Each device gets it’s own public/private key pair. When a message is sent, it is encrypted with a message-specific key. This key itself is encrypted with the public key of each client participating in the chat.

So if you have multiple devices, the message-key is encrypted for each of your devices. Same holds true for all devices of the person you are communicating with.

If you are in a group chat, then the message-key is encrypted for everyone’s device participating in the group. For decryption each device decrypts the message-key, then takes the message-key and decrypts the potentially long message.

This procedure enables bandwidth-efficient multi-user multi-device encryption. Each device public key can be represented via a fingerprint.

from Jabber & Omemo Getting Started by Marcel Ackermann Jun 26, 2017