Internet DRAFT - draft-schaad-cose-x509
draft-schaad-cose-x509
Network Working Group J. Schaad
Internet-Draft August Cellars
Intended status: Informational December 26, 2018
Expires: June 29, 2019
CBOR Object Signing and Encryption (COSE): Headers for carrying and
referencing X.509 certificates
draft-schaad-cose-x509-03
Abstract
The CBOR Encoded Message (COSE) structure syntax uses the COSE Key
structure for placing keys in a message. This document extends the
way that keys can be identified and transported by providing
parameters that refer to or contain X.509 certificates in messages
and in the COSE Key structure.
This document defines a set of hash algorithms for COSE. These
algorithms are needed in order to have X.509 certificates referred to
by a thumbprint.
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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Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 29, 2019.
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Terminology . . . . . . . . . . . . . . . . 3
1.2. Open Questions . . . . . . . . . . . . . . . . . . . . . 3
2. X.509 COSE Headers . . . . . . . . . . . . . . . . . . . . . 3
3. X.509 certificates and static-static ECDH . . . . . . . . . . 6
4. Hash Algorithm Identifiers . . . . . . . . . . . . . . . . . 7
4.1. SHA-2 256-bit Hash . . . . . . . . . . . . . . . . . . . 7
4.2. SHA-2 256-bit Hash trucated to 64 bits . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5.1. COSE Header Parameter Registry . . . . . . . . . . . . . 8
5.2. COSE Header Algorithm Parameter Registry . . . . . . . . 8
5.3. COSE Algorithm Registry . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
In the process of writing [RFC8152] discussions where held on the
question of X.509 certificates [RFC5280] and if there was a needed to
provide for them. 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 constrained device situations are being used where an
X.509 PKI is already installed. One of these situations is the 6tish
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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 management to be used
which is well understood.
When [RFC8152] was written, there were no requirements for hash
algorithms to be included in the algorithm registry. The use of
thumbprints to refer to X.509 certificates is defined in this
document which requires the use of hash algorithms. There have also
been other working groups in the IETF that have expressed a
requirement for hash algorithms to do have sections of content be
provided by reference rather than including it in the main message.
This document defines a set of hash algorithms for both of these
purposes.
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.
1.2. Open Questions
Should we define an extended key usage?
Are there any special certificate valiation text to be added?
List of other hash algorithms to be added.
Specific security considerations issues.
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) [RFC8392].
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It is not necessarily expected that constrained devices will fully
support the evaluation 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.
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.
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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.
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:
* application/pkix-cert [RFC2585]
* application/pkcs7-mime; smime-type="certs-only"
[I-D.ietf-lamps-rfc5751-bis]
* application/x-pem-file [RFC7468] 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 and server
authentication. 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
certificate MUST NOT be trusted unless the server is configured as
trusted to provide new trust anchors. This will normally be the
situation when self-signed certificates are used.
The header parameters used in the following locations:
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o COSE_Signature and COSE_Sign0 objects, in these objects they
identify the key that was used for generating signature.
o COSE_recipient objects, in this location they may be used to
identify the certificate for the recipient of the message.
+---------+-------+---------------+---------------------------------+
| Name | Value | 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
Below is an equivalent CDDL [I-D.ietf-cbor-cddl] description of the
text above.
COSE_X509 = bstr / [ 2*certs: bstr ]
COSE_CertHash = [ hashAlg: (int / tstr), hashValue: bstr ]
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 ECDH key agreement
algorithms. In this section we define the algorithm specific
parameters that are used for identifying or transporting the senders
key for static-static key agreement algorithms.
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+-----------+------+--------------+------------------+--------------+
| Name | Valu | Type | Algorithm | Description> |
| | e | | | |
+-----------+------+--------------+------------------+--------------+
| static | TBD | COSE_CertHas | ECDH- | Thumbprint |
| key X.509 | | h | SS+HKDF-256, | for the |
| thumbprin | | | ECDH- | senders |
| t | | | SS+HKDF-512, | X.509 |
| | | | ECDH-SS+A128KW, | certificate |
| | | | ECDH- | |
| | | | SS+AES192KW, | |
| | | | ECDH-SS+AES256KW | |
| | | | | |
| static | TBD | uri | ECDH- | URL for the |
| key X.509 | | | SS+HKDF-256, | senders |
| URL | | | ECDH- | X.509 |
| | | | SS+HKDF-512, | certificate |
| | | | ECDH-SS+A128KW, | |
| | | | ECDH- | |
| | | | SS+AES192KW, | |
| | | | ECDH-SS+AES256KW | |
| | | | | |
| static | TBD | COSE_X509 | ECDH- | static key |
| key X.509 | | | SS+HKDF-256, | X.509 |
| cert | | | ECDH- | certificate |
| chain | | | SS+HKDF-512, | chain |
| | | | ECDH-SS+A128KW, | |
| | | | ECDH- | |
| | | | SS+AES192KW, | |
| | | | ECDH-SS+AES256KW | |
+-----------+------+--------------+------------------+--------------+
Table 2: Static ECDH Algorithm Values
4. 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.
4.1. SHA-2 256-bit Hash
Define an algorithm identifier for SHA-256.
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4.2. 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 truncated hash means that there
is now a high likelihood that collisions will occur, thus this hash
function cannot be used in situations where a unique items is
required to be identified. Luckily 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.
5. IANA Considerations
5.1. COSE Header Parameter Registry
IANA is requested to register the new COSE Header items in Table 1 in
the "COSE Header Parameters" registry.
5.2. COSE Header Algorithm Parameter Registry
IANA is requested to register the new COSE Header items in Table 2 in
the "COSE Header Algorithm Parameters" registry.
5.3. COSE Algorithm Registry
IANA is requested to register the following algorithms in the "COSE
Algorithms" registry.
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+------------+-------+--------------------+-----------+-------------+
| Name | Value | Description | Reference | Recommended |
+------------+-------+--------------------+-----------+-------------+
| SHA-256 | TBD | SHA-2 256-bit Hash | [This | Yes |
| | | | Document] | |
| | | | | |
| SHA-256/64 | TBD | SHA-2 256-bit Hash | [This | No |
| | | trucated to | Document] | |
| | | 64-bits | | |
+------------+-------+--------------------+-----------+-------------+
6. Security Considerations
There are security considerations:
Self-signed certificates and Trust Anchors
7. References
7.1. Normative References
[I-D.schaad-cose-rfc8152bis-struct]
Schaad, J., "CBOR Object Signing and Encryption (COSE) -
Structures and Process", draft-schaad-cose-rfc8152bis-
struct-00 (work in progress), August 2018.
[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>.
[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>.
7.2. Informative References
[I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR and JSON data structures", draft-ietf-cbor-
cddl-06 (work in progress), November 2018.
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[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-12
(work in progress), September 2018.
[I-D.selander-ace-cose-ecdhe]
Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
cose-ecdhe-10 (work in progress), September 2018.
[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>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC7468] Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
April 2015, <https://www.rfc-editor.org/info/rfc7468>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[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>.
Author's Address
Jim Schaad
August Cellars
Email: ietf@augustcellars.com
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