Question

I download a file and his checksum (generated by cksum Unix command).

So, I want, in my C# app test if the Checksum is in adequation with the app I downloaded.

I checked at the Unix man page of chsum:

  The cksum command calculates and prints to standard output a checksum
  for each named file, the number of octets in the file and the
  filename.

  cksum uses a portable algorithm based on a 32-bit Cyclic Redundancy
  Check.  This algorithm finds a broader spectrum of errors than the
  16-bit algorithms used by sum (see sum(1)).  The CRC is the sum of the
  following expressions, where x is each byte of the file.

       x^32 + x^26 + x^23 +x^22 + x^16 + x^12 + x^11 + x^10 + x^8 + x^7
       + x^5 + x^4 + x^2 + x^1 + x^0

  The results of the calculation are truncated to a 32-bit value.  The
  number of bytes in the file is also printed.

So i wrote a simple program that does the sum :

byte[] arr = File.ReadAllBytes(@"MyApp").ToArray();

int cksum = 0;

foreach (byte x in arr)
{
    cksum += (x ^ 32 + x ^ 26 + x ^ 23 + x ^ 22 + x ^ 16 + x ^ 12 + x ^ 11 + x ^ 10 + x ^ 8 + x ^ 7 + x ^ 5 + x ^ 4 + x ^ 2 + x ^ 1 + x ^ 0);
}

But checksums aren’t the same, how can I fix this?

Thanks

---

EDIT

1) The modified algorithm is:

uint cksum = 0;

foreach (byte b in arr)
{
    var x = (uint)b;

    cksum += (IntPow(x, 32)
        + IntPow(x, 26) + IntPow(x, 23) + IntPow(x, 22)
        + IntPow(x, 16) + IntPow(x, 12) + IntPow(x, 11) + IntPow(x, 10)
        + IntPow(x, 8) + IntPow(x, 7) + IntPow(x, 5) + IntPow(x, 4) + IntPow(x, 2) + IntPow(x, 1) + IntPow(x, 0));
}

2) I used the class Crc32 : HashAlgorithm

Given an Unix file where the Crc32 is : 2774111254

• 1) Gives me : 4243613712
• 2) Gives me : 3143134679 (with a seed of 0)

What I’m doing wrong !?

Answer 1

In C# the ^ symbol is the exclusive-or operator. You want the function Math.Pow.

This gives the piower of two floating point numbers, alteratives are suggested at How do you do *integer* exponentiation in C#?

So, your code would look something like:

cksum += Math.pow(x,32) + Math.pow(x,26)

Be aware also of the last statement:

The results of the calculation are truncated to a 32-bit value. The
number of bytes in the file is also printed.

Is this signed (int) or unsigned (uint)

you could of course use the following:
http://www.codeproject.com/Articles/35134/How-to-calculate-CRC-in-C