Security codes ("safety numbers" on Signal) are supposed to provide you with a means to detect malicious MITM attacks. Since most users don't check their contacts' numbers out-of-band -- or even verify their own codes regularly -- it's kind of a probabilistic mechanism. In theory these checks should dissuade attacks on the system, but in practice does it?

This post illustrates one of the many reasons the solution is less effective than I would like it to be. Specifically, when the system spits out weird results (changing safety numbers, mismatched numbers) it tends to be hard to diagnose the problem in a way that lets you verify, let alone prove to third parties, that there really was an attack. Since changes and mismatches "just happen", people tend to take warnings less seriously than they would if there was a path to diagnosing the problem. Moreover, from a hypothetical attacker's perspective there isn't much disincentive when most users will just shrug at these problems. I'm not sure there's a perfect solution to this, but I think it would be interesting to make these systems more robust.