Playfair cipher encode
Playfair cipher encode, classical cipher, a puzzle not a protection, computed on your device. Not reversible to the exact input: only letters survive (J folds into I), a doubled letter or a lone trailing letter gets an X filler, and output is uppercase pairs. "HELLO WORLD" comes back as "HELXLOWORLDX".
Playfair cipher encode pairs your letters on a 5 by 5 square keyed by key, default PLAYFAIR, swapping each pair for its row or column corners. A repeated letter within a pair, or a lone leftover letter, is padded with an X to keep every pair distinct. J folds into I first, sharing one square position.
Questions
How does the 5 by 5 square get built?
From key, default PLAYFAIR: its letters go into the square first, deduplicated, with J merged into I, then the rest of the alphabet fills the remaining cells.
What happens to a doubled letter?
An X is inserted between the two identical letters so every pair stays distinct, before the pairing runs.
What happens to J?
It is folded into I before the square is built, so the two letters share one cell and one output.
Is Playfair secure?
No, it is a classical puzzle cipher with well-known analytic attacks; use AES-GCM encrypt for anything that needs real protection.
How do I decode it?
Use Playfair cipher decode with the same key.
Can I use this from the command line?
Yes, the same code runs on the page, the CLI and MCP.