It can't, because of this: https://en.wikipedia.org/wiki/Double_Ratchet_Algorithm https://en.wikipedia.org/wiki/Double_Ratchet_Algorithm This gives us the guar

It can't, because of this: https://en.wikipedia.org/wiki/Double_Ratchet_Algorithm https://en.wikipedia.org/wiki/Double_Ratchet_Algorithm This gives us the guarantee of forward secrecy. When you pair a Signal app, it gets to know the current keys used to advance the ratchet. But, it cannot deduce keys used to encrypt prior messages. It can only decrypt future messages.