Tiny Families of Functions with Random Properties: A Quality {Size Trade {o for Hashing

摘要: We present three explicit constructions of hash functions, which exhibit a trade-o between the size of the family (and hence the number of random bits needed to generate a member of the family), and the quality (or error parameter) of the pseudo-random property it achieves. Unlike previous constructions, most notably universal hashing, the size of our families is essentially independent of the size of the domain on which the functions operate. The rst construction is for the mixing property {mapping a proportional part of any subset of the domain to any other subset. The other two are for the extraction property {mapping any subset of the domain almost uniformly into a range smaller than it. The second and third constructions handle (respectively) the extreme situations when the range is very large or very small.