CBOR Encoded Message Syntax (COSE): Headers for carrying and referencing X.509 certificates
draft-schaad-cose-x509-00

Abstract

This document defines the headers and usage for referring to and transporting X.509 certificates in the CBOR Encoded Message (COSE) Syntax.

Contributing to this document

The source for this draft is being maintained in GitHub. Suggested changes should be submitted as pull requests at <https://github.com/cose-wg/X509>. Instructions are on that page as well. Editorial changes can be managed in GitHub, but any substantial issues need to be discussed on the COSE mailing list.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on May 26, 2017.

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Terminology
• 2. X.509 COSE Headers
• 3. Hash Algorithm Identifiers
• 3.1. SHA-2 256-bit Hash
• 4. IANA Considerations
• 4.1. COSE Header Parameter Registry
• 4.2. COSE Algorithm Registry
• 5. Security Considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Author's Address

1. Introduction

In the process of writing RFCXXXX [I-D.ietf-cose-msg] discussions where held on the question of X.509 certificates [RFC5280] and if there were needed. At the time there were no use cases presented that appeared to hve a sufficient set of support to include these headers. Since that time a number of cases where X.509 certificate support is necessary have been defined. This document provides a set of headers that will allow applications to transport and refer to X.509 certificates in a consistent manner.

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 [RFC2119].

When the words appear in lower case, their natural language meaning is used.

2. X.509 COSE Headers

The use of X.509 certificates allows for an existing trust infrastructure to be used with COSE.

When the header parameters defined in this section are placed in a COSE_Signature or COSE_Sign0 object, they identify the key that was used for generating signature.

When the header parameters defined in this section are placed in a COSE_recipient structure, they identify the key that was used by the sender when used with static-static key agreement algorithms.

Certificates obtained from any of these methods MUST still be validated according to the PKIX rules in [RFC5280]. This includes matching against the trust anchors configured for the application. This applies certificates of a chain length of one as well as longer chains.

The header parameters defined in this document are:

x5c:

This header parameter 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 is used where:
  • The first element is a boolean value set to true if the certificates are arranged such that a each certificate is issued by the next certficate. In otherwords a chain of certificates is presented. The chain of certificates does not need to be complete and normally SHOULD omit the trust anchor certificate. If the first element is set to false, then the certificates are not ordered and can include certificates that are not needed to create a certificate chain from the end-entity certificate. This allows for a certificate with a key exchange algorithm to be carried in a signed message.
  • The second element is the end-entity certificate. This is true regardless of wheither the certificates are ordered or not. This permits the application to identify which certificate is the end-entity certificate without a second header attribute.
  • Elements three through the last are certificates.

x5t:

This header parameter provides the ability to identify an X.509 certificate by a hash value. The parameter is an array of two elements. The first element is an algorithm identifier which is a signed integer, an unsigned integer or a string containing the hash algorithm identifier. The second element is a binary string containing the hash value.
For interoperability, applications which use this header parameter MUST support the hash algorithm 'sha256', but can use other hash algorithms.

x5u:

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

• application/pkix-cert [RFC2585]
• application/pkcs7-mime; smime-type="certs-only" [I-D.ietf-lamps-rfc5751-bis]
• Should we support a PEM type? I cannot find a registered media type for one

The URL provided MUST provide integrity protection. For example, an HTTP GET request to retrieve a certificate MUST use TLS [RFC5246]. If the certificate does not chain to an existing trust anchor, the identity of the server MUST be configured as trusted to provide new trust anchors. This will normally be the situation when self-signed certificates are used.

COSE_X509 = bstr / [ ordered: bool, certs: +bstr ]
COSE_CertHash = [ hashAlg: (int / tstr), hashValue: bstr ]