Logjam

cryptography

A 2015 attack downgrading Diffie-Hellman key exchange to weak 512-bit export parameters.

Logjam tricked servers into using export-grade DH groups small enough to break, then computed the shared secret. It revealed how many servers reused the same well-known primes, making precomputation attacks practical.

Logjam attacked Diffie-Hellman key exchange, downgrading connections to export-grade parameters and exploiting a property of the mathematics: a large precomputation against a single commonly used prime makes individual exchanges using that prime cheap to break afterwards.

The important finding was about reuse. A small number of standard primes were hardcoded into widely deployed software and used by an enormous fraction of servers, which meant one very expensive precomputation would compromise a very large population. The researchers noted this was within reach of a well-resourced state actor, and observed that it would explain certain capabilities that had been described publicly without a plausible mechanism.

The remediation was to use larger parameters and to stop sharing them, and the deeper change was philosophical. Logjam is a good example of a vulnerability that is not a coding error at all: every implementation followed the specification correctly, and the weakness came from a defaults decision made years earlier for compatibility. Reviewing what your cryptography is actually configured with, rather than that it is present, is the practice this produced.

Also known as: Logjam, CVE-2015-4000

← All glossary entries