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I do not quite understand how universal hashing works. For example, when I insert an item into my hash table, I have to choose a random function from my universal family of hash functions. Now I want to retrieve said item. How will my hash table know which function it has to use to calculate the hash?
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[h(x) = h(y)] ≤ 1/M. We also say that a set H of hash functions is a universal hash function family if the procedure “choose h ∈ H at random” is universal. (Here we are identifying the set of functions with the uniform distribution over the set.)
2. Universal hash families. Family of hash functions H is 2-universal if for any x≠y, Pr[h(x)=h(y)] ≤ 1/n for random h∈H.
Some common hashing algorithms include MD5, SHA-1, SHA-2, NTLM, and LANMAN. MD5: This is the fifth version of the Message Digest algorithm. MD5 creates 128-bit outputs. MD5 was a very commonly used hashing algorithm.
Take into account the following hash algorithms – CRC-32, MD5, and SHA-1. These are common hash algorithms with varying levels of collision risk.
Because you'll use the same hash function for all the items in the table.
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Which hash function is used is random only in the sense that they are not predictable by an adversary but the choice is a function of the key. There is a nice write up at http://www.cs.ucsb.edu/~suri/cs130a/Hashing.txt The matrix method is easier to understand than other methods...
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