Substitution · Cipher · Challenge

The Monoalphabetic Substitution Cipher

Each letter of the alphabet is replaced by a different letter according to a fixed mapping. Unlike Caesar's cipher (which has only 25 keys), a general substitution cipher has 26! ≈ 4 × 10^26 possible keys, far too many to brute-force. So why is it still considered weak? Because it preserves letter frequencies. The structure of English shines right through.

Frequency Analysis: The Cryptanalyst's Lever

In English, E appears about 12.7% of the time, followed by T, A, O, I, N. The chart on the left shows how often each letter appears in your ciphertext (gold bars) compared to standard English (pale bars). The cipher letter with the tallest gold bar is most likely E. The next is probably T or A.

Strategy

Start with the highest-frequency cipher letters and try mapping them to E, T, A, O. Look for short common words: a single-letter word is almost always A or I. A three-letter word that appears repeatedly is probably THE. Two-letter words ending in your guessed E are likely HE, BE, WE, or ME.

How to Play

Click any input under a ciphertext letter and type the plaintext letter you think it represents. Every instance of that ciphertext letter in the puzzle will fill in at once. If you map two different cipher letters to the same plaintext letter (a conflict, since the cipher is one-to-one), the inputs turn red. Keep iterating until the message reads cleanly.

Scoring

Easy puzzles are worth 50 points, Medium 100, Hard 150, ZZZ Lore 125. Each hint reveals one cipher → plain mapping for −10 points (minimum 10 points). Submitting an incorrect solution costs no points but resets your streak.

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