OAuth Symmetric Proof of Posession for Code Extension

Public clients in OAuth 2.0 is suseptible to the code interception attack. The code interception attack is an attack that a malicious client intercepts the code returned from the authorization endpoint and uses it to obtain the access token. This is possible on a public client as there is no client secret associated for it to be sent to the token endpoint. This is especially true on some smartphone platform in which the code is returned to a redirect URI with a custom scheme as there can be multiple apps that can register the same scheme.Under this scenario, the mitigation strategy stated in section 4.4.1 of does not work as they rely on per-client instance secret or per client instance redirect uri. To mitigate this attack, this extension utilizes dynamically created cryptographically random key called 'code verifier'. The code verifier is created for every authorization request and its transformed value called code challenge is sent to the authorization server to obtain the authorization code. The code obtained is then sent to the token endpoint with the code verifier and the server compairs it with the previously received reqeust code so that it can perfom the proof of posession of the code verifier by the client. This works as the mitigation since the attacker would not know the one-time key.

In addition to the terms defined in OAuth 2.0, this specification defines the following terms.

a cryptographically random string with big enough entropy that is used to correlate the authorization request to the token request

either the code verifier itself or some transformation of it that is sent from the client to the server in the authorization request NOTE 1: The client and the server MAY use mutually agreed pre-negotiated algorithm such as base64url encoding of the left most 128bit of SHA256 hash. NOTE 2: If no algorithm has been negotiated, it is treated as the code verifier itself.

Before starting the authorization process, the client MUST make sure that the server supports this specification. It may be obtained out-of-band or through some other mechanisms such as the discovery document in OpenID Connect Discovery. The exact mechanism on how the client obtains this information is out of scope of this specification. The client that wishes to use this specification MUST stop proceeding if the server does not support this extension.

In this specification, the client sends the transformation of the code verifier to the authorization server in the front channel. The default transformation is not doing transformation at all. If the the server supports, the client MAY register its desired transformation algorithm to the server. If the algorithm is registered, the server MUST reject any request that does not conform to the algorithm. How does this client registers the algorithm is out of scope for this specification. Also, this specification does not define any transformation other than the default transformation.

The client then creates a code verifier, code_verifier, in the following manner. code_verifier = high entropy cryptographic random string of length less than 128 bytes NOTE: code verifier MUST have high enough entropy to make it inpractical to guess the value.

Then, the client creates a code challenge, code_challenge, by applying the pre-negotiated algorithm between the client and the server. The default behavior is no transofrmation, i.e., code_challenge == code_verifier. The authorization server MUST support this 'no transformation' algorithm. The client sends the code challenge with the following parameter with the OAuth 2.0 Authorization Request: REQUIRED. code challenge.

When the server issues a code, it MUST associate the code_challenge value with the code so that it can be used later. Typically, the code_challenge value is stored in encrypted form in the code, but it could as well be just stored in the server in association with the code. The server MUST NOT include the code_challenge value in the form that any entity but itself can extract it.

Upon receipt of the code, the client sends the request to the token endpoint. In addition to the parameters defined in OAuth 2.0, it sends the following parameter: REQUIRED. code verifier

Upon receipt of the request at the token endpoint, the server verifies it by calculating the code challenge from code_verifier value and comparing it with the previously associated code_challenge. If they are equal, then the successful response SHOULD be returned. If the values are not equal, an error response indicating invalid_grant as described in section 5.2 of OAuth 2.0 SHOULD be returned.

This specification makes a registration request as follows:

This specification registers the following parameters in the IANA OAuth Parameters registry defined in OAuth 2.0. Parameter name: code_verifier Parameter usage location: Access Token Request Change controller: OpenID Foundation Artifact Binding Working Group - openid-specs-ab@lists.openid.net Specification document(s): this document Related information: None Parameter name: code_challenge Parameter usage location: Authorization Request Change controller: OpenID Foundation Artifact Binding Working Group - openid-specs-ab@lists.openid.net Specification document(s): this document Related information: None

The security model relies on the fact that the code verifier is not learned or guessed by the attacker. It is vitally important to adhear to this principle. As such, the code verifier has to be created in such a manner that it is cryptographically random and has high entropy that it is not practical for the attacker to guess, and if it is to be returned inside code, it has to be encrypted in such a manner that only the server can decrypt and extract it. If the no transformation algorithm, which is the default algorithm, is used, the client MUST make sure that the request channel is adequately protected. On a platform that it is not possible, the client and the server SHOULD utilize a transformation algorithm that makes it reasonably hard to recalculate the code verifier from the code challenge. All the OAuth security analysis presented in applies so readers SHOULD carefully follow it.

The initial draft of this specification was created by the OpenID AB/Connect Working Group of the OpenID Foundation, by most notably of the following people: Naveen Agarwal, Google Dirk Belfanz, Google Sergey Beryozkin John Bradley, Ping Identity Brian Campbell, Ping Identity Phill Hunt, Oracle Ryo Ito, mixi Michael B. Jones, Microsoft Torsten Lodderstadt, Deutsche Telekom Breno de Madeiros, Google Prateek Mishra, Oracle Anthony Nadalin, Microsoft Axel Nenker, Deutsche Telekom Nat Sakimura, Nomura Research Institute

-02 Changed title. Changed parameter names. Changed the default transformation algorithm and added crypto agility. More text in the security consideration. Now references RFC 6819. Recorded more contributors. -01 Minor editorial changes. -00 Initial version.