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.

4 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?

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