How is the cipher alphabet built from the keyword?

The unique letters of the keyword (in order of first appearance) are placed at the start of the cipher alphabet, then the remaining unused letters follow in normal alphabetical order. Each plain letter maps to the cipher letter in the matching position.

What happens to repeated letters in the keyword?

Only the first occurrence of each letter is used. For the keyword PEPPER the unique letters are P, E, R, so the alphabet starts PER and then continues with A, B, C and the rest.

Does it keep capitalisation?

Yes. Upper and lower case are preserved, and digits, spaces, and punctuation pass through unchanged.

How do I decrypt a message I enciphered?

Switch to decrypt and enter the same keyword. The tool inverts the mapping, so the original keyword always recovers the plain text exactly.

Is a keyword substitution cipher secure?

No. Monoalphabetic ciphers are easily broken by frequency analysis because each letter always maps to the same replacement. Use it for puzzles, teaching, and fun — not for protecting real secrets.

Is anything uploaded?

No — the cipher runs entirely in your browser and nothing is sent to a server.

Substitution Cipher Encoder & Decoder

A monoalphabetic substitution cipher replaces every letter with a fixed partner. This tool seeds that partner mapping from a keyword, making it easy to share a cipher with someone who only needs to remember a single word — handy for puzzles, escape rooms, classroom cryptography, and casual note-hiding.

How it works

The cipher alphabet is built in two steps:

Keyword letters first. Take the keyword’s unique letters in the order they first appear and place them at the front of a new 26-letter alphabet.
Fill the rest. Append every remaining letter of A–Z that the keyword did not use, in normal order.

Each plain letter A, B, C… then maps to the cipher letter in the same position. Encrypting applies this mapping; decrypting applies its inverse. Case is preserved, and anything that is not a letter passes through untouched.

Example

Keyword ZEBRA produces the cipher alphabet:

Plain:  A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Cipher: Z E B R A C D F G H I J K L M N O P Q S T U V W X Y

So HELLO enciphers to FAJJM. Decrypting FAJJM with the same keyword returns HELLO.

Note this is not real-world encryption — letter frequencies leak the mapping. Everything runs in your browser; nothing is uploaded.