Compute the most common byte

most_common_byte.py

from pwn import *

import warnings
warnings.filterwarnings('ignore')

context.arch = 'amd64'
context.log_level = 'critical'

# Start the challenge process
r = process('/challenge/run')

# Send the assembled code to the challenge
r.send(asm('''

.intel_syntax noprefix

push rbp
mov rbp, rsp
sub rsp, 520

mov r8, rdi
mov r9, rsi

lea rdi, [rbp - 512]
mov rcx, 256
xor eax, eax
rep stosw

xor rcx, rcx
process_loop:
    cmp rcx, r9
    jae find_max
    movzx eax, byte ptr [r8 + rcx]
    movzx edx, al
    lea rdi, [rbp - 512]
    inc word ptr [rdi + rdx * 2]
    inc rcx
    jmp process_loop

find_max:
xor rcx, rcx
xor rax, rax
xor rdx, rdx
find_max_loop:
    cmp rcx, 256
    je end_find_max
    lea rdi, [rbp - 512]
    movzx r10, word ptr [rdi + rcx * 2]
    cmp r10w, ax
    jbe skip_update
    mov ax, r10w
    mov dx, cx
skip_update:
    inc rcx
    jmp find_max_loop
end_find_max:
movzx eax, dx

mov rsp, rbp
pop rbp
ret
'''))

# Receive and print the result from the challenge
print(r.recvline_contains('pwn.college').decode())