How Trail of Bits helps verify the integrity of your Signal chats

What it is

Trail of Bits' operational writeup of the auditor it runs for Signal's Automatic Key Verification (AKV). The summary covers the auditor architecture; this page adds the end-user enablement steps, the failure behavior, and the operational commitments Trail of Bits makes as an auditor.

What to do

Enable AKV in Signal under Settings > Privacy > Advanced. In a supported chat, open the safety number verification screen and click "Verify Automatically." If automatic verification fails, fall back to manual safety number comparison — AKV does not replace it, it is a convenience layer over it.

Preconditions / Affected

• AKV "often does not support chats where you started the conversation by searching for a recipient's username." For those chats, expect manual safety-number verification only.
• The Signal app runs a periodic self-check that all keys in the global map for your account belong to your devices. If it can't verify the log, or finds unexpected keys, it shows "Automatic Key Verification is currently unavailable for your device." That warning has multiple possible causes (per Signal's docs), so it is not by itself proof of an attack.

Caveats

• The bound is a detection window, not prevention: a fully malicious server can maintain a split view for up to one week before clients warn (signatures from all three auditors — Signal, Cloudflare, Trail of Bits — must be within the last seven days). It does not detect faster attacks or protect during that window.
• Trail of Bits' auditor is one of three; the security model assumes the three operators do not collude and that clients enforce all three signatures.
• Trail of Bits states it will update the blog post itself for substantive signing-policy changes — resetting auditor state or rotating its signing key. If you rely on this auditor, that blog post is the channel to watch; the current public key is published there.

Why it matters to act on

This is a live, deployed feature you can turn on now, not a proposal. The actionable takeaway for an engineer is the enablement path plus the correct interpretation of failures: the "unavailable" warning is ambiguous and the seven-day window means AKV is a monitoring backstop, so keep manual safety-number comparison in your threat model rather than treating AKV as complete.