Playfair cipher decode
Playfair cipher decode, classical cipher, a puzzle not a protection, computed on your device. The X filler the encoder inserted for doubled or trailing letters cannot be told from a real X, and all spacing is gone: expect uppercase letters only, occasionally with an extra X.
Playfair cipher decode reverses the pairing, reading digraphs off the 5 by 5 square built from key and swapping each back to its opposite corners. An odd letter count after cleanup is refused, since Playfair only handles whole pairs. Any X the encoder padded in for a doubled or trailing letter looks identical to a real one.
Questions
What if my ciphertext has an odd number of letters?
It is refused, since Playfair only ever encodes complete letter pairs.
Can I tell a real X from a filler X?
No. The X the encoder inserted for a doubled or trailing letter looks exactly like a genuine X in the decoded output.
What if my key does not match the encoder?
The letters come back wrong with no error, since a valid but mismatched key still produces valid-looking pairs.
What happens to J?
It never appears in the output; every J position decodes as I, since the square merges the two.
Can I use this from the command line?
Yes, it runs the same on the page, the CLI and MCP.