Tutorial :Use LINQ to XOR Bytes Together



Question:

How can I use LINQ to to XOR bytes together in an array? I'm taking the output of an MD5 hash and would like to XOR every four bytes together so that I can get a 32 bit int out of it. I could easily do this with a loop, but I thought it was an interesting problem for LINQ.

public static byte[] CompressBytes(byte[] data, int length)  {      byte[] buffer = new byte[length];      for (int i = 0; i < data.Length; i++)      {          for (int j = 0; j < length; j++)          {              if (i * length + j >= data.Length)                  break;              buffer[j] ^= data[i * length + j];          }      }      return buffer;  }  

Slightly off topic, but is this even a good idea? If I need a int, would I be better off with a different hash function, or should I just take the first 4 bytes of the MD5 because XORing them all wouldn't help any? Comments are welcome.


Solution:1

You can use the IEnumerable.Aggregate function (not actually LINQ, but most people refer to the LINQ-related extension methods as LINQ) to perform a custom aggregate. For example, you could compute the total XOR of a list of bytes like this:

var xor = list.Aggregate((acc, val) => (byte)(acc ^ val));  

You can create a virtually unreadable chain of extension method calls to do what you're after:

int integer = BitConverter.ToInt32(Enumerable.Range(0, 3).                Select(i => data.Skip(i * 4).Take(4).                    Aggregate((acc, val) => (byte)(acc ^ val))).ToArray(), 0)  


Solution:2

To address the "off topic" part, I'd suggest just lopping off the first 32 bits of the MD5 hash. Or consider a simpler non-crypto hash such as CRC32.

Like other cryptographic hashes, MD5 is supposed to appear as random as possible, so XOR'ing other bytes won't really make a difference, IMO.


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