What is a good Hash function? I saw a lot of hash function and applications in my data structures courses in college, but I mostly got that it's pretty hard to make a good hash function. As a thumble rule to avoid collisions my professor said that:

function Hash(key)
  return key mod PrimeNumber

(mod is the % operator in C and similar languages)

with the primenumber to be the size of the hash table. I get that is a somewhat good function to avoid collisions and a fast one, but how can I make a better one? Is there better hash functions for string keys against numeric keys?

+1  A: 

I'd say that the main rule of thumb is not to roll your own. Try to use something that has been thoroughly tested, e.g., SHA-1 or something along those lines.

+3  A: 

There is a bunch of information around hash functions in wikipedia and the bottom of this article has algorithms implemented in various languages.


I made this post because I think it's important to know how something works before using it everywhere. I know that I will never write a better hash function than those SHA, but I wanted to see some hash functions that are simple and better than the one I wrote so I can understend what causes collision so that might improve my algorithms that uses hash.

+10  A: 

There's no such thing as a “good hash function” for universal hashes (ed. yes, I know there's such a thing as “universal hashing” but that's not what I meant). Depending on the context different criteria determine the quality of a hash. Two people already mentioned SHA. This is a cryptographic hash and it isn't at all good for hash tables which you probably mean.

Hash tables have very different requirements. But still, finding a good hash function universally is hard because different data types expose different information that can be hashed. As a rule of thumb it is good to consider all information a type holds equally. This is not always easy or even possible. For reasons of statistics (and hence collision), it is also important to generate a good spread over the problem space, i.e. all possible objects. This means that when hashing numbers between 100 and 1050 it's no good to let the most significant digit play a big part in the hash because for ~ 90% of the objects, this digit will be 0. It's far more important to let the last three digits determine the hash.

Similarly, when hashing strings it's important to consider all characters – except when it's known in advance that the first three characters of all strings will be the same; considering these then is a waste.

This is actually one of the cases where I advise to read what Knuth has to say in The Art of Computer Programming, vol. 3. Another good read is Julienne Walker's The Art of Hashing.

Konrad Rudolph
Konrad, you're surely correct from a theoretical perspective, but have you ever tried using the Paul Hsieh hash function I mentioned in my comment? It's really quite good against a lot of different kind of data!
Chris Harris
+1  A: 

A good hash function has the following properties:

  1. Given a hash of a message it is computationally infeasible for an attacker to find another message such that their hashes are identical.

  2. Given a pair of message, m' and m, it is computationally infeasible to find two such that that h(m) = h(m')

The two cases are not the same. In the first case, there is a pre-existing hash that you're trying to find a collision for. In the second case, you're trying to find any two messages that collide. The second task is significantly easier due to the birthday "paradox."

Where performance is not that great an issue, you should always use a secure hash function. There are very clever attacks that can be performed by forcing collisions in a hash. If you use something strong from the outset, you'll secure yourself against these.

Don't use MD5 or SHA-1 in new designs. Most cryptographers, me included, would consider them broken. The principle source of weakness in both of these designs is that the second property, which I outlined above, does not hold for these constructions. If an attacker can generate two messages, m and m', that both hash to the same value they can use these messages against you. SHA-1 and MD5 also suffer from message extension attacks, which can fatally weaken your application if you're not careful.

A more modern hash such as Whirpool is a better choice. It does not suffer from these message extension attacks and uses the same mathematics as AES uses to prove security against a variety of attacks.

Hope that helps!

Simon Johnson
+1  A: 

Hoffmann, could you please specify the application domain in your question? The current formulation leads the answerers astray. If, as your question implies, you're interested in hashes for hash tables, then this has nothing to do with cryptography.

If you're interested in cryptographic applications then your question is misleading.

Konrad Rudolph
+4  A: 

There are two major purposes of hashing functions:

  • to disperse data points uniformly into n bits.
  • to securely identify the input data.

It's impossible to recommend a hash without knowing what you're using it for.

If you're just making a hash table in a program, then you don't need to worry about how reversible or hackable the algorithm is... SHA-1 or AES is completely unnecessary for this, you'd be better off using a variation of FNV. FNV achieves better dispersion (and thus fewer collisions) than a simple prime mod like you mentioned, and it's more adaptable to varying input sizes.

If you're using the hashes to hide and authenticate public information (such as hashing a password, or a document), then you should use one of the major hashing algorithms vetted by public scrutiny. The Hash Function Lounge is a good place to start.

Myrddin Emrys
+1  A: 

This is an example of a good one and also an example of why you would never want to write one. It is a Fowler / Noll / Vo (FNV) Hash which is equal parts computer science genius and pure voodoo:

unsigned fnv_hash ( void *key, int len ) {
    unsigned char *p = key;
    unsigned h = 2166136261;
    int i;

    for ( i = 0; i < len; i++ )
      h = ( h * 16777619 ) ^ p[i];

   return h;

2166136261 and 16777619 really? WTF?

You may want to look at this site for some information on why these values are chosen:
+3  A: 

For doing "normal" hash table lookups on basically any kind of data - this one by Paul Hsieh is the best I've ever used.

If you care about cryptographically secure or anything else more advanced, then YMMV. If you just want a kick ass general purpose hash function for a hash table lookup, then this is what you're looking for.

Chris Harris
Thanks for the informative link! I know *a few* analyses by Bob Jenkins and others which point to quite good universally acceptable hash functions but I haven't come across this one yet.
Konrad Rudolph