railfence

Railfence, or the rail fence cipher, is a classical transposition cipher used in cryptography. It does not substitute letters but reorders them by writing the plaintext in a zigzag across a fixed number of rails (rows). After the text is laid out, the letters are read off row by row to produce the ciphertext.

Encryption involves choosing the number of rails. The plaintext is written diagonally down the rails, moving from the top rail to the bottom and then back up, in a repeating zigzag until all characters are placed. The ciphertext is then obtained by concatenating the letters from rail 1, then rail 2, and so on. Typically, spaces and punctuation are removed in practice.

Example: with three rails and the plaintext WEAREDISCOVER (no spaces), placing letters in a zigzag yields rail 1: W E C R, rail 2: E R D S O E, rail 3: A I V. Reading the rows in order gives the ciphertext WECRERDSOEAIV.

Decryption reverses the process: given the number of rails and the ciphertext, reconstruct the zigzag pattern and fill in letters by rail in order, then read along the zigzag path to retrieve the plaintext.

Security and usage: the rail fence cipher offers little real security and can be broken by simple analysis or brute force, especially for short messages. It is mainly of historical and instructional value, used to illustrate transposition techniques and to serve in puzzles.

History and variants: the exact origins are unclear, but the method appears in classical and later cryptography texts as a simple teaching tool. Variants include different rail counts and multi-pass schemes to increase complexity modestly.