Question

Please answer this problem without using iostream and without strings or arrays.

Just use stdio.h and math.h.

57677 1 Caesar's Cipher (70 points) The Caesar's cipher is a cryptographic method for encrypting text such that it becomes unreadable to a party with- out access to the cryptographic key. It is named after Julius Caesar, who allegedly used it to protect messages of military significance. The encryption and decryption operations are simple shifts of the alphabet letters in a cyclic fashion. Mathematically, for a key k, the encryption and decryption are defined as: Encryption: c = (1 + k) mod 26 , Decryption: z = (c-k) mod 26. To llustrate the use of the Ceasar's cipher, consider the encryption of "fry" with key k -3. Letters f, r, y correspond to the 5th, 16th, and 24th letter of the alphabet (letter a being the Oth) f:(5 + 3) r: (16 + 3) y: (24 + 3) mod 26 = 8→i mod 26 19 → u mod 26-1 → b Similarly, the decryption of i,u,b follows the reverse process í: (8-3) u: (19-3) b:(1-3) mod 26 mod 26 mod 26 5-1 16 → r 24→ = NOTE: The modulo operation for negative numbers is different from the % arithmetic operator in C Write a C program that decerypts a fle named "encrypted.txt" and places the decryption outpat to a ile called decrypted.txt". A file encrypted with k 3 is provided with the assignment. You can use it to test your decryption function. You will know when you have succeeded because the text becomes readable. In your program . Ask the user to enter the decryption key . Repeat the request until the right key is entered.

Answer 1

#include<stdio.h>
#include<stdlib.h>

int mod(int x, int y) {
        while(x < 0) {
                x += y;
        }
        return x % y;
}

char decrypt_fun(int shift, char l) {
        if(l <= 'z' && l >= 'a') {
                return 'a' + mod((int)(l - 'a') - shift, 26);
        }
        if(l <= 'Z' && l >= 'A') {
                return 'A' + mod((int)(l - 'A') - shift, 26);
        }
        return l;
}

char encrypt_fun(int shift, char l) {
        if(l <= 'z' && l >= 'a') {
                return 'a' + mod((int)(l - 'a') + shift, 26);
        }
        if(l <= 'Z' && l >= 'A') {
                return 'A' + mod((int)(l - 'A') + shift, 26);
        }
        return l;
}

int main(void) {
        int key = 3;

        int input;
        do {
                printf("Enter the decryption key: ");
                scanf("%d", &input);
                if(input != key) {
                        printf("Wrong key!\n");
                }
        } while(input != key);

        // remove the new line in buffer
        while (getchar () != '\n' );


        char code;
        FILE* inputFile = fopen("encrypted.txt", "r");
        FILE* outputFile = fopen("decrypted.txt", "w");

        //exit program if file doesn’t open
        if(inputFile==NULL || outputFile==NULL) {
                printf("File could not be open!!");
                exit(0);
        }

        do {
                code = (char)fgetc(inputFile);
                if(code != EOF) {
                        putc(decrypt_fun(key, code),outputFile);
                }
        } while(code != EOF);
        fclose(inputFile);
        fclose(outputFile);

        printf("File decrypted!\n");
        printf("Do you want to re-encrypt the file(y/n): ");
        scanf("%c", &code);
        if(code == 'y') {
                printf("Enter an encryption key: ");
                scanf("%d", &key);

                inputFile = fopen("decrypted.txt", "r");
                outputFile = fopen("encrypted.txt", "w");

                //exit program if file doesn’t open
                if(inputFile==NULL || outputFile==NULL) {
                        printf("File could not be open!!\n");
                        exit(0);
                }

                do {
                        code = (char)fgetc(inputFile);
                        if(code != EOF) {
                                putc(encrypt_fun(key, code),outputFile);
                        }
                } while(code != EOF);
                fclose(inputFile);
                fclose(outputFile);
                printf("File encrypted!\n");
        }
        printf("Goodbye!\n");
}

Hi. please find the code.. i have given code comments so that it is very easy for you to understand the flow. In case of any doubts, please ask in comments. If the answer helps you, please upvote. Thanks!