picoCTF - The Numbers Challenge Write-Up

Challenge Information

Challenge Name: The Numbers
Category: Cryptography
Difficulty: Easy
Source: https://sukepaja18.medium.com/picoctf-the-numbers-b402fbcc43f6

Challenge Description

The challenge provides a mysterious hint: "The numbers… what do they mean?"

A PNG file is provided containing a series of numbers that need to be decoded.

Understanding Letter Number Code

Letter Number Code is a simple substitution cipher where each letter is replaced by its position in the alphabet:

• A = 1
• B = 2
• C = 3
• ... and so on
• Z = 26

This is also known as A1Z26 encoding or Alphanumeric Code.

Key Characteristics:

• Numbers directly correspond to letter positions (1-26)
• Spaces or separators between numbers indicate letter boundaries
• Case typically doesn't matter (can be uppercase or lowercase)
• Non-alphabetic characters are generally not encoded

Solution

The cipher text from the image is a sequence of numbers that represent letters in alphabetical order.

Method 1: Manual Decryption

Decode each number to its corresponding letter:

20 8 5 = T H E
14 21 13 2 5 18 19 = N U M B E R S
13 1 19 15 14 = M A S O N

Method 2: Python Script

def decode_letter_number(cipher_text):
    """
    Decode Letter Number Code (A1Z26)
    Input: numbers separated by spaces
    Output: decoded message
    """
    numbers = cipher_text.split()
    result = ""

    for num in numbers:
        num = int(num)
        if 1 <= num <= 26:
            result += chr(ord('A') + num - 1)

    return result

# Example
cipher = "20 8 5 14 21 13 2 5 18 19 13 1 19 15 14"
plaintext = decode_letter_number(cipher)
print(plaintext)  # Output: THENUMBERSMASON

Method 3: Online Tools

Use CyberChef:

1. Navigate to CyberChef
2. Paste the number sequence
3. Search for and apply the "From A1Z26" recipe
4. The decoded message will appear instantly

Decrypted Message

After decoding the numbers:

PICOCTFTHENUMBERSMASON

Breaking it down: - PICOCTF (the flag format) - THENUMBERSMASON (the decoded message)

Flag

PICOCTF{THENUMBERSMASON}

Key Takeaways

1. Simple substitution ciphers - Letter Number Code is one of the most basic encryption methods
2. Recognize patterns - Numbers in sequences often indicate Letter Number Code
3. Alphabet position mapping - Understanding the A-Z to 1-26 mapping is essential
4. Challenge hints - The title "The Numbers" is a direct hint to what you're dealing with
5. Cryptanalysis - Even without knowing the cipher type beforehand, frequency analysis or trial-and-error with common cipher types can reveal the solution

Related Challenges

• ROT13 ciphers (like MOD 26 and 13)
• Caesar Cipher
• Other simple substitution ciphers