Proof-of-Possession Key Semantics for CBOR Web Tokens (CWTs)Microsoftmbj@microsoft.comhttps://self-issued.info/CombitechDjaeknegatan 31Malmö211 35Swedenludwig.seitz@combitech.seEricsson ABKista164 80Swedengoran.selander@ericsson.comSpotifyerdtman@spotify.comArm Ltd.6060Hall in TirolAustriaHannes.Tschofenig@arm.com
Security
ACECBOR Web TokenCWTProof-of-PossessionHolder-of-Key
This specification describes how to declare in a CBOR Web Token (CWT)
(which is defined by RFC 8392)
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
being the holder-of-key.
This specification provides equivalent functionality to
"Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)" (RFC 7800)
but using Concise Binary Object Representation (CBOR) and CWTs
rather than JavaScript Object Notation (JSON) and JSON Web Tokens (JWTs).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by
the Internet Engineering Steering Group (IESG). Further
information on Internet Standards is available in Section 2 of
RFC 7841.
Information about the current status of this document, any
errata, and how to provide feedback on it may be obtained at
.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
. Introduction
. Terminology
. Representations for Proof-of-Possession Keys
. Confirmation Claim
. Representation of an Asymmetric Proof-of-Possession Key
. Representation of an Encrypted Symmetric Proof-of-Possession Key
. Representation of a Key ID for a Proof-of-Possession Key
. Specifics Intentionally Not Specified
. Security Considerations
. Privacy Considerations
. Operational Considerations
. IANA Considerations
. CBOR Web Token Claims Registration
. Registry Contents
. CWT Confirmation Methods Registry
. Registration Template
. Initial Registry Contents
. References
. Normative References
. Informative References
Acknowledgements
Authors' Addresses
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 described as
being the holder-of-key.
This specification provides equivalent functionality to
"Proof-of-Possession Key Semantics for JSON Web Tokens (JWTs)"
but using Concise Binary Object Representation (CBOR)
and CWTs
rather than JavaScript Object Notation (JSON)
and JSON Web Tokens (JWTs) .
Terminology
The key words "MUST", "MUST NOT",
"REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are
to be interpreted as
described in BCP 14
when, and only when, they appear in all capitals, as shown here.
This specification uses terms defined in
the CBOR Web Token (CWT) ,
CBOR Object Signing and Encryption (COSE) , and
Concise Binary Object Representation (CBOR)
specifications.
These terms are defined by this specification:
Issuer
Party that creates the CWT and binds the claims about the subject to the
proof-of-possession key.
Presenter
Party that proves possession of a private key (for asymmetric key cryptography)
or secret key (for symmetric key cryptography) to a recipient of a CWT.
In the context of OAuth, this party is also called the OAuth Client.
Recipient
Party that receives the CWT containing the proof-of-possession key information from the presenter.
In the context of OAuth, this party is also called the OAuth Resource Server.
This specification provides examples in CBOR extended diagnostic
notation, as defined in .
The examples include line breaks for readability.
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
(which is defined in )
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.
For instance, some applications may use
the CWT sub (subject) claim
to identify the presenter.
Other applications may use
the iss (issuer) claim
to identify the presenter.
In some applications, the subject identifier might be relative to
the issuer identified by the iss claim.
The actual mechanism used is dependent upon the application.
The case in which the presenter is the subject of the CWT is analogous to
Security Assertion Markup Language (SAML) 2.0 SubjectConfirmation
usage.
Confirmation Claim
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 design is
analogous to the SAML 2.0 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.
establishes the
IANA "CWT Confirmation Methods" registry for CWT cnf
member values 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. At most one of the COSE_Key
and Encrypted_COSE_Key confirmation values defined
in 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 (CWT) Claims" registry .
Summary of the cnf Names, Keys, and Value Types
Name
Key
Value type
COSE_Key
1
COSE_Key
Encrypted_COSE_Key
2
COSE_Encrypt or COSE_Encrypt0
kid
3
binary string
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
representing the corresponding asymmetric public key.
The following example demonstrates such a declaration
in the CWT Claims Set of a CWT:
{
/iss/ 1 : "coaps://server.example.com",
/aud/ 3 : "coaps://client.example.org",
/exp/ 4 : 1879067471,
/cnf/ 8 :{
/COSE_Key/ 1 :{
/kty/ 1 : /EC2/ 2,
/crv/ -1 : /P-256/ 1,
/x/ -2 : h'd7cc072de2205bdc1537a543d53c60a6acb62eccd890c7fa27c9
e354089bbe13',
/y/ -3 : h'f95e1d4b851a2cc80fff87d8e23f22afb725d535e515d020731e
79a3b4e47120'
}
}
}
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 .
If the CWT is not encrypted, the symmetric key MUST
be encrypted as described in . This procedure is equivalent to
the one defined in .
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
representing the symmetric key
encrypted to a key known to the recipient
using COSE_Encrypt or COSE_Encrypt0.
The following example
illustrates a symmetric key that could subsequently be encrypted for use in the
Encrypted_COSE_Key member:
{
/kty/ 1 : /Symmetric/ 4,
/alg/ 3 : /HMAC 256-256/ 5,
/k/ -1 : h'6684523ab17337f173500e5728c628547cb37df
e68449c65f885d1b73b49eae1'
}
The COSE_Key representation
is used as the plaintext when encrypting the key.
The following example CWT Claims Set of a CWT
illustrates the use of an encrypted symmetric key as the
Encrypted_COSE_Key member value:
{
/iss/ 1 : "coaps://server.example.com",
/sub/ 2 : "24400320",
/aud/ 3: "s6BhdRkqt3",
/exp/ 4 : 1311281970,
/iat/ 5 : 1311280970,
/cnf/ 8 : {
/Encrypted_COSE_Key/ 2 : [
/protected header/ h'A1010A' /{ \alg\ 1:10 \AES-CCM-16-64-128\}/,
/unprotected header/ { / iv / 5: h'636898994FF0EC7BFCF6D3F95B'},
/ciphertext/ h'0573318A3573EB983E55A7C2F06CADD0796C9E584F1D0E3E
A8C5B052592A8B2694BE9654F0431F38D5BBC8049FA7F13F'
]
}
}
The example above was generated with the key:
h'6162630405060708090a0b0c0d0e0f10'
Representation of a Key ID for a Proof-of-Possession Key
The proof-of-possession key can also be identified using
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
the presenter's proof of possession of the key by including a
cnf claim in the CWT
whose value is a CBOR map containing a kid member
identifying the key.
The following example demonstrates such a declaration
in the CWT Claims Set of a CWT:
{
/iss/ 1 : "coaps://as.example.com",
/aud/ 3 : "coaps://resource.example.org",
/exp/ 4 : 1361398824,
/cnf/ 8 : {
/kid/ 3 : h'dfd1aa976d8d4575a0fe34b96de2bfad'
}
}
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.
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 .
In cases where the Key ID is not a cryptographic value derived from the key
or where not all of the parties involved are validating the cryptographic derivation,
implementers should expect collisions where different keys are assigned the same Key ID.
Recipients of a CWT with a PoP key linked through only a Key ID should be prepared to handle
such situations.
In the world of constrained Internet of Things (IoT) devices,
there is frequently a restriction on the size of Key IDs,
either because of 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 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.
Specifics Intentionally Not Specified
Proof of possession is often 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".
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
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 other means of proving possession of the key
exist, which could be used in conjunction with a CWT's confirmation key.
Applications making use of such alternate means are encouraged
to register them in the IANA "CBOR Web Token (CWT) Confirmation Methods" registry
established in .
Security Considerations
All the security considerations that
are discussed in 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 ,
because it provides additional protections.
Audience restriction can be used by recipients to reject messages intended for different recipients.
(Of course, applications not using proof of possession can also benefit
from using audience restriction to reject messages intended for different recipients.)
CBOR Web Tokens with proof-of-possession keys are used in context of an architecture,
such as the ACE OAuth Framework ,
in which protocols are used by a presenter to request these tokens
and to subsequently use them with recipients.
Proof of possession only provides the intended security gains when the
proof is known to be current and not subject to replay attacks;
security protocols using mechanisms such as nonces and timestamps can be used to
avoid the risk of replay when performing proof of possession for a
token.
Note that a discussion of the architecture or specific protocols that
CWTs with proof-of-possession keys are used with is beyond the scope of this specification.
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 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.
As described in Section Key
Identification and Appendix Notes on Key Selection of , it is important to make
explicit trust decisions about the keys.
Proof-of-possession signatures made with keys
not meeting the application's trust criteria MUST NOT be relied upon.
Privacy Considerations
A proof-of-possession key can be used as a correlation handle if the same key
is used on multiple occasions.
Thus, for privacy reasons, it is recommended that different proof-of-possession keys
be used when interacting with different parties.
Operational Considerations
The use of CWTs with proof-of-possession keys requires additional information
to be shared between the involved parties in order to ensure correct processing.
The recipient needs to be able to use credentials to verify the authenticity and
integrity of the CWT. Furthermore, the recipient may need to be able to decrypt
either the whole CWT or the encrypted parts thereof (see ).
This requires the recipient to know information about the issuer.
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 make sure that
it does not issue another CWT with different claims containing the same Key ID
within the lifetime of the CWTs, unless intentionally desired.
Failure to do so may allow one party to impersonate another party,
with the potential to gain additional 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 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
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 be one strategy to mitigate this risk.
IANA Considerations
The following registration procedure is used for all the
registries established by this specification.
Values are registered on a Specification Required
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.
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 directly to IANA's attention
(using the iana@iana.org mailing list) for resolution.
Designated experts should determine whether a registration request contains
enough information for the registry to be populated with the new values and
whether the proposed new functionality already exists.
In the case of an incomplete registration
or an attempt to register already existing functionality,
the designated experts should ask for corrections or reject the registration.
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 expert,
that expert should defer to the judgment of the other experts.
CBOR Web Token Claims Registration
This specification registers the cnf claim in the IANA
"CBOR Web Token (CWT) Claims" registry ,
established by .
Registry Contents
Claim Name: cnf
Claim Description: Confirmation
JWT Claim Name: cnf
Claim Key: 8
Claim Value Type(s): map
Change Controller: IESG
Specification Document(s): of RFC 8747
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.
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 the "JSON Web Token Claims" subregistry in
the "JSON Web Token (JWT)" registry .
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:
For Standards Track RFCs, list the "IESG". For others, give the name of the
responsible party.
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.
Note that the designated experts and IANA must be able to obtain
copies of the specification document(s) to perform their work.
Initial Registry Contents
Confirmation Method Name: COSE_Key
Confirmation Method Description: COSE_Key Representing Public
Key
Confirmation Value Type(s): COSE_Encrypt or COSE_Encrypt0
structure (with an optional corresponding COSE_Encrypt or
COSE_Encrypt0 tag)
Change Controller: IESG
Specification Document(s): of RFC 8747
Confirmation Method Name: kid
Confirmation Method Description: Key Identifier
JWT Confirmation Method Name: kid
Confirmation Key: 3
Confirmation Value Type(s): binary string
Change Controller: IESG
Specification Document(s): of RFC 8747
ReferencesNormative ReferencesCBOR Web Token ClaimsIANAKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Concise Binary Object Representation (CBOR)The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack.Guidelines for Writing an IANA Considerations Section in RFCsMany protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA).To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.This is the third edition of this document; it obsoletes RFC 5226.CBOR Object Signing and Encryption (COSE)Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need for the ability to have basic security services defined for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR.Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.CBOR Web Token (CWT)CBOR Web Token (CWT) is a compact means of representing claims to be transferred between two parties. The claims in a CWT are encoded in the Concise Binary Object Representation (CBOR), and CBOR Object Signing and Encryption (COSE) is used for added application-layer security protection. A claim is a piece of information asserted about a subject and is represented as a name/value pair consisting of a claim name and a claim value. CWT is derived from JSON Web Token (JWT) but uses CBOR rather than JSON.Informative ReferencesAuthentication and Authorization for Constrained Environments (ACE) using the OAuth 2.0 Framework (ACE-OAuth)This specification defines a framework for authentication and authorization in Internet of Things (IoT) environments called ACE- OAuth. The framework is based on a set of building blocks including OAuth 2.0 and CoAP, thus making a well-known and widely used authorization solution suitable for IoT devices. Existing specifications are used where possible, but where the constraints of IoT devices require it, extensions are added and profiles are defined.Work in ProgressJSON Web Token (JWT)IANAJSON Web Signature (JWS)JSON Web Signature (JWS) represents content secured with digital signatures or Message Authentication Codes (MACs) using JSON-based data structures. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and an IANA registry defined by that specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) specification.JSON Web Token (JWT)JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted.Assertions and Protocol for the OASIS Security Assertion Markup Language (SAML) V2.0Internet2cantor.2@osu.eduNokiaJohn.Kemp@nokia.comRSA Securityrphilpott@rsasecurity.comSun Microsystemseve.maler@sun.comProof-of-Possession Key Semantics for JSON Web Tokens (JWTs)This specification describes how to declare in a JSON Web Token (JWT) that the presenter of the JWT possesses a particular proof-of- possession key and how the recipient can cryptographically confirm proof of possession of the key by the presenter. Being able to prove possession of a key is also sometimes described as the presenter being a holder-of-key.The JavaScript Object Notation (JSON) Data Interchange FormatJavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data.This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance.Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data StructuresThis document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON.Acknowledgements
Thanks to the following people for their reviews of the specification:
,
,
,
,
,
,
,
,
and
.
and worked on this document as part of
the CelticPlus projects CyberWI and CRITISEC, with funding from Vinnova.Authors' AddressesMicrosoftmbj@microsoft.comhttps://self-issued.info/CombitechDjaeknegatan 31Malmö211 35Swedenludwig.seitz@combitech.seEricsson ABKista164 80Swedengoran.selander@ericsson.comSpotifyerdtman@spotify.comArm Ltd.6060Hall in TirolAustriaHannes.Tschofenig@arm.com