Scott Hanselman's ComputerZen.com

Excellent idea using a long with InterlockedIncrement64. Guids are not only expensive to generate, but are suprisingly large when serialized. Guid was the low-hanging fruit for me.

We had a long discussion on the clean-up. We were already using a charting solution that generates alot of temp files, so we already had a mechanism that cleans up after that. We toyed with having the persister clean up the files, but ran into too many problems where we wanted to refresh. That was probably more due to us debugging than real users. We've also been very selective on which pages use this mechanism to keep disk usage low.

Hi,

a combination of not cleaning up the files immediately and using sequentially incremented numbers can open up a security hole.

if i request a page and get a 'ViewState ID' of 5 - what would the webapp return if i manually set the ID to 4 or 6, e.g. ??? The ViewState of another user?

bye
dominick

i just wanted to point it out (sorry, i'm a security guy blabla :)

when i do courses on web app security, i always teach the students to watch out for something that looks sequential, like

?userid=4

...

the idea is so reasonable that people might incorporate that and i wouldn't want to see a mail on securityfocus BugTraq

"predictable viewstate id vulnerability in xy" :)

hmm - i did some tests - i also tried RNGCryptoServiceProvider (even more expensive)

but do you think that guid gen is _really_ that expensive (compared to all the other stuff going on in ASP.NET) ?

i am currently thinking about another way to achieve decent randomness....but nothing 'secure' comes to my mind....

well - if you want to have good random numbers, there's a price to pay i guess

or - what about generating a large random number once, store that in a session and increment this number...?

or - deleting the old viewstates immediately....

bye
dominick

thats what i did:

Test 1 (GUID)

using System;


public class MyClass
{
[System.Runtime.InteropServices.DllImport("KERNEL32")]
private static extern bool QueryPerformanceCounter(ref long lpPerformanceCount);
[System.Runtime.InteropServices.DllImport("KERNEL32")]
private static extern bool QueryPerformanceFrequency(ref long lpFrequency);


public static void RunTest()
{
for (int i = 0; i < 100000; i++)
{
string id = Guid.NewGuid().ToString();
}
}

public static void Main()
{
long frequency = 0;
QueryPerformanceFrequency(ref frequency);

long startTime = 0;
QueryPerformanceCounter(ref startTime);

RunTest();

long endTime = 0;
QueryPerformanceCounter(ref endTime);

float elapsed = (float)(endTime - startTime) / frequency;

Console.WriteLine("GUID {0:0000.000}ms ", elapsed);
Console.ReadLine();
}
}

Test 2 (RNG)

using System;
using System.Security.Cryptography;

public class MyClass
{
[System.Runtime.InteropServices.DllImport("KERNEL32")]
private static extern bool QueryPerformanceCounter(ref long lpPerformanceCount);
[System.Runtime.InteropServices.DllImport("KERNEL32")]
private static extern bool QueryPerformanceFrequency(ref long lpFrequency);


public static void RunTest()
{
byte[] rng = new byte[16];
for (int i = 0; i < 100000; i++)
{
new RNGCryptoServiceProvider().GetBytes(rng);
string id = Convert.ToBase64String(rng);
}
}

public static void Main()
{
long frequency = 0;
QueryPerformanceFrequency(ref frequency);

long startTime = 0;
QueryPerformanceCounter(ref startTime);

RunTest();

long endTime = 0;
QueryPerformanceCounter(ref endTime);

float elapsed = (float)(endTime - startTime) / frequency;

Console.WriteLine("RNG {0:0000.000}ms ", elapsed);
Console.ReadLine();
}
}

I don't really buy the "GUIDs are expensive" thing either. I think that may have been the case a long time ago when they were based on MAC addresses and that jazz, but GUIDs nowadays are randomly generated anyway. (Here's an article that covers the GUID format pretty well - http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnnetcomp/html/PPCGuidGen.asp )