CVE-2016-6313

What it actually does

The RNG mixing functions leak partial state, allowing recovery of 160 bits of output after observing 4640 prior bits.

Why it matters to the business

Attackers may recover portions of cryptographic material generated by affected GnuPG or Libgcrypt versions, weakening key secrecy and encrypted data protection across many systems.

How an attack unfolds

An attacker who can collect a long sequence of RNG output from a vulnerable process uses the known bias in the mixing functions to mathematically recover 160 bits of later output, reducing the search space for keys or nonces derived from that RNG.

How to fix it

Upgrade Libgcrypt to 1.5.6 or later, 1.6.6 or later, or 1.7.3 or later and GnuPG to 1.4.21 or later; rebuild and restart all dependent applications; regenerate any keys created while running vulnerable versions.

How to confirm the fix worked

Confirm installed package versions meet or exceed the fixed releases listed in the advisory; run regression tests that exercise the RNG mixing functions; monitor for any regenerated cryptographic material.

Typical attack path

1. Identify a process or service using the vulnerable Libgcrypt or GnuPG RNG
2. Capture at least 4640 bits of consecutive RNG output via side-channel or log exposure
3. Apply the published bias equations to compute 160 bits of subsequent output
4. Use recovered bits to narrow brute-force or dictionary attacks against generated keys or nonces

How to detect it

Review application and system logs for processes linking against vulnerable Libgcrypt versions; search package databases for pre-fix releases; examine any captured RNG output streams for statistical anomalies consistent with the published bias.

What your security team should do

• Prioritize patching of all GnuPG and Libgcrypt instances given the multi-year exposure window
• Audit and replace any keys or certificates generated with affected RNG versions
• Add package version checks to build and deployment pipelines to prevent reintroduction of vulnerable releases