I need a SHA3-255 or 511. What if I simply truncate a standard SHA3-256 or 512? Apart from the doubled probability of hash collision, are there any other things I should be aware of? I could also truncate one byte instead of one bit, if useful.
What I need is being able to store something different than the hash, in the same 32 bytes, or 64 bytes, so I need to sacrifice one bit to mark when the bytes represent a hash or something else.
Alternatively I could say that if the first byte, not bit, is 0xff, then the remaining ones represent something else. This should reduce the probability of hash collision, but what I would have a probability of 1/256 that a hash starts with 0xff, generating ambiguity in my encoding. I could say that if the for 4 bytes are 0xffffffff, then I would end up with probability of generating an ambiguous encoding of 1/2^32, but I would prefer having a well-defined encoding under any circumstances.
Any well known approach I'm not aware of?