Internet Engineering Task Force (IETF) J. Schaad

Internet Engineering Task Force (IETF) J. Schaad
Request for Comments: 9360 August Cellars
Category: Standards Track February 2023
ISSN: 2070-1721

CBOR Object Signing and Encryption (COSE): Header Parameters for
Carrying and Referencing X.509 Certificates

Abstract

The CBOR Object Signing and Encryption (COSE) message structure uses
references to keys in general. For some algorithms, additional
properties are defined that carry parameters relating to keys as
needed. The COSE Key structure is used for transporting keys outside
of COSE messages. This document extends the way that keys can be
identified and transported by providing attributes that refer to or
contain X.509 certificates.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9360.

Copyright Notice

Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Requirements Terminology
2. X.509 COSE Header Parameters
3. X.509 Certificates and Static-Static ECDH
4. IANA Considerations
4.1. COSE Header Parameters Registry
4.2. COSE Header Algorithm Parameters Registry
4.3. Media Type application/cose-x509
5. Security Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgements
Author's Address

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.

1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

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.

+=========+=======+===============+=====================+
| Name | Label | Value Type | Description |
+=========+=======+===============+=====================+
| x5bag | 32 | COSE_X509 | An unordered bag of |
| | | | X.509 certificates |
+---------+-------+---------------+---------------------+
| x5chain | 33 | COSE_X509 | An ordered chain of |
| | | | X.509 certificates |
+---------+-------+---------------+---------------------+
| x5t | 34 | COSE_CertHash | Hash of an X.509 |
| | | | certificate |
+---------+-------+---------------+---------------------+
| x5u | 35 | uri | URI pointing to an |
| | | | X.509 certificate |
+---------+-------+---------------+---------------------+

Table 1: X.509 COSE Header Parameters

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'.

+==============+=====+=============+===================+===========+
|Name |Label|Type | Algorithm |Description|
+==============+=====+=============+===================+===========+
|x5t-sender |-27 |COSE_CertHash| ECDH-SS+HKDF-256, |Thumbprint |
| | | | ECDH-SS+HKDF-512, |for the |
| | | | ECDH-SS+A128KW, |sender's |
| | | | ECDH-SS+A192KW, |X.509 |
| | | | ECDH-SS+A256KW |certificate|
+--------------+-----+-------------+-------------------+-----------+
|x5u-sender |-28 |uri | ECDH-SS+HKDF-256, |URI for the|
| | | | ECDH-SS+HKDF-512, |sender's |
| | | | ECDH-SS+A128KW, |X.509 |
| | | | ECDH-SS+A192KW, |certificate|
| | | | ECDH-SS+A256KW | |
+--------------+-----+-------------+-------------------+-----------+
|x5chain-sender|-29 |COSE_X509 | ECDH-SS+HKDF-256, |static key |
| | | | ECDH-SS+HKDF-512, |X.509 |
| | | | ECDH-SS+A128KW, |certificate|
| | | | ECDH-SS+A192KW, |chain |
| | | | ECDH-SS+A256KW | |
+--------------+-----+-------------+-------------------+-----------+

Table 2: Static ECDH Algorithm Values

4. IANA Considerations

4.1. COSE Header Parameters Registry

IANA has registered the new COSE Header parameters in Table 1 in the
"COSE Header Parameters" registry. The "Value Registry" field is
empty for all of the items. For each item, the "Reference" field
points to this document.

4.2. COSE Header Algorithm Parameters Registry

IANA has registered the new COSE Header Algorithm parameters in
Table 2 in the "COSE Header Algorithm Parameters" registry. For each
item, the "Reference" field points to this document.

4.3. Media Type application/cose-x509

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.

IANA has registered the following media type [RFC6838]:

Type name: application

Subtype name: cose-x509

Required parameters: N/A

Optional parameters: usage

* Can be absent to provide no further information about the
intended meaning of the order in the CBOR sequence of
certificates.

* Can be set to "chain" to indicate that the sequence of data
items is to be interpreted as a certificate chain.

Encoding considerations: binary

Security considerations: See the Security Considerations section of
RFC 9360.

Interoperability considerations: N/A

Published specification: RFC 9360

Applications that use this media type: Applications that employ COSE
and use X.509 as a certificate type.

Fragment identifier considerations: N/A

Additional information:

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

Person & email address to contact for further information:
[email protected]

Intended usage: COMMON

Restrictions on usage: N/A

Author: COSE WG

Change controller: IESG

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.

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.

[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.

[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/info/rfc9052>.

6.2. Informative References

[Constrained-BRSKI]
Richardson, M., van der Stok, P., Kampanakis, P., and E.
Dijk, "Constrained Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", Work in Progress, Internet-Draft,
draft-ietf-anima-constrained-voucher-19, 2 January 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-anima-
constrained-voucher-19>.

[EDHOC] Selander, G., Preuß Mattsson, J., and F. Palombini,
"Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
Progress, Internet-Draft, draft-ietf-lake-edhoc-19, 3
February 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-lake-edhoc-19>.

[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP",
RFC 2585, DOI 10.17487/RFC2585, May 1999,
<https://www.rfc-editor.org/info/rfc2585>.

[RFC2634] Hoffman, P., Ed., "Enhanced Security Services for S/MIME",
RFC 2634, DOI 10.17487/RFC2634, June 1999,
<https://www.rfc-editor.org/info/rfc2634>.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.

[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.

[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", RFC 8551, DOI 10.17487/RFC8551,
April 2019, <https://www.rfc-editor.org/info/rfc8551>.

[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.

[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.

[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.

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.

Author's Address

Jim Schaad
August Cellars