I'm not convinced that concatenating hashes is safer than a single hash function either. Let's say you have an entirely broken hash function for which hash(A)

I'm not convinced that concatenating hashes is safer than a single hash function either. Let's say you have an entirely broken hash function for which hash(A) = A. Clearly if you cocatenate hash(A) + sha256(A), you still end up with something is broken. I think if you concatenate the hash function, you may well end up with something which is strictly worse than using either hash function (as breaking one will break both). Also, if you have the result of both hash functions, then you may have provided data which warps the probability chart, as you now have much more information on the output state of the data, which isn't combined in a clearly useful way. Their might be a way to combine two hashes in a way which does preserve the properties of a hash function, but I think it would be significantly tricky that you would have essentially created a new hash function using other hash functions as building blocks. (That might be good grounds for generating a hash function resistant to attack - combine multiple different styles of hashing, but it doesn't seem to be common when hash functions are designed which suggest to me there are fundamental problems. (e.g. most hash functions repeat the same functional blocks, rather than having a large selection of different blocks).)