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Author: Erwinvandervalk

astro/src/content/docs/identitymodel/endpoints/client-assertions.md

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+---
+title: Client Assertions
+description: How to use client assertions (private_key_jwt / client_secret_jwt) for client authentication in protocol requests.
+sidebar:
+  order: 8
+  label: Client Assertions
+---
+
+Client assertions are an alternative to client secrets for authenticating
+confidential clients at token endpoints. Instead of sending a shared secret,
+the client creates a signed JWT (or SAML assertion) and includes it in the
+request. This is defined in
+[RFC 7523 — JSON Web Token (JWT) Profile for OAuth 2.0 Client Authentication](https://datatracker.ietf.org/doc/html/rfc7523)
+and is commonly known as the `private_key_jwt` or `client_secret_jwt`
+authentication methods defined in
+[OpenID Connect Core §9](https://openid.net/specs/openid-connect-core-1_0.html#ClientAuthentication).
+
+All protocol request types that derive from `ProtocolRequest` expose two
+properties for setting client assertions: `ClientAssertion` and
+`ClientAssertionFactory`.
+
+## ClientAssertion
+
+The `ClientAssertion` property lets you attach a pre-built assertion to any
+protocol request. Set its `Type` and `Value` and they will be included as the
+`client_assertion_type` and `client_assertion` parameters:
+
+```csharp
+var response = await client.RequestClientCredentialsTokenAsync(
+    new ClientCredentialsTokenRequest
+    {
+        Address = "https://demo.duendesoftware.com/connect/token",
+        ClientId = "client",
+
+        ClientAssertion =
+        {
+            Type = OidcConstants.ClientAssertionTypes.JwtBearer,
+            Value = mySignedJwt
+        },
+
+        ClientCredentialStyle = ClientCredentialStyle.PostBody
+    });
+```
+
+:::note
+When using a client assertion, set `ClientCredentialStyle` to
+`ClientCredentialStyle.PostBody`. Client assertions are not compatible with
+`AuthorizationHeader` style and an `InvalidOperationException` will be thrown if
+both are combined with a `ClientId`.
+:::
+
+## ClientAssertionFactory
+
+*Added in IdentityModel 7.2.0*
+
+The `ClientAssertionFactory` property accepts a `Func<Task<ClientAssertion>>`
+— a factory function that creates a **fresh** `ClientAssertion` on demand. This
+was introduced to support scenarios where a protocol request may need to be
+**retried**, and each attempt requires a new assertion with unique `jti` and
+`iat` claims.
+
+The primary motivating scenario is **DPoP** (Demonstrating Proof of Possession).
+When a DPoP token request receives a `use_dpop_nonce` error, the HTTP handler
+retries the request with an updated DPoP proof. If the client assertion were
+static, the server could reject the retry because it has already seen that
+assertion's `jti`. The factory solves this by generating a new assertion for
+each attempt.
+
+```csharp
+var response = await client.RequestClientCredentialsTokenAsync(
+    new ClientCredentialsTokenRequest
+    {
+        Address = "https://demo.duendesoftware.com/connect/token",
+        ClientId = "client",
+
+        ClientAssertionFactory = () => Task.FromResult(new ClientAssertion
+        {
+            Type = OidcConstants.ClientAssertionTypes.JwtBearer,
+            Value = CreateSignedJwt() // generates a fresh JWT each time
+        }),
+
+        ClientCredentialStyle = ClientCredentialStyle.PostBody
+    });
+```
+
+When `ClientAssertionFactory` is set, the factory is stored on the
+`HttpRequestMessage.Options` so that DPoP retry handlers (and other delegating
+handlers in the pipeline) can invoke it to obtain a new assertion on each
+attempt.
+
+:::note
+If both `ClientAssertion` and `ClientAssertionFactory` are set, the factory
+takes precedence during request preparation.
+:::
+
+### Usage with Duende.IdentityModel.OidcClient
+
+Both the `ClientAssertion` and `ClientAssertionFactory` properties exist on
+`ProtocolRequest` to support
+[Duende.IdentityModel.OidcClient](/identitymodel-oidcclient/). The OidcClient
+library builds on IdentityModel's protocol requests internally, and when
+configured with client assertion-based authentication, it sets these properties
+on the underlying requests it creates.
+
+When `ClientAssertionFactory` is set, it is used during both:
+
+- **Pushed Authorization Requests (PAR)** — the factory is invoked to produce a
+  fresh assertion for the PAR endpoint request.
+- **Token requests** — the factory is invoked again to produce a fresh assertion
+  for the token endpoint request.
+
+This ensures each request carries its own unique assertion, which is essential
+when the authorization server enforces `jti` uniqueness across requests.