How is the cipher alphabet built from the keyword?

The keyword is uppercased and stripped of non-letters, then each letter is added once in order of first appearance. After that, every remaining letter of the standard alphabet that has not yet been used is appended in A-to-Z order. The result is a 26-letter cipher alphabet.

What does the keyword MONARCHY produce?

MONARCHY first contributes M, O, N, A, R, C, H, Y. The remaining unused letters B, D, E, F, G, I, J, K, L, P, Q, S, T, U, V, W, X, Z follow in order. So plaintext A maps to M, B maps to O, and so on down the keyed alphabet.

Does it keep case and punctuation?

Yes. Uppercase and lowercase letters are substituted while preserving their case, and any non-letter such as a space, digit, or punctuation mark is passed through unchanged.

How do I decrypt a message?

Use the same keyword and choose the decode mode. Decoding simply reverses the substitution, mapping each ciphertext letter back to its position in the plain alphabet.

Is a keyword cipher secure?

No. It is a simple monoalphabetic substitution, so it preserves letter frequencies and falls quickly to frequency analysis on any message of reasonable length. Use it for puzzles and education only.

Keyword Substitution Cipher

A keyword substitution cipher is a classic monoalphabetic cipher in which the ciphertext alphabet is reordered using a memorable keyword. The keyword seeds the start of the alphabet, and the letters it does not contain fill the rest in normal order, producing a fixed one-to-one substitution.

How it works

The cipher alphabet is constructed in two stages, then mapped against the plain alphabet:

keyword  = MONARCHY
deduped  = M O N A R C H Y
remainder= B D E F G I J K L P Q S T U V W X Z
cipher   = MONARCHYBDEFGIJKLPQSTUVWXZ
plain    = ABCDEFGHIJKLMNOPQRSTUVWXYZ

To encode, find each plaintext letter’s position in the plain alphabet and emit the cipher-alphabet letter at the same position; to decode, do the reverse lookup.

Example and notes

With the keyword MONARCHY, the word FLEE encodes to IGAA because F→I, L→G, E→A, E→A. Decoding IGAA with the same keyword returns FLEE. Because the mapping is fixed for the whole message, the cipher keeps the frequency fingerprint of the language — E is still the most common letter under its substitute — which is exactly what makes it easy to break with frequency analysis on longer texts.