Signal has introduced Automatic Key Verification (AKV), a new security feature that helps detect if someone has secretly tampered with encryption keys to intercept messages. While Signal already uses end-to-end encryption and safety numbers (cryptographic fingerprints), a sophisticated attacker could theoretically corrupt the centralized account directory and pose as another user in a man-in-the-middle attack. The new AKV system addresses this vulnerability by creating a verifiable record of all public encryption keys.
The feature works through a transparent ledger system that records every change users make to their account information, such as linked phone numbers or usernames. This ledger lives on an open-source key transparency server that Signal developed specifically for AKV. When users register or modify their accounts, Signal records these changes in a log tree and creates searchable prefix trees that function as index books. Signal automatically searches these indexes on behalf of users to verify that contact information is current and unaltered.
To ensure the key transparency server itself has not been compromised, Signal enlisted third-party auditors Cloudflare and Trail of Bits. These auditors regularly check the index to confirm that entries have not been altered. When their checks pass, the auditors sign the response to indicate that the keys being provided are identical for both parties in a conversation. This auditing process, combined with user monitoring, creates a complete detection system where auditors guarantee both users see the same data, while monitoring ensures that data is regularly checked for accuracy.
The system has some limitations. Users must actively tap the "Verify automatically" button in a contact's profile to confirm their encryption key matches what the key transparency system expects. Additionally, AKV only works if users have their contact's phone number either through Signal or in their phone's address book. Without this information, the automatic verification cannot function, and users must fall back on traditional safety number or QR code verification methods.
Signal users who prefer not to involve third parties in their verification process can disable AKV entirely. The feature represents a significant step forward in preventing sophisticated interception attacks, but it still requires user participation to be effective. Users concerned about the highest level of security should make a habit of verifying contacts before sensitive conversations, particularly when communicating with new contacts or after any account changes.
Source: https://www.theregister.com/security/2026/08/11/signal-adds-an-extra-layer-of-security-to-make-sure-youre-actually-chatting-with-the-right-person/5286461