I know that message collisions are supposed to be hard to calculate for a good cryptographic hash.
Given 1k worth of data, it's hard to find another 1k worth of data that collides with the same hash.
But doesn't it become much easier when the message becomes variable length? I.e. it's easier to find a collision in 0-1G of data to 1k of data than it is to find one that is also exactly 1k of data.
Does knowing the intended size of the message increase the strength of USING the hashing function? I.e. a message has to be between 1k and 4k.