sub.c

// This program allows the user to implement a
// substitution cipher. That is, at runtime, the
// user enters a substitution key, and a string
// of plaintext to convert to ciphertext.

// includes
#include <stdio.h>
#include <cs50.h>
#include <string.h>
#include <ctype.h>

// function prototypes
int validateKey(int args, string argument);
string encrypt(string key, string plain);


// MAIN FUNCTION

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

    string keyInput = argv[1];                      // user's key input
    int errorCode = validateKey(argc, argv[1]);     // key validation

    // if the error code is 1, throw an exception, and exit
    if (errorCode == 1)
    {
        return errorCode;
    }

    // get some plain text from the user
    string plainText = get_string("plaintext: ");
    // printf("\n");

    // store encrypted output in a string
    string encrypted = encrypt(keyInput, plainText);

    // print encrypted text
    printf("ciphertext: %s\n", encrypted);

    // end program
    return 0;
}


// ENCRYPTOR
string encrypt(string key, string plain)
{
    char alphabet[26] = "abcdefghijklmnopqrstuvwxyz";  // alphabet reference

    // iterate through the plaintext
    for (int i = 0, len = strlen(plain); i < len; i++)
    {
        bool alphabetic = false;    // set up a variable to record if current char is alphabetic

        // iterate through the alphabet
        for (int j = 0, length = strlen(alphabet); j < length; j++)
        {
            // check if current char is lower-case letter
            if (plain[i] == alphabet[j])
            {
                // if the key is in upper-case, convert to lower-case
                if (isupper(key[j]))
                {
                    plain[i] = tolower(key[j]);
                }
                else
                {
                    plain[i] = key[j];
                }
                alphabetic = true;      // record that it's an alphabetic char
                j = strlen(alphabet);   // skip the rest of the alphabet scan
            }
            else if (plain[i] == toupper(alphabet[j]))
            {
                plain[i] = toupper(key[j]);
                alphabetic = true;              // record that it's an alphabetic char
                j = strlen(alphabet);           // skip the rest of the alphabet scan
            }
        }

        // if not alphabetic
        if (alphabetic == false)
        {
            plain[i] = plain[i];
        }
    }

    // returned the ciphertext, now stored in the original plain variable
    return plain;
}


// KEY VALIDATOR
int validateKey(int args, string key)
{
    // if not enough arguments
    if (args < 2)
    {
        printf("Try typing in a cipher key after ./substitution.\n");
        return 1;
    }
    // if too many arguments
    else if (args > 2)
    {
        printf("Try typing only one argument after ./substitution.\n");
        return 1;
    }
    // if right number of arguments
    else
    {
        int length = strlen(key);

        // but wrong length
        if (length < 26 || length > 26)
        {
            printf("Try using *exactly 26* upper-case or lower-case letters in your key.\n");
            return 1;
        }

        // if right length
        else
        {
            // iterate through the key
            for (int i = 0; i < length; i++)
            {
                // if it contains non-alphabetic characters
                if ((key[i] < 65 || key[i] > 90) && (key[i] < 97 || key[i] > 122))
                {
                    printf("Try using exactly 26 upper-case or lower-case LETTERS in your key.\n");
                    return 1;
                }
            }

            // declare a variable to keep track of the current character
            char currentChar;

            // iterate through the key
            for (int i = 0; i < length; i++)
            {
                // set currentChar value to currently selected character in key
                currentChar = key[i];
                // initialize a counter variable
                int counter = 0;

                // scan through the key
                for (int j = 0; j < length; j++)
                {
                    // if the currently selected character matches any in the key, increment counter
                    if (currentChar == key[j])
                    {
                        counter++;
                    }

                    // if the counter is greater than 1, return an error with a console warning
                    if (counter > 1)
                    {
                        printf("Try not repeating any letters in the key.\n");
                        return 1;
                    }
                }
            }

            // if the key is valid by all the above criteria
            return 0;
        }
    }
}