What Is Logjam?

Transport Layer Security, or TLS, is the protocol commonly used in HTTPS connections.   Logjam is the code name for a cryptographic weakness in the Diffie-Helman key exchange algorithm used by TLS. The Diffie-Helman Exchange (DHE) allows two parties – a browser and server in our case – to exchange prime numbers in a secure manner which are then used to create a shared secret used to encrypt a session.

A team of computer scientists and security researchers found that precomputing the prime number groups that DHE uses allows faster computation of the discrete logs used to find the shared secret. With academic-level resources, the researchers precomputed a 512-bit group used by 82% of vulnerable servers. The researchers posit that nation-state level resources could precompute 1024-bit Diffie-Helman groups, affecting even larger swaths of the internet.

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