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Revision #2 to TR12-182 | 20th October 2015 12:09

#### Hardness Preserving Reductions via Cuckoo Hashing

Revision #2
Authors: Itay Berman, Iftach Haitner, Ilan Komargodski, Moni Naor
Accepted on: 20th October 2015 12:09
Keywords:

Abstract:

The focus of this work is hardness-preserving transformations of somewhat limited pseudorandom functions families (PRFs) into ones with more versatile characteristics. Consider the problem of domain extension of pseudorandom functions: given a PRF that takes as input elements of some domain $\mathcal{U}$, we would like to come up with a PRF over a larger domain. Can we do it with little work and without significantly impacting the security of the system? One approach is to first hash the larger domain into the smaller one and then apply the original PRF. Such a reduction, however, is vulnerable to a "birthday attack": after $\sqrt{|\mathcal{U}|}$ queries to the resulting PRF, a collision (i.e., two distinct inputs having the same hash value) is very likely to occur. As a consequence, the resulting PRF is insecure against an attacker making this number of queries.

In this work we show how to go beyond the aforementioned birthday attack barrier by replacing the above simple hashing approach with a variant of cuckoo hashing, a hashing paradigm that resolves collisions in a table by using two hash functions and two tables, cleverly assigning each element to one of the two tables. We use this approach to obtain: (i) a domain extension method that requires just two calls to the original PRF, can withstand as many queries as the original domain size, and has a distinguishing probability that is exponentially small in the amount of non-cryptographic work; and (ii) a security-preserving reduction from non-adaptive to adaptive PRFs.

Revision #1 to TR12-182 | 24th December 2012 22:17

#### Hardness Preserving Reductions via Cuckoo Hashing

Revision #1
Authors: Itay Berman, Iftach Haitner, Ilan Komargodski, Moni Naor
Accepted on: 24th December 2012 22:17
Keywords:

Abstract:

A common method for increasing the usability and uplifting the security of pseudorandom function families (PRFs) is to hash" the inputs into a smaller domain before applying the PRF. This approach, known as Levin's trick", is used to achieve PRF domain extension" (using a short, e.g., fixed, input length PRF to get a variable-length PRF), and more recently to transform non-adaptive PRFs to adaptive ones. Such reductions, however, are vulnerable to a birthday attack": after $\sqrt{|\mathcal U|}$ queries to the resulting PRF, where $\mathcal U$ being the hash function range, a collision (i.e., two distinct inputs have the same hash value) happens with high probability. As a consequence, the resulting PRF is insecure against an attacker making this number of queries.

In this work we show how to go beyond the birthday attack barrier, by replacing the above simple hashing approach with a variant of cuckoo hashing --- a hashing paradigm typically used for resolving hash collisions in a table, by using two hash functions and two tables, and cleverly assigning each element into one of the two tables. We use this approach to obtain: (i) A domain extension method that requires just two calls to the original PRF, can withstand as many queries as the original domain size and has a distinguishing probability that is exponentially small in the non cryptographic work. (ii) A security-preserving reduction from non-adaptive to adaptive PRFs.

### Paper:

TR12-182 | 24th December 2012 21:51

#### Hardness Preserving Reductions via Cuckoo Hashing

TR12-182
Authors: Itay Berman, Iftach Haitner, Ilan Komargodski, Moni Naor
Publication: 24th December 2012 22:11
A common method for increasing the usability and uplifting the security of pseudorandom function families (PRFs) is to hash" the inputs into a smaller domain before applying the PRF. This approach, known as Levin's trick", is used to achieve PRF domain extension" (using a short, e.g., fixed, input length PRF to get a variable-length PRF), and more recently to transform non-adaptive PRFs to adaptive ones. Such reductions, however, are vulnerable to a birthday attack": after $\sqrt{|\mathcal U|}$ queries to the resulting PRF, where $\mathcal U$ being the hash function range, a collision (i.e., two distinct inputs have the same hash value) happens with high probability. As a consequence, the resulting PRF is insecure against an attacker making this number of queries.