public class MessageIdentity
{
    public Guid NameSpace;
    private readonly byte[] _namespaceBytes;

    public MessageIdentity(Guid guidNameSpace)
    {
        NameSpace = guidNameSpace;
        _namespaceBytes = guidNameSpace.ToByteArray();
        SwapByteOrder(_namespaceBytes);
    }

    public Guid Create(byte[] input)
    {
        byte[] hash;
        using (var algorithm = SHA1.Create())
        {
            algorithm.TransformBlock(_namespaceBytes, 0, _namespaceBytes.Length, null, 0);
            algorithm.TransformFinalBlock(input, 0, input.Length);
            hash = algorithm.Hash;
        }

        var newGuid = new byte[16];
        Array.Copy(hash, 0, newGuid, 0, 16);

        newGuid[6] = (byte)((newGuid[6] & 0x0F) | (5 << 4));
        newGuid[8] = (byte)((newGuid[8] & 0x3F) | 0x80);

        SwapByteOrder(newGuid);
        return new Guid(newGuid);
    }

    private static void SwapByteOrder(byte[] guid)
    {
        SwapBytes(guid, 0, 3);
        SwapBytes(guid, 1, 2);
        SwapBytes(guid, 4, 5);
        SwapBytes(guid, 6, 7);
    }

    private static void SwapBytes(byte[] guid, int left, int right)
    {
        var temp = guid[left];
        guid[left] = guid[right];
        guid[right] = temp;
    }
}