CBOR Web Token (CWT)Nexus TechnologySwedenerik.wahlstrom@nexusgroup.comhttps://www.nexusgroup.comMicrosoftmbj@microsoft.comhttp://self-issued.info/ARM Ltd.6060Hall in TirolAustriaHannes.Tschofenig@arm.com
Security
ACE Working GroupInternet-DraftJSON Web TokenJWTClaimsCBORCOSEOAuthACE
CBOR Web Token (CWT) is a compact means of representing claims to be
transferred between two parties. CWT is a profile of the JSON Web
Token (JWT) that is optimized for constrained devices. 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.
The JSON Web Token (JWT) is a standardized security token format
that has found use in OAuth 2.0 and OpenID Connect deployments, among other applications.
JWT uses JSON Web Signatures (JWS) and
JSON Web Encryption (JWE) to secure the contents of the JWT,
which is a set of claims represented in JSON .
The use of JSON for encoding information is popular for
Web and native applications, but it is considered inefficient for some
Internet of Things (IoT) systems that use low power radio technologies.
In this document an alternative encoding of claims is defined. Instead
of using JSON, as provided by JWTs, this specification uses
CBOR and calls this new structure "CBOR Web Token (CWT)", which is a
compact means of representing secured claims to be transferred between two
parties. CWT is closely related to JWT. It references the JWT claims
and both its name and pronunciation are derived from JWT. To protect the
claims contained in CWTs, the CBOR Object Signing and Encryption (COSE)
specification is used.
The suggested pronunciation of CWT is the same as the English word
"cot".
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 "Key words for use in
RFCs to Indicate Requirement Levels" .
This document reuses terminology from JWT
and COSE .
The "Type3StringOrURI" term has the same meaning, syntax, and
processing rules as the "StringOrUri" term defined in Section 2
of JWT , except that Type3StringOrURI uses
CBOR major type 3 instead of a JSON string value.
FIXME: Use tag 32 for URIs?
The "Type6NumericDate" term has the same meaning, syntax, and
processing rules as the "NumericDate" term defined in Section 2
of JWT , except that Type6NumericDate uses
CBOR major type 6, with tag value 1, instead of a numeric JSON
value.
The key used to identify a claim value.
The set of claims 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 claims in particular ways. However, in
the absence of such requirements, all claims that are not understood
by implementations MUST be ignored.
To keep CWTs as small as possible, the CBOR encoded claim keys are
represented using CBOR major type 0.
summaries all keys used to
identity the claims defined in this document.
None of the claims defined below are intended to be
mandatory to use or implement. They rather provide
a starting point for a set of useful, interoperable claims.
Applications using CWTs should define which specific claims they
use and when they are required or optional.
The iss (issuer) claim has the same meaning, syntax, and
processing rules as the iss claim defined in Section 4.1.1
of JWT , except that the format MUST be a
Type3StringOrURI. The CBOR encoded claim key 1 MUST be used to
identify this claim.
The sub (subject) claim has the same meaning, syntax, and
processing rules as the sub claim defined in Section 4.1.2
of JWT , except that the format MUST be a
Type3StringOrURI. The CBOR encoded claim key 2 MUST be used to
identify this claim.
The aud (audience) claim has the same meaning, syntax, and
processing rules as the aud claim defined in Section 4.1.3
of JWT , except that the format MUST be a
Type3StringOrURI. The CBOR encoded claim key 3 MUST be used to
identify this claim.
The exp (expiration time) claim has the same meaning, syntax,
and processing rules as the exp claim defined in Section
4.1.4 of JWT , except that the format
MUST be a Type6NumericDate. The CBOR encoded claim key 4 MUST be
used to identify this claim.
The nbf (not before) claim has the same meaning, syntax,
and processing rules as the nbf claim defined in Section
4.1.5 of JWT , except that the format
MUST be a Type6NumericDate. The CBOR encoded claim key 5 MUST be
used to identify this claim.
The iat (issued at) claim has the same meaning, syntax,
and processing rules as the iat claim defined in Section
4.1.6 of JWT , except that the format
MUST be a Type6NumericDate. The CBOR encoded claim key 6 MUST be
used to identify this claim.
The cti (CWT ID) claim has the same meaning, syntax,
and processing rules as the jti claim defined in Section
4.1.7 of JWT , except that the format
MUST be of major type 3 with a case-sensitive string value. The
CBOR encoded claim key 7 MUST be used to identify this claim.
Note: Claims defined by the OpenID Foundation have not yet been
included in the table above.
The security of the CWT is dependent on the protection offered by COSE. Without protecting the claims contained in a CWT an adversary is able to modify, add or remove claims. Since the claims conveyed in a CWT are used to make authorization decisions it is not only important to protect the CWT in transit but also to ensure that the recipient is able to authenticate the party that collected the claims and created the CWT. Without trust of the recipient in the party that created the CWT no sensible authorization decision can be made. Furthermore, the creator of the CWT needs to carefully evaluate each claim value prior to including it in the CWT so that the recipient can be assured about the correctness of the provided information.
This section will create a registry for CWT claims, possibly relating them to the JWT Claims Registry.
Three examples of CWTs follow.
A CWT used in the context of ACE requires at least the aud and a
cks claim (defined elsewhere).
This means that iss, alg, key_ops and
others are pre-established and assumed.
This would look like this non-normative JSON.
Using the CBOR encoded claim keys according to
and COSE
makes a CWT with aud and a
symmetric key look like this in CBOR diagnostic notation:
Defined in CBOR.
Size of the CWT with a symmetric key of 10 bytes is 45 bytes. This is
then packaged signed and encrypted using COSE.
Token with aud set to "coap://light.example.com" and an EC key with
kid set to 11.
Using the CBOR encoded claim keys according to
and COSE
makes a CWT with aud and an EC key
look like this in CBOR diagnostic notation:
Defined in CBOR.
Size of the CWT with an EC key is 109 bytes. This is
then packaged signed and encrypted using COSE.
CWT using all claims defined by this specification, plus extensions for AIF and an EC key.
Using the CBOR encoded claim keys according to
and COSE
makes a full CWT look like this
in CBOR diagnostic notation:
Defined in CBOR.
Size of the CWT with an EC key is 194 bytes. This is
then packaged signed and encrypted using COSE.
A straw man proposal of CWT was written in the draft
"Authorization for the Internet of Things using OAuth 2.0"
with the help of
Ludwig Seitz, Göran Selander, and Samuel Erdtman.
[[ to be removed by the RFC Editor before publication as an RFC ]]
-00
Created the initial version based on draft-wahlstroem-oauth-cbor-web-token-00.
Now reference the JWT claim definitions, rather than repeating them.