Every Signal chat begins when a client requests a contact's public key from the Signal server. A compromised server could supply a false key, enabling a man-in-the-middle attack. Previously, users had to verify safety numbers manually. Signal's new Automatic Key Verification uses key transparency to create a globally consistent view of public keys, making mismatch attacks harder to hide. The Signal app performs periodic self-checks; if verification fails, users see a warning that Automatic Key Verification is unavailable.

Trail of Bits operates one of three independent auditors—alongside Signal and Cloudflare—that validate the system. Their auditor, written from scratch as an independent implementation, continuously checks that the user-to-public-key map is globally consistent and well-formed. It stores the map as a Merkle tree and periodically signs the tree head with a private signing key, committing to a single consistent lineage. Clients require valid signatures from all three auditors within the last seven days; otherwise, Automatic Key Verification fails.

Users can enable the feature via Settings > Privacy > Advanced. In supported chats, they can click "Verify Automatically" on the safety number screen. If automatic verification fails, users should fall back to manual safety number comparison. Trail of Bits publishes its current public key and will update its signing policy as needed.