Summary

The Rail Fence Cipher is a type of transposition cipher that rearranges the order of letters in a plaintext message based on a key that determines the number of rows used in the encryption process. This cipher derives its name from the way letters are written in a zigzag pattern, resembling a rail fence. It offers a basic level of security by using a key to vary the number of rows, making the encryption slightly more challenging to decipher without knowledge of the key.

How it works: Encryption

Begin by writing the message in a zigzag pattern across multiple rows. The number of rows is determined by the key. For example, with a key of 3, you write the first letter in the top row, the second in the second row, and continue in a downward fashion until you reach the bottom row. Then, the direction changes, and you continue placing letters diagonally upwards until reaching the top row again, repeating this pattern until the entire message is written out.
Once the message is laid out in this zigzag pattern, the ciphertext is formed by reading each row sequentially from top to bottom. For example, if the plaintext is "defend the east wall" and the key is 3, the letters are written in three rows with zigzagging lines, and the ciphertext is read off as the sequence of letters from each row.

How it works: Decryption

To decrypt the message, you need to reconstruct the zigzag pattern used during encryption. This involves knowing the key (number of rails) used to encrypt the message. First, estimate or determine how many letters occupy each row by knowing the total length of the ciphertext and the encryption key. Start placing the ciphertext letters back into the zigzag pattern. Begin with the top row and fill it with the correct number of letters, then proceed to the next row until all rows are filled according to the pattern established during encryption. Once the zigzag grid is filled with the ciphertext letters, read the message by following the zigzag pattern from the start to the end of the grid to reconstruct the original plaintext message!.

If this is not clear, you can always try this cipher out and click the button "display grid" that is in the key container. This will display the grid of the text you have encrypted!
Note that in this code we are not removing the empty space, so whatever empty space is inputted it will count as a character

References

[1] Wikipedia, "Rail Fence Cipher" - Online source.
[1] Cryptop Corner, "Rail Fence Cipher" - Online source.