Question

The problem

I have a byte[] that is converted to a hex string, and then that string is parsed like this BigInteger.Parse(thatString,NumberSyles.Hexnumber).

This seems wasteful since BigInteger is able to accept a byte[], as long as the two’s complement is accounted for.

---

An working (inefficient) example

According to MSDN the most significant bit of the last byte should be zero in order for the following hex number be a positive one. The following is an example of a hex number that has this issue:

byte[] ripeHashNetwork = GetByteHash();
foreach (var item in ripeHashNetwork)
{
   Console.Write(item + "," );
} 

// Output:  
//      0,1,9,102,119,96,6,149,61,85,103,67,158,94,57,248,106,13,39,59,238,214,25,103,246

// Convert to Hex string using this http://stackoverflow.com/a/624379/328397
// Output: 
//       00010966776006953D5567439E5E39F86A0D273BEED61967F6` 

Okay, let’s pass that string into the static method of BigInteger:

 BigInteger bi2 = BigInt.Parse(thatString,NumberSyles.Hexnumber);

// Output bi2.ToString() ==
//                {25420294593250030202636073700053352635053786165627414518}

Now that I have a baseline of data, and known conversions that work, I want to make it better/faster/etc.

---

A not working (efficient) example

Now my goal is to round-trip a byte[] into BigInt and make the result look like 25420294593250030202636073700053352635053786165627414518. Let’s get started:

So according to MSDN I need a zero in my last byte to avoid my number from being seen as a two’s compliment. I’ll add the zero and print it out to be sure:

foreach (var item in ripeHashNetwork)
{
   Console.Write(item + "," );
} 

// Output:                            
//    0,1,9,102,119,96,6,149,61,85,103,67,158,94,57,248,106,13,39,59,238,214,25,103,246,0 

Okay, let’s pass that byte[] into the constructor of BigInteger:

 BigInteger bi2 = new BigInteger(ripeHashNetwork);

// Output bi2.ToString() ==
//                {1546695054495833846267861247985902403343958296074401935327488}

What I skipped over is the sample of what bigInt does to my byte array if I don’t add the trailing zero. What happens is that I get a negative number which is wrong. I’ll post that if you want.

So what am I doing wrong?

Answer 1

When you are going via the hex string, the first byte of your array is becoming the most significant byte of the resulting BigInteger.

When you are adding a trailing zero, the last bye of your array is the most significant.

I’m not sure which case is right for you, but that’s why you’re getting different answers.