What's the best way to create a short hash, similar to what tiny Url does?

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 1:

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

Update 2:

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

Answer 1

.NET string object has a GetHashCode() function. It returns an integer. Convert it into a hex and then to an 8 characters long string.

Like so:

string hashCode = String.Format("{0:X}", sourceString.GetHashCode()); 

More on that: http://msdn.microsoft.com/en-us/library/system.string.gethashcode.aspx

UPDATE: Added the remarks from the link above to this answer:

The behavior of GetHashCode is dependent on its implementation, which might change from one version of the common language runtime to another. A reason why this might happen is to improve the performance of GetHashCode.

If two string objects are equal, the GetHashCode method returns identical values. However, there is not a unique hash code value for each unique string value. Different strings can return the same hash code.

Notes to Callers

The value returned by GetHashCode is platform-dependent. It differs on the 32-bit and 64-bit versions of the .NET Framework.

Answer 2

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).