Wrong. Collisions can be found in MD5 in 2^21 time due to an attack by Xie and Feng. 2^64 is a very respectable number and is not practical for people to do on their home machines. 2^21 is.
You are right, of course. I wrote that as 'ideal digest' instead of MD5 then rewrote it. Specific digests always lose a few bits in real life, or in MD5's case, most of the bits...
Clarify? Collision attacks by definition do not feature an existing digest as input so they are not useful for breaking into a system secured with a digest.
Ah, I misunderstood. By "collision attack" you meant "find two plaintexts that hash to the same digest", I interpreted it as "find one plaintext that hashes to a specific digest", and "preimage attack" as "find the plaintext that was hashed to this digest".
