c#,loops,byte,bytearray , How to incrementally iterate through all possible values of a byte array of size n.

Question:

Tag: c#,loops,byte,bytearray

For my question n=16, but a generic answer would be appreciated too.

So I have a byte array:

``````byte[] key;
``````

My problem is that I want to iterate through all possible values of each element in this array, combined. I know this will take ages, and I'm not looking to actually complete this loop, just to make a loop which will at least attempt this.

So e.g.:

First iteration:

``````//Math.Pow(2,128) is the max no. of iterations right?
byte[] key;
for(int i = 0; i < Math.Pow(2,128); i++)
{
key = new byte[16] {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
}
``````

Second iteration:

``````//Math.Pow(2,128) is the max no. of iterations right?
byte[] key;
for(int i = 0; i < Math.Pow(2,128); i++)
{
key = new byte[16] {1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
}
``````

Third iteration:

``````//Math.Pow(2,128) is the max no. of iterations right?
byte[] key;
for(int i = 0; i < Math.Pow(2,128); i++)
{
key = new byte[16] {2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
}
``````

Final iteration:

``````//Math.Pow(2,128) is the max no. of iterations right?
byte[] key;
for(int i = 0; i < Math.Pow(2,128); i++)
{
key = new byte[16] {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255};
}
``````

Obviously I have just hardcoded the array above. I need a way of doing this in a proper way. Again, I know there are many different combinations. All I need is a way to start iterating through all possible values. How can I do this in my loop?

I.e. what should I replace the body of my loop with, in order to iterate through all possible values of a byte array of size 16.

What I have tried:

In the body of the loop I have tried the following:

``````key = new byte[16] { (byte)i, (byte)i, (byte)i, (byte)i, (byte)i, (byte)i, (byte)i, (byte)i, (byte)i, (byte)i, (byte)i, (byte)i, (byte)i, (byte)i, (byte)i, (byte)i };
``````

Obviously wrong, will only test a small subset of possible values. Will just try i= 0,...,255 and then start over for when i=256 --> (byte)i = 0.

I suspect I need some more nesting. Possibly up to 16 nested loops, which sounds insane and probably wrong? I can't get my head around this problem, any help would be much appreciated!

Purpose: The purpose of this question is to demonstrate how inefficient brute force cryptanalysis is in practice. The rest of my program works, I'm just stuck in this loop.

In case you don't realize: 16 bytes is the size of a Guid or the size of a standard cryptographic key size. There are so many combinations that you cannot enumerate even a fraction. Maybe you can enumerate the last 8 bytes if you parallelize across 1000 machines and wait a year.

You could do that easily by running a for loop from 0 to `ulong.MaxValue`. I'm submitting this as an answer because this very simple idea allows you to start enumerating and essentially never come to a point where you finish.

``````for (ulong i = 0; i < ulong.MaxValue; i++) {
var bytes = new [] {
0, 0, 0, 0, 0, 0, 0, 0
, (byte)(i >> (7 * 8))
, (byte)(i >> (6 * 8))
, (byte)(i >> (5 * 8))
//...
, (byte)(i >> (0 * 8)) };
}
``````

Or, just use 16 nested for loops. I don't think that's insane at all because it is so simple that it's clearly correct.

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