How is the cipher alphabet built from the keyword?

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

Does it keep capitalisation?

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

Is anything uploaded?

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

Substitution Cipher Encoder & Decoder

Keyword-seeded monoalphabetic substitution, all in your browser.

Substitution cipher encoder and decoder

A monoalphabetic substitution cipher maps every letter to a fixed replacement. This tool builds the cipher alphabet from a keyword: its unique letters come first, then the rest of the alphabet in order. The cipher alphabet is shown so you can see exactly how each letter is mapped.

Choose encrypt or decrypt, enter a keyword, and type your text — everything runs in your browser.