Question

I try to parse encrypted &RQ history, but i really can’t understand asm code. It is embedded to Delphi function.

Can somebody help me to understand this?

procedure decritt(var s:string; key:integer);
 asm
 mov ecx, key
 mov dl, cl
 shr ecx, 20
 mov dh, cl

 mov esi, s
 mov esi, [esi]
 or  esi, esi    // nil string
 jz  @OUT

// now esi points to the first character of the string

 mov ah, 10111000b

 mov ecx, length(s)
 or  ecx, ecx
 jz  @OUT
@IN:
 mov al, [esi]
 xor al, ah
 rol al, 3
 xor al, dh
 sub al, dl

 mov [esi], al
 inc esi
 ror ah, 3
 dec ecx
 jnz @IN
@OUT:
 end; // decritt

Thanks.

Answer 1

This c++ code would be equivalent:

void encrypt(std::string s, int key) {

    const char dl = key;
    const char dh = key >> 20;
    char ah = 0xB8;

    for (int i=0; i<s.length(); ++i) {
        char c = s[i];
        c ^= ah;
        c = (c<<3)|(c>>5); // rotate a char three bits left
        c ^= dh;
        c -= dl;
        s[i] = c;
        ah = (ah>>3)|(ah<<5);
    }
}