rail fence cipher

The rail fence cipher is a simple method of transposition cipher used in cryptology. In this cipher, plaintext letters are written in a zigzag pattern across a set number of "rails," and then the rows are read sequentially to form the ciphertext. For example, a depth-two rail fence would write letters alternating between two rows. This method is a basic form of route cipher, where the key is the pattern of writing and reading the letters. A limitation of such simple ciphers is that frequency analysis can reveal that letters appear with the same frequency as in average plaintext, suggesting a rearrangement rather than substitution.1