Simplify AuthServerRespond signature by removing redundant client_public_key parameter

[rezkam]

The current AuthServerRespond function in the OPAQUE specification receives client_public_key as a separate parameter, even though this same value is already available in the cleartext_credentials parameter. This creates unnecessary redundancy in the API.

Current implementation:

def AuthServerRespond(cleartext_credentials, server_private_key,
                     client_public_key, ke1, credential_response):
    # client_public_key used only in:
    dh3 = DiffieHellman(server_private_keyshare, client_public_key)

Proposed change:

def AuthServerRespond(cleartext_credentials, server_private_key,
                     ke1, credential_response):
    # Use the client_public_key from cleartext_credentials:
    dh3 = DiffieHellman(server_private_keyshare, 
                        cleartext_credentials.client_public_key)

This affects the GenerateKE2 function as well:

Current:

def GenerateKE2(...):
    cleartext_credentials = CreateCleartextCredentials(
        server_public_key,
        record.client_public_key,
        server_identity, 
        client_identity
    )
    auth_response = AuthServerRespond(
        cleartext_credentials, 
        server_private_key,
        record.client_public_key,  # redundant parameter
        ke1,
        credential_response
    )

Proposed:

def GenerateKE2(...):
    cleartext_credentials = CreateCleartextCredentials(
        server_public_key,
        record.client_public_key,
        server_identity, 
        client_identity
    )
    auth_response = AuthServerRespond(
        cleartext_credentials, 
        server_private_key,
        ke1,
        credential_response
    )

Benefits:

1. Cleaner API: Reduces parameter count in AuthServerRespond
2. Removes Redundancy: Eliminates passing the same value twice
3. Better Maintainability: Reduces the chance of errors where the two values might accidentally differ
4. Simpler Implementation: Makes implementation cleaner across different languages
5. No Security Impact: Maintains all security properties of the protocol

This change is purely structural and doesn't affect the protocol's security properties. The same client_public_key value is used, just accessed from cleartext_credentials instead of as a separate parameter.