[Cryptography] Collisions w/SHA-1 ~$100,000 TODAY

[Philipp Jovanovic]

> Can anyone provide a pointy-eared boss description of what a *freestart* collision is?

The input to the inner layer of a hash function based on the Merkle-Damgard construction (which is used by SHA1) are
- a message block and
- a chaining value.

For the first block there is no previous chaining value though and thus a fixed initialisation vector (IV) is used.

In a freestart collision an attacker can choose the IV.

That’s it.

All the best,
Philipp
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