End-to-end encryption, explained

What it does for you

• Only the phones in a conversation can read it; the server stores and delivers copies it cannot open.
• One stolen key does not unlock the rest of the conversation.
• You can check that nobody is in between by comparing a safety number.

How it works

Every device has its own keys; the server holds only the public halves, which other phones fetch to start a chat. The first message sets up a shared secret with the Signal Protocol’s key agreement, and from then on every message moves the keys forward (the Double Ratchet), so a key taken from one moment does not open the messages before or after it.

In groups, each member has a sender key that is shared through the one-to-one encrypted sessions; when someone joins or leaves, the keys are replaced. Photos, files and voice messages are encrypted with a fresh key each, which travels inside the encrypted message.

If the other person has no keys yet, the app sends nothing rather than falling back to plain text.

Where it stops

Translation needs the words, so a translated message is decrypted on your phone and sent to the translation service. The Security Center lists every place where end-to-end encryption does not apply.

Under the hood

Signal Protocol · Double Ratchet · libsignal