On 23 February 2017, researchers at CWI Amsterdam and Google showed the world the first real collision for the SHA-1 hash function. The project was called SHAttered, and this domain has been its home ever since. The two files that prove the break are still here to download.

What a collision actually is

A cryptographic hash takes any file and returns a short fixed-length fingerprint. The promise is simple: change a single bit of the file and the fingerprint changes too, and no two different files should ever share one. A collision breaks that promise. It is a pair of distinct inputs that produce the exact same hash. For a function used to sign software, certificates and documents, a collision is not a curiosity. It is a crack in the foundation.

What SHAttered produced

The team did not just argue that SHA-1 was weak on paper. They built the evidence. Two PDF files, visibly different and carrying different content, share one identical SHA-1 value: 38762cf7f55934b34d179ae6a4c80cadccbb7f0a. Run either file through SHA-1 and the answer matches. Run them through SHA-256 and the answer differs, which is how anyone can confirm they are genuinely two separate files.

What the attack cost

The break was expensive, and that was part of the point. Producing the collision took roughly nine quintillion SHA-1 computations, the work of about 6,500 CPU-years and 110 GPU-years run in parallel. That scale kept the attack out of reach for a casual attacker in 2017, yet it ran thousands of times faster than trying every possibility by brute force. The direction of travel was clear: the cost would only fall.

Why SHA-1 had to retire

Once a working collision exists, trust in a hash erodes quickly. Within months the result pushed browsers, certificate authorities and version-control systems to drop SHA-1 for anything security-sensitive. Git added collision detection. TLS certificates signed with SHA-1 were phased out. The lesson reached far past one algorithm: a function can look safe for years and still fall the moment the maths and the hardware line up.

The research and the people behind it

SHAttered was the work of Marc Stevens and Pierre Karpman at CWI Amsterdam, together with Elie Bursztein, Ange Albertini and Yarik Markov at Google. It built on years of earlier cryptanalysis of the SHA-1 design. The full technical paper that documents the method is preserved here.

Beyond the collision

The same questions that drove SHAttered run through everything we cover here: how hashing works, where it is used, and how systems prove they have not cheated. These guides pick up where the research leaves off.