CBOR Object Signing and Encryption (COSE): Headers for carrying and referencing X.509 certificates
draft-ietf-cose-x509-03
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Jim Schaad
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Network Working Group J. Schaad
Internet-Draft August Cellars
Intended status: Informational August 18, 2019
Expires: February 19, 2020
CBOR Object Signing and Encryption (COSE): Headers for carrying and
referencing X.509 certificates
draft-ietf-cose-x509-03
Abstract
The CBOR Signing And Encrypted Message (COSE) structure uses
references to keys in general. For some algorithms, additional
properties are defined which carry parts of 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.
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 https://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 February 19, 2020.
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Copyright Notice
Copyright (c) 2019 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 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
1.2. Open Questions . . . . . . . . . . . . . . . . . . . . . 3
2. X.509 COSE Headers . . . . . . . . . . . . . . . . . . . . . 3
3. X.509 certificates and static-static ECDH . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
4.1. COSE Header Parameter Registry . . . . . . . . . . . . . 7
4.2. COSE Header Algorithm Parameter Registry . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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 need for these attributes. Since that
time a number of cases where X.509 certificate support is necessary
have been defined. This document provides a set of attributes 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
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
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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.
Example COSE messages for the various headers defined below can be
found at https://github.com/cose-wg/Examples. THIS IS NOT YET DONE
BUT SHOULD BE COMING NOT LONG AFTER THE F2F MEETING.
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?
Revocation info?
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].
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.
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The header attributes defined in this document are:
x5bag: This header attributes 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 attribute can be in either a protected or
unprotected header attribute.
This header attribute 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 its own bstr.
x5chain: This header attribute 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. This means that the relying
party is still required to do path building, but that a
candidate path is proposed in this attribute.
As this header element does not provide any trust, the
header attribute can be in either a protected or
unprotected header attribute.
This header attribute allows for a single 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 bstr.
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* If multiple certificates are conveyed, a CBOR array of
bstrs is used. Each certificate being in it's own slot.
x5t: This header attribute provides the ability to identify an
X.509 certificate by a hash value. The attribute is an
array of two elements. The first element is an algorithm
identifier which is an 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 attribute can be in either a protected or
unprotected header attribute.
For interoperability, applications which use this header
attribute MUST support the hash algorithm 'sha256', but can
use other hash algorithms.
x5u: This header attribute provides the ability to identify an
X.509 certificate by a URI. The referenced resource can be
any of the following media types:
* application/pkix-cert [RFC2585]
* application/pkcs7-mime; smime-type="certs-only"
[RFC8551]
As this header attribute implies a trust relationship, the
attribute MUST be in the protected attributes.
The URI provided MUST provide integrity protection and
server authentication. For example, an HTTP or CoAP GET
request to retrieve a certificate MUST use TLS [RFC8446] or
DTLS [I-D.ietf-tls-dtls13]. 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 attributes are used in the following locations:
* COSE_Signature and COSE_Sign0 objects, in these objects they
identify the key that was used for generating signature.
* COSE_recipient objects, in this location they may be used to
identify the certificate for the recipient of the message.
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+---------+-------+---------------+---------------------+
| 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 | URI pointing to an |
| | | | X.509 certificate |
+---------+-------+---------------+---------------------+
Table 1: X.509 COSE Headers
Below is an equivalent CDDL [RFC8610] 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 attributes 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.
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 attribute contains the chain of
certificates starting with the sender's key exchange
certificate. The structure is the same as 'x5bag'.
x5t-sender: This header attribute contains the hash value for
the sender's key exchange certificate. The
structure is the same as 'x5t'.
x5u-sender: This header attribute contains a URI for the
sender's key exchange certificate. The structure
and processing are the same as 'x5u'.
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+---------------+-----+-------------+-------------------+-----------+
| Name |Value|Type | Algorithm |Description|
+===============+=====+=============+===================+===========+
| x5t-sender |TBD |COSE_CertHash| ECDH-SS+HKDF-256, |Thumbprint |
| | | | ECDH-SS+HKDF-512, |for the |
| | | | ECDH-SS+A128KW, |senders |
| | | | ECDH- SS+AES192KW, |X.509 |
| | | | ECDH-SS+AES256KW |certificate|
+---------------+-----+-------------+-------------------+-----------+
| x5u-sender |TBD |uri | ECDH-SS+HKDF-256, |URI for the|
| | | | ECDH-SS+HKDF-512, |senders |
| | | | ECDH-SS+A128KW, |X.509 |
| | | | ECDH- SS+AES192KW, |certificate|
| | | | ECDH-SS+AES256KW | |
+---------------+-----+-------------+-------------------+-----------+
| x5chain-sender|TBD |COSE_X509 | ECDH-SS+HKDF-256, |static key |
| | | | ECDH-SS+HKDF-512, |X.509 |
| | | | ECDH-SS+A128KW, |certificate|
| | | | ECDH- SS+AES192KW, |chain |
| | | | ECDH-SS+AES256KW | |
+---------------+-----+-------------+-------------------+-----------+
Table 2: Static ECDH Algorithm Values
4. IANA Considerations
4.1. COSE Header Parameter Registry
IANA is requested to register the new COSE Header items in Table 1 in
the "COSE Header Parameters" registry.
4.2. 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. Security Considerations
Establishing trust in a certificate is a vital part of processing.
Trust cannot be assumed whenever a new self-signed certificate
appears on the client, instead a well defined process is required.
One common way for a new trust anchor to be added (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
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certificates during path building. Doing the network access can
consume resources dealing with power and network bandwidth. On the
other hand, an oracle can potentially be built based on if the
network resources are only accessed if the signature validation
passes. In any event, both the signature and certificate validation
MUST be checked before acting on any requests.
As called out in the COSE algorithms document
[I-D.ietf-cose-rfc8152bis-algs] basic checking on the keys in a
certificate needs to be performed prior to using them. These can
include validating that points are on curves for elliptical curve
algorithms and that sizes of keys are acceptable for RSA. The use of
unvalidated keys can lead either to loss of security or excessive
consumption of resources.
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>.
[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>.
6.2. Informative References
[I-D.ietf-cose-rfc8152bis-algs]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-03
(work in progress), June 10, 2019,
<https://www.ietf.org/archive/id/draft-ietf-cose-
rfc8152bis-algs-03>.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-32 (work in progress), July 8,
Schaad Expires February 19, 2020 [Page 8]
Internet-Draft COSE X.509 August 2019
2019, <https://www.ietf.org/archive/id/
draft-ietf-tls-dtls13-32>.
[I-D.selander-ace-cose-ecdhe]
Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
cose-ecdhe-13 (work in progress), March 11, 2019,
<https://www.ietf.org/archive/id/draft-selander-ace-cose-
ecdhe-13>.
[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>.
[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>.
[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>.
Author's Address
Jim Schaad
August Cellars
Email: ietf@augustcellars.com
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