Signal doesn’t keep any record of the lookups it’s performed and allows you to satisfy yourself that it doesn’t by giving you access to its source code: There is a problem with this scheme (to quote Signal’s developers Open Whisper Systems) because the “pre-image space” for 10-digit numbers is small, “inverting these hashes is basically a straightforward dictionary attack”, which is another way of saying that it’s feasible for a computer to make guesses quickly and cheaply enough to compromise the security of the hashes. If it doesn’t match anything, guess another number, and another, and another… and so on until you find a match. Guess a number, run it through the hashing algorithm and see if it matches one that you’ve stolen. ![]() The only way for a hacker with stolen hashes to figure out what telephone numbers they’ve got is to guess. The privacy of this design is that anyone intercepting the traffic or hacking the directory will see hashes rather than real telephone numbers. It connects users where it finds matches. Right now Signal counteracts this by turning every number in a user’s address book into a truncated SHA256 hash, which is transmitted and checked against a central database of hashes. This is a privacy compromise because it means that while the service’s own encryption stops it from reading your messages (or letting intelligence agencies that later ask for access to this data read them either) it can end up knowing a lot about who you know. This requires that apps check who else among a person’s contacts uses it by consulting a central “social graph” of how people are connected. It sounds esoteric, but it fixes an important privacy weakness that has dogged end-to-end encrypted messaging: users want to know who else they know that uses the same service. Signal, arguably the world’s most respected secure messaging app, plans to use the DRM (Digital Rights Management) secure enclave built into Intel’s Skylake chips as a way of hiding away how people are connected.
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