docs: def _tls12_SHA256PRF(secret, label, seed, req_len):
      """
      Provides the implementation of TLS 1.2 PRF function as
      defined in section 5 of RFC 5246:
  
      PRF(secret, label, seed) = P_SHA256(secret, label + seed)
  
      Parameters are:
  
      - secret: the secret used by the HMAC in the 2 expansion
                functions (S1 and S2 are the halves of this secret).
      - label: specific label as defined in various sections of the RFC
               depending on the use of the generated PRF keystream
      - seed: the seed used by the expansion functions.
      - req_len: amount of keystream to be generated
      """
      return _tls_P_SHA256(secret, label + seed, req_len)