[messaging] Forward secrecy and multiple devices

[Nadim Kobeissi]

Hi everyone,
I've been wondering about how to make asynchronous forward-secret 
messaging systems work when the user is accessing message history from 
multiple devices.

Say I send a bunch of messages from computer A to another user's 
computer U.
Later, I buy myself a new computer B on which I want to download and 
decrypt my message history.

If the messages I sent all relied on my long-term identity, then I can 
just use my long-term key pair to decrypt the messages on computer B and 
there wouldn't be a problem.

However, I am wondering how that would work in case I was using 
forward-secret session keys that changed message by message. How would 
the session secrets be communicated across devices? How would computer B 
be able to decrypt my forward-secret messages sent from computer A?

It would be great to hear the opinion of the many experts on this list 
regarding this matter.

Regards,
NK