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Re: draft-ietf-ipsec-<many>-00.txt
> Now, we have to agree on which other transforms to write up! I favor
> triple DES, and DES-CBC with CRC.
> Perry likes MD5 instead of CRC, but I think that the CRC would improve
> speed, and be good enough when encrypted afterward.
If you can XOR a specific pattern into the DES-CBC IV, on decrypting
the resultant plaintext has that pattern XORed into the first data
block. Since the IV is longer than the CRC and I know the CRC
polynomial, I can choose an XOR pattern that is divisible by the
polynomial, and thus
undetected by it.
Thus, I can make a large number of undetected changes (defined as XORs)
to the first 8 bytes of a message. If I know what they are to start
with, I can possibly change them to desired values.
Is this a problem?
CBC is ciphertext[i] = encrypt(plaintext[i] ^ ciphertext[i-1]), so
plaintext[i] = ciphertext[i-1] ^ decrypt(ciphertext[i]), so
plaintext[0] = decrypt(ciphertext[0]) ^ IV.
--
-Colin
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