Trail of Bits Audits Signal's New Automatic Key Verification System

Signal Introduces Automatic Key Verification

Signal has rolled out Automatic Key Verification (AKV), a new feature that automates the process of verifying the integrity of chat keys. Previously, users had to manually compare safety numbers with their contacts to detect potential server compromises that could lead to messages being encrypted to an attacker. AKV aims to address this vulnerability by providing an automated verification method.

How Automatic Key Verification Works

Automatic Key Verification utilizes a "key transparency" mechanism to establish a globally consistent view of public keys associated with each phone number. The Signal application now performs regular self-checks to confirm that all keys stored in the global map for a user's account belong to their devices. If the system detects inconsistencies or is unable to verify the log, users receive a warning indicating that AKV is unavailable for their device.

Trail of Bits' Role as an Auditor

Trail of Bits developed and operates one of the three external auditors crucial for the trustworthiness of Signal's Automatic Key Verification system. This independent auditor, implemented from scratch, continuously monitors the AKV system to ensure its honest operation. The other two auditors are run by Signal and Cloudflare.

Auditor Functionality

The Trail of Bits auditor provides external verification that the user-to-public key map is globally consistent, well-formed, and transparent. Each time a new entry is added to the map, the auditor updates its local copy, stored as a Merkle tree. Periodically, the auditor signs the head of this tree using a unique signing key. This process ensures that clients see the same set of public keys as everyone else in the system, as clients require signatures from all three auditors to validate the key transparency server's data.