Decoding and understanding assembly code

Question

So a little background. I am a beginner with c and assembly code, we have an \"bomb\" assignment (written in c)which calls methods that require certain passwords, but the code i

Answer 1

Here is a C equivalent of phase2:

int t[6];
read_six_numbers (t);
if ((t[0] != 0) || (t[1] != 1)) {
    explode_bomb();
}

for (int i = 2; i < 6; i++) {
        if (t[i] != t[i - 2] + t[i - 1]) {
            explode_bomb();
    }
}

So the password is 0, 1, 1, 2, 3, 5.

How did I do this ? By gradually replacing the assembly with C.

You'll note that the stack pointer (rsp) never changes. You can see the stack as an array t of 32 bits numbers. That is each time you move by 4 bytes you move to the next element. i.e. 0(%rsp), 4(%rsp), ... are equivalent to t[0], t[1], ...

I'll show you a possible gradual transformation of the bit you have trouble with:

                lea    0x8(%rsp),%rbx
                lea    0x18(%rsp),%rbp
<phase_2+42>:   mov    -0x8(%rbx),%eax
                add    -0x4(%rbx),%eax
                cmp    %eax,(%rbx)
                je     <phase_2+57>
                callq  explode_bomb
<phase_2+57>:   add    $0x4,%rbx
                cmp    %rbp,%rbx
                jne    phase_2+42
------------------------------------------------------
                    rbx = rsp + 8;
                    rbp = rsp + 24;
<phase_2+42>:       eax = [rbx - 8];
                    eax += [rbx - 4];
                    if (eax == [rbx]) goto <phase_2+57>;
                    explode_bomb();
<phase_2+57>:       rbx += 4;
                    if (rbx != rbp) goto phase_2+42;
------------------------------------------------------
rbx = rsp + 8;
rbp = rsp + 24;
do {
    eax = [rbx - 8] + [rbx - 4];
        if (eax != [rbx]) {
        explode_bomb();
    }
        rbx += 4;
} while (rbx != rbp);
------------------------------------------------------
rbx = 8;
do {
    eax = [rsp + rbx - 8] + [rsp + rbx - 4];
        if (eax != [rsp + rbx]) {
        explode_bomb();
    }
        rbx += 4;
} while (rbx < 24);
------------------------------------------------------
i = 2;
do {
    eax = t[i - 2] + t[i - 1];
        if (eax != t[i]) {
        explode_bomb();
    }
        i += 1;
} while (i < 6);
------------------------------------------------------
for (int i = 2; i < 6; i++) {
    if (t[i] != t[i - 2] + t[i - 1]) {
            explode_bomb();
        }
}

If you take the time to understand these transformations you'll be able to transform and understand any piece of assembly.