any of the following media types:¶

• application/pkix-cert [RFC2585]¶
• application/pkcs7-mime; smime-type="certs-only" [RFC8551]¶
• application/cose-x509 (Section 4.3)¶
• application/cose-x509; usage=chain (Section 4.3)¶

When the application/cose-x509 media type is used, the data is a CBOR sequence of single-entry COSE_X509 structures (encoding "bstr"). If the parameter "usage" is set to "chain", this sequence indicates a certificate chain.¶

The end-entity certificate MUST be integrity protected by COSE. This can, for example, be done by sending the 'x5u' in the unprotected or protected header combined with an 'x5t' in the protected header, or including the end-entity certificate in the external_aad. As the end-entity certificate is integrity protected by COSE, the URI does not need to provide any protection.¶

If a retrieved certificate does not chain to an existing trust anchor, that certificate MUST NOT be trusted unless the URI provides integrity protection and server authentication and the server is configured as trusted to provide new trust anchors or if an out-of-band confirmation can be received for trusting the retrieved certificate. If an HTTP or Constrained Application Protocol (CoAP) GET request is used to retrieve a certificate, TLS [RFC8446], DTLS [RFC9147], or Object Security for Constrained RESTful Environments (OSCORE) [RFC8613] SHOULD be used.¶

The header parameters are used in the following locations:¶

COSE_Signature and COSE_Sign1 objects:

In these objects, the parameters identify the certificate to be used for validating the signature.¶

COSE_recipient objects:

In this location, the parameters identify the certificate for the recipient of the message.¶

The labels assigned to each header parameter can be found in Table 1.¶

Below is an equivalent Concise Data Definition Language (CDDL) description (see [RFC8610]) of the text above.¶

COSE_X509 = bstr / [ 2*certs: bstr ]
COSE_CertHash = [ hashAlg: (int / tstr), hashValue: bstr ]

The contents of "bstr" are the bytes of a DER-encoded certificate.¶

3. X.509 Certificates and Static-Static ECDH

The header parameters defined in the previous section are used to identify the recipient certificates for the Elliptic Curve Diffie-Hellman (ECDH) key agreement algorithms. In this section, we define the algorithm-specific parameters that are used for identifying or transporting the sender's 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 parameter contains the chain of certificates starting with the sender's key exchange certificate. The structure is the same as 'x5chain'.¶

x5t-sender:

This header parameter contains the hash value for the sender's key exchange certificate. The structure is the same as 'x5t'.¶

x5u-sender:

This header parameter contains a URI for the sender's key exchange certificate. The structure and processing are the same as 'x5u'.¶

4. IANA Considerations

5. Security Considerations

Establishing trust in a certificate is a vital part of processing. A major component of establishing trust is determining what the set of trust anchors are for the process. A new self-signed certificate appearing on the client cannot be a trigger to modify the set of trust anchors, because a well-defined trust-establishment process is required. One common way for a new trust anchor to be added to (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. The resulting network access can consume power and network bandwidth. On the other hand, if the certificates are validated after the signature is validated, an oracle can potentially be built based on detecting the network resources, which is only done if the signature validation passes. In any event, both the signature validation and the certificate validation MUST be completed successfully before acting on any requests.¶

Unless it is known that the Certificate Authority (CA) required proof of possession of the subject's private key to issue an end-entity certificate, the end-entity certificate MUST be integrity protected by COSE. Without proof of possession, an attacker can trick the CA into issuing an identity-misbinding certificate with someone else's "borrowed" public key but with a different subject. An on-path attacker can then perform an identity-misbinding attack by replacing the real end-entity certificate in COSE with such an identity-misbinding certificate.¶

End-entity X.509 certificates contain identities that a passive on-path attacker eavesdropping on the conversation can use to identify and track the subject. COSE does not provide identity protection by itself, and the 'x5t' and 'x5u' header parameters are just alternative permanent identifiers and can also be used to track the subject. To provide identity protection, COSE can be sent inside another security protocol providing confidentiality.¶

Before using the key in a certificate, the key MUST be checked against the algorithm to be used, and any algorithm-specific checks need to be made. These checks can include validating that points are on curves for elliptical curve algorithms and that the sizes of RSA keys are within an acceptable range. The use of unvalidated keys can lead to either loss of security or excessive consumption of resources (for example, using a 200K RSA key).¶

When processing the 'x5u' header parameter, the security considerations of [RFC3986], and specifically those defined in Section 7.1 of [RFC3986], also apply.¶

Regardless of the source, certification path validation is an important part of establishing trust in a certificate. Section 6 of [RFC5280] provides guidance for the path validation. The security considerations of [RFC5280] are also important for the correct usage of this document.¶

Protecting the integrity of the 'x5bag', 'x5chain', and 'x5t' contents by placing them in the protected header bucket can help mitigate some risks of a misbehaving CA (cf. Section 5.1 of [RFC2634]).¶

The security of the algorithm used for 'x5t' does not affect the security of the system, as this header parameter selects which certificate that is already present on the system should be used, but it does not provide any trust.¶

Acknowledgements

Jim Schaad passed on 3 October 2020. This document is primarily his work. Ivaylo Petrov served as the document editor after Jim's untimely death, mostly helping with the approval and publication processes. Jim deserves all credit for the technical content.¶

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