What is DES-X?

Question

Hi,

What is DES-X?

And

DES-X and DES, are they backwards compatible?

Answer 1

DES and DES-X are both block ciphers. See http://en.wikipedia.org/wiki/DES-X for more details. In short, DES-X adds key whitening.

Answer 2

Here's the wikipedia article on DES-X. DES-X increases the key size by appending XOR'd versions of the key before and after the encryption.

Answer 3

Well, DES-X is a variant of the DES block cipher (as I'm sure you already knew).

The reason for the introduction of the DES-X was an attempt to increase the security of the original DES algorithm (which was limited to a 56bit key). The proposed solution with DEX-X was to use two more 64-bit keys which would be applied to make it harder for an attacker to guess the key of the DES algorithm. Basically, the first additional key is XORed to the plain text which is then encrypted with DES. The second additional key is XORed to the resulting cypher.

However, as far the backwards compatibility.. I'm not sure what you mean by that? If you're asking if you can use DES to decrypt DES-X messages then NO (it the strict sense). If you're asking whether a DES-X implementation could be configured to encrypt/decrypt DES messages then the answer is YES.

Here's an example:

    DES(msg) = CYPHER
    DES-X(msg) = K2 X DES(K1 x msg) = CYPER-X

If you choose K2 and K1 to be the all 0 then:

    DES-x(msg) = K2 x DES(K1 x msg) [where K1 = 0, K2 =0] = DES(msg)

It should be pointed out that what I mean by making K1 and K2 0 is actually choosing a key which is 64 bits of 0 = {0,0,0,0,0...0} (64 times). Such a key does not modify the plaintext of the cypher at all when the XOR operation is applied.

Answer 4

The summary of this paper says that DES-X is "compatible." However, I'm not sure if that includes backwards-compatibility.

http://www.cs.ucdavis.edu/~rogaway/papers/cryptobytes.ps