2
$\begingroup$

I came across this question regarding the security of Quadruple DES (hypothetically DES done four times) in CBC encryption mode vs security of AES-128 in CBC encryption mode. What parameters help to decide the security, assuming 4 different keys (or other case with k1, k2, k3, k1) so the key length would be 224bits (3DES provides 100-bit security level) vs 128bits for AES.

$\endgroup$
  • 1
    $\begingroup$ Specify which keys you're using. The same four time? Four different? K1,K2,K3,K1? $\endgroup$ – Binou Jul 27 at 11:27
  • 2
    $\begingroup$ Welcome to Cryptography. Where did you see this question? What is the source of this question? 3DES has more than 100-bit security, it has 112-bit security. See here Why is triple-DES using three different keys vulnerable to a meet-in-the-middle-attack?. 4DES with four independent key can have 112-bit security which is still less than AES's 128 bit security. $\endgroup$ – kelalaka Jul 27 at 13:34
4
$\begingroup$

In some applications, a serious limiting factor for the security of 4DES is its 64-bit block size. In common modes of operation, that limits the security to data sizes that are insufficient for many application nowadays. It makes 4DES much less secure than AES-128 is.

For example, assume a VPN in CBC mode using a fixed key. Assume an adversary injects known traffic consisting of a known repeated byte representing 99.9% of the payload for 64 GiB of reference ciphertext, then gets another 64 GiB of actual ciphertext. With good probability there is at least one match between one of the $2^{33}$ blocks (of 8 bytes) of the reference traffic and the $2^{33}$ blocks of the actual traffic. And any such matching block can be deciphered correctly with >99% probability.

If we want residual risk at most $\epsilon$ that this kind of attack succeeds, we need to limit the traffic with the same key to about $2^{36}\,\sqrt\epsilon$ bytes. For a residual risk of one in a million, that's about 64 MiB, not a lot by modern standards.

While DES-CTR with non-overlapping counter is less susceptible to this attack than DES-CBC, such mode is unusual, impractical, and remains vulnerable to a distinguishing attack at similar data size threshold.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.