Bifid cipher decode
Bifid cipher decode, classical cipher, a puzzle not a protection, computed on your device. Not reversible to the exact input: J was folded into I at encode time and all spacing, digits and punctuation were dropped, so plaintext comes back as uppercase letters only.
Runs on your device. The file is never uploaded.
Bifid cipher decode maps each ciphertext letter back to its row and column on the 5 by 5 square. Each block of numbers is split into rows then columns and paired again into plaintext. The keyword box is blank by default and the period is 5; a wrong square returns other letters and no error.
Questions
What do I need to decode?
The same square arrangement used to encode. The tool reads the letters back to coordinates, unfolds the two rows and pairs them again. A different square gives different letters with no error.
Why is my text uppercase with no spaces?
Because Bifid encodes only the 25 letters of its square. Case, spacing, digits and punctuation were dropped at encode time and cannot be recovered here.
Where did my J go?
A 5 by 5 square has 25 cells for 26 letters, so J and I share one. Every J in the original comes back as I. Read around it.
How do I check I have the right square?
Decode a short known fragment first. Bifid fails silently, so a phrase you already know is the fastest confirmation that both sides agree.