Downfall of "fortress SHA1"
As we all know about the most common and used hashing technique SHA1 hash technique. we use hashing technique in browser security, managing code repositories, or even just detecting duplicate files in storage.SHA-1 was designed in 1995 by the National Security Agency (NSA) as a part of the Digital Signature Algorithm. Like other hashes, SHA-1 also converts any input message to a long string of numbers and letters that serve as a cryptographic fingerprint for that particular message.
Collision attacks appear when the same hash value (fingerprint) is produced for two different messages, which then can be exploited to forge digital signatures, allowing attackers to break communications encoded with SHA-1.The explanation is technologically tricky, but you can think of it as attackers who surgically alters their fingerprints in order to match yours, and then uses that to unlock your smartphone.
On February 23rd-2017 more than 20 years after of SHA-1 was first introduced, google announced the first practical technique for generating a collision. This represents the culmination of two years of research that sprung from a collaboration between the CWI Institute in Amsterdam and Google. As a proof of the attack, google released two PDFs that have identical SHA-1 hashes but different content.
Collision attacks appear when the same hash value (fingerprint) is produced for two different messages, which then can be exploited to forge digital signatures, allowing attackers to break communications encoded with SHA-1.The explanation is technologically tricky, but you can think of it as attackers who surgically alters their fingerprints in order to match yours, and then uses that to unlock your smartphone.
On February 23rd-2017 more than 20 years after of SHA-1 was first introduced, google announced the first practical technique for generating a collision. This represents the culmination of two years of research that sprung from a collaboration between the CWI Institute in Amsterdam and Google. As a proof of the attack, google released two PDFs that have identical SHA-1 hashes but different content.
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
"This attack required over 9,223,372,036,854,775,808 SHA1 computations. This took the equivalent processing power as 6,500 years of single-CPU computations and 110 years of single-GPU computations," the researcher explains.
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.
Previous warnings
Linus Torvalds was warned in 2005 that the use of the SHA-1 hash to sign code in Linux and Git was insecure and urged to shift to something better protected, but rejected the advice outright.
Free software evangelist John Gilmore warned Torvalds ten years ago that "SHA1 has been broken; it's possible to generate two different blobs that hash to the same SHA1 hash".
Gilmore penned his warning to Torvalds in April 2005, when MD5 had already been cracked and SHA1 remained "hard to crack" - but still crackable.
90-days for Services to Migrate to Safer Cryptographic Hashes
Despite declared insecure by researchers over a decade ago and Microsoft in November 2013, announcing it would not accept SHA1 certificates after 2016, SHA1 has widely been used over the Internet.
So, it's high time to migrate to safer cryptographic hashes such as SHA-256 and SHA-3.
Google is planning to release the proof-of-concept (PoC) code in 90 days, which the company used for the collision attack, meaning anyone can create a pair of PDFs that hash to the same SHA-1 sum given two distinct images with some pre-conditions.
Therefore, an unknown number of widely used services that still rely on the insecure SHA1 algorithm have three months to replace it with the more secure one.
