Assembly code. Encryption to Decryption Routine

Question

Could someone take a look at this for me and help me work out a decryption that would reverse a string that has been input by the user. I don't mean just doing the reverse of this procedure.
```
  push edx 
  push ecx 
  not eax 
  add eax,0x04 
  mov edx,eax 
  pop eax 
  xor eax,edx 
  pop edx 
  rol al,1 
  rol al,1
  rol al,1 
  sub al,0x02 
  ret
```
*

The registers are: Inwards- ecx: encryption key. eax: character to be encrypted.

Outwards- eax: encrypted character

Thank you for taking the time to look.

Do you have any *specific* questions? "help me" isn't all that specific. — Drew McGowen, Jul 25 '13 at 14:41
possible duplicate of [How can I decrypt this encryption routine?](http://stackoverflow.com/questions/11677101/how-can-i-decrypt-this-encryption-routine) — harold, Jul 25 '13 at 15:10
It's exactly the same code. And after almost exactly a year. Is it from a university course or something? — harold, Jul 25 '13 at 15:13
Thanks for the feedback everyone. I'm actually doing a short course for my new job. Thanks @harold it is exactly the same as that problem a year ago. I have tried the solution from that in visual studio however the program keeps breaking and i get: **Unhandled exception at 0x772c15de** and **0xC0000005: Access violation.** In the same error message and it goes to **crtexe.c** Anyone any ideas on this? — user2619128, Jul 26 '13 at 11:53
The program is to encrypt a 6 character string and then decrypt. So i'm looking for the decryption routine in assembly code on this one — user2619128, Jul 26 '13 at 11:56
Here is the same error i'm getting but i just can't work out where i have an uninitialised pointer [http://stackoverflow.com/questions/5886103/why-could-i-get-an-unhandled-exception-access-violation-writing-in-c] — user2619128, Jul 26 '13 at 12:16
The solution from my answer there does not even use pointers to begin with, so that would be odd.. what function is at 0x772c15de? — harold, Jul 26 '13 at 12:26
Well in the debugger it would break there, and show the surrounding code etc.. — harold, Jul 26 '13 at 13:06
Ah right, of course. This is where the code breaks in the **crtexe.c** `/* * The /GS security cookie must be initialized before any exception * handling targetting the current image is registered. No function * using exception handling can be called in the current image until * after __security_init_cookie has been called. */ __security_init_cookie(); return __tmainCRTStartup(); }` — user2619128, Jul 26 '13 at 13:14
Well, I don't know. Maybe post a new question just for the separate problem — harold, Jul 26 '13 at 19:45

nio · Answer 1 · 2013-07-25T15:22:29.483

The algorithm is symmetric because i could decrypt back every character and key combination.

These two functions were tested by a loop for any error in decrypting the value back:

#include <iostream>
using namespace std;

unsigned char enc(unsigned char ch, unsigned char key)
{
    unsigned char tmp = key^(~ch+(unsigned char)0x04);
    return (( (tmp<<3) | (tmp>>5) ) & 0xff)-0x02;
}

unsigned char dec(unsigned char ch, unsigned char key)
{
    unsigned char tmp = (ch+0x02);
    tmp = ((tmp>>3) | (tmp<<5)) & 0xff;
    return ~((tmp^key )-(unsigned char)0x04);
}

int main()
{
    // single encryption test
    char c = 'A';
    char key = 'K';
    cout << "single test:" << (char)enc(c, key) << endl;

    bool problem = false;
    int k, ch;
    for(k=0;k<256;k++)
    {

        for(ch=0;ch<256;ch++)
        {
            if( enc( dec((unsigned char)ch, (unsigned char)k), k) != ch )
            {
                problem = true;
                cout << "error k=" << k << "c=" << ch
                     << "result=" <<  (unsigned int)enc( dec((unsigned char)ch, (unsigned char)key), (unsigned char)key) << endl;

            }
        }
    }
    if(problem) cout << "There was a problem." << endl;
    else cout << "There was no problem." << endl;
}

Assembly code. Encryption to Decryption Routine

1 Answers1