Invented by Felix Delastelle around 1901, the Bifid cipher fractionates plaintext letters into two digits each using a 5×5 Polybius square, then rearranges the digits before recombining them into letters. The key parameters are the square (usually built from a keyword) and the period — the block size over which the transposition happens. Because J is merged with I, the alphabet fits into 25 cells.
Rows and columns are numbered 1–5. Each letter maps to a (row, column) pair. You can edit the keyword above to rearrange the square.
For the current plaintext: each letter is replaced by its row/column digits, rows are listed then columns, and the digits are re-paired to form the ciphertext letters.