Rail fence cipher
The rail fence cipher (also called a zigzag cipher) is a form of transposition cipher. It derives its name from the way in which it is encoded.
In the rail fence cipher, the plaintext is written downwards and diagonally on successive "rails" of an imaginary fence, then moving up when we reach the bottom rail. When we reach the top rail, the message is written downwards again until the whole plaintext is written out. The message is then read off in rows. For example, if we have 3 "rails" and a message of 'WE ARE DISCOVERED. FLEE AT ONCE', the cipherer writes out:
W . . . E . . . C . . . R . . . L . . . T . . . E . E . R . D . S . O . E . E . F . E . A . O . C . . . A . . . I . . . V . . . D . . . E . . . N . .
Then reads off to get the ciphertext:
WECRL TEERD SOEEF EAOCA IVDEN
Problems with the rail fence cipher
The rail fence cipher is not very strong; the number of practical keys is small enough that a cryptanalyst can try them all by hand.
The term zigzag cipher may refer to the rail fence cipher as described above. However, it may also refer to a different type of cipher system that looks like a zigzag line going from the top of the page to the bottom. As described in Fletcher Pratt's Secret and Urgent, it is "written by ruling a sheet of paper in vertical columns, with a letter at the head of each column. A dot is made for each letter of the message in the proper column, reading from top to bottom of the sheet. The letters at the head of the columns are then cut off, the ruling erased and the message of dots sent along to the recipient, who, knowing the width of the columns and the arrangement of the letters at the top, reconstitutes the diagram and reads what it has to say."
- Helen Fouché Gaines, Cryptanalysis, a study of ciphers and their solution, Dover, 1956, ISBN 0-486-20097-3