Increment a byte[]

Question

I have a byte[] testKey = new byte[8];

This obviously starts with all bytes as 0. I want to go through all the bytes and increment by 1 on each iteration of the loop so eventually I go through all possibilities of the byte array. I also want to do this as FAST as possible. Yes I am trying to write a brute forcer.

Update I got the unsafe method working, and it is the quickest. However, by my calculations, it is going to take 76,000,000 years to loop through doing DES encryption on each key using the .Net DESCryptoServiceProvider. 10,000 encryptions takes 1.3 seconds. Thanks for all the awesome answers to the most useless question ever!

Answer 1

btw; it takes a lot of processing to check 2^64 options...

Well, the fastest way may be to just use an Int64 (aka long) or UInt64 (ulong), and use ++? Do you really need the byte[]?

As a hacky alternative, how about:

Array.Clear(data, 0, data.Length);
while (true)
{
  // use data here
  if (++data[7] == 0) if (++data[6] == 0)
    if (++data[5] == 0) if (++data[4] == 0)
      if (++data[3] == 0) if (++data[2] == 0)
        if (++data[1] == 0) if (++data[0] == 0) break;
}

The only other approach I can think of would be to use unsafe code to talk to an array as though it is an int64... messy.

unsafe static void Test() {
    byte[] data = new byte[8];
    fixed (byte* first = data) {
        ulong* value = (ulong*)first;
        do {
            // use data here
            *value = *value + 1;
        } while (*value != 0);
    }
}

Answer 2

This is how you increase the value in the array:

int index = testKey.Length - 1;
while (index >= 0) {
   if (testKey[index] < 255) {
      testKey[index]++;
      break;
   } else {
      testKey[index--] = 0;
   }
}

When index is -1 after this code, you have iterated all combinations.

This will be slightly faster than using BitConverter, as it doesn't create a new array for each iteration.

Edit:
A small performance test showed that this is about 1400 times faster than using BitConverter...

Answer 3

What a great question! Here's a way to do it without unsafe code:

public struct LongAndBytes
{
    [FieldOffset(0)]
    public ulong UlongValue;
    [FieldOffset(0)]
    public byte Byte0;
    [FieldOffset(1)]
    public byte Byte1;
    [FieldOffset(2)]
    public byte Byte2;
    [FieldOffset(3)]
    public byte Byte3;
    [FieldOffset(4)]
    public byte Byte4;
    [FieldOffset(5)]
    public byte Byte5;
    [FieldOffset(6)]
    public byte Byte6;
    [FieldOffset(7)]
    public byte Byte7;

    public byte[] ToArray()
    {
        return new byte[8] {Byte0, Byte1, Byte2, Byte3, Byte4, Byte5, Byte6, Byte7};
    }
}


// ...

    LongAndBytes lab = new LongAndBytes();

    lab.UlongValue = 0;
    do {
        // stuff
        lab.UlongValue++;
    } while (lab.ULongValue != 0);

Each of the members Byte0...Byte7 overlap the ulong and share its members. It's not an array - I tried dinking around with that and had unsatisfactory results. I bet someone knows the magic declaration to make that happen. I can do that for a P/Invoke, but not for use in .NET as an array is an object.

Answer 4

byte[8] is essentially an ulong but if you really need it to be byte[8] you can use

byte[] bytes = new byte[8];
ulong i = 0;
bytes = BitConverter.GetBytes(i);

Answer 5

You can extract the bytes using bit operators:

byte[] bytes = new byte[8];
for (ulong u = 0; u < ulong.MaxValue; u++)
{
    bytes[0] = (byte)(u & 0xff);
    bytes[1] = (byte)((u >> 8) & 0xff);
    bytes[2] = (byte)((u >> 16) & 0xff);
    bytes[3] = (byte)((u >> 24) & 0xff);
    bytes[4] = (byte)((u >> 32) & 0xff);
    bytes[5] = (byte)((u >> 40) & 0xff);
    bytes[6] = (byte)((u >> 48) & 0xff);
    bytes[7] = (byte)((u >> 56) & 0xff);
    // do your stuff...
}

This is less 'hackish', since it operates on an unsigned 64-bit integer first and then extract the bytes. However beware CPU endianess.

Answer 6

for (UInt64 i = 0; i < UInt64.MaxValue; i++)
{
    byte[] data = BitConverter.GetBytes(i)
}

Answer 7

byte[] array = new byte[8];
int[] shifts = new int[] { 0, 8, 16, 24, 32, 40, 48, 56 };    
for (long index = long.MinValue; index <= long.MaxValue; index++)
{
    for (int i = 0; i < 8; i++)
    {
        array[i] = (byte)((index >> shifts[i]) & 0xff);
    }
    // test array
}

Answer 8

for (int i = 0; i < bytes.Length & 0 == ++bytes[i]; i++);

Should be as fast as the unsafe method and allows arrays of any size.