Подозреваемый (Suspect)

Platform: CUSTOM | Category: Crypto | Type: Challenge | Difficulty: Easy | OS: N/A | **Author: XESXOR Techniques: Integer to bytes conversion, Big-endian byte order decoding, Hex string to bytes conversion

Summary

Walkthrough of the CUSTOM Crypto Подозреваемый (Suspect). Translation: "The suspect was silent for a long time, then suddenly said a number... We've never encountered this before. Help us understand what he's trying to tell us."

Description

"Подозреваемый долго молчал, а потом внезапно назвал число... Мы никогда с таким не встречались. Помоги понять нам, что он пытается до нас донести."

Translation: "The suspect was silent for a long time, then suddenly said a number... We've never encountered this before. Help us understand what he's trying to tell us."

Analysis

The file sus.txt contains one large number:

6446982264000635652948556502623658209996049063529575549

Key observations:

1. Number is too small for RSA — doesn't look like a cryptographic modulus
2. Description mentions "number as message" — the suspect "said a number"
3. File name "sus" — suspicious ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Hypothesis: Text is encoded as a large integer (text-to-int encoding). Each ASCII character is represented as a byte, and the entire byte sequence is interpreted as one large number.

Solution

Method 1: int.to_bytes()

 python read-only • copy allowed Copy
#!/usr/bin/env python3

n = 6446982264000635652948556502623658209996049063529575549

# Calculate required number of bytes
byte_length = (n.bit_length() + 7) // 8

# Convert number to bytes (big-endian)
result = n.to_bytes(byte_length, 'big')

print(result)
# b'CODEBY{REDACTED}'

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

Method 2: Via hex string

 python read-only • copy allowed Copy
#!/usr/bin/env python3

n = 6446982264000635652948556502623658209996049063529575549

# Convert to hex, remove '0x' prefix
hex_str = hex(n)[2:]

# If odd number of characters, add leading zero
if len(hex_str) % 2:
    hex_str = '0' + hex_str

# Convert hex to bytes, then to string
result = bytes.fromhex(hex_str).decode('utf-8')

print(result)
# 'CODEBY{REDACTED}'

Verification of reverse conversion

 python read-only • copy allowed Copy
# How text is encoded into a number:
text = "CODEBY{REDACTED}"
encoded = int.from_bytes(text.encode(), 'big')
print(encoded)
# 6446982264000635652948556502623658209996049063529575549

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

Mathematics

Text-to-int encoding works as follows:

• String "ABC" = bytes [0x41, 0x42, 0x43]
• Big-endian interpretation: 0x41 * 256² + 0x42 * 256 + 0x43 = 4276803

Reverse conversion:

• Number is divided by 256, remainders give bytes from right to left
• Or use int.to_bytes() in Python

Alternative Names for This Technique

• Text-to-int encoding — encoding text as a number
• Integer message encoding — integer encoding of a message
• Big integer text representation — representing text as a large number
• Bytes-to-int conversion — converting bytes to an integer ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Key Takeaways / Lessons

• Mastering Integer to bytes conversion, Big-endian byte order decoding, Hex string to bytes conversion was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

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Crafted by XESXOR Research • 2025-12-30 • CUSTOM • Подозреваемый (Suspect) • Difficulty: Easy