def _derive_padding_crypto(self, seed, pad_string): # XXX consider secret_seed
        secret = self.mac(pad_string,
                          seed,
                          self.shared_secret)
        return aes.AES_CTR_128(secret[:KEYLEN], secret[KEYLEN:],
                               counter_wraparound=True)
Derive and return an obfs2 padding key using the pad string in 'pad_string'.
dataset/ETHPy150Open isislovecruft/obfsproxy/obfsproxy/transports/obfs2.py/Obfs2Transport._derive_padding_crypto