Privacy focussed messenger

Guiding principles

Everyone is blocked by default
Can only message contacts where there is mutual agreement (i.e. both have added each other)
Text only
Transit over open protocols
Mandatory encryption of communication stream
Mandatory encryption of data (E2EE)
No read receipts
No way to know if someone has added/removed you from their contacts
Messaging someone who hasn't added you to contacts = message silently deleted

How to avoid a centralised server
Important to prevent abuse of the service
Only allow connections if both people make a request (don't show incoming requests)
What transport mechanism should be used?
How can users send messages over a p2p protocol? Don't want a centralised server.
End to end encryption mandatory - client certificates? Public keys?
How to migrate / sync messages to another phone or device?
How to backup messages?
What if a user has multiple devices and wants to check messages on all of them?
How would group chats work?
How to deliver a message when a user is offline?
Could Tor be used for message delivery?
How to ensure each user has a unique ID? Use their email address?
What prevents a user from using someone else's ID?
If a user is removed as a contact, do not tell the other party and simply drop their messages.
No read receipts, might offer delivery receipts. This can leak information so need to be careful.
Double Ratchet Algorithm
Perfect Forward Secrecy?
How can we stop messages from being decrypted in future when computational power makes it feasible to brute force?

Text only: Simple to implement, compresses well, less scope for abuse (e.g. can't send images)