On Tue, Dec 07, 2004 at 06:46:20PM -0700, Joel Maslak wrote: > The short-term fix seems to be something I've been recommending for a > while: > > Compute hashes with both SHA-1 and MD5. > > The chance of one algorithm becoming compromised in the mid-term is > relatively high IMHO (I was responsible for a PKI system which had to keep > integrity for 20 year periods of time - not an easy task considering what > we don't know about the future). The chance of two becoming compromised > is relatively less. The chance of a problem with MD5 and SHA-1 allowing > two different files to have collisions in both algorithms in *BOTH* is > very very small. You might want to read this: http://www.mail-archive.com/cryptography@xxxxxxxxxxxx/msg02611.html Essentially you can use Joux's attack to generate 2^64 collisions for SHA-1 with about 64*2^80 effort; out of those 2^64 collisions you have an even chance that at least one of them will also collide in MD5. That's still 2^86 hash computations, nothing to sneeze at, but nowhere near the expected 2^144 operations. That workfactor might be doable in a 20 year timeframe. It actually looks like it's better to generate 2^80 MD5 collisions instead of 2^64 SHA-1 collisions, since the initial collisions would be trivial to generate, and even if MD5 wasn't broken 80*2^64 + 2^80 << 64*2^80 + 2^64. So in fact you can generate a MD5||SHA collision with only a tiny bit more work than generating a SHA collision. So this really doesn't buy you anything. Jack
