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How can I decrypt this encryption routine?

I have been on this program for a week now and I am so confused. I tried everything and anything
but I didn’t manage to make it work. I would really appreciate it if you could give me some help with my program.
thank you.

I have the following code and I need to write the Decryption routine for it.

OChars = Original char -> is the word which the user types in.           
EKey = The Encryption Key (one letter)              
Length = The length of characters that the user needs to put             
EChars = Stores the Encrypted characters in so the decryption routine can use it to decrypt it.

    void encrypt_chars (int length, char EKey){
char temp_char;                 // original/encrypted char temporary store

for (int i = 0; i < length; i++){
    temp_char = OChars [i];     // get next char from original string
    __asm {                     // call the encrypt subroutine
        push   eax              // save register values on stack to be safe
        push   ecx
        movsx  ecx,temp_char    // enregister the source character
        movsx  eax,EKey         // and encryption key.
        call   encryptB         // calls the encryption subroutingencrypt the character
        mov    temp_char,al     // only need lower byte of EAX to return encrypted char
        pop    ecx              // restore original register values from stack
        pop    eax
    }
    EChars [i] = temp_char;     // Store encrypted char in the encrypted chars array
   }
     return;

encryption routine ASM          

     __asm {

encryptB: push edx          //saves register value edx on stack
          push ecx          //saves register value ecx on stack
          not  eax          //
          add  eax,0x04     //add 4 to eax register
          mov  edx,eax      //move eax to edx
          pop  eax          //brings eax back to
          xor  eax,edx      //clear values to zero
          pop  edx          //bring edx back 
          rol  al,3         //three times.
          sub  al,0x02      //subtracts 2 from al
          ret 
}               
Here ends the encryption part            
The decryption routine will start as follows             

void decrypt_chars (int length, char EKey){
  char temp_char;                       

for (int i = 0; i < length; i++){
    temp_char = EChars [i];         
    __asm {                         


     }

      DChars [i] = temp_char;           
}
 return;

decryption routine ASM

__asm {


      }

Answer 1

Because it XOR’s with the f(Key) where f is some function, all computations have to be undone except the ones that make up f – they have to be done normally.

So, something like this: (not tested)

; eax = EKey, cl = char
decryptB:
  add ecx, 2   // undo sub 2
  ror cl, 3    // undo rol
  not eax      // actually do not
  add eax, 4   // actually do add 4
  xor eax, ecx // undo xor
  ret