C代码索引计算解析及C#复现key4解密逻辑的技术问询

Let's break down that tricky pointer arithmetic in your C code and figure out how to replicate it cleanly in C#.

First: How Pointer Arithmetic Works in C #

In C, array indexing is just syntactic sugar for pointer operations. When you write inputData[i], it’s exactly equivalent to *(inputData + i) — regardless of whether i is positive or negative. The compiler only cares if the resulting memory address is valid (which it is here, since it points to stack memory your program owns).

Decoding decoder - (char*)data #

The expression decoder - (char*)data calculates the byte offset between the start of the decoder array and the start of the data array. Since decoder is a local stack variable and data is a function parameter, decoder lives at a lower memory address than data — hence the negative offset you saw (like -300 or -436). This offset is fixed for the duration of the function call, since both arrays’ memory positions don’t change.

Simplifying inputData[decoder - (char*)data] #

Let’s substitute what we know to demystify this line:

• inputData starts as (char*)data, and increments by 1 each loop iteration. After counter steps, it’s equivalent to (char*)data + counter.
• Let fixedOffset = decoder - (char*)data (the negative value you measured, e.g., -300).

So:

inputData[fixedOffset] = *(inputData + fixedOffset) = *( (char*)data + counter + fixedOffset )

But since fixedOffset = decoder - (char*)data, we can substitute that in:

*( (char*)data + counter + (decoder - (char*)data) ) = *( decoder + counter )

Which is exactly decoder[counter]!

The negative index is just a roundabout way to access the counter-th element of the decoder array. The C compiler doesn’t bat an eye at negative indexes here — it just computes the memory address and reads from it.

Replicating This in C# #

Now that we’ve simplified the logic, you can ditch the confusing negative indexes and replicate the behavior directly:

1. First, replicate the decoder array exactly as in the C code:

   // Match your original C decoderKey values
   char[] decoderKey = { -20, 100, -50, -70, 40, /* ... rest of your values ... */ };
   char[] decoder = new char[44];
   
   // Copy first 40 elements from decoderKey (matches memcpy in C)
   Array.Copy(decoderKey, decoder, 40);
   // Set decoder[40] to decoderKey[40] (matches the explicit assignment)
   decoder[40] = decoderKey[40];
   
   // Note: decoder[41], decoder[42], decoder[43] are uninitialized stack garbage in C
   // If your decryption relies on these values, you’ll need to capture their exact bytes from the original C program
   // and add them to the decoder array in C#.
   

2. Then compute key4 using the simplified logic:

   // Assume inputData is your 128-byte array in C#
   for (int counter = 0; counter < cbSize; counter++)
   {
       // This is exactly equivalent to the C code's key4 calculation
       char key4 = inputData[counter] ^ decoder[counter];
       
       // Apply the rest of the decryption logic as in the original code
       inputData[counter] = key4;
       if (counter % 4 == 0)
           inputData[counter] = (char)(key1 ^ key4);
       if (counter % 4 == 1)
           inputData[counter] ^= key3;
   }
   

Edge Case: When counter >= 44 #

In the original C code, decoder is only 44 elements long. When counter exceeds 43, decoder[counter] actually reads uninitialized stack memory beyond the array. If your decryption depends on these values, you’ll need to:

• Run the original C program, capture the exact bytes stored in the stack right after the decoder array, then extend your C# decoder array to include those bytes.
• If your cbSize is 44 or less, you don’t need to worry about this.

内容的提问来源于stack exchange，提问作者Malcolm McCaffery