code: def _tls_decrypt(alg, c):
      
      p = TLSPlaintext()
      p.type = c.type
      p.version = c.version
      p.data = alg.decrypt(c.data)
      p.len = len(p.data)
      return p
docs: Provided with a TLSCiphertext instance c, and a stream or block cipher alg,
      the function decrypts c.data and returns a newly created TLSPlaintext.