A collision occurs when two distinct pieces of data—a document, a binary, or a website’s certificate—hash to the same digest as shown above. In practice, collisions should never occur for secure hash functions. However if the hash algorithm has some flaws, as SHA-1 does, a well-funded attacker can craft a collision. The attacker could then use this collision to deceive systems that rely on hashes into accepting a malicious file in place of its benign counterpart. For example, two insurance contracts with drastically different terms.
Finding the SHA-1 collision
In 2013, Marc Stevens published a paper that outlined a theoretical approach to create a SHA-1 collision. We started by creating a PDF prefix specifically crafted to allow us to generate two documents with arbitrary distinct visual contents, but that would hash to the same SHA-1 digest. In building this theoretical attack in practice we had to overcome some new challenges. We then leveraged Google’s technical expertise and cloud infrastructure to compute the collision which is one of the largest computations ever completed.
Here are some numbers that give a sense of how large scale this computation was:
Nine quintillion (9,223,372,036,854,775,808) SHA1 computations in total
6,500 years of CPU computation to complete the attack first phase
110 years of GPU computation to complete the second phase
While those numbers seem very large, the SHA-1 shattered attack is still more than 100,000 times faster than a brute force attack which remains impractical.
Mitigating the risk of SHA-1 collision attacks
Moving forward, it’s more urgent than ever for security practitioners to migrate to safer cryptographic hashes such as SHA-256 and SHA-3. Following Google’s vulnerability disclosure policy, we will wait 90 days before releasing code that allows anyone to create a pair of PDFs that hash to the same SHA-1 sum given two distinct images with some pre-conditions. In order to prevent this attack from active use, we’ve added protections for Gmail and GSuite users that detects our PDF collision technique. Furthermore, we are providing a free detection system to the public.
You can find more details about the SHA-1 attack and detailed research outlining our techniques here.
About the team
This result is the product of a long-term collaboration between the CWI institute and Google’s Research security, privacy and anti-abuse group.
Marc Stevens and Elie Bursztein started collaborating on making Marc’s cryptanalytic attacks against SHA-1 practical using Google infrastructure. Ange Albertini developed the PDF attack, Pierre Karpman worked on the cryptanalysis and the GPU implementation, Yarik Markov took care of the distributed GPU code, Alex Petit Bianco implemented the collision detector to protect Google users and Clement Baisse oversaw the reliability of the computations.
