[gxti]

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.

[StavrosK]

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".

Please disregard my comment above.

[Xk]

I guess he could mean that you could find a plaintext that had the same hash value through use of a collision ... but that's just finding a preimage.