Polybius Square Cipher

How it is calculated

The 5×5 grid holds the alphabet row by row with I and J sharing one cell: row 1 is A B C D E, row 2 F G H I/J K, row 3 L M N O P, row 4 Q R S T U, row 5 V W X Y Z. To encode, each letter is replaced by its row and column digits (both from 1 to 5), and the pairs are separated by spaces. J is treated as I. Anything that is not an A–Z letter (spaces, digits, punctuation, accented letters) is skipped. To decode, every two consecutive digits are read as row and column; pairs containing a digit outside 1–5 are ignored. Decoding returns capital letters, so J cannot be recovered.

Common mistakes

• Expecting the letter J to survive; it is encoded as I, so decoding gives I.
• Using the cipher for real security; it is a simple substitution and is broken at a glance with letter frequencies.
• Typing digits or accented characters in encode mode and expecting them in the output; they are dropped.
• Decoding with the mode left on encode, or entering digit pairs that include 0, 6, 7, 8 or 9.
• Reading the pair as column then row; the first digit is the row.