"When Signal users register, change their phone number or username, or re-create their account, Signal records the changes in a log tree ('the ledger') and facilitates searching through the log tree with prefix trees ('the index books')," Signal said in the announcement.
Automatic Key Verification: a button, a green checkmark, and what it means
Signal has added Automatic Key Verification (AKV), a user-facing feature that simplifies checking whether a contact's public encryption key matches what Signal's systems expect. From the app, users tap a contact’s profile, go to the “View Safety Number” screen, and hit the “Verify automatically” button. If the verification succeeds, the app shows a green checkmark indicating the public key presented matches Signal’s key transparency record.
Signal presented AKV as an attempt to close a specific risk: even with end-to-end encryption and safety numbers, a centralized directory could be corrupted so encrypted messages were routed to the wrong recipient — an interception that would still appear encrypted but would end up in a different place.
Signal’s key transparency server, the ledger, and the index
AKV relies on a new architecture Signal describes as a ledger of public keys. Every change a user makes to their account information — for example, linking a phone number — is recorded as a new iteration in that ledger. Accompanying the ledger is an index that Signal calls prefix trees or “index books,” which facilitate searching through the ledger.
Signal said the ledger lives on an “open-source key transparency server” created for AKV. The ledger plus index lets the system detect tampering that would require altering a user’s public key — and thus the safety number — to mount a man-in-the-middle interception.
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Get a security leadCloudflare and Trail of Bits as third-party auditors — what they can and cannot do
Signal named Cloudflare and security firm Trail of Bits as the AKV third-party auditors. Their role is to check Signal’s key transparency server and the index to ensure entries do not appear to have been altered. When their checks are clear, the auditor signs the response to indicate the keys being provided are the same for both users.
Signal acknowledged a limitation: auditors can guarantee the index and the key transparency server have not been tampered with, but they cannot verify the accuracy of the data contained in those records. In other words, auditing can confirm consistency between copies and detect alteration of entries, but it cannot independently certify that a given entry corresponds to the person it claims to represent.
Monitoring, Alice and Bob, and remaining user responsibilities
Signal described monitoring as the complementary layer to auditing. The app allows each user to look up someone else’s address and to look up their own ledger entry; Signal said that monitoring “requires Alice and Bob to do both of these things on a regular basis, each detecting a different kind of tampering.”
The app automatically verifies a user’s own phone number and username data in the log, Signal said. For verifying someone else, the app will comb the index on the user's behalf to ensure the information retrieved is the most up-to-date. But the company makes clear this does not remove user responsibility: “If you want to be truly sure your contact is who they say they are, you’ll need to hit that verify button every time you want to chat.”
AKV has practical limits. To verify someone else, you must have their phone number in Signal or a matching entry in your phone’s address book. AKV can be disabled by users who do not want a third party involved; Signal recommends reverting to manual safety-number or QR-code verification in that case.
What this means for technologists, end users, and adversaries
- Technologists and security teams: AKV adds an architectural layer — a public ledger plus an index and third-party auditing — designed to detect tampering with public keys in a centralized directory. Teams will need to assess how that ledger and auditing model fits operational threat models and whether reliance on auditors aligns with their risk posture.
- End users and the general public: The feature simplifies verification to a single tap and a visible green checkmark, but verification still requires either the contact’s phone number or manual QR/safety-number checks. Users who disable AKV remove a layer of third-party assurance and are advised to revert to manual verification.
- Adversaries and interceptors: The ledger-plus-audit design aims to make undetected changes to public keys harder to achieve at scale, but Signal’s own account — that auditors cannot vouch for data accuracy — highlights that certain targeted or out-of-band manipulation remains a theoretical risk unless users actively verify identities.
Signal’s Automatic Key Verification is a practical attempt to close a narrow but consequential surveillance gap: encrypted messages could be intercepted if the directory mapping identities to public keys were altered. The company has combined an open-source key transparency server, immutable ledger updates, an index, third-party auditors, and app-side monitoring to create a layered detection system. The trade-offs are explicit in Signal’s announcement — auditors can verify consistency, users must still verify accuracy, and phone-number requirements and opt-outs leave gaps for some conversations.
