Method decrypt

keychain.py:117–148

(self,encr)

    # no match is found, None is returned, otherwise the method returns
    # a two items array: [key_name, decripted_packet].
    def decrypt(self,encr):
        if len(encr) < 11 + 1 + 10:
            return None # Min length is 11 (header) + some data + 10 (HMAC).

        copy = bytearray(encr)
        copy[1] = copy[1] & (0xff^MessageFlagsRelayed) # Clear Relayed.
        copy[6] = 0 # TTL. Set to zero for HMAC.
        padlen = copy[-1] & 0x0f # Padding length.
        copy[-1] = copy[-1] & 0xf0 # Clear padding len field.
        hm = copy[-10:] # The HMAC part: we will check it against our HMAC.

        # Test every key for a matching HMAC.
        for key_name in self.keys:
            key = self.keys[key_name]

            # Derive the encryption and HMAC keys.
            aes_key,hmac_key = self.derive_keys(key)
            my_hm = bytearray(HMAC_SHA256(hmac_key,copy[:-10])[:10])
            my_hm[-1] = my_hm[-1] & 0xf0
            if hm != my_hm: continue # No match.

            # Decrypt the payload
            iv = self.sha16(copy[:11])
            plain = cryptolib.aes(aes_key,2,iv).decrypt(encr[11:-10])

            # Compose the final decrypted packet removing the IV
            # field, the padding and the HMAC.
            orig = bytearray(7 + len(plain) - padlen)
            orig[:7] = encr[:7]
            orig[7:] = plain if padlen == 0 else plain[:-padlen]
            return (key_name,orig)
        return None

if __name__ == "__main__":
    kc = Keychain()