Converter

Platform: spbctf | Category: pwn | Type: Challenge | Difficulty: Easy | OS: NA | Author: XESXOR | Date: 2026-03-11 | Status: Solved Techniques: ascii_art_parsing, automated_interaction, number_format_conversion, struct_packing

Summary

Task: answer ~112 number format conversion questions (hex, octal, big/little endian 64-bit) presented with ASCII art format names. Solution: automate parsing of format names and numbers, then respond with hex/oct string representations or struct-packed binary for endian formats.

Converter — SPB CTF Pwn Season

Description

"Конвертируйте числа в разные форматы"
(Convert numbers to different formats)

Server: nc 109.233.56.90 11573
Binary: None provided (black-box)
Points: 10

Analysis

The server sends numbers and asks to convert them to different formats displayed as ASCII art: Hex, Octal, Big Endian 64, Little Endian 64.

Format Requirements:

• Hex/Octal: Send string representation (e.g., "0xff", "0o377")
• Big Endian 64: Send raw packed bytes using struct.pack('>Q', num)
• Little Endian 64: Send raw packed bytes using struct.pack('<Q', num) ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Must answer approximately 112 questions correctly.

Solution

 python read-only • copy allowed Copy
from pwn import *
import struct

io = remote('109.233.56.90', 11573)

def parse_ascii_art(data):
    # Pattern matching for ASCII art format names
    if b'HEX' in data:
        return 'hex'
    elif b'OCT' in data:
        return 'octal'
    elif b'BIG' in data:
        return 'big'
    elif b'LITTLE' in data:
        return 'little'
    return None

for _ in range(112):
    data = io.recvuntil(b'Answer: ')
    
    # Extract number from output
    num = int(io.recvline().strip())
    
    # Determine format from ASCII art
    fmt = parse_ascii_art(data)
    
    if fmt == 'hex':
        io.sendline(hex(num).encode())
    elif fmt == 'octal':
        io.sendline(oct(num).encode())
    elif fmt == 'big':
        io.send(struct.pack('>Q', num))
    elif fmt == 'little':
        io.send(struct.pack('<Q', num))
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io.interactive()

‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍