Is 3DES Broken?

[Jerrold Leichter]

| > >>I think you meant ECB mode?
| > 
| > >No, I meant CBC -- there's a birthday paradox attack to watch out for.
| > 
| > Yep.  In fact, there's a birthday paradox problem for all the standard
| > chaining modes at around 2^{n/2}.
| > 
| > For CBC and CFB, this ends up leaking information about the XOR of a couple
| > plaintext blocks at a time; for OFB and counter mode, it ends up making the
| > keystream distinguishable from random.  Also, most of the security proofs
| > for block cipher constructions (like the secure CBC-MAC schemes) limit the
| > number of blocks to some constant factor times 2^{n/2}.
| 
| I'm surprised that no-one has said that ECB mode is "unsafe at any speed".
Picking nits, but:  ECB mode is "unsafe at any speed" to encrypt an arbitrary 
data stream.  If the data stream is known to have certain properties - e.g., 
because it has undergone some kind of transform before being fed into ECB - 
then ECB is as good as any other mode.

After all, CBC is just ECB applied to a datastream transformed through a
particular unkeyed XOR operation.

There's a paper - by Ron Rivest and others? - that examines this whole issue,
and carefully separates the roles of the unkeyed and keyed transformations.
(I think this may be the paper where all-or-nothing transforms were 
introduced.)
							-- Jerry


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