draft-ietf-ace-cwt-proof-of-possession-06.txt   draft-ietf-ace-cwt-proof-of-possession-07.txt 
ACE M. Jones ACE M. Jones
Internet-Draft Microsoft Internet-Draft Microsoft
Intended status: Standards Track L. Seitz Intended status: Standards Track L. Seitz
Expires: August 25, 2019 RISE SICS Expires: March 21, 2020 RISE SICS
G. Selander G. Selander
Ericsson AB Ericsson AB
S. Erdtman S. Erdtman
Spotify Spotify
H. Tschofenig H. Tschofenig
ARM Ltd. Arm Ltd.
February 21, 2019 September 18, 2019
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-06 draft-ietf-ace-cwt-proof-of-possession-07
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)
that the presenter of the CWT possesses a particular proof-of- that the presenter of the CWT possesses a particular proof-of-
possession key. Being able to prove possession of a key is also possession key. Being able to prove 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)" (RFC 7800), but using CBOR and Semantics for JSON Web Tokens (JWTs)" (RFC 7800) but using CBOR and
CWTs rather than JSON and JWTs. CWTs rather than JSON and JWTs.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 25, 2019. This Internet-Draft will expire on March 21, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 25 skipping to change at page 2, line 25
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Representations for Proof-of-Possession Keys . . . . . . . . 3 3. Representations for Proof-of-Possession Keys . . . . . . . . 3
3.1. Confirmation Claim . . . . . . . . . . . . . . . . . . . 4 3.1. Confirmation Claim . . . . . . . . . . . . . . . . . . . 4
3.2. Representation of an Asymmetric Proof-of-Possession Key . 5 3.2. Representation of an Asymmetric Proof-of-Possession Key . 5
3.3. Representation of an Encrypted Symmetric Proof-of- 3.3. Representation of an Encrypted Symmetric Proof-of-
Possession Key . . . . . . . . . . . . . . . . . . . . . 5 Possession Key . . . . . . . . . . . . . . . . . . . . . 5
3.4. Representation of a Key ID for a Proof-of-Possession Key 6 3.4. Representation of a Key ID for a Proof-of-Possession Key 6
3.5. Specifics Intentionally Not Specified . . . . . . . . . . 8 3.5. Specifics Intentionally Not Specified . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8 4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9 5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
6. Operational Considerations . . . . . . . . . . . . . . . . . 10 6. Operational Considerations . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7.1. CBOR Web Token Claims Registration . . . . . . . . . . . 11 7.1. CBOR Web Token Claims Registration . . . . . . . . . . . 10
7.1.1. Registry Contents . . . . . . . . . . . . . . . . . . 11 7.1.1. Registry Contents . . . . . . . . . . . . . . . . . . 10
7.2. CWT Confirmation Methods Registry . . . . . . . . . . . . 11 7.2. CWT Confirmation Methods Registry . . . . . . . . . . . . 11
7.2.1. Registration Template . . . . . . . . . . . . . . . . 11 7.2.1. Registration Template . . . . . . . . . . . . . . . . 11
7.2.2. Initial Registry Contents . . . . . . . . . . . . . . 12 7.2.2. Initial Registry Contents . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . 13 8.1. Normative References . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . 13 8.2. Informative References . . . . . . . . . . . . . . . . . 13
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 14 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 14
Document History . . . . . . . . . . . . . . . . . . . . . . . . 14 Document History . . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
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 CBOR Semantics for JSON Web Tokens (JWTs)" [RFC7800] but using CBOR
[RFC7049] and CWTs [RFC8392] rather than JSON [RFC8259] and JWTs [RFC7049] and CWTs [RFC8392] rather than JSON [RFC8259] and JWTs
[JWT]. [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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
skipping to change at page 3, line 26 skipping to change at page 3, line 26
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
to the proof-of-possession key. to the proof-of-possession key.
Presenter Presenter
Party that proves possession of a private key (for asymmetric key Party that proves possession of a private key (for asymmetric key
cryptography) or secret key (for symmetric key cryptography) to a cryptography) or secret key (for symmetric key cryptography) to a
recipient. recipient of a CWT.
In context of OAuth this party is also called OAuth Client. In the context of OAuth, this party is also called the OAuth
Client.
Recipient Recipient
Party that receives the CWT containing the proof-of-possession key Party that receives the CWT containing the proof-of-possession key
information from the presenter. information from the presenter.
In context of OAuth this party is also called OAuth Resource In the context of OAuth, this party is also called the OAuth
Server. Resource Server.
This specification provides examples in CBOR extended diagnostic
notation, as defined in Appendix G of [RFC8610]. The examples
include line breaks for readability.
3. Representations for Proof-of-Possession Keys 3. Representations for Proof-of-Possession Keys
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
and the members of that map identify the proof-of-possession key. 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, The presenter can be identified in one of several ways by the CWT,
skipping to change at page 5, line 9 skipping to change at page 5, line 19
| Encrypted_COSE_Key | 2 | COSE_Encrypt or COSE_Encrypt0 | | Encrypted_COSE_Key | 2 | COSE_Encrypt or COSE_Encrypt0 |
| kid | 3 | binary string | | kid | 3 | binary string |
\--------------------+-----+-------------------------------/ \--------------------+-----+-------------------------------/
Figure 1: Summary of the cnf names, keys, and value types Figure 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 (using corresponding asymmetric public key. The following example
CBOR diagnostic notation) demonstrates such a declaration in the CWT demonstrates such a declaration in the CWT Claims Set of a 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 : 1361398824, /exp/ 4 : 1879067471,
/cnf/ 8 :{ /cnf/ 8 :{
/COSE_Key/ 1 :{ /COSE_Key/ 1 :{
/kty/ 1 : /EC/ 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
skipping to change at page 5, line 46 skipping to change at page 6, line 7
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. key MUST be encrypted as described in Section 3.3.
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 (using CBOR diagnostic notation, with The following example illustrates a symmetric key that could
linebreaks for readability) 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 : /HMAC256/ 5, /alg/ 3 : /HMAC256//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 (using CBOR diagnostic The following example CWT Claims Set of a CWT illustrates the use of
notation, with linebreaks for readability) illustrates the use of an an encrypted symmetric key as the "Encrypted_COSE_Key" member value:
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 : {
/COSE_Encrypt0/ 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 by the use of a The proof-of-possession key can also be identified using a Key ID
Key ID instead of communicating the actual key, provided the instead of communicating the actual key, provided the recipient is
recipient is able to obtain the identified key using the Key ID. In able to obtain the identified key using the Key ID. In this case,
this case, the issuer of a CWT declares that the presenter possesses the issuer of a CWT declares that the presenter possesses a
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" 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 claim in the CWT whose value is a CBOR map with the CBOR map
containing a "kid" member identifying the key. containing a "kid" member identifying the key.
The following example (using CBOR diagnostic notation) demonstrates The following example demonstrates such a declaration in the CWT
such a declaration in the CWT Claims Set of a CWT: Claims Set of a CWT:
{ {
/iss/ 1 : "coaps://server.example.com", /iss/ 1 : "coaps://as.example.com",
/aud/ 3 : "coaps://client.example.org", /aud/ 3 : "coaps://resource.example.org",
/exp/ 4 : 1361398824, /exp/ 4 : 1361398824,
/cnf/ 8 : { /cnf/ 8 : {
/kid/ 2 : 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-possesion 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. Where the Key ID is not a cryptographic value derived Section 6. In cases where the Key ID is not a cryptographic value
from the key or where all of the parties involved are not validating derived from the key or where not all of the parties involved are
the cryptographic derivation, it is possible to get into situations validating the cryptographic derivation, implementers should expect
where the same Key ID is being used for multiple keys. The collisions, where different keys are assigned the same Key ID.
implication of this is that a recipient may have multiple keys known Recipients of a CWT with a PoP key linked through only a Key ID
to it that have the same Key ID, and thus it might not know which should be prepared to handle such situations.
proof-of-possession key is associated with the CWT.
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. These table constraints or a desire to keep message sizes small.
restrictions are going to protocol dependent. For example, DTLS can
use a Key ID of any size. However, if the key is being used with
COSE encrypted message, then the length of the key needs to be
minimized and may have a limit as small as one byte.
Note that the value of a Key ID is not always the same for different
parties. When sending a COSE encrypted message with a shared key,
the Key ID may be different on both sides of the conversation, with
the appropriate one being included in the message based on the
recipient of the message.
For symmetric keys, the Key ID is normally going to be generated by
the CWT issuer. This means that enforcing a rule that Key ID values
only match if CWTs have the same issuer works for matching Key IDs
between CWTs. In this case, the issuer can ensure that there are no
collisions between currently active symmetric keys for all CWTs that
it has issued. This allows for a recipient to use the pair of issuer
and Key ID for matching keys.
For asymmetric keys, the Key ID value is normally going to be
generated by the CWT recipient, thus the possibility of collisions is
greater. For instance, recipients might start by assigning a Key ID
of 0, given that Key IDs are frequently only needed to be unique and
meaningful to the recipient. This problem can be addressed in a
couple of different ways, depending on how the Key ID value is going
to be used:
o The issuer can assign a new unique Key ID the first time it sees
the key. Depending on the protocol being used, the new value may
then need to be transported to the presenter by the protocol used
to issue CWTs. In this case, the rule of requiring that the
issuer, Key ID pair be used for matching works.
o The issuer can use a different confirmation method if a collision
might be unavoidable.
o A recipient can decide not to use a CWT based on a created Key ID
if it does not fit the recipient's requirements.
o If an issuer is going to use the Key ID confirmation method and is
not going to guarantee that serial number uniqueness is going to
be preserved, the recipient needs to have that information
configured into it so that appropriate actions can be taken.
3.5. Specifics Intentionally Not Specified 3.5. Specifics Intentionally Not Specified
Proof of possession is often demonstrated by having the presenter Proof of possession is often demonstrated by having the presenter
sign a value determined by the recipient using the key possessed by sign a value determined by the recipient using the key possessed by
the presenter. This value is sometimes called a "nonce" or a the presenter. This value is sometimes called a "nonce" or a
"challenge". "challenge". There are, however, also other means to demonstrate
freshness of the exchange and to link the proof-of-possession key to
the participating parties, as demonstrated by various authentication
and key exchange protocols.
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 another means of proving possession of the key when it is a Note that other means of proving possession of the key exist, which
symmetric key is to encrypt the key to the recipient. The means of could be used in conjunction with a CWT's confirmation key.
obtaining a key for the recipient is likewise protocol specific. Applications making use of such alternate means are encouraged to
register them in the IANA "CWT Confirmation Methods" registry
established in Section 7.2.
4. Security Considerations 4. Security Considerations
All of the security considerations that are discussed in [RFC8392] All the security considerations that are discussed in [RFC8392] also
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 4.1.3 of [JWT], as it audience restriction, as described in Section 4.1.3 of [JWT], as it
provides additional protections. Audience restriction can be used by provides additional protections. Audience restriction can be used by
recipients to reject messages intended for different recipients. recipients to reject messages intended for different recipients.
A recipient might not understand the "cnf" claim. Applications that A recipient might not understand the "cnf" claim. Applications that
require the proof-of-possession keys communicated with it to be use proof-of-possession keys in CWTs with the "cnf" claim MUST ensure
understood and processed MUST ensure that the parts of this that the parts of this specification that they use are implemented by
specification that they use are implemented. the intended recipient.
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
[I-D.ietf-ace-oauth-authz], in which protocols are used by a [I-D.ietf-ace-oauth-authz], in which protocols are used by a
presenter to request these tokens and to subsequently use them with presenter to request these tokens and to subsequently use them with
recipients. To avoid replay attacks when the proof-of-possession recipients. Proof of possession only provides the intended security
tokens are sent to presenters, a security protocol, which uses gains when the proof is known to be current and not subject to replay
mechansims such as nonces or timestamps, has to be utilized. Note attacks; security protocols using mechanisms such as nonces and
that a discussion of the architecture or specific protocols that CWT timestamps can be used to avoid the risk of replay when performing
proof-of-possession tokens are used with is beyond the scope of this proof of possession for a token. Note that a discussion of the
specification. architecture or specific protocols that CWT proof-of-possession
tokens are used with is beyond the scope of 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 (e.g., either by encrypting the "cnf"
element, as specified in Section 3.3, or by encrypting the whole CWT,
as specified in [RFC8392]).
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
upon. upon.
5. Privacy Considerations 5. Privacy Considerations
A proof-of-possession key can be used as a correlation handle if the 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, same key is used on multiple occasions. Thus, for privacy reasons,
it is recommended that different proof-of-possession keys be used it is recommended that different proof-of-possession keys be used
when interacting with different parties. when interacting with different parties.
6. Operational Considerations 6. Operational Considerations
The use of CWTs with proof-of-possession keys requires additional The use of CWTs with proof-of-possession keys requires additional
information to be shared between the involved parties in order to information to be shared between the involved parties in order to
ensure correct processing. The recipient needs to be able to use ensure correct processing. The recipient needs to be able to use
credentials to verify the authenticity, integrity, and potentially credentials to verify the authenticity and integrity of the CWT.
the confidentiality of the CWT and its content. This requires the Furthermore, the recipient may need to be able to decrypt either the
recipient to know information about the issuer. Likewise, there whole CWT or the encrypted parts thereof (see Section 3.3). This
needs to be agreement between the issuer and the recipient about the requires the recipient to know information about the issuer.
claims being used (which is also true of CWTs in general). Likewise, there needs to be agreement between the issuer and the
recipient about the claims being used (which is also true of CWTs in
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 containing the same Key make sure that it does not issue another CWT with different claims
ID with a different content, or for a different subject, within the containing the same Key ID within the lifetime of the CWTs, unless
lifetime of the CWTs, unless intentionally desired. Failure to do so intentionally desired. Failure to do so may allow one party to
may allow one party to impersonate another party, with the potential impersonate another party, with the potential to gain additional
to gain additional privileges. Likewise, if PoP keys are used for 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.,
extended scope) for the same recipient, but wants to continue using
an existing security association (e.g., a DTLS session) bound to the
key 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. wrong kind of CWT. Using an audience restriction for the CWT would
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,
skipping to change at page 10, line 49 skipping to change at page 10, line 21
the RFC Editor: The name of the mailing list should be determined in the RFC Editor: The name of the mailing list should be determined in
consultation with the IESG and IANA. Suggested name: cwt-reg- consultation with the IESG and IANA. Suggested name: cwt-reg-
review@ietf.org. ]] 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 to the IESG's attention period longer than 21 days can be brought to the IESG's attention
(using the iesg@ietf.org mailing list) for resolution. (using the iesg@ietf.org mailing list) for resolution.
Criteria that should be applied by the Designated Experts include Designated Experts should determine whether a registration request
determining whether the proposed registration duplicates existing contains enough information for the registry to be populated with the
functionality, determining whether it is likely to be of general new values and whether the proposed new functionality already exists.
applicability or whether it is useful only for a single application, In the case of an incomplete registration or an attempt to register
and evaluating the security properties of the item being registered already existing functionality, the Designated Experts should ask for
and whether the registration makes sense. 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
skipping to change at page 12, line 13 skipping to change at page 11, line 36
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. Other details (e.g., postal name of the responsible party.
address, email address, home page URI) may also be included.
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. included but is not required. Note that the Designated Experts
and IANA must be able to obtain copies of the specification
document(s) to perform their work.
7.2.2. Initial Registry Contents 7.2.2. Initial Registry Contents
o Confirmation Method Name: "COSE_Key" o Confirmation Method Name: "COSE_Key"
o Confirmation Method Description: COSE_Key Representing Public Key o Confirmation Method Description: COSE_Key Representing Public Key
o JWT Confirmation Method Name: "jwk" o JWT Confirmation Method Name: "jwk"
o Confirmation Key: 1 o Confirmation Key: 1
o Confirmation Value Type(s): COSE_Key structure o Confirmation Value Type(s): COSE_Key structure
o Change Controller: IESG o Change Controller: IESG
o Specification Document(s): Section 3.2 of [[ this document ]] o Specification Document(s): Section 3.2 of [[ this document ]]
skipping to change at page 14, line 30 skipping to change at page 14, line 5
[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>.
[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>.
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, Michael Richardson, and Jim Schaad. specification: Roman Danyliw, Benjamin Kaduk, Michael Richardson, and
Jim Schaad.
Ludwig Seitz and Goeran Selander worked on this document as part of Ludwig Seitz and Goeran Selander worked on this document as part of
the CelticPlus project CyberWI, with funding from Vinnova. the CelticPlus project CyberWI, with funding from Vinnova.
Document History Document History
[[ to be removed by the RFC Editor before publication as an RFC ]] [[ to be removed by the RFC Editor before publication as an RFC ]]
-07
o Addressed Area Director review by Benjamin Kaduk.
-06 -06
o Corrected nits identified by Roman Danyliw. o Corrected nits identified by Roman Danyliw.
-05 -05
o Added text suggested by Jim Schaad describing considerations when o Added text suggested by Jim Schaad describing considerations when
using the Key ID confirmation method. using the Key ID confirmation method.
-04 -04
skipping to change at page 16, line 19 skipping to change at page 16, line 4
Email: ludwig@ri.se Email: ludwig@ri.se
Goeran Selander Goeran Selander
Ericsson AB Ericsson AB
Faeroegatan 6 Faeroegatan 6
Kista 164 80 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 Hall in Tirol 6060
Austria Austria
Email: Hannes.Tschofenig@arm.com Email: Hannes.Tschofenig@arm.com
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