Another nail in the coffin of SHA1

This recent paper “From Collisions to Chosen-Prefix Collisions – Application to Full SHA-1” by G. Leurent and T. Peyrin puts another nail in the coffin of SHA1. The authors present a chosen-prefix collisions attack to SHA1 which allows client impersonation in TLS 1.2 and peer impersonation in IKEv2 with an expected cost between 1.2 and 7 Million US$. The authors expect that soon it will be possible to bring down the cost to 100.000 US$ per collision.

For what concerns CA certificates, the attack allows, at the same cost, to create a rogue CA and fake certificates in which the signature includes the use of SHA1 hash, but only if the true CA does not randomize the serial number field in the certificates.

It is almost 15 years that it is known that SHA1 is not secure: NIST deprecated it in 2011, it should not have been used from 2013 and substituted with SHA2 or SHA3. By 2017 all browsers should have removed support for SHA1, but the problem is always with legacy applications that still use it: how many of them are still out there?

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