What is the Bifid cipher?

Bifid was invented by Félix Delastelle around 1901. Each letter is found in a 5 by 5 Polybius square and written as a row and a column number. All the row numbers are written out first, then all the column numbers, and the combined list is read back in pairs as new letters. That mixing spreads each letter's information over two letters of the result.

Enter the keyword used to build the square, or type out all 25 letters of the square if the puzzle gives it to you. Some versions work in blocks of a set size, called the period. If the result is almost right, try a period like 5 or 7.

Example

UAEOLWRINS with the square BGWKZQPNDSIOAXEFCLUMTHYVR decodes to FLEEATONCE.