What is the Playfair cipher?

Playfair encrypts letters two at a time using a 5 by 5 grid built from a keyword, with I and J sharing a square. Pairs in the same row shift right, pairs in the same column shift down, and other pairs swap corners of the rectangle they form. It was used by the British military well into the 20th century.

Before encrypting, an X goes between doubled letters and at the end if there is an odd letter left over. When you decode, those X letters are still in the result, so skip them as you read.

Example

BMODZBXDNABEKUDMUIXMMOUVIF with the keyword PLAYFAIR EXAMPLE decodes to HIDE THE GOLD IN THE TREXE STUMP, which reads as "hide the gold in the tree stump."