MonoAlphabeticCipher.c

/* Monoalphabetic substitution cipher basically replaces each letter of a plaintext by another letter or symbol, depending only on the letter.
   The same plaintext letter is always replaced by the same ciphertext letter and the same letter can be replaced by letter itself.
   Example if a is there we replace it with B to Z and so on for the others.*/

#include<stdio.h>
#include<string.h>

//this is the encription function
void monoalphebaticcipher(char input[100],int size){
     
     char output[size-1]; 
     char cipherer[26] = {'Z','Y','X','W','V','U','T','S','R','Q','P','O','N','M','L','K','J','I','H','G','F','E','D','C','B','A'};     
     char list[26] = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o','p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};  
     char space = ' ';
     //iterate through input and cipher the plane text as output
     for(int i=0;i<size-1;i++){     
         for(int j=0;j<26;j++){
             if(input[i]==list[j])
                output[i] = cipherer[j];
             else if( input[i] == space)
                output[i] = space;    
         } 
     } 
  
     printf("The Cipher text :");
     for(int j=0;j<size-1;j++)
         printf("%c",output[j]);  
}

int main(void){

     char input[100];
     printf("Give the plain text here in small letters :");
     //Taking input from user
     fgets(input,100,stdin);
     int size = strlen(input);
     //calling monoalphabeticcipher
     monoalphebaticcipher(input,size);
     return 0;
}
/*
  Sample I/O :
  Give the plain text here in small letters :hello world
  The Cipher text :SVOOL DLIOW
*/