Based picoCTF 2019 Solution

Description

1. Step 1Understand the challenge format
   The server sends you encoded strings one at a time in binary, octal, or hex format and asks you to convert them to the word they represent. You must respond correctly within a short time limit.
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   The three bases you need to handle:

   • Binary (base 2): digits 0 and 1. E.g., 01110000 = 112 = 'p'
   • Octal (base 8): digits 0-7. E.g., 160 = 112 = 'p'
   • Hexadecimal (base 16): digits 0-9 and a-f. E.g., 70 = 112 = 'p'

   In Python: int('01110000', 2) for binary, int('160', 8) for octal, int('70', 16) for hex. Then chr() to get the character.

2. Step 2Write a pwntools script to automate responses
   The time limit is tight, so write a Python script using pwntools to automate the conversion. Read each encoded string, detect the base from context, convert it, and send the answer.
   bash

   pip3 install pwntools

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   python

   python3 << 'EOF'
   from pwn import *
   
   r = remote('<HOST>', <PORT_FROM_INSTANCE>)
   
   def decode_word(encoded, base):
       parts = encoded.strip().split()
       chars = []
       for p in parts:
           chars.append(chr(int(p, base)))
       return ''.join(chars)
   
   for _ in range(3):
       line = r.recvuntil(b'?').decode()
       print(line)
       if 'binary' in line.lower():
           encoded = r.recvline().decode().strip()
           answer = decode_word(encoded, 2)
       elif 'octal' in line.lower():
           encoded = r.recvline().decode().strip()
           answer = decode_word(encoded, 8)
       else:
           encoded = r.recvline().decode().strip()
           answer = decode_word(encoded, 16)
       r.sendline(answer.encode())
   
   r.interactive()
   EOF

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   pwntools is a Python library designed for CTF challenges involving network connections and binary exploitation. The remote() function creates a TCP connection. recvuntil() reads until a specific byte sequence appears, and sendline() sends data followed by a newline.

3. Step 3Receive the flag
   After correctly converting all encoded strings, the server outputs the flag.
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   Number base conversion is fundamental to computer science. Computers store all data in binary; hexadecimal is a compact human-readable notation for binary (each hex digit represents 4 bits); octal (each octal digit represents 3 bits) was common in early Unix systems.

Alternate Solution

Flag

Automate binary/octal/hex to ASCII conversion using pwntools to respond within the server's time limit.