draft-ietf-ace-cwt-proof-of-possession-11.txt   rfc8747.txt 
ACE M. Jones Internet Engineering Task Force (IETF) M. Jones
Internet-Draft Microsoft Request for Comments: 8747 Microsoft
Intended status: Standards Track L. Seitz Category: Standards Track L. Seitz
Expires: May 3, 2020 RISE SICS ISSN: 2070-1721 Combitech
G. Selander G. Selander
Ericsson AB Ericsson AB
S. Erdtman S. Erdtman
Spotify Spotify
H. Tschofenig H. Tschofenig
Arm Ltd. Arm Ltd.
October 31, 2019 March 2020
Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs) Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs)
draft-ietf-ace-cwt-proof-of-possession-11
Abstract Abstract
This specification describes how to declare in a CBOR Web Token (CWT) This specification describes how to declare in a CBOR Web Token (CWT)
(which is defined by RFC 8392) that the presenter of the CWT (which is defined by RFC 8392) that the presenter of the CWT
possesses a particular proof-of-possession key. Being able to prove possesses a particular proof-of-possession key. Being able to prove
possession of a key is also sometimes described as being the holder- possession of a key is also sometimes described as being the holder-
of-key. This specification provides equivalent functionality to of-key. This specification provides equivalent functionality to
"Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)" (RFC "Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)" (RFC
7800) but using Concise Binary Object Representation (CBOR) and CWTs 7800) but using Concise Binary Object Representation (CBOR) and CWTs
rather than JavaScript Object Notation (JSON) and JSON Web Tokens rather than JavaScript Object Notation (JSON) and JSON Web Tokens
(JWTs). (JWTs).
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
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-
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Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
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Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on May 3, 2020. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8747.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology
3. Representations for Proof-of-Possession Keys . . . . . . . . 3 3. Representations for Proof-of-Possession Keys
3.1. Confirmation Claim . . . . . . . . . . . . . . . . . . . 4 3.1. Confirmation Claim
3.2. Representation of an Asymmetric Proof-of-Possession Key . 5 3.2. Representation of an Asymmetric Proof-of-Possession Key
3.3. Representation of an Encrypted Symmetric Proof-of- 3.3. Representation of an Encrypted Symmetric
Possession Key . . . . . . . . . . . . . . . . . . . . . 6 Proof-of-Possession Key
3.4. Representation of a Key ID for a Proof-of-Possession Key 7 3.4. Representation of a Key ID for a Proof-of-Possession Key
3.5. Specifics Intentionally Not Specified . . . . . . . . . . 8 3.5. Specifics Intentionally Not Specified
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8 4. Security Considerations
5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9 5. Privacy Considerations
6. Operational Considerations . . . . . . . . . . . . . . . . . 9 6. Operational Considerations
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 7. IANA Considerations
7.1. CBOR Web Token Claims Registration . . . . . . . . . . . 11 7.1. CBOR Web Token Claims Registration
7.1.1. Registry Contents . . . . . . . . . . . . . . . . . . 11 7.1.1. Registry Contents
7.2. CWT Confirmation Methods Registry . . . . . . . . . . . . 11 7.2. CWT Confirmation Methods Registry
7.2.1. Registration Template . . . . . . . . . . . . . . . . 11 7.2.1. Registration Template
7.2.2. Initial Registry Contents . . . . . . . . . . . . . . 12 7.2.2. Initial Registry Contents
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 8. References
8.1. Normative References . . . . . . . . . . . . . . . . . . 12 8.1. Normative References
8.2. Informative References . . . . . . . . . . . . . . . . . 13 8.2. Informative References
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 14 Acknowledgements
Document History . . . . . . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction 1. Introduction
This specification describes how a CBOR Web Token (CWT) [RFC8392] can This specification describes how a CBOR Web Token (CWT) [RFC8392] can
declare that the presenter of the CWT possesses a particular proof- declare that the presenter of the CWT possesses a particular proof-
of-possession (PoP) key. Proof of possession of a key is also of-possession (PoP) key. Proof of possession of a key is also
sometimes described as being the holder-of-key. This specification sometimes described as being the holder-of-key. This specification
provides equivalent functionality to "Proof-of-Possession Key provides equivalent functionality to "Proof-of-Possession Key
Semantics for JSON Web Tokens (JWTs)" [RFC7800] but using Concise Semantics for JSON Web Tokens (JWTs)" [RFC7800] but using Concise
Binary Object Representation (CBOR) [RFC7049] and CWTs [RFC8392] Binary Object Representation (CBOR) [RFC7049] and CWTs [RFC8392]
rather than JavaScript Object Notation (JSON) [RFC8259] and JSON Web rather than JavaScript Object Notation (JSON) [RFC8259] and JSON Web
Tokens (JWTs) [JWT]. Tokens (JWTs) [JWT].
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
This specification uses terms defined in the CBOR Web Token (CWT) This specification uses terms defined in the CBOR Web Token (CWT)
[RFC8392], CBOR Object Signing and Encryption (COSE) [RFC8152], and [RFC8392], CBOR Object Signing and Encryption (COSE) [RFC8152], and
Concise Binary Object Representation (CBOR) [RFC7049] specifications. Concise Binary Object Representation (CBOR) [RFC7049] specifications.
These terms are defined by this specification: These terms are defined by this specification:
Issuer Issuer
Party that creates the CWT and binds the claims about the subject Party that creates the CWT and binds the claims about the subject
skipping to change at page 4, line 9 skipping to change at line 143
By including a "cnf" (confirmation) claim in a CWT, the issuer of the By including a "cnf" (confirmation) claim in a CWT, the issuer of the
CWT declares that the presenter possesses a particular key and that CWT declares that the presenter possesses a particular key and that
the recipient can cryptographically confirm that the presenter has the recipient can cryptographically confirm that the presenter has
possession of that key. The value of the "cnf" claim is a CBOR map possession of that key. The value of the "cnf" claim is a CBOR map
(which is defined in Section 2.1 of [RFC7049]) and the members of (which is defined in Section 2.1 of [RFC7049]) and the members of
that map identify the proof-of-possession key. that map identify the proof-of-possession key.
The presenter can be identified in one of several ways by the CWT, The presenter can be identified in one of several ways by the CWT,
depending upon the application requirements. For instance, some depending upon the application requirements. For instance, some
applications may use the CWT "sub" (subject) claim [RFC8392], to applications may use the CWT "sub" (subject) claim [RFC8392] to
identify the presenter. Other applications may use the "iss" identify the presenter. Other applications may use the "iss"
(issuer) claim [RFC8392] to identify the presenter. In some (issuer) claim [RFC8392] to identify the presenter. In some
applications, the subject identifier might be relative to the issuer applications, the subject identifier might be relative to the issuer
identified by the "iss" claim. The actual mechanism used is identified by the "iss" claim. The actual mechanism used is
dependent upon the application. The case in which the presenter is dependent upon the application. The case in which the presenter is
the subject of the CWT is analogous to Security Assertion Markup the subject of the CWT is analogous to Security Assertion Markup
Language (SAML) 2.0 [OASIS.saml-core-2.0-os] SubjectConfirmation Language (SAML) 2.0 [OASIS.saml-core-2.0-os] SubjectConfirmation
usage. usage.
3.1. Confirmation Claim 3.1. Confirmation Claim
The "cnf" claim in the CWT is used to carry confirmation methods. The "cnf" claim in the CWT is used to carry confirmation methods.
Some of them use proof-of-possession keys while others do not. This Some of them use proof-of-possession keys, while others do not. This
design is analogous to the SAML 2.0 [OASIS.saml-core-2.0-os] design is analogous to the SAML 2.0 [OASIS.saml-core-2.0-os]
SubjectConfirmation element in which a number of different subject SubjectConfirmation element in which a number of different subject
confirmation methods can be included (including proof-of-possession confirmation methods can be included (including proof-of-possession
key information). key information).
The set of confirmation members that a CWT must contain to be The set of confirmation members that a CWT must contain to be
considered valid is context dependent and is outside the scope of considered valid is context dependent and is outside the scope of
this specification. Specific applications of CWTs will require this specification. Specific applications of CWTs will require
implementations to understand and process some confirmation members implementations to understand and process some confirmation members
in particular ways. However, in the absence of such requirements, in particular ways. However, in the absence of such requirements,
all confirmation members that are not understood by implementations all confirmation members that are not understood by implementations
MUST be ignored. MUST be ignored.
This specification establishes the IANA "CWT Confirmation Methods" Section 7.2 establishes the IANA "CWT Confirmation Methods" registry
registry for these members in Section 7.2 and registers the members for CWT "cnf" member values and registers the members defined by this
defined by this specification. Other specifications can register specification. Other specifications can register other members used
other members used for confirmation, including other members for for confirmation, including other members for conveying proof-of-
conveying proof-of-possession keys using different key possession keys using different key representations.
representations.
The "cnf" claim value MUST represent only a single proof-of- The "cnf" claim value MUST represent only a single proof-of-
possession key. At most one of the "COSE_Key" and possession key. At most one of the "COSE_Key" and
"Encrypted_COSE_Key" confirmation values defined in Figure 1 may be "Encrypted_COSE_Key" confirmation values defined in Table 1 may be
present. Note that if an application needs to represent multiple present. Note that if an application needs to represent multiple
proof-of-possession keys in the same CWT, one way for it to achieve 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 this is to use other claim names (in addition to "cnf") to hold the
additional proof-of-possession key information. These claims could additional proof-of-possession key information. These claims could
use the same syntax and semantics as the "cnf" claim. Those claims use the same syntax and semantics as the "cnf" claim. Those claims
would be defined by applications or other specifications and could be would be defined by applications or other specifications and could be
registered in the IANA "CBOR Web Token Claims" registry registered in the IANA "CBOR Web Token (CWT) Claims" registry
[IANA.CWT.Claims]. [IANA.CWT.Claims].
/--------------------+-----+-------------------------------\ +--------------------+-----+-------------------------------+
| Name | Key | Value type | | Name | Key | Value type |
|--------------------+-----+-------------------------------| +====================+=====+===============================+
| COSE_Key | 1 | COSE_Key | | COSE_Key | 1 | COSE_Key |
| Encrypted_COSE_Key | 2 | COSE_Encrypt or COSE_Encrypt0 | +--------------------+-----+-------------------------------+
| kid | 3 | binary string | | Encrypted_COSE_Key | 2 | COSE_Encrypt or COSE_Encrypt0 |
\--------------------+-----+-------------------------------/ +--------------------+-----+-------------------------------+
| kid | 3 | binary string |
+--------------------+-----+-------------------------------+
Figure 1: Summary of the cnf names, keys, and value types Table 1: Summary of the "cnf" Names, Keys, and Value Types
3.2. Representation of an Asymmetric Proof-of-Possession Key 3.2. Representation of an Asymmetric Proof-of-Possession Key
When the key held by the presenter is an asymmetric private key, the When the key held by the presenter is an asymmetric private key, the
"COSE_Key" member is a COSE_Key [RFC8152] representing the "COSE_Key" member is a COSE_Key [RFC8152] representing the
corresponding asymmetric public key. The following example corresponding asymmetric public key. The following example
demonstrates such a declaration in the CWT Claims Set of a CWT: demonstrates such a declaration in the CWT Claims Set of a CWT:
{ {
/iss/ 1 : "coaps://server.example.com", /iss/ 1 : "coaps://server.example.com",
/aud/ 3 : "coaps://client.example.org", /aud/ 3 : "coaps://client.example.org",
/exp/ 4 : 1879067471, /exp/ 4 : 1879067471,
/cnf/ 8 :{ /cnf/ 8 :{
/COSE_Key/ 1 :{ /COSE_Key/ 1 :{
/kty/ 1 : /EC2/ 2, /kty/ 1 : /EC2/ 2,
/crv/ -1 : /P-256/ 1, /crv/ -1 : /P-256/ 1,
/x/ -2 : h'd7cc072de2205bdc1537a543d53c60a6acb62eccd890c7fa27c9 /x/ -2 : h'd7cc072de2205bdc1537a543d53c60a6acb62eccd890c7fa27c9
e354089bbe13', e354089bbe13',
/y/ -3 : h'f95e1d4b851a2cc80fff87d8e23f22afb725d535e515d020731e /y/ -3 : h'f95e1d4b851a2cc80fff87d8e23f22afb725d535e515d020731e
79a3b4e47120' 79a3b4e47120'
} }
} }
} }
The COSE_Key MUST contain the required key members for a COSE_Key of 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 that key type and MAY contain other COSE_Key members, including the
"kid" (Key ID) member. "kid" (Key ID) member.
The "COSE_Key" member MAY also be used for a COSE_Key representing a 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 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 not revealed to unintended parties. The means of encrypting a CWT is
explained in [RFC8392]. If the CWT is not encrypted, the symmetric explained in [RFC8392]. If the CWT is not encrypted, the symmetric
key MUST be encrypted as described in Section 3.3. This procedure is key MUST be encrypted as described in Section 3.3. This procedure is
equivalent to the one defined in section 3.3 of [RFC7800]. equivalent to the one defined in Section 3.3 of [RFC7800].
3.3. Representation of an Encrypted Symmetric Proof-of-Possession Key 3.3. Representation of an Encrypted Symmetric Proof-of-Possession Key
When the key held by the presenter is a symmetric key, the When the key held by the presenter is a symmetric key, the
"Encrypted_COSE_Key" member is an encrypted COSE_Key [RFC8152] "Encrypted_COSE_Key" member is an encrypted COSE_Key [RFC8152]
representing the symmetric key encrypted to a key known to the representing the symmetric key encrypted to a key known to the
recipient using COSE_Encrypt or COSE_Encrypt0. recipient using COSE_Encrypt or COSE_Encrypt0.
The following example illustrates a symmetric key that could The following example illustrates a symmetric key that could
subsequently be encrypted for use in the "Encrypted_COSE_Key" member: subsequently be encrypted for use in the "Encrypted_COSE_Key" member:
{ {
/kty/ 1 : /Symmetric/ 4, /kty/ 1 : /Symmetric/ 4,
/alg/ 3 : /HMAC 256-256/ 5, /alg/ 3 : /HMAC 256-256/ 5,
/k/ -1 : h'6684523ab17337f173500e5728c628547cb37df /k/ -1 : h'6684523ab17337f173500e5728c628547cb37df
e68449c65f885d1b73b49eae1' e68449c65f885d1b73b49eae1'
} }
The COSE_Key representation is used as the plaintext when encrypting The COSE_Key representation is used as the plaintext when encrypting
the key. the key.
The following example CWT Claims Set of a CWT illustrates the use of The following example CWT Claims Set of a CWT illustrates the use of
an encrypted symmetric key as the "Encrypted_COSE_Key" member value: an encrypted symmetric key as the "Encrypted_COSE_Key" member value:
{ {
/iss/ 1 : "coaps://server.example.com", /iss/ 1 : "coaps://server.example.com",
/sub/ 2 : "24400320", /sub/ 2 : "24400320",
/aud/ 3: "s6BhdRkqt3", /aud/ 3: "s6BhdRkqt3",
/exp/ 4 : 1311281970, /exp/ 4 : 1311281970,
/iat/ 5 : 1311280970, /iat/ 5 : 1311280970,
/cnf/ 8 : { /cnf/ 8 : {
/Encrypted_COSE_Key/ 2 : [ /Encrypted_COSE_Key/ 2 : [
/protected header/ h'A1010A' /{ \alg\ 1:10 \AES-CCM-16-64-128\}/, /protected header/ h'A1010A' /{ \alg\ 1:10 \AES-CCM-16-64-128\}/,
/unprotected header/ { / iv / 5: h'636898994FF0EC7BFCF6D3F95B'}, /unprotected header/ { / iv / 5: h'636898994FF0EC7BFCF6D3F95B'},
/ciphertext/ h'0573318A3573EB983E55A7C2F06CADD0796C9E584F1D0E3E /ciphertext/ h'0573318A3573EB983E55A7C2F06CADD0796C9E584F1D0E3E
A8C5B052592A8B2694BE9654F0431F38D5BBC8049FA7F13F' A8C5B052592A8B2694BE9654F0431F38D5BBC8049FA7F13F'
] ]
} }
} }
The example above was generated with the key: The example above was generated with the key:
h'6162630405060708090a0b0c0d0e0f10' h'6162630405060708090a0b0c0d0e0f10'
3.4. Representation of a Key ID for a Proof-of-Possession Key 3.4. Representation of a Key ID for a Proof-of-Possession Key
The proof-of-possession key can also be identified using a Key ID The proof-of-possession key can also be identified using a Key ID
instead of communicating the actual key, provided the recipient is instead of communicating the actual key, provided the recipient is
able to obtain the identified key using the Key ID. In this case, able to obtain the identified key using the Key ID. In this case,
the issuer of a CWT declares that the presenter possesses a the issuer of a CWT declares that the presenter possesses a
particular key and that the recipient can cryptographically confirm particular key and that the recipient can cryptographically confirm
proof of possession of the key by the presenter by including a "cnf" the presenter's proof of possession of the key by including a "cnf"
claim in the CWT whose value is a CBOR map with the CBOR map claim in the CWT whose value is a CBOR map containing a "kid" member
containing a "kid" member identifying the key. identifying the key.
The following example demonstrates such a declaration in the CWT The following example demonstrates such a declaration in the CWT
Claims Set of a CWT: Claims Set of a CWT:
{ {
/iss/ 1 : "coaps://as.example.com", /iss/ 1 : "coaps://as.example.com",
/aud/ 3 : "coaps://resource.example.org", /aud/ 3 : "coaps://resource.example.org",
/exp/ 4 : 1361398824, /exp/ 4 : 1361398824,
/cnf/ 8 : { /cnf/ 8 : {
/kid/ 3 : h'dfd1aa976d8d4575a0fe34b96de2bfad' /kid/ 3 : h'dfd1aa976d8d4575a0fe34b96de2bfad'
}
} }
}
The content of the "kid" value is application specific. For The content of the "kid" value is application specific. For
instance, some applications may choose to use a cryptographic hash of instance, some applications may choose to use a cryptographic hash of
the public key value as the "kid" value. the public key value as the "kid" value.
Note that the use of a Key ID to identify a proof-of-possession key Note that the use of a Key ID to identify a proof-of-possession key
needs to be carefully circumscribed, as described below and in needs to be carefully circumscribed, as described below and in
Section 6. In cases where the Key ID is not a cryptographic value Section 6. In cases where the Key ID is not a cryptographic value
derived from the key or where not all of the parties involved are derived from the key or where not all of the parties involved are
validating the cryptographic derivation, implementers should expect validating the cryptographic derivation, implementers should expect
collisions, where different keys are assigned the same Key ID. collisions where different keys are assigned the same Key ID.
Recipients of a CWT with a PoP key linked through only a Key ID Recipients of a CWT with a PoP key linked through only a Key ID
should be prepared to handle such situations. should be prepared to handle such situations.
In the world of constrained Internet of Things (IoT) devices, there In the world of constrained Internet of Things (IoT) devices, there
is frequently a restriction on the size of Key IDs, either because of is frequently a restriction on the size of Key IDs, either because of
table constraints or a desire to keep message sizes small. table constraints or a desire to keep message sizes small.
Note that the value of a Key ID for a specific key is not necessarily Note that the value of a Key ID for a specific key is not necessarily
the same for different parties. When sending a COSE encrypted the same for different parties. When sending a COSE encrypted
message with a shared key, the Key ID may be different on both sides message with a shared key, the Key ID may be different on both sides
skipping to change at page 8, line 24 skipping to change at line 342
The means of communicating the nonce and the nature of its contents The means of communicating the nonce and the nature of its contents
are intentionally not described in this specification, as different are intentionally not described in this specification, as different
protocols will communicate this information in different ways. protocols will communicate this information in different ways.
Likewise, the means of communicating the signed nonce is also not Likewise, the means of communicating the signed nonce is also not
specified, as this is also protocol specific. specified, as this is also protocol specific.
Note that other means of proving possession of the key exist, which Note that other means of proving possession of the key exist, which
could be used in conjunction with a CWT's confirmation key. could be used in conjunction with a CWT's confirmation key.
Applications making use of such alternate means are encouraged to Applications making use of such alternate means are encouraged to
register them in the IANA "CWT Confirmation Methods" registry register them in the IANA "CBOR Web Token (CWT) Confirmation Methods"
established in Section 7.2. registry established in Section 7.2.
4. Security Considerations 4. Security Considerations
All the security considerations that are discussed in [RFC8392] also All the security considerations that are discussed in [RFC8392] also
apply here. In addition, proof of possession introduces its own apply here. In addition, proof of possession introduces its own
unique security issues. Possessing a key is only valuable if it is unique security issues. Possessing a key is only valuable if it is
kept secret. Appropriate means must be used to ensure that kept secret. Appropriate means must be used to ensure that
unintended parties do not learn private key or symmetric key values. unintended parties do not learn private key or symmetric key values.
Applications utilizing proof of possession SHOULD also utilize Applications utilizing proof of possession SHOULD also utilize
audience restriction, as described in Section 3.1.3 of [RFC8392], as audience restriction, as described in Section 3.1.3 of [RFC8392],
it provides additional protections. Audience restriction can be used because it provides additional protections. Audience restriction can
by recipients to reject messages intended for different recipients. be used by recipients to reject messages intended for different
(Of course, applications not using proof of possession can also recipients. (Of course, applications not using proof of possession
benefit from using audience restriction to reject messages intended can also benefit from using audience restriction to reject messages
for different recipients.) intended for different recipients.)
CBOR Web Tokens with proof-of-possession keys are used in context of CBOR Web Tokens with proof-of-possession keys are used in context of
an architecture, such as the ACE OAuth Framework an architecture, such as the ACE OAuth Framework [ACE-OAUTH], in
[I-D.ietf-ace-oauth-authz], in which protocols are used by a which protocols are used by a presenter to request these tokens and
presenter to request these tokens and to subsequently use them with to subsequently use them with recipients. Proof of possession only
recipients. Proof of possession only provides the intended security provides the intended security gains when the proof is known to be
gains when the proof is known to be current and not subject to replay current and not subject to replay attacks; security protocols using
attacks; security protocols using mechanisms such as nonces and mechanisms such as nonces and timestamps can be used to avoid the
timestamps can be used to avoid the risk of replay when performing risk of replay when performing proof of possession for a token. Note
proof of possession for a token. Note that a discussion of the that a discussion of the architecture or specific protocols that CWTs
architecture or specific protocols that CWT proof-of-possession with proof-of-possession keys are used with is beyond the scope of
tokens are used with is beyond the scope of this specification. this specification.
As is the case with other information included in a CWT, it is As is the case with other information included in a CWT, it is
necessary to apply data origin authentication and integrity necessary to apply data origin authentication and integrity
protection (via a keyed message digest or a digital signature). Data protection (via a keyed message digest or a digital signature). Data
origin authentication ensures that the recipient of the CWT learns origin authentication ensures that the recipient of the CWT learns
about the entity that created the CWT since this will be important about the entity that created the CWT, since this will be important
for any policy decisions. Integrity protection prevents an adversary for any policy decisions. Integrity protection prevents an adversary
from changing any elements conveyed within the CWT payload. Special from changing any elements conveyed within the CWT payload. Special
care has to be applied when carrying symmetric keys inside the CWT care has to be applied when carrying symmetric keys inside the CWT
since those not only require integrity protection but also since those not only require integrity protection but also
confidentiality protection. confidentiality protection.
As described in Section 6 (Key Identification) and Appendix D (Notes As described in Section 6 (Key Identification) and Appendix D (Notes
on Key Selection) of [JWS], it is important to make explicit trust on Key Selection) of [JWS], it is important to make explicit trust
decisions about the keys. Proof-of-possession signatures made with decisions about the keys. Proof-of-possession signatures made with
keys not meeting the application's trust criteria MUST NOT be relied keys not meeting the application's trust criteria MUST NOT be relied
skipping to change at page 9, line 51 skipping to change at line 417
recipient about the claims being used (which is also true of CWTs in recipient about the claims being used (which is also true of CWTs in
general). general).
When an issuer creates a CWT containing a Key ID claim, it needs to When an issuer creates a CWT containing a Key ID claim, it needs to
make sure that it does not issue another CWT with different claims make sure that it does not issue another CWT with different claims
containing the same Key ID within the lifetime of the CWTs, unless containing the same Key ID within the lifetime of the CWTs, unless
intentionally desired. Failure to do so may allow one party to intentionally desired. Failure to do so may allow one party to
impersonate another party, with the potential to gain additional impersonate another party, with the potential to gain additional
privileges. A case where such reuse of a Key ID would be intentional privileges. A case where such reuse of a Key ID would be intentional
is when a presenter obtains a CWT with different claims (e.g., is when a presenter obtains a CWT with different claims (e.g.,
extended scope) for the same recipient, but wants to continue using extended scope) for the same recipient but wants to continue using an
an existing security association (e.g., a DTLS session) bound to the existing security association (e.g., a DTLS session) bound to the key
key identified by the Key ID. Likewise, if PoP keys are used for identified by the Key ID. Likewise, if PoP keys are used for
multiple different kinds of CWTs in an application and the PoP keys multiple different kinds of CWTs in an application and the PoP keys
are identified by Key IDs, care must be taken to keep the keys for are identified by Key IDs, care must be taken to keep the keys for
the different kinds of CWTs segregated so that an attacker cannot the different kinds of CWTs segregated so that an attacker cannot
cause the wrong PoP key to be used by using a valid Key ID for the cause the wrong PoP key to be used by using a valid Key ID for the
wrong kind of CWT. Using an audience restriction for the CWT would wrong kind of CWT. Using an audience restriction for the CWT would
be one strategy to mitigate this risk. be one strategy to mitigate this risk.
7. IANA Considerations 7. IANA Considerations
The following registration procedure is used for all the registries The following registration procedure is used for all the registries
established by this specification. established by this specification.
Values are registered on a Specification Required [RFC8126] basis Values are registered on a Specification Required [RFC8126] basis
after a three-week review period on the cwt-reg-review@ietf.org after a three-week review period on the <cwt-reg-review@ietf.org>
mailing list, on the advice of one or more Designated Experts. mailing list, on the advice of one or more designated experts.
However, to allow for the allocation of values prior to publication, However, to allow for the allocation of values prior to publication,
the Designated Experts may approve registration once they are the designated experts may approve registration once they are
satisfied that such a specification will be published. [[ Note to satisfied that such a specification will be published.
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 Registration requests sent to the mailing list for review should use
an appropriate subject (e.g., "Request to Register CWT Confirmation an appropriate subject (e.g., "Request to Register CWT Confirmation
Method: example"). Registration requests that are undetermined for a Method: example"). Registration requests that are undetermined for a
period longer than 21 days can be brought directly to IANA's period longer than 21 days can be brought directly to IANA's
attention (using the iana@iana.org mailing list) for resolution. attention (using the iana@iana.org mailing list) for resolution.
Designated Experts should determine whether a registration request Designated experts should determine whether a registration request
contains enough information for the registry to be populated with the contains enough information for the registry to be populated with the
new values and whether the proposed new functionality already exists. new values and whether the proposed new functionality already exists.
In the case of an incomplete registration or an attempt to register In the case of an incomplete registration or an attempt to register
already existing functionality, the Designated Experts should ask for already existing functionality, the designated experts should ask for
corrections or reject the registration. corrections or reject the registration.
It is suggested that multiple Designated Experts be appointed who are It is suggested that multiple designated experts be appointed who are
able to represent the perspectives of different applications using able to represent the perspectives of different applications using
this specification in order to enable broadly informed review of this specification in order to enable broadly informed review of
registration decisions. In cases where a registration decision could registration decisions. In cases where a registration decision could
be perceived as creating a conflict of interest for a particular be perceived as creating a conflict of interest for a particular
Expert, that Expert should defer to the judgment of the other expert, that expert should defer to the judgment of the other
Experts. experts.
7.1. CBOR Web Token Claims Registration 7.1. CBOR Web Token Claims Registration
This specification registers the "cnf" claim in the IANA "CBOR Web This specification registers the "cnf" claim in the IANA "CBOR Web
Token Claims" registry [IANA.CWT.Claims] established by [RFC8392]. Token (CWT) Claims" registry [IANA.CWT.Claims], established by
[RFC8392].
7.1.1. Registry Contents 7.1.1. Registry Contents
o Claim Name: "cnf" * Claim Name: "cnf"
o Claim Description: Confirmation
o JWT Claim Name: "cnf" * Claim Description: Confirmation
o Claim Key: TBD (maybe 8)
o Claim Value Type(s): map * JWT Claim Name: "cnf"
o Change Controller: IESG
o Specification Document(s): Section 3.1 of [[ this document ]] * Claim Key: 8
* Claim Value Type(s): map
* Change Controller: IESG
* Specification Document(s): Section 3.1 of RFC 8747
7.2. CWT Confirmation Methods Registry 7.2. CWT Confirmation Methods Registry
This specification establishes the IANA "CWT Confirmation Methods" This specification establishes the IANA "CWT Confirmation Methods"
registry for CWT "cnf" member values. The registry records the registry for CWT "cnf" member values. The registry records the
confirmation method member and a reference to the specification that confirmation method member and a reference to the specification that
defines it. defines it.
7.2.1. Registration Template 7.2.1. Registration Template
Confirmation Method Name: Confirmation Method Name:
The human-readable name requested (e.g., "kid"). The human-readable name requested (e.g., "kid").
Confirmation Method Description: Confirmation Method Description:
Brief description of the confirmation method (e.g., "Key Brief description of the confirmation method (e.g., "Key
Identifier"). Identifier").
JWT Confirmation Method Name: JWT Confirmation Method Name:
Claim Name of the equivalent JWT confirmation method value, as Claim Name of the equivalent JWT confirmation method value, as
registered in [IANA.JWT.Claims]. CWT claims should normally have registered in the "JSON Web Token Claims" subregistry in the "JSON
a corresponding JWT claim. If a corresponding JWT claim would not Web Token (JWT)" registry [IANA.JWT]. CWT claims should normally
make sense, the Designated Experts can choose to accept 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". registrations for which the JWT Claim Name is listed as "N/A".
Confirmation Key: Confirmation Key:
CBOR map key value for the confirmation method. CBOR map key value for the confirmation method.
Confirmation Value Type(s): Confirmation Value Type(s):
CBOR types that can be used for the confirmation method value. CBOR types that can be used for the confirmation method value.
Change Controller: Change Controller:
For Standards Track RFCs, list the "IESG". For others, give the For Standards Track RFCs, list the "IESG". For others, give the
name of the responsible party. name of the responsible party.
Specification Document(s): Specification Document(s):
Reference to the document or documents that specify the parameter, Reference to the document or documents that specify the parameter,
preferably including URIs that can be used to retrieve copies of preferably including URIs that can be used to retrieve copies of
the documents. An indication of the relevant sections may also be the documents. An indication of the relevant sections may also be
included but is not required. Note that the Designated Experts included but is not required. Note that the designated experts
and IANA must be able to obtain copies of the specification and IANA must be able to obtain copies of the specification
document(s) to perform their work. document(s) to perform their work.
7.2.2. Initial Registry Contents 7.2.2. Initial Registry Contents
o Confirmation Method Name: "COSE_Key" * Confirmation Method Name: "COSE_Key"
o Confirmation Method Description: COSE_Key Representing Public Key * Confirmation Method Description: COSE_Key Representing Public Key
o JWT Confirmation Method Name: "jwk" * JWT Confirmation Method Name: "jwk"
o Confirmation Key: 1 * Confirmation Key: 1
o Confirmation Value Type(s): COSE_Key structure * Confirmation Value Type(s): COSE_Key structure
o Change Controller: IESG * Change Controller: IESG
o Specification Document(s): Section 3.2 of [[ this document ]] * Specification Document(s): Section 3.2 of RFC 8747
o Confirmation Method Name: "Encrypted_COSE_Key" * Confirmation Method Name: "Encrypted_COSE_Key"
o Confirmation Method Description: Encrypted COSE_Key * Confirmation Method Description: Encrypted COSE_Key
o JWT Confirmation Method Name: "jwe" * JWT Confirmation Method Name: "jwe"
o Confirmation Key: 2 * Confirmation Key: 2
o Confirmation Value Type(s): COSE_Encrypt or COSE_Encrypt0 * Confirmation Value Type(s): COSE_Encrypt or COSE_Encrypt0
structure (with an optional corresponding COSE_Encrypt or structure (with an optional corresponding COSE_Encrypt or
COSE_Encrypt0 tag) COSE_Encrypt0 tag)
o Change Controller: IESG * Change Controller: IESG
o Specification Document(s): Section 3.3 of [[ this document ]] * Specification Document(s): Section 3.3 of RFC 8747
o Confirmation Method Name: "kid" * Confirmation Method Name: "kid"
o Confirmation Method Description: Key Identifier * Confirmation Method Description: Key Identifier
o JWT Confirmation Method Name: "kid" * JWT Confirmation Method Name: "kid"
o Confirmation Key: 3 * Confirmation Key: 3
o Confirmation Value Type(s): binary string * Confirmation Value Type(s): binary string
o Change Controller: IESG * Change Controller: IESG
o Specification Document(s): Section 3.4 of [[ this document ]] * Specification Document(s): Section 3.4 of RFC 8747
8. References 8. References
8.1. Normative References 8.1. Normative References
[IANA.CWT.Claims] [IANA.CWT.Claims]
IANA, "CBOR Web Token Claims", IANA, "CBOR Web Token Claims",
<http://www.iana.org/assignments/cwt>. <https://www.iana.org/assignments/cwt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>. October 2013, <https://www.rfc-editor.org/info/rfc7049>.
skipping to change at page 13, line 28 skipping to change at line 588
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>. May 2018, <https://www.rfc-editor.org/info/rfc8392>.
8.2. Informative References 8.2. Informative References
[I-D.ietf-ace-oauth-authz] [ACE-OAUTH]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0 Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-21 Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
(work in progress), February 2019. draft-ietf-ace-oauth-authz-21, 14 February 2019,
<https://tools.ietf.org/html/draft-ietf-ace-oauth-authz-
21>.
[IANA.JWT.Claims] [IANA.JWT] IANA, "JSON Web Token (JWT)",
IANA, "JSON Web Token Claims", <https://www.iana.org/assignments/jwt>.
<http://www.iana.org/assignments/jwt>.
[JWS] Jones, M., Bradley, J., and N. Sakimura, "JSON Web [JWS] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <http://www.rfc-editor.org/info/rfc7515>. 2015, <https://www.rfc-editor.org/info/rfc7515>.
[JWT] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token [JWT] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<http://www.rfc-editor.org/info/rfc7519>. <https://www.rfc-editor.org/info/rfc7519>.
[OASIS.saml-core-2.0-os] [OASIS.saml-core-2.0-os]
Cantor, S., Kemp, J., Philpott, R., and E. Maler, Cantor, S., Kemp, J., Philpott, R., and E. Maler,
"Assertions and Protocol for the OASIS Security Assertion "Assertions and Protocol for the OASIS Security Assertion
Markup Language (SAML) V2.0", OASIS Standard saml-core- Markup Language (SAML) V2.0", OASIS Standard saml-core-
2.0-os, March 2005, 2.0-os, March 2005, <https://docs.oasis-
<http://docs.oasis-open.org/security/saml/v2.0/>. open.org/security/saml/v2.0/saml-core-2.0-os.pdf>.
[RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of- [RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of-
Possession Key Semantics for JSON Web Tokens (JWTs)", Possession Key Semantics for JSON Web Tokens (JWTs)",
RFC 7800, DOI 10.17487/RFC7800, April 2016, RFC 7800, DOI 10.17487/RFC7800, April 2016,
<https://www.rfc-editor.org/info/rfc7800>. <https://www.rfc-editor.org/info/rfc7800>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259, Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017, DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>. <https://www.rfc-editor.org/info/rfc8259>.
skipping to change at page 14, line 25 skipping to change at line 635
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data [RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>. June 2019, <https://www.rfc-editor.org/info/rfc8610>.
Acknowledgements Acknowledgements
Thanks to the following people for their reviews of the Thanks to the following people for their reviews of the
specification: Roman Danyliw, Christer Holmberg, Benjamin Kaduk, specification: Roman Danyliw, Christer Holmberg, Benjamin Kaduk,
Mirja Kuehlewind, Yoav Nir, Michael Richardson, Adam Roach, Eric Mirja Kühlewind, Yoav Nir, Michael Richardson, Adam Roach, Éric
Vyncke, and Jim Schaad. Vyncke, and Jim Schaad.
Ludwig Seitz and Goeran Selander worked on this document as part of Ludwig Seitz and Göran Selander worked on this document as part of
the CelticPlus projects CyberWI and CRITISEC, with funding from the CelticPlus projects CyberWI and CRITISEC, with funding from
Vinnova. Vinnova.
Document History
[[ to be removed by the RFC Editor before publication as an RFC ]]
-11
o Addressed remaining IESG review comment by Mirja Kuehlewind.
-10
o Addressed IESG review comments by Adam Roach and Eric Vyncke.
-09
o Addressed Gen-ART review comments by Christer Holmberg and SecDir
review comments by Yoav Nir.
-08
o Addressed remaining Area Director review comments by Benjamin
Kaduk.
-07
o Addressed Area Director review by Benjamin Kaduk.
-06
o Corrected nits identified by Roman Danyliw.
-05
o Added text suggested by Jim Schaad describing considerations when
using the Key ID confirmation method.
-04
o Addressed additional WGLC comments by Jim Schaad and Roman
Danyliw.
-03
o Addressed review comments by Jim Schaad, see https://www.ietf.org/
mail-archive/web/ace/current/msg02798.html
o Removed unnecessary sentence in the introduction regarding the use
any strings that could be case-sensitive.
o Clarified the terms Presenter and Recipient.
o Clarified text about the confirmation claim.
-02
o Changed "typically" to "often" when describing ways of performing
proof of possession.
o Changed b64 to hex encoding in an example.
o Changed to using the RFC 8174 boilerplate instead of the RFC 2119
boilerplate.
-01
o Now uses CBOR diagnostic notation for the examples.
o Added a table summarizing the "cnf" names, keys, and value types.
o Addressed some of Jim Schaad's feedback on -00.
-00
o Created the initial working group draft from draft-jones-ace-cwt-
proof-of-possession-01.
Authors' Addresses Authors' Addresses
Michael B. Jones Michael B. Jones
Microsoft Microsoft
Email: mbj@microsoft.com Email: mbj@microsoft.com
URI: http://self-issued.info/ URI: https://self-issued.info/
Ludwig Seitz Ludwig Seitz
RISE SICS Combitech
Scheelevaegen 17 Djaeknegatan 31
Lund 223 70 SE-211 35 Malmö
Sweden Sweden
Email: ludwig@ri.se Email: ludwig.seitz@combitech.se
Goeran Selander Göran Selander
Ericsson AB Ericsson AB
Faeroegatan 6 SE-164 80 Kista
Kista 164 80
Sweden Sweden
Email: goran.selander@ericsson.com Email: goran.selander@ericsson.com
Samuel Erdtman Samuel Erdtman
Spotify Spotify
Email: erdtman@spotify.com Email: erdtman@spotify.com
Hannes Tschofenig Hannes Tschofenig
Arm Ltd. Arm Ltd.
Hall in Tirol 6060 6060 Hall in Tirol
Austria Austria
Email: Hannes.Tschofenig@arm.com Email: Hannes.Tschofenig@arm.com
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