Bifid Cipher — Bench tool
Published 20 Oct 2026 ·
Felix Delastelle was a French amateur who, in the 1890s, worked out how to get the diffusion of a digraph cipher without any of the arithmetic. The Bifid is a fractionating cipher: it splits every letter into a row number and a column number, mixes the numbers, and reassembles them into different letters. It is one of the strongest pencil-and-paper ciphers ever devised, and it is almost unknown outside cryptology courses.
- Era
- Modern
- Type
- Substitution
- Key space
- square keyword x period
- Level
- Specialist
IWhat this tool does
Write the coordinates of the plaintext in the Polybius square, then split them into two streams — all the rows, then all the columns — and re-pair the digits to read off new letters.
Square with keyword BRASS:
| 1 | 2 | 3 | 4 | 5 | |
|---|---|---|---|---|---|
| 1 | B | R | A | S | C |
| 2 | D | E | F | G | H |
| 3 | I | K | L | M | N |
| 4 | O | P | Q | T | U |
| 5 | V | W | X | Y | Z |
Encipher LOOM (a single period of four, though the classical period is five):
| Step | Result |
|---|---|
| coordinates of L, O, O, M | 31, 41, 41, 33 |
| row stream | 3 4 4 3 |
| column stream | 1 1 1 3 |
| concatenated | 3 4 4 3 1 1 1 3 |
| re-paired | 34, 43, 11, 13 |
| letters | M, T, B, A |
LOOM became MTBA. Every letter changed value, and the change depended on the letters around it — the same diffusion property that makes Playfair more than a substitution, achieved with counting instead of a square rule.
IIThe key
Two parameters compose the key: the keyword that builds the square, and the period — the block length over which fractionation runs.
The keyword behaves as it does everywhere else: a few thousand plausible squares from memorable words. The period is the interesting part. A period of five is the classical choice, and it is the weakest useful setting; longer periods diffuse more and cost the analyst more.
The Bifid's period is a security parameter in the modern sense: a single dial that trades speed for strength, and one that the operator can turn without changing any other part of the system.
That is a surprisingly modern idea to find in an 1895 amateur's notebook. It is the same shape as the number of rounds in a modern block cipher.
Delastelle also invented the trifid cipher, which uses a 3 x 3 x 3 cube and coordinates in three digits rather than two, and the four-square cipher. All three are fractionating, and all three were ignored for most of a century because they were French, amateur, and published in journals nobody read.
IIIWhat it will not protect
Historical ciphers are for study and recreation and are not suitable for protecting real information. The full analysis is in the cipher entry.
Read the full entry, with deciphering steps and weakness analysis →