Create a cryptographically secure random GUID in .NET

Question

I want to create a cryptographically secure GUID (v4) in .NET.

.NET's Guid.NewGuid() function is not cryptographically secure, but .NET does provide the System.Security.Cryptography.RNGCryptoServiceProvider class.

I would like to be able to pass a random number function as a delegate to Guid.NewGuid (or even pass some class that provides a generator interface) but it doesn't look as though that is possible with the default implementation.

Can I create a cryptographically secure GUID by using System.GUID and System.Security.Cryptography.RNGCryptoServiceProvider together?

Answer 1

Yes you can, Guid allows you to create a Guid using a byte array, and RNGCryptoServiceProvider can generate a random byte array, so you can use the output to feed a new Guid:

public Guid CreateCryptographicallySecureGuid() 
{
    using (var provider = new RNGCryptoServiceProvider()) 
    {
        var bytes = new byte[16];
        provider.GetBytes(bytes);

        return new Guid(bytes);
    }
}

Answer 2

If anyone is interested here is the above sample code adjusted for .NET Core 1.0 (DNX)

public Guid CreateCryptographicallySecureGuid()
{
    using (var provider = System.Security.Cryptography.RandomNumberGenerator.Create())
    {
        var bytes = new byte[16];
        provider.GetBytes(bytes);

        return new Guid(bytes);
    }
}

Answer 3

https://tools.ietf.org/html/rfc4122 says there are a few bits that should be fixed in order to indicate that this GUID is a version-4 (random) one. Here is the code altered to set/unset these bits.

public Guid CreateCryptographicallySecureGuid()
{
    using (var provider = new RNGCryptoServiceProvider())
    {
        var bytes = new byte[16];
        provider.GetBytes(bytes);
        bytes[8] = (byte)(bytes[8] & 0xBF | 0x80);
        bytes[7] = (byte)(bytes[7] & 0x4F | 0x40);
        return new Guid(bytes);
    }
}

Answer 4

If you are using at least c# 7.2 and netcoreapp2.1 (or System.Memory), this is the fastest/most efficient approach.

public static Guid CreateCryptographicallySecureGuid()
{
    Span<byte> bytes = stackalloc byte[16];
    RandomNumberGenerator.Fill(bytes);
    return new Guid(bytes);
}

I created a benchmark comparing this to the accepted answer. I modified it to use a static implementation of RandomNumberGenerator since GetBytes() is thread safe. (although the only guarantee I see is that RNGCryptoServiceProvider has a thread safe implementation...it's possible other implementations do not)

[MemoryDiagnoser]
public class Test
{
    private static readonly RandomNumberGenerator _rng = RandomNumberGenerator.Create();

    [Benchmark]
    public void Heap()
    {
        var bytes = new byte[16];
        _rng.GetBytes(bytes);
        new Guid(bytes);
    }

    [Benchmark]
    public void Fill()
    {
        Span<byte> bytes = stackalloc byte[16];
        RandomNumberGenerator.Fill(bytes);
        new Guid(bytes);
    }
}

| Method |     Mean |     Error |    StdDev | Gen 0/1k Op | Gen 1/1k Op | Gen 2/1k Op | Allocated Memory/Op |
|------- |---------:|----------:|----------:|------------:|------------:|------------:|--------------------:|
|   Heap | 129.4 ns | 0.3074 ns | 0.2725 ns |      0.0093 |           - |           - |                40 B |
|   Fill | 116.5 ns | 0.3440 ns | 0.2872 ns |           - |           - |           - |                   - |