pwntools-level-2.6

Objective:

This level requires you to write an assembly code snippet to meet the following condition to bypass the check, and use the asm API from pwntools to compile the assembly code and complete the challenge. The condition for this challenge is:

Write a `for` loop to calculate a specific formula.(refer to the `trace` method of the `ASMChallenge` class)

In short: We have to write asm that do sum from 1 to rcx

Core Code:

def trace(self):
    self.start()
    return self.rax == sum(range(self.init_rcx + 1))

Solution Code:

from pwn import *

context.arch = "amd64"
context.os = "linux"
context.log_level = "debug"

binary = "/challenge/pwntools-tutorials-level2.6"
p = process(binary)

payload = asm("""
    mov rax, rcx
    inc rcx
    imul rax, rcx
    shr rax, 1
""")

p.sendafter(b"Please give me your assembly in bytes", payload)
print(p.recvall().decode())

I directly used sum of natural number formuale for it.

mov     rax, rcx        ; rax = rcx
inc     rax             ; rax = rcx + 1
imul    rax, rcx        ; rax = rcx * (rcx + 1)
shr     rax, 1          ; rax /= 2

Flag: