This article describes an error that occurs during the authentication process. When attempting to authenticate, the following error message is displayed:

{"error": "invalid_client", "error_description": "The client_assertion signature is invalid."}

NOTE: This refers to the authentication step to call the token endpoint when using private_key_jwt/client_secret_jwt as the token endpoint auth method.

• OpenID Connect/OAuth 2.0 application
• Private_jey_jwt or client_secret_jwt client authentication method
• /token endpoint
• Okta Classic Engine
• Okta Identity Engine (OIE)

The root cause of this error is an invalid signature within the client assertion provided during the request. This typically indicates that the private key used to sign the assertion does not match the public key registered for the client or the signature format is incorrect.

Follow these steps to resolve the signature error:

1. Decode the client assertion JWT to inspect the header (for example, jwt.io).
2. Identify the algorithm (alg) and key ID (kid)[mandatory] in the header.
3. Verify that the algorithm matches the expected signing method (for example, RS256).
4. Confirm that the key ID (kid) matches the public key registered in the application.
5. Ensure the private key used to sign the assertion corresponds to the registered public key.
6. Generate a new client assertion if the keys do not match.
7. Upload the correct public key to the app if the current one is incorrect.

Related References

• Client authentication methods | Okta Developer
• Implement OAuth for Okta with a service app | Okta Developer