bobbyyankou's comments

On: Announcing the first SHA-1 collision

[bobbyyankou]

Can someone help me understand what the major distinction is between this accomplishment (SHAttered) and the same team's The SHAppening (2015)?

It looks like the did the same thing or something similar in 2^57.5 SHA1 calculations back then versus 2^63 SHA1 calculations this time.

[detaro → bobbyyankou]

SHAppening was a freestart collision, this is a "full" one. This stack overflow answer looks like a good description of freestart collisions: https://crypto.stackexchange.com/questions/29695/what-is-a-f...

My attempt at TL;DR: SHA-1 works on blocks, and each block is processed and it's data "mixed" with a previous intermediate result based on the previous blocks (Merkle-Damgård construction). A freestart collision only shows blocks and values for the intermediate results which lead to a collision. For a full collision you still have to figure out what sequence of blocks gets the intermediate results to the necessary values.

[bobbyyankou → detaro]

Ah, cool. I didn't notice the distinction between freestart and full. Thanks for that link; it was very helpful.

[hiddencost → bobbyyankou]

https://en.wikipedia.org/wiki/SHA-1#The_SHAppening