Bifid cipher encode
Bifid cipher encode, classical cipher, a puzzle not a protection, computed on your device. Not reversible to the exact input: the classic 5x5 Polybius square has no J, so J is folded into I, and everything that is not a letter, including spaces, digits and punctuation, is dropped. Output is uppercase. "The fox jumps, 42 times!" comes back as "THEFOXIUMPSTIMES".
Runs on your device. The file is never uploaded.
Bifid cipher encode reads each letter as a row and a column on a 5 by 5 square. It writes all the rows of a block, then all its columns, and re-pairs those numbers into fresh letters. Blocks hold 5 letters unless you change the period, and both sides must share the keyword and the period.
Questions
What does Bifid do?
It puts each letter on a 5 by 5 Polybius square, splits the coordinates into two rows, then reads them back together to make new letters. Because it mixes coordinates from separate positions, it spreads each letter across the message instead of substituting one for one.
Why did my text lose characters?
A 5 by 5 square holds 25 letters, so one letter is folded into another, and only letters are encoded. The repository records the measured round trip: "The fox jumps, 42 times!" comes back as "THEFOXIUMPSTIMES". Digits, spaces and case are gone, and J has become I.
What is the key?
The arrangement of the square, and the period if the variant uses one. Both sides need the same setup or the coordinates fall in different places.
Is it stronger than a simple substitution?
Stronger, and still broken. Fractionation defeats naive letter-frequency counting, which is why it survived into field use, but it is a classical cipher with known analytic attacks. Use AES-GCM encrypt for anything real.