logjam

A logjam is a pile or accumulation of logs in a river, stream, or other waterway that obstructs flow and can impede navigation. They may form naturally when fallen trees and debris collect in channels, or result from human logging operations, especially during log drives when freshly felled timber is floated downstream to mills. Logjams can raise water levels upstream, alter sediment transport, and pose safety hazards to boats and communities. To manage them, crews use booms, divers, or winches to remove logs, or allow logs to break up naturally during floods. In historical North American and European logging, log drives were a major spring activity, and logjams required systematic river policing.

Logjam (cryptography) refers to a vulnerability in TLS/SSL that can allow an attacker to downgrade the key exchange from a strong Diffie-Hellman group to a weaker 512-bit export-grade group, enabling traffic decryption under certain conditions. The attack relies on the use of shared Diffie-Hellman primes and the ability to perform discrete-log computations; widespread support for export-grade cipher suites and common prime parameters made the vulnerability practical. Mitigations include disabling export cipher suites, ensuring servers and clients use sufficiently large DH parameters (e.g., 2048 bits or larger), or switching to elliptic-curve Diffie-Hellman (ECDH/ECDHE) which avoids the weak primes; applying software updates and configuration changes and using TLS across modern protocols. The issue highlighted the risks of parameter reuse and the importance of forward secrecy and up-to-date crypto libraries.

In a broader sense, the term logjam is used metaphorically to describe any obstruction or congestion that blocks progress, whether in transport networks, information systems, or organizational processes.