Signal >> Blog >> Introducing Automatic Key Verification Introducing Automatic Key Verification<br>Katherine Yen on 11 Aug 2026

Signal now offers a feature called “automatic key verification” which complements the existing safety number system. Signal is always end-to-end encrypted, and automatic key verification provides an additional, streamlined way to confirm that there’s no unexpected party between you and the other “end” of an end-to-end encrypted session.It works through a system of verifications performed by you, your Signal connections, and third-party auditors that together provide the same assurance as manually verifying safety numbers. Unlike safety numbers, these verifications are done independently and do not require an in-person meeting or a secondary communication channel.This system of verifications ensures that the association between a phone number or username and its public encryption key is globally consistent and transparent to all participants in Signal’s ecosystem. This protects against scenarios where a key is swapped out without the key owner’s knowledge — for example, if a malicious party compromised Signal and associated a different key with your connection’s phone number.To see this feature in action, go to a Signal connection’s profile, hit “View Safety Number,” and tap on the “Verify automatically” button under the “Automatic key verification” heading. The button will show a green checkmark and “Encryption verified” when the feature is available and verification succeeds. Over time, this verification, combined with the ones continually performed by your Signal connection and third-party auditors, ensures the consistency of this Signal connection’s key across the Signal ecosystem.This feature is built on a concept known as “key transparency,” which is the term we’ll use throughout the rest of this post as we explain why we built a key transparency system and give a high-level overview of how it works.A primer on public and private keys<br>Asymmetric cryptography is what enables you to send messages to your friend that only your friend can read. It involves a pair of mathematically-linked keys called the public key and private key, which can be used in a variety of applications including sending and receiving messages.Imagine you have a locked mailbox with a slot. Your public key is like your mailbox address — you can share it with anyone who wants to send mail to you. Your private key is like the mailbox key. You’re the only person that has it, and you can use it to read messages sent to you. Anyone can drop mail in your mailbox (encrypt with your public key), but only you can unlock and read it (decrypt with your private key). Your mailbox address must be publicly available; you can’t receive mail if no one knows where to send it.When you sign up for Signal, the Signal app generates a public/private key pair for you as part of the registration process1. The private key stays on your device — only you, not Signal, not anyone else, have access to this private key. Your public key is sent to Signal, which acts as a central directory for all users’ public keys. When you want to send a message to another Signal user, you ask Signal for your connection’s public key and encrypt your message with the key that Signal hands back and your private key.Mallory in the middle attack<br>Sending messages requires fetching public keys for recipients, and fetching public keys means relying on a central directory. In theory, a malicious directory operator could carry out something called a “Mallory in the middle” attack, though doing so would require an outsider bypassing the security of major cloud providers or a privileged insider deliberately targeting a specific account. But even though this is a very advanced and unlikely attack, it’s one we still want to guard against. We’ll continue using the mailbox analogy to illustrate how this attack would hypothetically work.Imagine Bob wants to send an invitation for coffee to his friend Alice, so Bob looks up her mailbox address in the central directory. If the directory is somehow compromised by an adversary (Mallory), it might direct Bob to Mallory’s mailbox instead of Alice’s. When Bob sends his mail to what he thinks is Alice’s mailbox, it actually goes to Mallory’s mailbox instead. Mallory then uses her own mailbox key to retrieve Bob’s message, reads it, modifies it if she chooses, puts it in a new envelope, and forwards it to Alice’s mailbox.In her modification, Mallory could change the coffee shop location or meeting time for example, causing Alice to show up at the wrong place or time. Alice would have no indication that the message had been intercepted or tampered with, so to her, it looks like the message came directly from Bob. Meanwhile, Bob would be left waiting fruitlessly for Alice to show up. And even if Mallory chose not to alter Bob’s message, her ability to read it at all constitutes a serious breach of communication...