[messaging] Forward secrecy and multiple devices

[Michael Rogers]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

On 31/10/14 16:58, Nadim Kobeissi wrote:
>> Protocols are different from the applications that use them. I
>> said "can use," because it depends on the application. Maybe the
>> application stores nothing, so there's nothing to transmit, but
>> all future messages will ephemerally appear synchronized.
> Right, but protocols don't operate in an ether. It is the case
> that, in order to maintain any modicum of
> efficiency/speed/usability, generic messaging applications will in
> fact need to store some message history. The protocol doesn't
> dictate the application's needs outside of cryptographic
> guarantees.

It seems to me that you've answered your own question. No breakthrough
is needed to sync message history between devices while maintaining
whatever forward secrecy properties your app already provides. Just
use the same forward-secret protocol for syncing messages between each
user's devices that you already use for exchanging messages between
users. Then on each device, apply whatever retention policy you
already apply on a single device.

Cheers,
Michael
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.12 (GNU/Linux)

iQEcBAEBCAAGBQJUU8JSAAoJEBEET9GfxSfMZoQIAIzUSx3aEihB8r+lMNSIv+US
Y4ZMk4wh6rdp5QEya7PBnX4bhdFSHryO1Ys5GKbe17getD6lbOVQi50I4btARCqj
78Ox3OJ8tbS4VI0j+s6H4nB6ID4x3EsXLij3R6ctNmGCdgm3PzbqPbknfoFso+x9
t+ZGYfc1a9azfB4Q73pIw9lvcPlKmUYs93GP9aW9YchST+tS9IOaEtRF/6h/C+wL
53OXJTumUScxkGWok99g7mcwkYb8GH2b30WobcroDwZjlyKKfwUyNmvCrjjakCh/
lBHB0nAuATii9l8HA2B5X2Y061hsyeXrbJHGabI13D8FhKe1REVjSfVLJNLmt1o=
=N8dE
-----END PGP SIGNATURE-----