Thread: Signed unsigned ints?

ffffffc2 is unsigned... It sounds like underflow, but I am not seeing any operation that would cause an underflow.

Code:

cyphertext += plaintext.at(i) ^ (S.at((S.at(j) + S.at(k)) % 256));

That line looks dubiously like the culprit. But I am compilerless at the moment, so yeah...

I'm not familiar with ARC, but what's wrong with what you're getting? (As master points out, those are unsigned numbers.)

His result is supposed to be a single byte value... That is what is wrong. However, I am going to say as a pragmatist, if the code can successfully decode its output, what the hell is the problem? You aren't trying to compress data, you are trying to obscure it.

Originally Posted by Blackroot

Signed values are numbers without the ability to contain negative values. Unsigned values have this ability.

You have this backwards.

Considering that the string class is really a typedef'd basic_string<char>, every time you call the at() method it's probably returning a signed char, and somewhere in the Encrypt function a sign extension happens.

I did a quick test by typedef'ing basic_string<unsigned char> ustring, and then converting all your strings to ustrings. Unfortunately this gave me issues with getline(), so I just switched to C-style input.

Does this work for you?

Code:

#include <cstdio>
#include <string>
#include <vector>

using namespace std;

typedef basic_string<unsigned char> ustring;

ustring Encrypt(const ustring &plaintext, const ustring &key);

int main()
{
   unsigned char buf[1024];
   ustring cyphertext, plaintext, key;
   
   fputs("Enter string to be encrypted/decrypted: ", stdout);
   fgets((char*)buf, sizeof(buf), stdin);
   plaintext.assign(buf);
   
   fputs("Enter encryption key: ", stdout);
   fgets((char*)buf, sizeof(buf), stdin);
   key.assign(buf);
   
   cyphertext = Encrypt(plaintext, key);
   
   for(unsigned int i = 0; i < cyphertext.length(); ++i)
      printf("%x ", cyphertext.at(i));

   return 0;
}

ustring Encrypt(const ustring &plaintext, const ustring &key)
{
   unsigned int i(0);
   unsigned int j(0);
   unsigned int k(0);
   vector<unsigned int> S(256, 0);
   ustring cyphertext;
   
   //BEGIN KEY-SCHEDULING ALGORITHM
   for(i = 0; i < 256; ++i)
   {
      S.at(i) = i;   //initialize s-box S[0] = 0...S[255] = 255 
   }
   
   for(i = 0; i < 256; ++i)
   {
      //transpose s-box values based on key
      j = (j + S.at(i) + key.at(i % key.length())) % 256;
      swap(S.at(i), S.at(j));
   }
   //END KEY-SCHEDULING ALGORITHM

   //BEGIN PSEUDO-RANDOM GENORATION ALGORITHM
   for(i = 0, j = 0; i < plaintext.length(); ++i)
   {
      //transpose s-box values based on s-box values
      j = (j + 1) % 256;
      k = (k + S.at(j)) % 256;
      swap(S.at(j), S.at(k));
      //encryption happens here
      cyphertext += plaintext.at(i) ^ (S.at((S.at(j) + S.at(k)) % 256));
   }
   //END PSEUDO-RANDOM GENORATION ALGORITHM
   
   return cyphertext;
}

Code:

>a.exe
Enter string to be encrypted/decrypted: Hello.
Enter encryption key: 1234
39 2a 9c 93 ce 20 ed

Originally Posted by master5001

ffffffc2 is unsigned... It sounds like underflow, but I am not seeing any operation that would cause an underflow.

Code:

cyphertext += plaintext.at(i) ^ (S.at((S.at(j) + S.at(k)) % 256));

That line looks dubiously like the culprit. But I am compilerless at the moment, so yeah...

Well, the math is being done correctly. That is, the xor operation is providing the correct results. So, for example, when it's time to encrypt the '.', you have:

Code:

cyphertext += "." ^ 236;
            =  46 ^ 236
            = 194

That (unsigned) 194 is being stored in cyphertext as the signed int (char? wchar?) -62. This is what I don't know how to fix. I've tried substituting basic_string<unsigned char> for string, but that just breaks everything (like getline(), which I guess I don't know how to implement with basic_string<>).

I could rewrite the entire thing with c-style string, I suppose, and I imagine it would work without a hitch, but that defeats the purpose.

Originally Posted by Mostly Harmless

I read your post (which came in while I was writing mine) and had to look over my shoulder to make sure you weren't standing behind me reading what I was typing. Creepy.

Thanks for getting it to work with the c-style input, but now I'm on a mission to find a way to get it to work with C++ strings. Ugh.

Alternatively, you could just cast the cyphertext to an unsigned char before casting to the int...

Code:

...
	for(unsigned int i = 0; i < text_length; ++i)
		cout << hex << static_cast<unsigned int>(static_cast<unsigned char>(cyphertext.at(i))) << " ";
...

Unfortunately, this gives different results than using unsigned char strings, which is why I think there's sign extension going on somewhere. Maybe there's another reason, but I'm tired and can't think too hard about it right now...

Originally Posted by Mostly Harmless

Thanks for getting it to work with the c-style input, but now I'm on a mission to find a way to get it to work with C++ strings. Ugh.

Perhaps you could compile char as unsigned if for some reason what arpsmack posted is insufficient.

Oh MAN. I've been super lame recently. Your function works fine. The reason I was getting different output is because fgets() doesn't remove the trailing newline.

All you have to do is convert the char to an unsigned char, and then to an int before printing it. This just avoids sign extension.