The Client shall always prove possession of a UserIdentityToken when it passes it to the Server. Some tokens include a secret such as a password which the Server will accept as proof. In order to protect these secrets the Token may be encrypted before it is passed to the Server. Other types of tokens allow the Client to create a signature with the secret associated with the Token. In these cases, the Client proves possession of a UserIdentityToken by creating a signature with the secret and passing it to the Server.

Each UserIdentityToken allowed by an Endpoint shall have a UserTokenPolicy specified in the EndpointDescription. The UserTokenPolicy specifies what SecurityPolicy to use when encrypting or signing. If this SecurityPolicy is omitted then the Client uses the SecurityPolicy in the EndpointDescription. If the matching SecurityPolicy is set to None then no encryption or signature is required. The possible SecurityPolicies are defined in OPC 10000-7.

It is recommended that applications never set the SecurityPolicy to None for UserIdentityTokens that include a secret because these secrets could be used by an attacker to gain access to the system.

The encrypted secret and Signature are embedded in a ByteString which is part of the UserIdentityToken. The format of this ByteString depends on the type of UserIdentityToken and the SecurityPolicy.

The legacy token secret format defined in is not extensible and provides only encryption but the encrypted data is not signed. It is used together with the USERNAME_1 UserIdentityToken. The password secret exchanged with this format shall not exceed 64 bytes.

The EncryptedSecret format defined in provides an extensible secret format together with the definition how the secret is signed and encrypted. It allows for the layout to be updated as new token types are defined or new SecurityPolicies are added.

The UserIdentityToken types and the token formats supported by the Endpoint are identified by the UserTokenPolicy defined in 7.37.