caesar.c

// Encrypt a message using the Caesar cypher, with a user-provided key.

// This is CS50x pset2 caesar.c

//

// Author: david@newtongwc.org

//

// Usage: caesar <secret>

//

// Reads the (new-line-terminated) clear-text message from stdin, writes the

// crypt-text message to stdout.

#include <stdio.h>

#include <ctype.h>

#include <string.h>

#include <cs50.h>

#define true 1

#define false 0

int ArgsAreValid(int argc, string argv[]) {

    if (argc != 2) {

        printf("Error: You must supply exactly 1 argument to %s\n", argv[0]);

        return false;

    }

    if (atoi(argv[1]) < 0) {

        printf("Error: The ceaser secret must be a non-negative integer\n");

        return false;

    }

    return true;

}

int GetSecret(int argc, string argv[]) {

    return atoi(argv[1]);

}

// Encrypt a single character by shifting it some number ("secret") of places

// up the alphabet, wrapping around from 'z' to 'a'. Works for both upper- and

// lower-case letters, staying within the same case. Leaves non-alphabetic

// characters unchanged.

char EncryptChar(char clear, int secret) {

    char crypt = 0;

    if (isupper(clear)) {

        crypt = (clear - 'A' + secret) % 26 + 'A';

    } else if (islower(clear)) {

        crypt = (clear - 'a' + secret) % 26 + 'a';

    } else {

        crypt = clear;

    }

    return crypt;

}

// Encrypts a full message, one character at a time, using the provided

// encryption secret.

string Encrypt(string clear_text, int secret) {

    string crypt_text = malloc(strlen(clear_text) * sizeof(char));

    int num_chars = strlen(clear_text);

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

        crypt_text[i] = EncryptChar(clear_text[i], secret);

    }

    return crypt_text;

}

int main(int argc, string argv[]) {

    if (!ArgsAreValid(argc, argv)) {

        return 1;

    }

    int secret = GetSecret(argc, argv);

    string clear_text = GetString();

    string crypt_text = Encrypt(clear_text, secret);

    printf("%s\n", crypt_text);

    free(clear_text);

    free(crypt_text);

    return 0;

}