In certain situations, clients need to authenticate with identityserver, e.g.

  • confidential applications (aka clients) requesting tokens at the token endpoint
  • APIs validating reference tokens at the introspection endpoint

For that purpose you can assign a list of secrets to a client or an API resource.

Secret parsing and validation is an extensibility point in identityserver, out of the box it supports shared secrets as well as transmitting the shared secret via a basic authentication header or the POST body.

Creating a shared secret

The following code sets up a hashed shared secret:

var secret = new Secret("secret".Sha256());

This secret can now be assigned to either a Client or an ApiResource. Notice that both do not only support a single secret, but multiple. This is useful for secret rollover and rotation:

var client = new Client
    ClientId = "client",
    ClientSecrets = new List<Secret> { secret },

    AllowedGrantTypes = GrantTypes.ClientCredentials,
    AllowedScopes = new List<string>
        "api1", "api2"

In fact you can also assign a description and an expiration date to a secret. The description will be used for logging, and the expiration date for enforcing a secret lifetime:

var secret = new Secret(
    "2016 secret",
    new DateTime(2016, 12, 31));

Authentication using a shared secret

You can either send the client id/secret combination as part of the POST body:

POST /connect/token


..or as a basic authentication header:

POST /connect/token

Authorization: Basic xxxxx


You can manually create a basic authentication header using the following C# code:

var credentials = string.Format("{0}:{1}", clientId, clientSecret);
var headerValue = Convert.ToBase64String(Encoding.UTF8.GetBytes(credentials));

var client = new HttpClient();
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", headerValue);

The IdentityModel library has helper classes called TokenClient and IntrospectionClient that encapsulate both authentication and protocol messages.

Beyond shared secrets

There are other techniques to authenticate clients, e.g. based on public/private key cryptography. IdentityServer includes support for private key JWT client secrets (see RFC 7523).

Secret extensibility typically consists of three things:

  • a secret definition
  • a secret parser that knows how to extract the secret from the incoming request
  • a secret validator that knows how to validate the parsed secret based on the definition

Secret parsers and validators are implementations of the ISecretParser and ISecretValidator interfaces. To make them available to IdentityServer, you need to register them with the DI container, e.g.:


Our default private key JWT secret validator expects the full (leaf) certificate as base64 on the secret definition. This certificate will then be used to validate the signature on the self-signed JWT, e.g.:

var client = new Client
    ClientId = "client.jwt",
    ClientSecrets =
        new Secret
            Type = IdentityServerConstants.SecretTypes.X509CertificateBase64,
            Value = "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"

    AllowedGrantTypes = GrantTypes.ClientCredentials,
    AllowedScopes = { "api1", "api2" }

You could implement your own secret validator (or extend ours) to implement e.g. chain trust validation instead.