The Substituted Truth
🔍 Details
| Attribute | Details |
|---|---|
| Challenge Name | The Substituted Truth |
| Category | Cryptography |
| Difficulty | 🟢 Easy |
| Flag | F4H{pmTrixy27Y**********} |
This is the only flag in the competition, which is case insensitive.
📝 Description
A mysterious text has been uncovered, but the letters seem out of place.
The only clue left behind is a single word: KEYWORDABCFGHIJLMNPQSTUVXZ
📥 Download encoded_message.txt
🧩 Hints
- Common letters appear more often than others—try a frequency attack. An online tool might help!
💡 Solution
This challenge is based on a monoalphabetic substitution cipher using a custom alphabet as the key:
KEYWORDABCFGHIJLMNPQSTUVXZ
This type of cipher replaces each letter in the plaintext with a corresponding letter from the key alphabet. The order of the alphabet has been rearranged, and certain letters (like E or K) might appear in non-standard positions.
Steps to solve:
- Analyze the frequency of letters in the ciphertext.
- Use the provided key alphabet to map each encrypted character back to its original.
- Because the flag is case-insensitive, you don’t need to worry about uppercase/lowercase matching.
- Tools like Boxentriq Monoalphabetic Solver (opens in a new tab) or dCode Monoalphabetic Cipher (opens in a new tab) can help automate the decryption.
📚 Insights
Monoalphabetic substitution ciphers use a fixed one-to-one mapping between the plaintext and ciphertext alphabets. While simple in design, they can be quite effective unless the key is known or a frequency analysis is performed.