How do messengers, like signal or XMPP-omemo messengers, store their messages? Since the session key is only valid for the current session, how can such extensions like MAM (message archiving on XMPP server) or storing the messages encrypted on the disk work? Futher, you often want plausible deniability with "malleable encryption" and publishing your MAC-Keys. If this happens, how can you be sure that integrity and authentication is still valid?