Proof Key for Code Exchange

Introduction

OAuth 2.0 public clients utilizing the Authorization Code Grant are susceptible to the authorization code interception attack. RFC7636 describes the attack as well as a technique to mitigate against the threat through the use of Proof Key for Code Exchange (PKCE, pronounced “pixy”).

Configuration

You can set code_challenge_length and code_challenge_method. Both have defaults:

• code_challenge_length: 64 and

• code_challenge_method: S256

S256 is mandatory to implement so there should be good reasons for not choosing it. To other defined method is plain. plain should only be used when you rely on the operating system and transport security not to disclose the request to an attacker.

The security model relies on the fact that the code verifier is not learned or guessed by the attacker. It is vitally important to adhere 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.

The client SHOULD create a “code_verifier” with a minimum of 256 bits of entropy. This can be done by having a suitable random number generator create a 32-octet sequence.

code_challenge_length is the length of that sequence.

Example

"add_ons": {
    "pkce": {
        "function": "oidcrp.oauth2.add_on.pkce.add_support",
        "kwargs": {
            "code_challenge_length": 64,
            "code_challenge_method": "S256"
        }
    }
}