'Derive and return an obfs3 key using the pad string in \'pad_string\'.'

def _derive_crypto(self, pad_string):

    secret = hmac_sha256.hmac_sha256_digest(self.shared_secret, pad_string)
    return aes.AES_CTR_128(secret[:KEYLEN], secret[KEYLEN:], counter_wraparound=True)
