At 11:18 PM 7/7/03 -0700, Sarad AV wrote: >Lets say we are using SHA-1 and i hash 2^80 >messages.What I am looking for is a compression >function such that the chances of collision in the >message digest obtained by hashing these 2^80 messages >is collision free or very low probability of >collision.How do we make such a compression function?
You don't need one with a good hash function. Consider a hash made out of a block cipher digesting the input. A block cipher has the property that any one bit difference in input will change half the output bits on average. There's nothing gained by increasing the input entropy (compressing).
