(credit: Bob Embleton)
For more than six years, the SHA1 cryptographic hash function underpinning Internet security has been at death’s door. Now it’s officially dead, thanks to the submission of the first known instance of a fatal exploit known as a “collision.”
Despite more than a decade of warnings about the lack of security of SHA1, the watershed moment comes as the hash function remains widely used. Git, the world’s most widely used system for managing software development among multiple people, relies on it for data integrity. The GnuPG e-mail encryption program still deems SHA1 safe. And hundreds if not thousands of big-name software packages rely on SHA1 signatures to ensure installation and update files distributed over the Internet haven’t been maliciously altered.
A collision occurs when the two different files or messages produce the same cryptographic hash. The most well-known collision occurred sometime around 2010 against the MD5 hash algorithm, which is even weaker than SHA1. A piece of nation-sponsored espionage malware known as Flame used the attack to hijack the Windows update mechanism Microsoft uses to distribute patches to hundreds of millions of customers. By forging the digital signature used to cryptographically prove the authenticity of Microsoft servers, Flame was able to spread from one infected computer to another inside targeted networks.