The Authenticate method finalizes the login sequence and generates an authentication token.
string Authenticate( byte challenge, byte proof )
Authentication token if login was successful, null otherwise.
The password proof value is computed as follows:
proof = SHA-256(challenge, SHA-256(passwordSalt, UTF-8(password)))
Here password is the plain-text user password, passwordSalt is the password salt obtained with a call to GetChallenge, and challenge is the challenge value returned by the same call to GetChallenge.