/*

# Railfence Cipher

Railfence cipher is a cryptographic technique which is a form of transposition cipher.

Given a plain-text message and a numeric key, cipher/de-cipher the given text using Rail Fence algorithm.

## Example:

Encryption

Input : "hello world"

Key = 3

Output : horel ollwd

Decryption

Input : horel ollwd

Key = 3

Output : hello world

## Note:

Initially The entire matrix is filled with "*"

## Output in C :


*/

#include<stdio.h>

#include<string.h>

int main(){

char a[100];

int key,len;

//Enter the text u want to convert to railfence

printf("Enter the input message \n");

gets(a);

len = strlen(a); //len is the length of input message

//specify the number of rows/rails

printf("Enter the number of rails \n");

scanf("%d",&key);

char matrix[30][30];

//code for obtaining railfence matrix

//length of input message is equal to the number of coloumns and number of rails equal to number of rows

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

for(int j=0;j<len;j++){

matrix[i][j] = '*';

}

}

int k=0,l=0,temp=0,flag=0;

//here the input message should be stored in railfence format.

for(int i=0;i<len;i++){

flag=0;

temp=0;

//if k value is equal to 0 or key-1 then convert the k value to negative so that it backtraverses the rows in railfence fashion.

if(k==0 || k == key-1){

k=k*(-1);

}

//since matrix index cannot be negative whenever k value is -ve convert to positive and after computation convert it back to original value

if(k<0){

temp=k;

k = k * (-1);

flag=1;

}

matrix[k][i] = a[l];

if(flag==1){

k=temp;

}

k++;

l++;

}

printf("\n");

//printing the railfence matrix obtained.Railfence matrix even stores white spaces if the input message is a sentence.

printf("RailFence Matrix is : \n");

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

for(int j=0;j<len;j++){

char k = matrix[i][j];

printf("%c \t",k);

}

printf("\n");

}

//code for encryption

char str[100];

int p=0;

//string the encrypted message from railfence matrix if the entry of the matrix is not '*'

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

{

for(int j=0;j<len;j++){

if(matrix[i][j] != '*'){

str[p++]=matrix[i][j];

}

}

}

//printing original message

printf("Original message is: \n");

puts(a);

//printing encrypted message

printf("Encrypted text is : \n");

for(int i=0;i<p ; i++){

printf("%c",str[i]);

}

printf("\n");

//code for decryption

char dstr[100];

int pp=0;

printf("Decrypted text is : \n");

int kk=0,temp1=0;

for(int i=0;i<len;i++){

temp1=0;

flag=0;

//if k value is equal to 0 or key-1 then convert the k value to negative so that it backtraverses the rows in railfence fashion.

if(kk==0 || kk==key-1){

kk = kk* (-1);

}

//since matrix index cannot be negative whenever k value is -ve convert to positive and after computation convert it back to original value

if(kk<0){

temp1=kk;

kk = kk * (-1);

flag=1;

}

char ct = matrix[kk][i];

//obtaining decrypted message from railfence matrix

dstr[pp++]=matrix[kk][i];

if(flag==1){

kk=temp1;

}

kk++;

}

//printing decrypted message

for(int i=0;i<pp ; i++){

printf("%c",dstr[i]);

}

printf("\n");

}