# Creating a hash of XOR'd blocks

Suppose a message $m$ is divided into blocks of length $160$ bits: $m > = M_1 || M_2 || ... || M_l$ And define $h(m) = M_1 \oplus M_2 \oplus ... \oplus M_l$

Which of the three desirable properties of a good cryptographic hash function does h satisfy? Show that it does not satisfy the other two.

Three desirable properties of a cryptographic hash function http://en.wikipedia.org/wiki/Cryptographic_hash_function#Properties

I feel like if there is only one, like the question states, it has to be the first one, preimage resistance. However, given a hash $h = 000....0$ I can find a bunch of $m$'s that could give that particular hash. Maybe since there are lots of solutions you can't pin point the specific $m$ that gave that value?

What do you think?

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I believe that this is a poorly written question: such an $h$ obviously doesn't have either preimage resistance, second preimage resistance or collision resistance.

The inability to rederive the specific value of $m$ based on its hash is not an interesting property; it's pretty much true of any function which generates an output shorter than its input.

I don't know where you're getting these questions from; based on this question, I'd suggest you look elsewhere.

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Old Exams, thanks for the confirmation of my worst fears! – Bobby S Dec 16 '11 at 17:43
1. ⊕ is a relatively simple operation, so $h(x)$ can be calculated quickly.

2. preimage resistant: No. Given $h(x)$, we can construct $M_1||M_2||...||M_l$ in an arbitrary manner. Since we know h(m), we have a 160 bit string of 0s and 1s. For each 0 in $h(M)$, we can assign an even number of 1s (or all 0s) to that position in the l blocks. For each 1 in $h(m)$, we can assign any odd number of 1s to that position in the l blocks.

3. Strongly collision-free: No. We are only looking for $M_1 \nleqq M_2$ but $h(M_1) = h(M_2)$. We do not have to base this on a given h(m). So, we can arbitrarily construct $M_1||M_2||...||M_l$ for some number of blocks $l$. We could change an even number of 1s from a given position to 0, from 0 to 1, etc. and this changes the message but not the hash value.

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Stackexchange uses markdown not latex for formatting posts. It has limited latex support for formulas enclosed in \$ (mathjax). – CodesInChaos Jul 9 at 20:40