Новогоднее письмо (New Year Letter)

Platform: hackerlab | Category: Crypto | Type: Challenge | Difficulty: Medium | OS: NA | Author: XESXOR | Date: 2026-01-02 | Status: Solved Techniques: binascii module for UU decoding, Python code deobfuscation, UUencode recognition from file structure

Summary

Task: Decode an obfuscated Python script and encoded flag.txt file. Solution: Recognized UUencode format from M-prefixed lines and 45-byte chunks, decoded using binascii.a2b_uu().

Новогоднее письмо (New Year Letter) — hackerlab

Description

Кажется, что на Новый Год кто-то прислал Вам подарок! Проверьте почту!

Files: task.zip (contains task.py and flag.txt)

Analysis

1. Obfuscated Python Script (task.py)

 python read-only • copy allowed Copy
I=isinstance
D=None
import sys,os,binascii as J
def A(in_file,out_file,name=D,mode=D,*,backtick=False):
    # ... obfuscated code ...
    while len(G)>0:C.write(J.some_func(G,backtick=backtick));G=A.read(45)
    # ...
if __name__=='__main__':B='input.txt';C='flag.txt';E='tmp.txt';F=438;A(B,C,name=E,mode=F,backtick=True)

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

Key observations:

• Uses binascii module (imported as J)
• Parameter backtick=True — specific to UUencode (b2a_uu function)
• Reads 45 bytes at a time — standard UUencode chunk size

2. Encoded Data (flag.txt)

MT*$@T)W0OM"RT8O0O"#0D]"^T+30OM"\+"#1@]"RT+#0MM"PT+70O-&+T+4@
MT+C0L]&`T+[0NM"X(-"_T+O0OM&)T+#0M-"ZT+@@(M"8T+/1@-&+(-":T+[0
...

UUencode indicators:

• Lines start with 'M' — marker for 45-byte encoded lines
• Characters in ASCII range 32-96 (space to backtick)
• Standard UUencode format structure ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Solution

 python read-only • copy allowed Copy
#!/usr/bin/env python3
"""
Новогоднее письмо (New Year Letter) - UUencode decoder
"""

import binascii

def decode_uuencode(filename):
    """Decode UUencoded file using binascii.a2b_uu()"""
    with open(filename, 'r') as f:
        lines = f.readlines()
    
    decoded = b''
    for line in lines:
        line = line.rstrip('\n')
        if line:
            try:
                decoded += binascii.a2b_uu(line)
            except binascii.Error as e:
                print(f"Warning: {e} on line: {line[:20]}...")
                continue
    
    return decoded
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

# Decode the file
result = decode_uuencode('flag.txt')
print(result.decode('utf-8'))

Result

Decoded message — a New Year greeting:

С Новым Годом, уважаемые игроки площадки "Игры Кодебай"!
Мы хотим поздравить Вас с Наступающим Новым Годом, а также подарить флаг к этому заданию - CODEBY{REDACTED}!
...

Theory: UUencode

UUencode (Unix-to-Unix encoding) — a method for encoding binary data into ASCII text:

1. Line format: First character indicates the length of decoded data

   • 'M' = chr(32 + 45) = 45 bytes (maximum)
   • Last line is usually shorter ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

2. Encoding: 3 bytes → 4 ASCII characters (32-95 or with backtick)

3. Python binascii:

   • binascii.b2a_uu(data, backtick=True) — encoding
   • binascii.a2b_uu(line) — decoding

Notes

The flag uu_3n_c0d3 — leetspeak for "uu_encode", confirming the UUencode technique. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍