| > > 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}.
| >  
| 
| It seems that the block size of an algorithm then
| is a severe limiting factor.  Is there anyway to
| expand the effective block size of an (old 8byte)
| algorithm, in a manner akin to the TDES trick,
| and get an updated 16byte composite that neuters
| the birthday trick?
Many people have tried to do this.  I know of no successes that are really
practical.  (I've played around with many "obviously good" ideas myself, and
have always managed to break them with a little more thought.  Everything 
that gives you the desired security ends up costing much more than twice
the cost of the underlying block algorithm for a double-size block.)

The block size appears to be a fairly basic and robust property of block
ciphers.  There's probably a theorem in there somewhere - probably one of
those that isn't hard to prove once you figure out exactly what it ought to
say!
                                                        -- Jerry


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