From http://jopinblog.wordpress.com/2009/02/04/a-shorter-friendlier-guiduuid-in-net/

a-shorter-friendlier-guiduuid-in-net

using System;
using System.Security.Cryptography;
 
public class UniqueIdGenerator
{
    private static readonly UniqueIdGenerator _instance = new UniqueIdGenerator();
    private static char[] _charMap = { // 0, 1, O, and I omitted intentionally giving 32 (2^5) symbols
        '2', '3', '4', '5', '6', '7', '8', '9', 
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'J', 'K', 'L', 'M', 'N', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'
    };
 
    public static UniqueIdGenerator GetInstance()
    {
        return _instance;
    }
 
    private RNGCryptoServiceProvider _provider = new RNGCryptoServiceProvider();
 
    private UniqueIdGenerator()
    {
    }
 
    public void GetNext(byte[] bytes)
    {
        _provider.GetBytes(bytes);
    }
 
    public string GetBase32UniqueId(int numDigits)
    {
        return GetBase32UniqueId(new byte[0], numDigits);
    }
     
    public string GetBase32UniqueId(byte[] basis, int numDigits)
    {
        int byteCount = 16;
        var randBytes = new byte[byteCount - basis.Length];
        GetNext(randBytes);
        var bytes = new byte[byteCount];
        Array.Copy(basis, 0, bytes, byteCount - basis.Length, basis.Length);
        Array.Copy(randBytes, 0, bytes, 0, randBytes.Length);
 
        ulong lo = (((ulong)BitConverter.ToUInt32(bytes, 8)) << 32) | BitConverter.ToUInt32(bytes, 12); // BitConverter.ToUInt64(bytes, 8);
        ulong hi = (((ulong)BitConverter.ToUInt32(bytes, 0)) << 32) | BitConverter.ToUInt32(bytes, 4);  // BitConverter.ToUInt64(bytes, 0);
        ulong mask = 0x1F;
 
        var chars = new char[26];
        int charIdx = 25;
 
        ulong work = lo;
        for (int i = 0; i < 26; i++)
        {
            if (i == 12)
            {
                work = ((hi & 0x01) << 4) & lo;
            }
            else if (i == 13)
            {
                work = hi >> 1;
            }
            byte digit = (byte)(work & mask);
            chars[charIdx] = _charMap[digit];
            charIdx--;
            work = work >> 5;
        }
 
        var ret = new string(chars, 26 - numDigits, numDigits);
        return ret;
    }
}