Horcrux Encrypted Messaging
A few points:<p>* The 0day argument (that using multiple apps reduces exposure) is a poor one. If we&#x27;re talking about 0days that allow message decryption from a MITM, this is true--but if we&#x27;re talking about RCE with (on mobile OSes) a local sandbox escape (e.g. <a href="https:&#x2F;&#x2F;threatpost.com&#x2F;whatsapp-zero-day-exploited-in-targeted-spyware-attacks&#x2F;144696&#x2F;" rel="nofollow">https:&#x2F;&#x2F;threatpost.com&#x2F;whatsapp-zero-day-exploited-in-target...</a>), then installing more apps <i>increases</i> the attack surface and thus the risk.<p>* Another argument for multiple services seems to be some implied DOS resistance. But of course exactly the opposite is true: if you split your message and deliver it via multiple channels, any one of those channels can be taken offline and thus thwart message delivery.<p>* The real argument for multiple services seems to be based on an assumed threat model of an attacker who primarily relies on lawful intercept orders, <i>not</i> on technical capabilities (e.g., a warrant, not cryptanalysis). Adding multiple services may do little or nothing for the latter threat, since many messaging services rely on the same underlying protocols (e.g. the Signal Protocol) or the same crypto primitives. This is important for my next point:<p>* If the threat model is about lawful intercepts, and you actually trust modern crypto, this entire scheme makes no sense. Just share a public key for a common, well-vetted algorithm. Encrypting messages using an air-gapped device makes sense.<p>* Further, if we trust crypto, this scheme&#x27;s sole value is effectively in contact (or public key) discovery: assuming you can&#x27;t meet face-to-face, the point of this scheme is really to allow multiple services to help you discover a contact&#x27;s public key (assuming we replace the unnecessary OTP approach with public key exchanges). But many (most?) of these services rely on the telephone system for identity--so there&#x27;s still a fairly trivial single point of failure, commonly controlled by the government.<p>* To beat a dead horse: if we don&#x27;t trust modern crypto, it&#x27;s likely an adversary can access all components of the message+OTP anyway (even without lawful intercept capabilities), so this scheme adds no value.<p>Summary:<p>This proposal would benefit by a more clearly stated threat model. As-is, it&#x27;s hard to come up with a coherent threat model where this proposal adds value.