code: def _tls_add_pad(p, block_size):
      
      padlen = -p.len % block_size
      padding = chb(padlen) * (padlen + 1)
      p.len += len(padding)
      p.data += padding
docs: Provided with cipher block size parameter and current TLSCompressed packet
      p (after MAC addition), the function adds required, deterministic padding
      to p.data before encryption step, as it is defined for TLS (i.e. not
      SSL and its allowed random padding). The function has no return value.