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IETF OAuth Proof-of-Possession

Hannes Tschofenig

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Status Finished various specifications, including

OAuth Core: RFC 6749 Bearer Tokens: RFC 6750 Security Threats: RFC 6819

Discussion about an enhancement to Bearer Token security (now called “Proof-of-Possession”) since the early days of the working group.

Design Team work late 2012/early 2013, which lead to requirements, use cases, and solution strawman proposals: Symmetric Key Solution: draft-ietf-oauth-v2-http-mac-05 Asymmetric Key Solution: draft-tschofenig-oauth-hotk-03

These two documents have now been replaced by others specs.

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JSON Web Token (JWT)

Standardized version of an access token (and ID Token).

Uses JSON-based encoding + JWE/JWS for encryption/signature + claims.

Described in https://ietf.org/doc/draft-ietf-oauth-json-web-token/

WGLC finishedProof-of-Possession Token extends JWT and

embeds either a Public key (or a fingerprint of it), or Symmetric key (encrypted)

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PoP Token: Asymmetric Key Example

{

"iss":"xas.xboxlive.com",

"aud":"http://auth.xboxlive.com",

"exp":"1361398824",

"nbf":"1360189224",

"cnf":{

"jwk":{

"kty":"EC",

"use":"sig",

"crv":"P-256",

"x":"18wHLeIgW9wVN6VD1Txgpqy2LszYkMf6J8njVAibvhM",

"y":"-V4dS4UaLMgP_4fY4j8ir7cl1TXlFdAgcx55o7TkcSA"

}

}

}

Binds a public key to the access token

[Keyed message digest/digital signature omitted.]

[Header omitted.]

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PoP Token: Symmetric Key Example{

"alg":"RSA1_5",

"enc":"A128CBC-HS256",

"cty":"jwk+json"

}

{

"iss": "https://server.example.com",

"sub": "24400320",

"aud": "s6BhdRkqt3",

"nonce": "n-0S6_WzA2Mj",

"exp": 1311281970,

"iat": 1311280970,

"cnf":{

"jwk":

"eyJhbGciOiJSU0ExXzUiLCJlbmMiOiJB MTI4Q0JDLUhTMjU2IiwiY3R5IjoiandrK

... (remainder of JWE omitted for brevity)"

}

}

{ "kty":"oct", "alg":"HS256", "k":"ZoRSOrFzN_FzUA5XKM YoVHyzff5oRJxl-IXRtztJ6uE" }

Binds a symmetric key to the access token

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I

Client

Authorization

Server

Resource

Server

II

III

Architecture

Relevant document:

http://datatracker.ietf.org/doc/draft-hunt-oauth-pop-architecture/

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I

Client

Authorization

Server

Resource

Server

II

III

Variants: • Key Distribution at Access Token Issuance• Key Distribution at Client Registration

AS <-> Client Interaction

Relevant specifications:http://datatracker.ietf.org/doc/draft-bradley-oauth-pop-key-distribution/http://datatracker.ietf.org/doc/draft-jones-oauth-proof-of-possession/

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Client

Authorization

Server

Resource

Server

AS <-> Client InteractionExample: Symmetric Key

Request access token.I support PoP tokens

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Client

Authorization

Server

Resource

Server

AS <-> Client InteractionExample: Symmetric Key

AS creates PoP-enabled access token

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Client

Authorization

Server

Resource

Server

AS <-> Client InteractionExample: Symmetric Key

AS sends access token to Client & symmetric key

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AS <-> Client Interaction AS needs to bind a key to the access token.

Key can be an fresh and unique symmetric key, or (ephemeral) public key

This requires two extensions: New elements within the JWT to include the (encrypted symmetric

key) or the public key. JWT is also integrity protected. Mechanism for conveying ephemeral key from AS to client and for

client to provide directives to AS.

Details: draft-bradley-oauth-pop-key-distribution-00 Transport symmetric key from AS to client. Transport (ephemeral) asymmetric key from AS to client. Transport public key from client to AS. Algorithm indication

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Dynamic Client Registration Attempt to simplify developer interaction with AS when they

deploy client applications. Today, developers need to register various parameters

(manually), such as Authentication mechanism & client authentication credentials Redirect URIs Grant types Meta data (client name, client logo, scopes, contact information, etc.)

Also allows meta-data, including public keys, to be uploaded to AS.

Two documents: draft-ietf-oauth-dyn-reg draft-ietf-oauth-dyn-reg-metadata

WGLC in progress.

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I

Client

Authorization

Server

Resource

Server

II

III

Building Blocks:a) Proof of possession of PoP keyb) Message integrity (+ Channel Binding)c) RS-to-client authentication

Client <-> RS Interaction

Relevant specification : http://datatracker.ietf.org/doc/draft-richer-oauth-signed-http-request/

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Client

Authorization

Server

Resource

Server

AS <-> Client InteractionExample: Symmetric Key

AS sends access token to Client & Authenticator

Authenticator= Keyed MessageDigest ComputedOver Request.

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Client

Authorization

Server

Resource

Server

AS <-> Client InteractionExample: Symmetric Key

RS “unwraps” access tokenand obtains symmetric key.RS verifies authenticator. Shared

LongTermKey

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Channel Binding

Channel bindings bind the application layer security to the underlying channel security mechanism.

Various approaches for providing channel bindings: PoP public key use in TLS (as described in HOTK draft) tls-unique: TLS Finish message tls-server-end-point: hash of the TLS server's certificate:

Currently, no channel bindings described in <draft-richer-oauth-signed-http-request>

Be aware: Recently, new attacks have been identified with TLS-based channel bindings, see http://www.ietf.org/proceedings/89/slides/slides-89-tls-3.pdf

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Client

Authorization

Server

Resource

Server

II

III

Variants:a) Token introspection b) Out-of-band

RS <-> AS Interaction [optional]

Relevant specification: http://datatracker.ietf.org/doc/draft-richer-oauth-introspection/

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Next Steps

Good time to review through the PoP specification bundle is now Provide feedback Ask questions

Re-chartering to include documents in the milestone list.

Implementation activities to see what works and what doesn’t.

Various open issues to resolve. Planning to schedule OAuth WG conference calls