The rail fence cipher (also called a zigzag cipher) is a classical type of transposition cipher. It derives its name from how encryption is performed, in analogy to a fence built with horizontal rails. Rails can be anywhere from two to more rails.
Encryption
Message: This is a secret message.
Rail: 2
Output: TIIAERTESG HSSSCMSAE
Decryption
Note: Knowing the number of "Rails" and the message length is enough for decryption.
Input: TIIAERTESG HSSSCMSAE
Rail: 2
Message Length: 20
Output: This is a secret message.