PKCE Support in IdentityServer and IdentityModel

PKCE stands for “Proof Key for Code Exchange” and is a way to make OAuth 2.0 and OpenID Connect operations using an authorization code more secure. It is specified in RFC 7636.

PKCE applies to authorization/token requests whenever the code grant type is involved – e.g. plain OAuth 2.0 authorization code flow as well as (the superior) OpenID Connect hybrid flow (e.g. code id_token).

It mitigates an attack where the authorization response can be intercepted and the “stolen” code can be used to request access tokens. It introduces a per-request secret between the legitimate client and the authorization server, that is unknown to an attacker that can only see authorization responses. This is mainly useful for mobile/native clients.

IdentityServer3 as of v2.4 fully supports PKCE and the authorization and token endpoint documentation has the new parameters. Also the discovery endpoint now includes the code_challenge_methods_supported entry.

IdentityModel v1.5 includes the client pieces to interact with PKCE. You can e.g. use the following code to construct an authorization request:

var nonce = CryptoRandom.CreateRandomKeyString(64);
var verifier = CryptoRandom.CreateRandomKeyString(64);
var challenge = verifier.ToCodeChallenge();
 
var request = new AuthorizeRequest("https://my.server/authorization");
var url = request.CreateAuthorizeUrl(
    clientId: "myclient",
    responseType: "code id_token",
    scope: "openid myapi",
    redirectUri: "https://my.client/cb",
    nonce: nonce,
    codeChallenge: challenge,
    codeChallengeMethod: OidcConstants.CodeChallengeMethods.Sha256);

and the TokenClient to exchange the code with the token:

var tokenClient = new TokenClient(
    "https://my.server/token",
    "client",
    "secret");
 
var response = await tokenClient.RequestAuthorizationCodeAsync(
    code: code,
    redirectUri: "https://my.client/cb",
    codeVerifier: verifier);
This entry was posted in IdentityServer, OAuth, OpenID Connect, Uncategorized. Bookmark the permalink.

8 Responses to PKCE Support in IdentityServer and IdentityModel

  1. Pingback: OAuth 2.0 and PKCE | Pedro Félix's shared memory

  2. reece says:

    Quick question – is it possible to use this via the OpenIdConnectAuthentication middleware? Would it just be a case of using the Notifications to add the necessary url parameters?

  3. olonga says:

    What about while refreshing tokens? It seems this spec does not say anything regarding using PKCE while using refresh token to renew access token.

  4. Rohit Vats says:

    We can combine authorization code with client credentials flow (i.e. same client can be used for both flows). Is it possible to combine PKCE flow with client credentials flow too? Can we use same client?

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