Four-square cipher decode
Four-square cipher decode, classical cipher, a puzzle not a protection, computed on your device. The X filler the encoder appended for an odd final letter cannot be told from a real X, and all spacing is gone: expect uppercase letters only.
Four-square cipher decode reverses the pairing: each ciphertext digraph is read across the two keyed squares back onto the plain unkeyed square. Both key1 and key2 have to match the ones used to encode, or the letters come back wrong with no error raised. The X filler inserted for an odd trailing letter cannot be told apart from a genuine X.
Questions
What keys do I need?
The exact key1 and key2 used to encode; a mismatch returns wrong letters silently rather than an error.
Can I tell a real trailing X from a filler X?
No, the X the encoder appended for an odd final letter is indistinguishable from a genuine one.
What if my ciphertext has an odd number of letters?
The tool refuses, since four-square only encodes complete letter pairs.
What happens to J?
It never appears; every J position decodes as I.
Can I use this from the command line?
Yes, it runs the same on the page, the CLI and MCP.