Bifid — Each letter becomes a pair of grid coordinates; the coordinates are written out in rows, then re-read in pairs within a fixed period.
Each letter becomes (row, column); the coordinates are recombined within a period. Experimental solver.
A worked example§
Encrypting a sample with the cipher itself, at build time:
MEET ME BY THE OLD CLOCK TOWER AT DAWNKey: key=MONARCHY, period=7
produces the ciphertext
NINCMUABYAYVORBOIMORVWMPNRTXUGThis example is generated by running the cipher when the page is built, and the round trip is checked, so it always matches what the solver does.
How it is broken§
Fractionation smears each plaintext letter across two ciphertext letters, so the period and the grid must be recovered together. The solver hill climbs per period and is honest that it is experimental.
History and context§
Felix Delastelle, around 1901 — the first practical cipher to combine substitution with fractionation, an idea that runs straight through to the rotor machines.
See also§
Playfair · Hill cipher (matrix) · Four-square · Trifid
Browse every cipher in the Polygraphic ciphers family, read the history of codebreaking, or return to the wiki main page.
Frequently asked questions§
How is the Bifid broken?
Fractionation smears each plaintext letter across two ciphertext letters, so the period and the grid must be recovered together. The solver hill climbs per period and is honest that it is experimental.
What key does the Bifid use?
keyword + period. The keyspace is unbounded in practice.
How much ciphertext do I need?
At least 80 characters for this solver to attempt it; short messages can be readable and still not be proof.
Can I break a Bifid on this page?
Not with the browser build, which targets the common puzzle families. The desktop version searches it: see the Windows app page.