(k-Rail Fence Cipher): The rail fence cipher is a transposition cipher, where a

Question

(k-Rail Fence Cipher): The rail fence cipher is a transposition cipher, where a plaintext message is enciphered into a ciphertext message by performing some sort of permutation on the plaintext letters. Using this encryption technique, a plaintext is written down as a sequence of diagonals and then reads off as a sequence of rows. For example, to encipher the message “Hello CISC 3580 Students!” with a rail fence of depth 2, we write the following: H   l   o    I    C   5   0   t    d   n   s   e   l   C   S   3   8   S   u   e    t

The resulting ciphertext is: HloIC50tdnselCS38Suet

The decryption function is performed as follows: - Compute the length of ciphertext. Let m be this length. - Let k be the number of rails (i.e., key). Reserve m placeholders on the k-rail fence in the same way a plaintext was placed on a k-rail fence. - Assign the ciphertext letters to those placeholders’ row by row, starting from the top one. - Read off the resulting plaintext as a sequence of diagonals. 1. Write an algorithm for a k-rail fence cipher, where k 2. 2. Show that your proposed algorithm is correct using an informal proof (i.e., discussion). 3. Give a program corresponding to your proposed algorithm, using your favorite programming language (preferably C++). Your program should simulate this cipher and display the ciphertext along with the corresponding plaintext and key. The implementation of this k-rail fence cipher software should be as structured as possible.

Explanation / Answer

Algorithm for K- rail fence cipher

Example : consider a plain text "I AM A STUDENT"

Arrange it as: I M S U E T

A A T D N

Cipher text is:IMSUETAATDN

Program for rail fence cipher

#include<stdio.h>

#include<string.h>

Void enmsg(char msg[],int key)

{

int mlen = strlen(msg),i,j,k=-1,row=0,col=0;

Char Matrix[key][mlen];

for(i=o;i<key;++i)

for(j=0;j<mlen;j++)

railMatrix[i][j]=' ';

for(i=0;i<mlen;i++)

{

railMatrix[row][col++]=msg[i];

If(row==0||rowkey-1)

Key=key*(-1);

row =row+k;

}

Printf(" encrypted cipher text :");

for(i=0;i<key;++i)

for(j=0;j<mlen;++j)

If(railMatrix[i][j]!=' ')

Printf("%c",railMatrix[i][j]);

}

void decmsg(char enmsg, int key)

{

int mlen=strlen(enmsg),I, j, k=-1,row=0,col=0,m=0;

char railMatrix[key][mlen];

for(i=0;i<key;++i)

for(j=0;j<mlen;++j)

railMatrix[i][j]=' ';

for(i=0;i<mlen;i++)

{

railMatrix[row][col++]='*';

if(row==0||row=key-1)

k=k*(-1);

row=row+k;

}

for(i=0;i<key;++i)

for(j=0:j<mlen;++j)

If(railMatrix[i][j]=enmsg[m++];

row=col=0;

k=-1;

Printf("decrypted message: ");

for(i=0;i<mlen;++I)

{

Printf("%c",railMatrix[row][col++]);

if(row==0||row=key-1)

k=k*(-1);

row=row+k;

}

int main(){

char msg[]="IM A STUDENT":

char enmsg[]="IMSUETAATDN";

interesting key =3;

printf("original msg :%",msg);

enmsg(msg, key) ;

decmsg(enmsg,key);

return 0;

}

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