The Signal protocol and the MLS (Message Layer Security) protocol are both designed for end-to-end encryption of messages.

The Signal protocol is particularly suited for one-on-one chats and small group. On the other hand, the MLS protocol is more appropriate for larger group.

Both can be implemented on Nostr.

If you are interested in understanding the basic principles of these protocols, I highly recommend the following two videos:

https://youtu.be/7uEeE3TUqmU

https://youtu.be/FESp2LHd42U