Cryptographic Algorithms

Introduction

The rail fence cipher is a form of transposition cipher. Rail fence cipher is also called as Zig-Zag cipher. It derives its name from the way in which it is encoded.

Example:

                        Encryption
                        Input : "attack at once"
                        Key = 2 
                        Output : atc toctaka ne 
                        Decryption
                        Input : "atc toctaka ne"
                        Key = 2
                        Output : attack at once

Encryption

In a transposition cipher, the order of the alphabets is re-arranged to obtain the cipher-text.

In the rail fence cipher, the plain-text is written downwards and diagonally on successive rails of an imaginary fence.
When we reach the bottom rail, we traverse upwards moving diagonally, after reaching the top rail, the direction is changed again. Thus the alphabets of the message are written in a zig-zag manner.
After each alphabet has been written, the individual rows are combined to obtain the cipher-text.

For example, if the message is “GeeksforGeeks” and the number of rails = 3 then cipher is prepared as:

Decryption

As we’ve seen earlier, the number of columns in rail fence cipher remains equal to the length of plain-text message. And the key corresponds to the number of rails.

Hence, rail matrix can be constructed accordingly. Once we’ve got the matrix we can figure-out the spots where texts should be placed (using the same way of moving diagonally up and down alternatively ).
Then, we fill the cipher-text row wise. After filling it, we traverse the matrix in zig-zag manner to obtain the original text.

                        Implementation: 
                        Let cipher-text = “GsGsekfrek eoe” , and Key = 3

Number of columns in matrix = len(cipher-text) = 13
Number of rows = key = 3

Hence original matrix will be of 3*13 , now marking places with text as ‘*’ we get

                        * _ _ _ * _ _ _ * _ _ _ *
                        _ * _ * _ * _ * _ * _ * 
                        _ _ * _ _ _ *  _ _ _ * _

Rail Fence Cipher

Implementation

Encryption

Decryption