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Versions: 00 01 02 03 draft-ietf-cose-x509
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
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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
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(http://trustee.ietf.org/license-info) in effect on the date of
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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.
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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.
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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:
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* 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 ]
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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.
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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.
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[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
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