Write a program to implement Vigenere Cipher. Information and Network Security (2170709) GTU

Algorithm/Steps :

1. Take plain text from file named input.txt
2. Take Key as input from key.txt
3. Add same number of characters as plain text from  key
4. Do encryption and save Encrypted text in output.txt
5. Hence decrypt the encrypted message and save decrypted text in input.txt

Code :

Encryption

#include<stdio.h>

#include<string.h>

int main()

{

            char plain[50],ci[50],key[50];

            FILE *key1 = fopen("key.txt","r");

            FILE *plain1 = fopen("input.txt","r");

            FILE *cipher = fopen("output.txt","w");

           

            fscanf(plain1, "%s", plain);

            printf("\n%s",plain);

           

            fscanf(key1, "%s", key);

            printf("\n%s",key);

            int len = strlen(key);

            int len1 = strlen(plain);

            int j=0,i=len;

            while(i != len1)

            {

                        l1:

                        key[len] = key[j];

                        if(j==(len-1))

                        {

                                    j=0;

                                    goto l1;

                        }

                        len++;

                        i++;

                        j++;

            }

            //printf("\nKey is: %s",key);

            i=0;j=0;

            while(plain[i] != '\0')

            {

                        ci[j] = (plain[i] + key[i]) % 26 + 'A';

                        //printf("%c",ci[j]);

                        i++;

                        j++;

            }

            ci[j] = '\0';

            printf("\nEncryted text is: %s",ci);

            fprintf(cipher,"%s",ci);

            return 0;

}

Decryption

#include<stdio.h>

#include<string.h>

int main()

{

            char plain[50],ci[50],key[50];

            FILE *key1 = fopen("key.txt","r");

            FILE *cipher = fopen("output.txt","r");

           

            fscanf(cipher, "%s", ci);

            printf("\n%s",ci);

            fscanf(key1, "%s", key);

           

            int len = strlen(key);

            int len1 = strlen(ci);

            int j=0,i=len;

            while(i != len1)

            {

                        l1:

                        key[len] = key[j];

                        if(j==(len-1))

                        {

                                    j=0;

                                    goto l1;

                        }

                        len++;

                        i++;

                        j++;

            }

            printf("\n%s",key);

            i=0;j=0;

            while(ci[i] != '\0')

            {

                        plain[j] = (((ci[i] - key[i])+26) % 26) + 'A';

                        i++;

                        j++;

            }

            plain[j] = '\0';

            printf("\nDecrypted text is: %s",plain);

            return 0;

}