What's the best way to create a short hash, si

Question

I'm currently using MD5 hashes but I would like to find something that will create a shorter hash that uses just [a-z][A-Z][0-9]. It only needs to be around 5-10 characters long.

Is there something out there that already does this?

Update:

I like the CRC32 hash. Is there a clean way of calculating it in .NET?

Update2:

I'm using the CRC32 function from the link Joe provided. How can I convert the uInt into the characters defined above?

Answer 1

You can use CRC32, it is 8 bytes long and similar to MD5. Unique values will be supported by adding timestamp to actual value.

So its will look like http://foo.bar/abcdefg12.

Answer 2

I dont think URL shortening services use hashes, I think they just have a running alphanumerical string that is increased with every new URL and stored in a database. If you really need to use a hash function have a look at this link: some hash functions Also, a bit offtopic but depending on what you are working on this might be interesting: Coding Horror article

Answer 3

Is your goal to create a URL shortener or to create a hash function?

If your goal is to create a URL shortener, then you don't need a hash function. In that case, you just want to pre generate a sequence of cryptographically secure random numbers, and then assign each url to be encoded a unique number from the sequence.

You can do this using code like:

using System.Security.Cryptography;

const int numberOfNumbersNeeded = 100;
const int numberOfBytesNeeded = 8;
var randomGen = RandomNumberGenerator.Create();
for (int i = 0; i < numberOfNumbersNeeded; ++i)
{
     var bytes = new Byte[numberOfBytesNeeded];
     randomGen.GetBytes(bytes);
}

Using the cryptographic number generator will make it very difficult for people to predict the strings you generate, which I assume is important to you.

You can then convert the 8 byte random number into a string using the chars in your alphabet. This is basically a change of base calculation (from base 256 to base 62).

Answer 4

If you're looking for a library that generates tiny unique hashes from inters, I can highly recommend http://hashids.org/net/. I use it in many projects and it works fantastically. You can also specify your own character set for custom hashes.

Answer 5

There's a wonderful but ancient program called btoa which converts binary to ASCII using upper- and lower-case letters, digits, and two additional characters. There's also the MIME base64 encoding; most Linux systems probably have a program called base64 or base64encode. Either one would give you a short, readable string from a 32-bit CRC.

Answer 6

If you don't care about cryptographic strength, any of the CRC functions will do.

Wikipedia lists a bunch of different hash functions, including length of output. Converting their output to [a-z][A-Z][0-9] is trivial.