Internet DRAFT - draft-jones-ace-cwt-proof-of-possession
draft-jones-ace-cwt-proof-of-possession
ACE Working Group M. Jones
Internet-Draft Microsoft
Intended status: Standards Track L. Seitz
Expires: January 1, 2018 RISE SICS
G. Selander
Ericsson AB
E. Wahlstroem
S. Erdtman
Spotify AB
H. Tschofenig
ARM Ltd.
June 30, 2017
Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs)
draft-jones-ace-cwt-proof-of-possession-01
Abstract
This specification describes how to declare in a CBOR Web Token (CWT)
that the presenter of the CWT possesses a particular proof-of-
possession key. Being able to prove possession of a key is also
sometimes described as the presenter being a holder-of-key. This
specification provides equivalent functionality to "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)" (RFC 7800), but
using CBOR and CWTs rather than JSON and JWTs.
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 January 1, 2018.
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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
(http://trustee.ietf.org/license-info) in effect on the date of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Representations for Proof-of-Possession Keys . . . . . . . . 3
3.1. Confirmation Claim . . . . . . . . . . . . . . . . . . . 4
3.2. Representation of an Asymmetric Proof-of-Possession Key . 5
3.3. Representation of an Encrypted Symmetric Proof-of-
Possession Key . . . . . . . . . . . . . . . . . . . . . 5
3.4. Representation of a Key ID for a Proof-of-Possession Key 6
3.5. Specifics Intentionally Not Specified . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. CBOR Web Token Claims Registration . . . . . . . . . . . 9
6.1.1. Registry Contents . . . . . . . . . . . . . . . . . . 9
6.2. CWT Confirmation Methods Registry . . . . . . . . . . . . 9
6.2.1. Registration Template . . . . . . . . . . . . . . . . 9
6.2.2. Initial Registry Contents . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12
Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Document History . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
This specification describes how a CBOR Web Token [CWT] can declare
that the presenter of the CWT possesses a particular proof-of-
possession (PoP) key. Proof of possession of a key is also sometimes
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described as the presenter being a holder-of-key. This specification
provides equivalent functionality to "Proof-of-Possession Key
Semantics for JSON Web Tokens (JWTs)" [RFC7800], but using CBOR
[RFC7049] and CWTs [CWT] rather than JSON [RFC7159] and JWTs [JWT].
1.1. Notational Conventions
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].
Unless otherwise noted, all the protocol parameter names and values
are case sensitive.
2. Terminology
This specification uses terms defined in the CBOR Web Token [CWT],
[I-D.ietf-cose-msg], and Concise Binary Object Representation (CBOR)
[RFC7049] specifications.
These terms are defined by this specification:
Issuer
Party that creates the CWT and binds the proof-of-possession key
to it.
Presenter
Party that proves possession of a private key (for asymmetric key
cryptography) or secret key (for symmetric key cryptography) to a
recipient.
Recipient
Party that receives the CWT containing the proof-of-possession key
information from the presenter.
3. Representations for Proof-of-Possession Keys
By including a "cnf" (confirmation) claim in a CWT, the issuer of the
CWT declares that the presenter possesses a particular key and that
the recipient can cryptographically confirm that the presenter has
possession of that key. The value of the "cnf" claim is a CBOR map
and the members of that map identify the proof-of-possession key.
The presenter can be identified in one of several ways by the CWT
depending upon the application requirements. If the CWT contains a
"sub" (subject) claim [CWT], the presenter is normally the subject
identified by the CWT. (In some applications, the subject identifier
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will be relative to the issuer identified by the "iss" (issuer) claim
[CWT].) If the CWT contains no "sub" claim, the presenter is
normally the issuer identified by the CWT using the "iss" claim. The
case in which the presenter is the subject of the CWT is analogous to
Security Assertion Markup Language (SAML) 2.0
[OASIS.saml-core-2.0-os] SubjectConfirmation usage. At least one of
the "sub" and "iss" claims is typically present in the CWT and some
use cases may require that both be present.
3.1. Confirmation Claim
The "cnf" claim is used in the CWT to contain members used to
identify the proof-of-possession key. Other members of the "cnf" map
may be defined because a proof-of-possession key may not be the only
means of confirming the authenticity of the token. This is analogous
to the SAML 2.0 [OASIS.saml-core-2.0-os] SubjectConfirmation element
in which a number of different subject confirmation methods can be
included (including proof-of-possession key information).
The set of confirmation members that a CWT must contain to be
considered valid is context dependent and is outside the scope of
this specification. Specific applications of CWTs will require
implementations to understand and process some confirmation members
in particular ways. However, in the absence of such requirements,
all confirmation members that are not understood by implementations
MUST be ignored.
This specification establishes the IANA "CWT Confirmation Methods"
registry for these members in Section 6.2 and registers the members
defined by this specification. Other specifications can register
other members used for confirmation, including other members for
conveying proof-of-possession keys using different key
representations.
The "cnf" claim value MUST represent only a single proof-of-
possession key; thus, at most one of the "COSE_Key" and
"Encrypted_COSE_Key" confirmation values defined below may be
present. Note that if an application needs to represent multiple
proof-of-possession keys in the same CWT, one way for it to achieve
this is to use other claim names, in addition to "cnf", to hold the
additional proof-of-possession key information. These claims could
use the same syntax and semantics as the "cnf" claim. Those claims
would be defined by applications or other specifications and could be
registered in the IANA "CBOR Web Token Claims" registry
[IANA.CWT.Claims].
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3.2. Representation of an Asymmetric Proof-of-Possession Key
When the key held by the presenter is an asymmetric private key, the
"COSE_Key" member is a COSE_Key [I-D.ietf-cose-msg] representing the
corresponding asymmetric public key. The following example (using
JSON notation) demonstrates such a declaration in the CWT Claims Set
of a CWT:
{
"iss": "https://server.example.com",
"aud": "https://client.example.org",
"exp": 1361398824,
"cnf":{
"COSE_Key":{
"kty": "EC",
"crv": "P-256",
"x": "18wHLeIgW9wVN6VD1Txgpqy2LszYkMf6J8njVAibvhM",
"y": "-V4dS4UaLMgP_4fY4j8ir7cl1TXlFdAgcx55o7TkcSA"
}
}
}
The COSE_Key MUST contain the required key members for a COSE_Key of
that key type and MAY contain other COSE_Key members, including the
"kid" (Key ID) member.
The "COSE_Key" member MAY also be used for a COSE_Key representing a
symmetric key, provided that the CWT is encrypted so that the key is
not revealed to unintended parties. The means of encrypting a CWT is
explained in [CWT]. If the CWT is not encrypted, the symmetric key
MUST be encrypted as described below.
3.3. Representation of an Encrypted Symmetric Proof-of-Possession Key
When the key held by the presenter is a symmetric key, the
"Encrypted_COSE_Key" member is an encrypted COSE_Key
[I-D.ietf-cose-msg] representing the symmetric key encrypted to a key
known to the recipient using COSE_Encrypt or COSE_Encrypt0.
The following example (using JSON notation) illustrates a symmetric
key that could subsequently be encrypted for use in the
"Encrypted_COSE_Key" member:
{
"kty": "oct",
"alg": "HS256",
"k": "ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE"
}
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The COSE_Key representation is used as the plaintext when encrypting
the key. The COSE_Key could, for instance, be encrypted using a
COSE_Encrypt0 representation using the AES-CCM-16-64-128 algorithm.
The following example CWT Claims Set of a CWT (using JSON notation)
illustrates the use of an encrypted symmetric key as the
"Encrypted_COSE_Key" member value:
{
"iss": "https://server.example.com",
"sub": "24400320",
"aud": "s6BhdRkqt3",
"exp": 1311281970,
"iat": 1311280970,
"cnf":{
"Encrypted_COSE_Key":
"(TBD)"
}
}
3.4. Representation of a Key ID for a Proof-of-Possession Key
The proof-of-possession key can also be identified by the use of a
Key ID instead of communicating the actual key, provided the
recipient is able to obtain the identified key using the Key ID. In
this case, the issuer of a CWT declares that the presenter possesses
a particular key and that the recipient can cryptographically confirm
proof of possession of the key by the presenter by including a "cnf"
claim in the CWT whose value is a CBOR map with the CBOR map
containing a "kid" member identifying the key.
The following example (using JSON notation) demonstrates such a
declaration in the CWT Claims Set of a CWT:
{
"iss": "https://server.example.com",
"aud": "https://client.example.org",
"exp": 1361398824,
"cnf":{
"kid": "dfd1aa97-6d8d-4575-a0fe-34b96de2bfad"
}
}
The content of the "kid" value is application specific. For
instance, some applications may choose to use a cryptographic hash of
the public key value as the "kid" value.
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3.5. Specifics Intentionally Not Specified
Proof of possession is typically demonstrated by having the presenter
sign a value determined by the recipient using the key possessed by
the presenter. This value is sometimes called a "nonce" or a
"challenge".
The means of communicating the nonce and the nature of its contents
are intentionally not described in this specification, as different
protocols will communicate this information in different ways.
Likewise, the means of communicating the signed nonce is also not
specified, as this is also protocol specific.
Note that another means of proving possession of the key when it is a
symmetric key is to encrypt the key to the recipient. The means of
obtaining a key for the recipient is likewise protocol specific.
4. Security Considerations
All of the security considerations that are discussed in [CWT] also
apply here. In addition, proof of possession introduces its own
unique security issues. Possessing a key is only valuable if it is
kept secret. Appropriate means must be used to ensure that
unintended parties do not learn private key or symmetric key values.
Applications utilizing proof of possession should also utilize
audience restriction, as described in Section 4.1.3 of [JWT], as it
provides different protections. Proof of possession can be used by
recipients to reject messages from unauthorized senders. Audience
restriction can be used by recipients to reject messages intended for
different recipients.
A recipient might not understand the "cnf" claim. Applications that
require the proof-of-possession keys communicated with it to be
understood and processed must ensure that the parts of this
specification that they use are implemented.
Proof of possession via encrypted symmetric secrets is subject to
replay attacks. This attack can, for example, be avoided when a
signed nonce or challenge is used since the recipient can use a
distinct nonce or challenge for each interaction. Replay can also be
avoided if a sub-key is derived from a shared secret that is specific
to the instance of the PoP demonstration.
As is the case with other information included in a CWT, it is
necessary to apply data origin authentication and integrity
protection (via a keyed message digest or a digital signature). Data
origin authentication ensures that the recipient of the CWT learns
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about the entity that created the CWT since this will be important
for any policy decisions. Integrity protection prevents an adversary
from changing any elements conveyed within the CWT payload. Special
care has to be applied when carrying symmetric keys inside the CWT
since those not only require integrity protection but also
confidentiality protection.
5. Privacy Considerations
A proof-of-possession key can be used as a correlation handle if the
same key is used with multiple parties. Thus, for privacy reasons,
it is recommended that different proof-of-possession keys be used
when interacting with different parties.
6. IANA Considerations
The following registration procedure is used for all the registries
established by this specification.
Values are registered on a Specification Required [RFC5226] basis
after a three-week review period on the cwt-reg-review@ietf.org
mailing list, on the advice of one or more Designated Experts.
However, to allow for the allocation of values prior to publication,
the Designated Experts may approve registration once they are
satisfied that such a specification will be published. [[ Note to
the RFC Editor: The name of the mailing list should be determined in
consultation with the IESG and IANA. Suggested name: cwt-reg-
review@ietf.org. ]]
Registration requests sent to the mailing list for review should use
an appropriate subject (e.g., "Request to Register CWT Confirmation
Method: example"). Registration requests that are undetermined for a
period longer than 21 days can be brought to the IESG's attention
(using the iesg@ietf.org mailing list) for resolution.
Criteria that should be applied by the Designated Experts include
determining whether the proposed registration duplicates existing
functionality, determining whether it is likely to be of general
applicability or whether it is useful only for a single application,
and evaluating the security properties of the item being registered
and whether the registration makes sense.
It is suggested that multiple Designated Experts be appointed who are
able to represent the perspectives of different applications using
this specification in order to enable broadly informed review of
registration decisions. In cases where a registration decision could
be perceived as creating a conflict of interest for a particular
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Expert, that Expert should defer to the judgment of the other
Experts.
6.1. CBOR Web Token Claims Registration
This specification registers the "cnf" claim in the IANA "CBOR Web
Token Claims" registry [IANA.CWT.Claims] established by [CWT].
6.1.1. Registry Contents
o Claim Name: "cnf"
o Claim Description: Confirmation
o JWT Claim Name: "cnf"
o Claim Key: TBD (maybe 8)
o Claim Value Type(s): map
o Change Controller: IESG
o Specification Document(s): Section 3.1 of [[ this document ]]
6.2. CWT Confirmation Methods Registry
This specification establishes the IANA "CWT Confirmation Methods"
registry for CWT "cnf" member values. The registry records the
confirmation method member and a reference to the specification that
defines it.
6.2.1. Registration Template
Confirmation Method Name:
The human-readable name requested (e.g., "kid").
Confirmation Method Description:
Brief description of the confirmation method (e.g., "Key
Identifier").
JWT Confirmation Method Name:
Claim Name of the equivalent JWT confirmation method value, as
registered in [IANA.JWT.Claims]. CWT claims should normally have
a corresponding JWT claim. If a corresponding JWT claim would not
make sense, the Designated Experts can choose to accept
registrations for which the JWT Claim Name is listed as "N/A".
Confirmation Key:
CBOR map key value for the confirmation method.
Confirmation Value Type(s):
CBOR types that can be used for the confirmation method value.
Change Controller:
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For Standards Track RFCs, list the "IESG". For others, give the
name of the responsible party. Other details (e.g., postal
address, email address, home page URI) may also be included.
Specification Document(s):
Reference to the document or documents that specify the parameter,
preferably including URIs that can be used to retrieve copies of
the documents. An indication of the relevant sections may also be
included but is not required.
6.2.2. Initial Registry Contents
o Confirmation Method Name: "COSE_Key"
o Confirmation Method Description: COSE_Key Representing Public Key
o JWT Confirmation Method Name: "jwk"
o Confirmation Key: 1
o Confirmation Value Type(s): map
o Change Controller: IESG
o Specification Document(s): Section 3.2 of [[ this document ]]
o Confirmation Method Name: "Encrypted_COSE_Key"
o Confirmation Method Description: Encrypted COSE_Key
o JWT Confirmation Method Name: "jwe"
o Confirmation Key: 2
o Confirmation Value Type(s): array (with an optional COSE_Encrypt
or COSE_Encrypt0 tag)
o Change Controller: IESG
o Specification Document(s): Section 3.3 of [[ this document ]]
o Confirmation Method Name: "kid"
o Confirmation Method Description: Key Identifier
o JWT Confirmation Method Name: "kid"
o Confirmation Key: 3
o Confirmation Value Type(s): binary string
o Change Controller: IESG
o Specification Document(s): Section 3.4 of [[ this document ]]
7. References
7.1. Normative References
[CWT] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", Work in Progress, draft-ietf-ace-
cbor-web-token-06, June 2017,
<https://tools.ietf.org/html/draft-ietf-ace-cbor-web-
token-06>.
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[I-D.ietf-cose-msg]
Schaad, J., "CBOR Object Signing and Encryption (COSE)",
draft-ietf-cose-msg-24 (work in progress), November 2016.
[IANA.CWT.Claims]
IANA, "CBOR Web Token Claims",
<http://www.iana.org/assignments/cwt>.
[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>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <http://www.rfc-editor.org/info/rfc3629>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[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>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <http://www.rfc-editor.org/info/rfc7049>.
7.2. Informative References
[IANA.JWT.Claims]
IANA, "JSON Web Token Claims",
<http://www.iana.org/assignments/jwt>.
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[JWT] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7159, May 2015,
<http://www.rfc-editor.org/info/rfc7519>.
[OASIS.saml-core-2.0-os]
Cantor, S., Kemp, J., Philpott, R., and E. Maler,
"Assertions and Protocol for the OASIS Security Assertion
Markup Language (SAML) V2.0", OASIS Standard saml-core-
2.0-os, March 2005,
<http://docs.oasis-open.org/security/saml/v2.0/>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)",
RFC 7800, DOI 10.17487/RFC7800, April 2016,
<http://www.rfc-editor.org/info/rfc7800>.
Acknowledgements
Thanks to the following people for their reviews of the
specification: Michael Richardson and Jim Schaad.
Open Issues
o Convert the examples from JSON/JWT to CBOR/CWT.
Document History
[[ to be removed by the RFC Editor before publication as an RFC ]]
-01
o Tracked CBOR Web Token (CWT) Claims Registry updates.
o Addressed review comments by Michael Richardson and Jim Schaad.
o Added co-authors.
-00
o Created the initial draft from RFC 7800.
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Authors' Addresses
Michael B. Jones
Microsoft
Email: mbj@microsoft.com
URI: http://self-issued.info/
Ludwig Seitz
RISE SICS
Scheelevaegen 17
Lund 223 70
Sweden
Email: ludwig@ri.se
Goeran Selander
Ericsson AB
Faeroegatan 6
Kista 164 80
Sweden
Email: goran.selander@ericsson.com
Erik Wahlstroem
Sweden
Email: erik@wahlstromstekniska.se
Samuel Erdtman
Spotify AB
Birger Jarlsgatan 61, 4tr
Stockholm 113 56
Sweden
Phone: +46702691499
Email: erdtman@spotify.com
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Hannes Tschofenig
ARM Ltd.
Hall in Tirol 6060
Austria
Email: Hannes.Tschofenig@arm.com
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