ALT-20 · Paper · 23 Oct 2026
Logjam: Diffie-Hellman, downgraded
Export-grade cryptography from the 1990s was still switched on in 2015, and an attacker could force a modern connection down to it. How a policy relic became a live attack — and what it suggested about 1024-bit keys.
- 1990sUS export rules limit cryptography sold abroad to weak, “export-grade” keys.
- May 2015Logjam disclosed (weakdh.org); browsers move to reject short Diffie-Hellman groups.
- Oct 2015“Imperfect Forward Secrecy” presented at ACM CCS.
- Top 1M HTTPS domains initially vulnerable
- 8.4%weakdh.org
- Vulnerable servers that used one single 512-bit group
- 82%Adrian et al., CCS 2015
How it worked
Breaking discrete logarithms has an expensive precomputation that depends only on the prime, not on the individual connection. With most servers sharing the same primes, one precomputation unlocked them all — and a downgrade forced clients into using them.
Why it mattered
The paper argued that the same economics put widely shared 1024-bit groups within reach of a state-level adversary. Old switches left on for compatibility became attack paths decades later.
Our reading
Analysis — Altumptic's interpretation of the sources below.
- Deliberately weakened cryptography never stays contained.
- Shared parameters concentrate risk: one break, many victims.
- Compatibility settings deserve an expiry date.
What it means for you
- Disable export ciphers and finite-field Diffie-Hellman under 2048 bits.
- Prefer elliptic-curve key exchange (X25519) in TLS configurations.
- Scan your own endpoints periodically; defaults drift.
Sources
- weakdh.org — Weak Diffie-Hellman and the Logjam attack
- Adrian et al. — Imperfect Forward Secrecy, CCS 2015 (Microsoft Research)
Altumptic was not involved in this research. Figures are as published by the sources above.
Cite this entry
Altumptic Lab (2026). ALT-20 — Logjam: Diffie-Hellman, downgraded. 23 Oct 2026. altumptic.com/research/logjam-2015