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From Twitter:

Marc Stevens released a new 76-step (out of 80) SHA-1 collision, and SIMD libdetectcoll https://marc-stevens.nl/research/

What is a 76-step collision, and how does it differ from a "regular" collision? Is it related to whatever a "freestart" collision is?

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migrated from security.stackexchange.com Feb 6 '17 at 11:13

This question came from our site for information security professionals.

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Analysis of a primitive often times is conducted on a reduced strength variant of the actual algorithm. This makes it easier to find the weak points of the design, which can become obscured after many iterations.

Assuming a "regular" collision means a collision over the entire 80 rounds, then a 76-step collision finds a collision after 76 rounds.

The paper for the result in question does mention that it is for a free-start collision. As for what this means, there is a Q/A here that does a goob job of explaining. Basically, it means that the attacker is allowed to select the initialization vector.

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