Type your text and a keyword, and the tool encrypts it in letter pairs using a 5×5 Playfair square built from that keyword (I and J merged).
A keyword fills a 5×5 grid (I and J share a cell). The message is split into letter pairs; each pair is transformed by the row/column rules — same row shifts right, same column shifts down, otherwise the pair forms a rectangle and swaps columns.
If a pair contains two identical letters (like the LL in HELLO) an X separates them, and a trailing X pads an odd-length message. This is why decrypted text may contain extra X letters where padding was added.
No. J is merged into I before encryption, so decrypted text shows I wherever the original had J. This is standard for the classic Playfair square.