Function decode_challenge_response 

pub fn decode_challenge_response(
    buf: &[u8],
) -> Result<ChallengeResponse, ProtocolError>

Decode a client’s auth challenge-response from a wire frame.

buf must contain the frame payload after the 4-byte length prefix has been stripped: [tag:u8][signature:64][pubkey:32].

Returns Err(UnknownTag) if the tag is not TAG_CHALLENGE_RESPONSE.