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A rail fence cipher is a type of written code or cipher that allows its users to transform text for the purposes of encoding, using only a pencil and paper.
In a rail fence cipher, letters are not changed, but only switched around regarding their positioning in the message. This type of cipher is often called a transposition cipher, because letters are simply transposed in terms of their placement. Transposition ciphers like the rail fence cipher are relatively weak forms of encoding, and can easily be broken, especially with today’s technology. These types of ciphers date back to the American Civil War, where soldiers would use the code to send encrypted messages.
In a rail fence cipher, the writer takes a message and writes it into descending lines or "rails." The rail fence cipher is sometimes called a zig zag cipher if the writer uses a zigzag or W pattern to represent text.
In order to encode the text, the user takes the letters in the top line, or rail, and puts them together. He or she then writes out the second line and the third line. The result is an encoded line of text. For example, using the phrase "hello world and a series of three rails, the result (for a linear descent) would be HLODEORLWL.