bitwise-and

Problem

This problem comes from pwn.college: Assembly Crash Course ⤴.

In this level, you will be working with registers. You will be asked to modify or read from registers.

We will set some values in memory dynamically before each run. On each run, the values will change. This means you will need to perform some type of formulaic operation with registers. We will tell you which registers are set beforehand and where you should put the result. In most cases, it’s rax.

In this level, you will be working with bit logic and operations. This will involve heavy use of directly interacting with bits stored in a register or memory location. You will also likely need to make use of the logic instructions in x86: and, or, not, xor.

Bitwise logic in assembly is yet another interesting concept! x86 allows you to perform logic operations bit by bit on registers.

For the sake of this example, say registers only store 8 bits.

The values in rax and rbx are:

• rax = 10101010
• rbx = 00110011

If we were perform a bitwise AND of rax and rbx using the and rax, rbx instruction, the result would be calculated by ANDing each bit pair one by one, hence why it’s called bitwise logic.

So from left right:

• 1 AND 0 = 0
• 0 AND 0 = 0
• 1 AND 1 = 1
• 0 AND 1 = 0
• …

Finally, we combine the results gether get:

• rax = 00100010

Here are some truth tables for reference:

AND

A | B | X
---+---+---
0 | 0 | 0
0 | 1 | 0
1 | 0 | 0
1 | 1 | 1

OR

A | B | X
---+---+---
0 | 0 | 0
0 | 1 | 1
1 | 0 | 1
1 | 1 | 1

XOR

A | B | X
---+---+---
0 | 0 | 0
0 | 1 | 1
1 | 0 | 1
1 | 1 | 0

Without using the following instructions: mov, xchg

Please perform the following:

Set rax to the value of (rdi AND rsi)

---

NOTE: rax will have all bits set 1. If it didn’t, this level would be trickier!

Solution

.intel_syntax noprefix
.global _start

_start:
  and rdi, rsi
  and rax, rdi

Bonus

What if rax didn’t have all bits set 1?

.intel_syntax noprefix
.global _start

_start:
  xor rax, rax
  not rax
  and rdi, rsi
  and rax, rdi

The solution just has two extra lines. The first line xor rax, rax ensures that rax is zeroed out. Then we perform not rax which sets all the bits 1, and proceed as normal.