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3

If we use $H_1(X) = H_0(X) \oplus firstnbits(X)$, this would seem to be trivial. EDIT: As Cédric Van Rompay pointed out, this is only a counterexample if $H_1$ winds up being preimage-resistant. This may not be a necessary consequence of $H_0$ being preimage-resistant, but I really only need one case where it is.


2

When the output of the hash function is $n$ bits, then there are $2^n$ possible outputs. For a preimage attack you are given a hash $h$ and you need to find a message $m$ where $h = H(m)$. Since there are $2^n$ possible outputs, the probability of guessing an input that that maps to the given output is $\dfrac{1}{2^n}$. So on average you need to try $2^n$ ...


1

Assuming the hash is strong, you will not be able to find a preimage any more easily when you know multiple hashes (whether they use the same function or another). In fact, with some hashes you will be able to derive $H(S||K)$ from just $H(S)$ and $K$ (known as the length extension attack), so it can't give you more information. Unless the hash is broken, ...



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