# A hash function for sets that from a hash of a set and a subset of it doesn't reveal the hash of the remaining elements in the set

I'm looking for a function to compute a hash of a set.

It needs to satisfy two properties:

1. If someone published a hash $$h(S)$$ of a set $$S$$ and a hash $$h(S')$$ of some subset $$S' \subset S$$ of it, I should not be able to compute the hash of $$S \setminus S'$$ from $$h(S)$$ and $$h(S')$$.

2. From the hashes $$h(S_1)$$ and $$h(S_2)$$ of two disjoint sets $$S_1$$ and $$S_2$$, I should be able to compute the hash $$h(S)$$ of their union $$S = S_1 \cup S_2$$.

Does such a hash function exist?

• is the answer below satisfactory? – kodlu Dec 25 '18 at 2:14
• I think hashing is actually a red herring here. What you really want is an easily computable commutative and associative binary operation $\otimes$ over some large but finite set $\Omega$ such that the inverse operation $(a \otimes b, a) \mapsto b$ is hard to compute. If you had one, you could just feed arbitrary inputs to a conventional cryptographic hash function and map the results to elements of $\Omega$. Unfortunately I can't really think of any way to actually construct such an operation. – Ilmari Karonen Apr 24 at 18:48

Here is an idea which mostly addresses what you appear to want; I'm not happy with it, however I thought I'd share it just in case.

Our 'hashes' are vectors of $$n$$ elements, each element being either a number between 0 and $$p-1$$, or the special symbol $$\bot$$.

The hash of a singleton element $$s$$ (that is, a set that consists of a single element) is computed by selecting the $$n$$ elements to be numbers deterministically (e.g. using the bits from SHAKE(s)), and then selecting $$b$$ of the $$n$$ indicies (again, deterministically), and setting each of those $$b$$ indicies to $$\bot$$.

And, to combine two hashes (using Ilmari's notation, to compute $$c = a \otimes b$$), you perform the following operation element-wise to the two hashes):

$$c_i = \bot$$ if $$a_i = \bot$$ or $$b_i = \bot$$

Otherwise, $$c_i = a_i + b_i \bmod p$$

It should be obvious that the above operator is both associative and commutative.

And, assuming $$p, n, b$$ are properly set, then given $$a \otimes b$$ and $$a$$, it should be highly probable that $$b$$ cannot be recovered (as there is likely be to some element $$i$$ in $$a$$ for which you have $$a_i = \bot$$ and $$b_i \ne \bot$$; in that case, you have no information on $$b_i$$, and so any of the $$n$$ possible values are equiprobable.

Of course, the hashes are quite large, and this noninvertability is only probable (and even that is true only if the sets $$a, b$$ are not too large).

• Nice Idea, I need to study it in detail! I have a concern regarding the size of the set. It seems like the sets should be pretty lare. For the first condition, assuming the set $S$ is known, if it's not large enough and therefore has a small powerset, the we could easily brute force the hash of $S \backslash S'$ by trying all possibility(assuming hash computation is efficient enough). Wouldn't this be the case for any such hash? – Marc Ilunga Apr 24 at 21:47
• I haven't gone through it in detail, but I suspect that if $S \ S'$ is small enough to brute force with this algorithm, then it wouldn't be that much work to brute force it against any such algorithm. – poncho Apr 24 at 22:24
• Yes I was thinking that for a $n$ elements sets brute-forcing takes at most $2^n$ steps where each step computes a hash of a subset, and verifies it against the one that was provided and if it matches, computes the difference and output the hash of the difference. And as you said tighter bounds can be achieved as a function of the size of $S'$. This is confusing to me because subsets of $S$ should also be part of the domain. In that case the first condition cannot really be achieved in general. – Marc Ilunga Apr 24 at 23:24

This is not really an answer but a collection of observations.

Condition 1) seems a bit problematic and confusing. It seems that it would only hold if the size of $$S$$ is large enough independently of the $$H(\cdot)$$ used. The reason is if $$n$$, the size of $$S$$ is not large enough then an attacker for condition one could try the following(assuming we also know $$S$$):

• let $$P = 2^S$$ be the powerset of $$S$$, i.e the set of all subsets of $$S$$
• inputs: $$S, H(S), H(S')$$
• let $$A$$ be an adversary for condition 1.
• On every element $$s \in P$$, $$A$$ computes the hash $$H(s)$$ and checks if $$H(s) = H(S')$$
• if check passes output $$H(S\setminus s)$$.

Now if the number of collisions is really small and the hash computation is assumed for free(efficient enough) then the complexity would be at most $$O(2^N)$$ with $$N$$ the size of $$S$$. Note: If we don't know S, we would also need that the number of maximal sets(sets that are not subsets of other sets) is large enough.

Furthermore we can actually get tighter bounds on the complexity assuming $$S$$ is known. If $$S'$$ has size $$k$$ then $$S'$$ has size $$k'= N-k$$. so the total number of steps would be $$\sum^{k'}_{i = 1} \binom{n}{k}$$. A closed formula is really welcomed! ;)

Finally what confuses me is that subsets of $$S$$ should be also in the domain of any such $$H(\cdot)$$, therefore this property would not hold for a small enough subset by the reasoning above.

Is it possible that such $$H(\cdot)$$ can never exits and satisfy condition 1 in general or am i missing something?

• The way I read the OP's condition 1, the attacker is not assumed to know the set $S$, but only its hash. But there's definitely some ambiguity there, and your attack is indeed feasible if $S$ is known and not too large. – Ilmari Karonen Apr 25 at 21:51

Let's formalize your encoding of the sets $$S$$ (assumed to be the subsets of some universal set $$\Omega=\{\omega_1,\ldots,\omega_n\}\leftrightarrow (1,\ldots,1)$$ as the string $$E(S)=(\mathbb{1}\{ w_k \in S\}: 1\leq k \leq n)$$ which has a $$1$$ in the $$k^{th}$$ position if and only if $$w_k$$ is an element of $$S.$$ So to hash a set you hash its encoding $$E(S).$$

Note that any good hash function should have the avalanche property; i.e., if a bit is flipped (say an element is added or removed from $$S$$ to obtain $$S'$$) the two hashes $$h(E(S)),$$ and $$h(E(S'))$$ should not be easy to determine from each other, the property you want should hold provided $$n$$ is large enough so that it can't be brute forced for a collision, say $$n>2^{512}.$$

If the universe is not so large, you may need to use some kind of salting to increase the strength of the hash function.

• Seems like I would not be able to to compute a hash of the union by only knowing the hashes of the two sets? – Ishamael Nov 1 '18 at 18:30
• Correct, since it would imply a weakness in the original hash $h(\cdot)$ as applied to strings – kodlu Nov 1 '18 at 23:09