Source: Schneier on Security
WhatsApp and Signal's convenience feature—syncing encrypted conversations across linked devices—creates a security perimeter problem that's harder to exploit than breaking encryption itself. Attackers can intercept messages by compromising the less-protected desktop client or network traffic between phone and computer, bypassing the cryptographic guarantees users assume they have. This exposes a core tension between usability (multi-device access) and threat modeling: the weakest link in a device chain becomes the actual security boundary, not the encryption algorithm.