docs: def server_session(user, password, salt, A, B, b):
      """
      Server session secret
          Both:  u = H(A, B)
  
          Host:  S = (Av^u) ^ b              (computes session key)
          Host:  K = H(S)
      """
      N, g, k = get_prime()
      u = get_scramble(A, B)
      v = get_verifier(user, password, salt)
      vu = pow(v, u, N)                       # v^u
      Avu = (A * vu) % N                      # Av^u
      session_secret = pow(Avu, b, N)         # (Av^u) ^ b
      K = hash_digest(hashlib.sha1, session_secret)
      if DEBUG_PRINT:
          print('server session_secret=', binascii.b2a_hex(long2bytes(session_secret)), end='\n')
          print('server session hash K=', binascii.b2a_hex(K))
  
      return K