Simple proof that GUID is not unique [closed]

Question

I'd like to prove that a GUID is not unique in a simple test program. I expected the following code to run for hours, but it's not working. How can I make it work?

BigInteger begin = new BigInteger((long)0);
BigInteger end = new BigInteger("340282366920938463463374607431768211456",10);  //2^128
for(begin; begin<end; begin++)
  Console.WriteLine(System.Guid.NewGuid().ToString());

I'm using C#.

Answer 1

Here's a nifty little extension method that you can use if you want to check guid uniqueness in many places in your code.

internal static class GuidExt
{
    public static bool IsUnique(this Guid guid)
    {
        while (guid != Guid.NewGuid())
        { }
        return false;
    }
}

To call it, simply call Guid.IsUnique whenever you generate a new guid...

Guid g = Guid.NewGuid();
if (!g.IsUnique())
{
    throw new GuidIsNotUniqueException();
}

...heck, I'd even recommend calling it twice to make sure it got it right in the first round.

Answer 2

For me.. the time it takes for a single core to generate a UUIDv1 guarantees it will be unique. Even in a multi core situation if the UUID generator only allows one UUID to be generated at a time for your specific resource (keep in mind that multiple resources can totally utilize the same UUIDs however unlikely since the resource inherently part of the address) then you will have more than enough UUIDs to last you until the timestamp burns out. At which point I really doubt you would care.

Answer 3

Have you tried begin = begin + new BigInteger((long)1) in place of begin++?

Answer 4

Since part of Guid generation is based on the current machine's time, my theory to get a duplicate Guid is:

1. Perform a clean installation of Windows
2. Create a startup script that resets the time to 2010-01-01 12:00:00 just as Windows boots up.
3. Just after the startup script, it triggers your application to generate a Guid.
4. Clone this Windows installation, so that you rule out any subtle differences that may occur in subsequent boot-ups.
5. Re-image the hard drive with this image and boot-up the machine a few times.

Answer 5

Aren't you all missing a major point?

I thought GUIDs were generated using two things which make the chances of them being Globally unique quite high. One is they are seeded with the MAC address of the machine that you are on and two they use the time that they were generated plus a random number.

So unless you run it on the actual machine and run all you guesses within the smallest amount of time that the machine uses to represent a time in the GUID you will never generate the same number no matter how many guesses you take using the system call.

I guess if you know the actual way a GUID is made would actually shorten the time to guess quite substantially.

Tony

Answer 6

Well if the running time of 83 billion years does not scare you, think that you will also need to store the generated GUIDs somewhere to check if you have a duplicate; storing 2^128 16-byte numbers would only require you to allocate 4951760157141521099596496896 terabytes of RAM upfront, so imagining you have a computer which could fit all that and that you somehow find a place to buy terabyte DIMMs at 10 grams each, combined they will weigh more than 8 Earth masses, so you can seriously shift it off the current orbit, before you even press "Run". Think twice!