Breaking DES

[smb at research.att.com]

	 My understanding of how an exhaustive search on the key space
	 can be used to break DES is that for every key, K, D(K,Cipher)
	 is applied until the output matches something legible.

	 Say that some random string, to be thrown out, is added
	 to the beginning of the plain text, and that DES is applied
	 in cbc mode, then how could such an attack work? 

	 My point, I don't see how DES can be broken if the initial
	 block is a grabage block, and cipher block chaining is used.
	 Please enlighten me (gently).

	 One other point... is the decision to encrypt - decrypt -encrypt
	 when applying triple des arbitrary? Why not just encrypt
	 with k1 and then encrypt with k2. Isn't the effect the same?

There are two reasons for that, one of which no longer applies.

The one that still matters is that if you set k1==k2, then the operation
is equivalent to single encryption with k1, thus providing backwards
compatibility.

The other reason is that it was initially feared that DES was a group.
That is, encryption with k1 and k2 might be equivalent to single encryption
with some unknown (to you and me) key k3.  But a cryptanalyst or a brute-
force cracker would neither know nor care that you double-encrypted.

It has now been proved that DES is not a group.  What isn't clear to me
is whether it's ``mostly closed'', though I suspect not.