The rail fence cipher is a form of transposition cipher. It derives its name from the way in which it is encoded.
Method
In the rail fence cipher, the plain text is written downwards and diagonally on successive "rails" of an imaginary fence, then moving up when the bottom rail is reached. When the top rail is reached, the message is written downwards again until the whole plaintext is written out. The message is then read off in rows. For example, if 3 "rails" and the message 'WE ARE DISCOVERED. FLEE AT ONCE' is used, 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: WECRLTEERDSOEEFEAOCAIVDEN Note that this particular example does NOT use spaces separating the words. The decipherer will need to add them based on context. If spaces are shown in the ciphertext, then they must be included in the count of letters to determine the width of the solution grid. Keys can also be used in this cipher e.g. In this example shown above have Key=3 which means there are three rails, or three lines of text.
Solution
This solution does not work on all Rail Fence Ciphers. Here is another example to see how to actually solve a Rail Fence cipher. We'll use a 3-rail fence to encode a new phrase and include spacing in between the words. Our ciphertext comes out as IA_EZS_ELYLK_UZERLIPL. Note that our ciphertext has a total of 21 units. This will be important later on as we try to decipher it. To solve the cipher, you must know the height and cycle of the puzzle. The height is simply the number of fence rails used to create it. In this example, we said that 3 fence rails were used, so the height is 3. The height will always be higher than 2 and no more than the number of letters in the ciphertext or else the phrase will not be properly encoded; the height can thus be discovered by process of elimination if not known. To determine the puzzle width, which will tell us how many total units will be in each row, you must determine the "cycle" of letters. A "cycle" of letters runs from the top row, down through each subsequent row, and then up again, but stopping before reaching the top row again. So a 2-rail puzzle has a "cycle" of 2 letters; a 3-rail puzzle has a "cycle" of 4 letters; a 4-rail puzzle has a "cycle" of 6 letters; etc. The math equation for this is: "Cycle" = - 2. 1. 1. 1. 1. 1 1... 1... 1... 1... 1 1..... 1..... 1..... 1..... 1 . 2. 2. 2. 2. . 2. 4. 2. 4. 2. 4. 2. 4. . 2... 6. 2... 6. 2... 6. 2... 6. .. 3... 3... 3... 3.. .. 3. 5... 3. 5... 3. 5... 3. 5.. ... 4..... 4..... 4..... 4... Our 3-rail fence example has a "cycle" of 4 units. So divide the total units by the cycle number and round down to the next whole number. There are 21 units in the example, so our "base puzzle width" is 5. It is important to realize that there are 5 "full cycles" plus a "partial cycle" of 1 more letter. Therefore, the top row has 6 units in it. The middle row has 10 units. The bottom row has 5 units. Take the first 6 units from our ciphertext and write them across the top row, leaving much space between the units: _ELYLK_UZERLIPL. I... A... _... E... Z... S The middle row takes the next 10 units and adds 1 unit just after and 1 unit just before each unit in the top row: IA_EZSRLIPL. I... A... _... E... Z... S . _. E. L. Y. L. K. _. U. Z. E. The bottom row gets the final 5 units written below and in between the pairs of units in the middle row: IA_EZS_ELYLK_UZE. I... A... _... E... Z... S . _. E. L. Y. L. K. _. U. Z. E. .. R... L... I... P... L.. Now just follow the down-up-down-up pattern to determine the original message: I_REALLY_LIKE_PUZZLES!
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.
Zigzag cipher
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."
