Rail Fence Cipher – Encryption and Decryption
Given a plain-text message and a numeric key, cipher/de-cipher the given text using Rail Fence algorithm.
The rail fence cipher (also called a zigzag cipher) is a form of transposition cipher. It derives its name from the way in which it is encoded.
Encryption Input : "GeeksforGeeks " Key = 3 Output : GsGsekfrek eoe Decryption Input : GsGsekfrek eoe Key = 3 Output : "GeeksforGeeks " Encryption Input : "defend the east wall" Key = 3 Output : dnhaweedtees alf tl Decryption Input : dnhaweedtees alf tl Key = 3 Output : defend the east wall Encryption Input : "attack at once" Key = 2 Output : atc toctaka ne Decryption Input : "atc toctaka ne" Key = 2 Output : attack at once
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:
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.
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
* _ _ _ * _ _ _ * _ _ _ * _ * _ * _ * _ * _ * _ * _ _ * _ _ _ * _ _ _ * _
Below is a program to encrypt/decrypt the message using the above algorithm.
atc toctaka ne GsGsekfrek eoe dnhaweedtees alf tl GeeksforGeeks attack at once delendfthe east wal
Time Complexity: O(row * col)
Auxiliary Space: O(row * col)
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