Key Transparency

When you send a message in a messenger like WhatsApp or Signal, it is made unreadable while in transit. Only your conversation partner’s device can make it readable again. For this to work, your app first fetches a kind of digital ID for the other person from the provider’s server. This is exactly where a risk lies: you have to trust that the server really sends you the ID of the right person. Key Transparency is a method that makes this trust unnecessary. The provider must record in a public, tamper-proof directory which ID it issues for which account — and anyone can check these entries.

The problem with a substituted key

Encryption reliably protects messages while they’re in transit. But it doesn’t protect you from talking to the wrong person from the very start. A provider could send your app its own ID instead of the real one. Then you would be encrypting for it, it would read along, and forward the message to the real recipient. Experts call this a man-in-the-middle attack, meaning an attacker positioned in the middle of the connection.

Until now, this could only be checked manually. In Signal or WhatsApp, there’s a long sequence of numbers or a QR code that two people compare in person for this purpose. This safety number is effective, but hardly anyone uses it. A method that only works if millions of people voluntarily do extra work protects, in practice, very few of them.

Key Transparency reverses the responsibility. It’s not the user who has to take action, but the provider who has to justify itself. This doesn’t make fraud impossible, but it leaves traces. And a company that advertises trust can ill afford a provable fraud. For journalists, lawyers, or people facing political persecution, this difference is significant.

A public directory that cannot be changed retroactively

The provider maintains a directory. It contains the currently valid key for every phone number or account. Every new entry is appended, old entries remain in place. The directory thus only grows, it is never rewritten. This is called an append-only log, meaning a record to which you can only add entries at the end.

To ensure no one changes anything secretly, the directory is secured with checksums. A checksum is a short sequence of numbers calculated from a data set that changes completely with any modification. These checksums are stacked on top of each other in a tree structure, until a single value at the very top represents the entire directory. If someone changes a single entry, this topmost value changes immediately and visibly.

Your app therefore doesn’t have to download the entire directory. It only fetches the topmost value and a short proof path for the entry it’s interested in. It also regularly checks whether its own key is correctly recorded. If the provider were to secretly assign you a second key, you would see this in your own app. A common misconception, by the way, is that this would make contact data public: the entries are first converted into unreadable identifiers, so the directory is not a list of phone numbers.

Messengers that already use it

WhatsApp introduced Key Transparency in 2023. When checking the safety number there, a checkmark now appears automatically, without you having to meet up with anyone. Apple uses a comparable method for iMessage, Google for its messaging app. Similar protocols are also being discussed for email encryption and software updates.

The model comes from another field. For years, certificates for websites have had to be recorded in public logs so that forged certificates can be spotted. This system is called Certificate Transparency and has repeatedly uncovered real forgeries. Key Transparency applies the same idea to private messages.

In news reports, the term usually comes up in connection with government demands for access to encrypted chats. An important distinction here: Key Transparency does not encrypt anything. It only ensures that an attack on key distribution does not go unnoticed. Anyone who disables the check entirely or ignores warnings remains vulnerable regardless.