What is the Bifid Cipher?

Choose a period length. Write the plaintext in groups of that length. Then choose a keyword and create a Polybius square with it. Write numbers 1 through 5 along the top and left. Write the row numbers on a line below the plaintext, and the column numbers in a line below that. Then, write the row numbers for a group followed by the column numbers for that group, followed by the row numbers for the next group, and so on. Use these as coordinates in the Polybius square with which to extract the ciphertext letters.

Example

Plaintext: Whoever has made a voyage up the Hudson must remember the Kaatskill mountains.

Period Length: 8

Keyword: RIPVANWINKLE

Polybius Square:

  1 2 3 4 5
1 R I P V A
2 N W K L E
3 B C D F G
4 H M O Q S
5 T U X Y Z

Encipher steps

Plaintext: whoeverh asmadeav oyageupt hehudson mustreme mberth ekaatski llmounta ins
Row #    : 24421214 14443211 45132515 42453442 45451242 432154 22115421 22445251 124
Column # : 21354511 55253554 34555231 15123531 22511525 215111 53551532 44232115 215

CT: 24 42 12 14 21 35 45 11 14 44 32 11 55 25 35 54 45 13 25 15 34 55 52 31 42 45 34 42 15 12 35 31 45 45 12 42 22 51 15 25 43 21 54 21 51 12 21 15 42 15 35 51 53 22 24 45 25 14 42 32 11 51 24 21 55

CT: L M I V N G S R V Q C R Z E G Y S P E A F Z U B M S F M A I G B S S I M W T A E O N Y N T I N A M A G T X W L S E V M C R T L N Z

CT: LMIVN GSRVQ CRZEG YSPEA FZUBM SFMAI GBSSI MWTAE ONYNT INAMA GTXWL SEVMC RTLNZ