Network Working Group J. Schaad Internet-Draft August Cellars Intended status: Informational May 23, 2017 Expires: November 24, 2017 CBOR Object Signing and Encryption (COSE): Headers for carrying and referencing X.509 certificates draft-schaad-cose-x509-01 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/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on November 24, 2017. Copyright Notice Copyright (c) 2017 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 Schaad Expires November 24, 2017 [Page 1]
Internet-Draft COSE X.509 May 2017 (http://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 Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Terminology . . . . . . . . . . . . . . . . 3 2. X.509 COSE Headers . . . . . . . . . . . . . . . . . . . . . 3 3. Hash Algorithm Identifiers . . . . . . . . . . . . . . . . . 6 3.1. SHA-2 256-bit Hash . . . . . . . . . . . . . . . . . . . 6 3.2. SHA-2 256-bit Hash trucated to 64 bits . . . . . . . . . 6 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 4.1. COSE Header Parameter Registry . . . . . . . . . . . . . 6 4.2. COSE Algorithm Registry . . . . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 6.1. Normative References . . . . . . . . . . . . . . . . . . 7 6.2. Informative References . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 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 have 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. Some of the constrainted 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 [I-D.selander-ace-cose-ecdhe] 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 managment to be used which is well understood. Schaad Expires November 24, 2017 [Page 2]
Internet-Draft COSE X.509 May 2017 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. 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) [I-D.ietf-ace-cbor-web-token]. It is not necessarily expected that constrainted devices will fully support the evalaluation 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 [RFC5280] 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 parameters defined in this document are: x5bag: This header parameters 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. Schaad Expires November 24, 2017 [Page 3]
Internet-Draft COSE X.509 May 2017 As this header element does not provide any trust, the header parameter can be in either a protected or unprotected header bag. 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 of bstrs is used. Each certificate being in it's own slot. 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 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. As this header element does not provide any trust, the header parameter can be in either a protected or unprotected header bag. 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 of bstrs is used. Each certificate being in it's own slot. 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 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 parameter can be in either a protected or unprotected header bag. 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: Schaad Expires November 24, 2017 [Page 4]
Internet-Draft COSE X.509 May 2017 * 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 As this header element implies a trust relationship, the header parameter MUST be in the protected header bag. The URL provided MUST provide integrity protection. For example, an HTTP or CoAP GET request to retrieve a certificate MUST use TLS [RFC5246] or DTLS. 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. The header paramters used in the following locations: o COSE_Signature and COSE_Sign0 objects, in these objects they identify the key that was used for generating signature. o COSE_recipient object, in this object they identify the key used by the sender for static-static key agreement algorithms. They would be used in place either XXXX or YYYY. +---------+-------+---------------+---------------------------------+ | name | label | 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 | URL pointing to an X.509 | | | | | certificate | +---------+-------+---------------+---------------------------------+ Table 1: X.509 COSE Headers COSE_X509 = bstr / [ *certs: bstr ] COSE_CertHash = [ hashAlg: (int / tstr), hashValue: bstr ] Schaad Expires November 24, 2017 [Page 5]
Internet-Draft COSE X.509 May 2017 3. Hash Algorithm Identifiers The core COSE document did have a need for a standalone hash algorithm, and thus did not define any. In this document, two hash algorithms are defined for use with the 'x5t' header parameter. 3.1. SHA-2 256-bit Hash Define an algorithm identifier for SHA-256. 3.2. SHA-2 256-bit Hash trucated to 64 bits This hash function uses the SHA-2 256-bit hash function as in the previous section, however it truncates the result to 64-bits for transmission. The fact that it is a trucated hash means that there is now a high likelyhood that colisions will occur, thus this hash function cannot be used in situations where a unique items is required to be identified. Luckly for the case of identifying a certificate that is not a requirement, the only requirement is that the number of potential certificates (and thus keys) to be tried is reduced to a small number. (Hopefully that number is one, but it can not be assumed to be.) After the set of certificates has been filtered down, the public key in each certificate will need to be tried for the operation in question. The certificate can be validated either before or after it has been checked as working. The trade-offs involved are: o Certificate validation before using the key will imply that more network traffic may be required in order to fetch certificates and do revocation checking. o Certificate validation after using the key means that bad keys can be used and, if not carefully checked, the result may be used prior to completing the certificate validation. Using unvalidated keys can expose the device to more timing and oracle attacks as the attacker would be able to see if the key operation succeeded or failed as no network traffic to validate the certificate would ensue. 4. IANA Considerations 4.1. COSE Header Parameter Registry Put in the registrations. Schaad Expires November 24, 2017 [Page 6]
Internet-Draft COSE X.509 May 2017 4.2. COSE Algorithm Registry Put in the registrations. 5. Security Considerations There are security considerations: 6. References 6.1. Normative References [I-D.ietf-cose-msg] Schaad, J., "CBOR Object Signing and Encryption (COSE)", draft-ietf-cose-msg-24 (work in progress), November 2016. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://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, <http://www.rfc-editor.org/info/rfc5280>. 6.2. Informative References [I-D.ietf-ace-cbor-web-token] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "CBOR Web Token (CWT)", draft-ietf-ace-cbor-web-token-04 (work in progress), April 2017. [I-D.ietf-lamps-rfc5751-bis] Schaad, J., Ramsdell, B., and S. Turner, "Secure/ Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 Message Specification", draft-ietf-lamps-rfc5751-bis-06 (work in progress), April 2017. [I-D.selander-ace-cose-ecdhe] Selander, G., Mattsson, J., and F. Palombini, "Ephemeral Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace- cose-ecdhe-06 (work in progress), April 2017. Schaad Expires November 24, 2017 [Page 7]
Internet-Draft COSE X.509 May 2017 [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, <http://www.rfc-editor.org/info/rfc2585>. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc5246>. Author's Address Jim Schaad August Cellars Email: ietf@augustcellars.com Schaad Expires November 24, 2017 [Page 8]