[messaging] Forward secrecy and multiple devices

[Ben Harris]

Computer A encrypts message with a personal key and stores the message in
the cloud. The personal key is never shared so it can't be attacked,
computer B gets the messages and decrypts with the personal key.w

On 31 October 2014 21:04, Nadim Kobeissi <nadim at nadim.computer> wrote:

> 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
>
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