Trail of Bits Launches Independent Auditor for Signal's Automatic Key Verification

Trail of Bits has announced the launch of an independent auditor designed to bolster the security of Signal's new Automatic Key Verification feature. This initiative aims to protect users from compromised servers that might attempt to substitute false public keys for their contacts, a critical vulnerability in end-to-end encrypted communication.

Signal's Automatic Key Verification is a form of "key transparency" designed to make "mismatch attacks"—where an attacker intercepts and substitutes public keys—more difficult to conceal. The system creates a globally consistent view of public keys associated with phone numbers. The Signal application now performs periodic self-checks to ensure that all keys stored in its global map for a user's account belong to their legitimate devices. If these checks fail or inconsistencies are found, the user is alerted that Automatic Key Verification is unavailable.

Trail of Bits' role is to provide external, independent verification of the key transparency system's integrity. Their auditor, built from scratch and released as open-source, continuously monitors the user-to-public-key map for global consistency and proper formation. Each time a new entry is added to the map, Trail of Bits updates its local copy, stored as a Merkle tree. Periodically, they sign the head of this tree with a private key known only to them.

This multi-auditor approach is crucial for user trust. Signal clients currently require signatures from three distinct auditors: one operated by Signal itself, one by Cloudflare, and the newly announced one by Trail of Bits. When Automatic Key Verification is active, the Signal client fetches Merkle tree heads from the Signal key transparency server and verifies that each tree head is part of a lineage endorsed by all registered auditors within the preceding seven days. This mechanism ensures that a malicious server cannot maintain a split view of the system for longer than a week before client applications begin displaying warnings.

By implementing their auditor independently from Signal's reference implementation, Trail of Bits provides a robust, adversarial perspective on the system's security. The open-source nature of their auditor allows for community scrutiny and contribution, further enhancing transparency and trust. The company has committed to providing updates if any substantive changes are made to their signing policy, such as resetting auditor state or rotating signing keys.

Users can enable Automatic Key Verification within Signal's privacy settings. For supported chats, the feature can be activated through the safety number verification screen. Signal notes that this feature may not be available for all chats, particularly those initiated via username searches. In cases where automatic verification fails, users are advised to revert to manual safety number comparison.

Trail of Bits emphasizes that they are operating this auditor service voluntarily, without payment from Signal or any other entity. Their motivation stems from a belief in the importance of free and private communication as a public good. They view public key integrity as a fundamental component of any secure end-to-end encryption system and offer their expertise to others looking to implement similar security measures.

The development and operation of this independent auditor by Trail of Bits represent a significant step in strengthening the security and trustworthiness of modern encrypted messaging platforms, offering users greater assurance that their communications are protected from sophisticated man-in-the-middle attacks.