| ACE Working Group | E. Wahlström |
| Internet-Draft | Nexus Technology |
| Intended status: Informational | M. Jones |
| Expires: June 6, 2016 | Microsoft |
| H. Tschofenig | |
| ARM Ltd. | |
| December 4, 2015 |
CBOR Web Token (CWT)
draft-wahlstroem-ace-cbor-web-token-00
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.
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The JSON Web Token (JWT) [5] 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) [3] and JSON Web Encryption (JWE) [4] to secure the contents of the JWT, which is a set of claims represented in JSON [5]. 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 [6] 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) [7] 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" [1].
This document reuses terminology from JWT [5] and COSE [7].
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. Section 4 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 [5], 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 [5], 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 [5], 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 [5], 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 [5], 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 [5], 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 [5], 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.
/---------+------------------------+--------------------------\ | Claim | CBOR encoded claim key | CBOR major type of value | |---------+------------------------+--------------------------| | iss | 1 | 3 | | sub | 2 | 3 | | aud | 3 | 3 | | exp | 4 | 6 tag value 1 | | nbf | 5 | 6 tag value 1 | | iat | 6 | 6 tag value 1 | | cti | 7 | 3 | \---------+------------------------+--------------------------/
Figure 1: Summary of the values, CBOR major types and encoded claim keys.
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.
{
"aud":"coap://light.example.com",
"cks":
[ // COSE_Key is a CBOR map with an array of keys
{
"kty":4, // symmetric key is indicated using kty 4
"k": "loremipsum" // the symmetric key
}
]
}
Figure 2: "aud" claim and symmetric key in non-normative JSON
Using the CBOR encoded claim keys according to Section 4 and COSE [7] makes a CWT with aud and a symmetric key look like this in CBOR diagnostic notation:
{
3: "coap://light.example.com",
8:
[
{
1: 4,
-1: "loremipsum"
}
]
}
Figure 3: CWT in CBOR diagnostic notation
Defined in CBOR.
a2 # map(2)
03 # unsigned(3)
78 18 # text(24)
636f61703a2f2f6c696768742e6578616d706c652e636f6d # "coap://light.example.com"
08 # unsigned(8)
81 # array(1)
a2 # map(2)
01 # unsigned(1)
04 # unsigned(4)
20 # negative(0)
6a # text(10)
6c6f72656d697073756d # "loremipsum"
Figure 4: CWT with "aud" and symmetric key 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.
{
"aud": "coap://light.example.com",
"cks":
[ // COSE_Key is a CBOR map with an array of keys
{
"kty": "EC",
"kid": "11",
"crv": 1, // using P-384
"x": h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a09eff',
"y": h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbfc117e'
}
]
}
Figure 5: "aud" claim and EC key in non-normative JSON
Using the CBOR encoded claim keys according to Section 4 and COSE [7] makes a CWT with aud and an EC key look like this in CBOR diagnostic notation:
{
3: "coap://light.example.com",
8:
[
{
1: 2,
2: "11",
-1: 1,
-2: h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a09eff',
-3: h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbfc117e'
}
]
}
Figure 6: CWT with EC key in CBOR diagnostic notation
Defined in CBOR.
a2 # map(2)
03 # unsigned(3)
78 18 # text(24)
636f61703a2f2f6c696768742e6578616d706c652e636f6d # "coap://light.example.com"
08 # unsigned(8)
81 # array(1)
a5 # map(5)
01 # unsigned(1)
02 # unsigned(2)
02 # unsigned(2)
62 # text(2)
3131 # "11"
20 # negative(0)
01 # unsigned(1)
21 # negative(1)
58 20 # bytes(32)
bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a09eff # "\xBA\xC5\xB1\x1C\xAD\x8F\x99\xF9\xC7+\x05\xCFK\x9E&\xD2D\xDC\x18\x9FtR(%Z!\x9A\x86\xD6\xA0\x9E\xFF"
22 # negative(2)
58 20 # bytes(32)
20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbfc117e # "\x13\x8B\xF8-\xC1\xB6\xD5b\xBE\x0F\xA5J\xB7\x80J:d\xB6\xD7,\xCF\xEDko\xB6\xED(\xBB\xFC\x11~"
Figure 7: CWT with EC 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.
{
"iss": "coap://as.example.com",
"aud": "coap://light.example.com",
"sub": "erikw",
"exp": 1444064944,
"nbf": 1443944944,
"iat": 1443944944,
"cti": 2929,
"cks":
[ // COSE_Key is a CBOR map with an array of keys
{
"kty": "EC",
"kid": "11",
"crv": 1, // using P-384
"x": h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a09eff',
"y": h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbfc117e'
}
],
"aif": [["/s/light", 1], ["/a/led", 5], ["/dtls", 2]]
}
Figure 8: All claims, "aif" and EC key in non-normative JSON
Using the CBOR encoded claim keys according to Section 4 and COSE [7] makes a full CWT look like this in CBOR diagnostic notation:
{
1: "coap://as.example.com",
3: "coap://light.example.com",
2: "erikw",
4: 1(1444064944),
5: 1(1443944944),
6: 1(1443944944),
7: 2929,
8: [
{
1: 2,
2: "11",
-1: 1,
-2: h'bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a09eff',
-3: h'20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbfc117e'
}
],
9: [["/s/light", 1], ["/a/led", 5], ["/dtls", 2]]
}
Figure 9: Full CWT with EC key in CBOR diagnostic notation
Defined in CBOR.
a9 # map(9)
01 # unsigned(1)
75 # text(21)
636f61703a2f2f61732e6578616d706c652e636f6d # "coap://as.example.com"
03 # unsigned(3)
78 18 # text(24)
636f61703a2f2f6c696768742e6578616d706c652e636f6d # "coap://light.example.com"
02 # unsigned(2)
65 # text(5)
6572696b77 # "erikw"
04 # unsigned(4)
c1 # tag(1)
1a 5612aeb0 # unsigned(1444064944)
05 # unsigned(5)
c1 # tag(1)
1a 5610d9f0 # unsigned(1443944944)
06 # unsigned(6)
c1 # tag(1)
1a 5610d9f0 # unsigned(1443944944)
07 # unsigned(7)
19 0b71 # unsigned(2929)
08 # unsigned(8)
81 # array(1)
a5 # map(5)
01 # unsigned(1)
02 # unsigned(2)
02 # unsigned(2)
62 # text(2)
3131 # "11"
20 # negative(0)
01 # unsigned(1)
21 # negative(1)
58 20 # bytes(32)
bac5b11cad8f99f9c72b05cf4b9e26d244dc189f745228255a219a86d6a09eff # "\xBA\xC5\xB1\x1C\xAD\x8F\x99\xF9\xC7+\x05\xCFK\x9E&\xD2D\xDC\x18\x9FtR(%Z!\x9A\x86\xD6\xA0\x9E\xFF"
22 # negative(2)
58 20 # bytes(32)
20138bf82dc1b6d562be0fa54ab7804a3a64b6d72ccfed6b6fb6ed28bbfc117e # "\x13\x8B\xF8-\xC1\xB6\xD5b\xBE\x0F\xA5J\xB7\x80J:d\xB6\xD7,\xCF\xEDko\xB6\xED(\xBB\xFC\x11~"
09 # unsigned(9)
83 # array(3)
82 # array(2)
68 # text(8)
2f732f6c69676874 # "/s/light"
01 # unsigned(1)
82 # array(2)
66 # text(6)
2f612f6c6564 # "/a/led"
05 # unsigned(5)
82 # array(2)
65 # text(5)
2f64746c73 # "/dtls"
02 # unsigned(2)
Figure 10: Full CWT with EC 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" [8] with the help of Ludwig Seitz, Göran Selander, and Samuel Erdtman.
[[ to be removed by the RFC Editor before publication as an RFC ]]
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