Your point about padding and length bytes is spot-on. I actually left this out of the explanation, although it's interesting, because I felt it a useless diversion: something that's safe except in an easily describable subset of cases is still not fit for purpose when specially designed tools without these problems exist.
Point being, although hash functions might work most of the time, their general construction does not make them safe for this purpose. You can't say something is 'mostly secure' because it works 'most of the time'. The times is doesn't work will affect someone, and for them the system is not secure at all.
Plus, implementations with weird edge cases make for horrible debugging, especially when it comes to security.
Point being, although hash functions might work most of the time, their general construction does not make them safe for this purpose. You can't say something is 'mostly secure' because it works 'most of the time'. The times is doesn't work will affect someone, and for them the system is not secure at all.
Plus, implementations with weird edge cases make for horrible debugging, especially when it comes to security.