Encryption

#include  <iostream>

using namespace std;

int check_enc_key(string encryption_key) {
    int length = encryption_key.length();
    const string ABC = "abcdefghijklmnopqrstuvwxyz";

    // check length
    if(encryption_key.length() != 26) {
        cout << "Error! The encryption key must contain 26 characters." << endl;
        return EXIT_FAILURE;
    }

    // check chars, loop ABC
    for(int i = 0; i < length; i++) {

        bool found = false;

        // check if ABC char i is found from enc key
        for(int j = 0; j < length; j++) {
            if(ABC[i] == encryption_key[j]) {
                found = true;

                // check for uppercase while at it
                if(j >= 'A' && j <= 'Z') {
                    cout << "Error! The encryption key must contain only lower case characters." << endl;
                    return EXIT_FAILURE;
                }
            }
        }

        // all ABC letters must be found from encryption_key
        if (found == false) {
            cout << "Error! The encryption key must contain all alphabets a-z." << endl;
            return EXIT_FAILURE;
        }
    }
    return 0;
}

int encrypt(string encryption_key, string plain_text) {
    int length = plain_text.length();
    string enc_text;
    cout << encryption_key << endl;

    /*
     * Edelleen ASCII-arvon selvittämisen jälkeen saat kirjaimen järjestysluvun aakkostossa,
     * kun vähennät kirjaimen ASCII-arvosta ‘a’-kirjaimen ASCII-arvon (97). Tämä järjestysluku sopii
     *  indeksiksi, kun etsit salausavaimesta salattavan tekstin kirjaimia.
     */
    for(int i = 0; i < length; i++) {
        char encrypted_char = encryption_key[int(plain_text[i])-97];
        // cout << plain_text[i] << " == " << encrypted_char << endl;
        enc_text += encrypted_char;
    }

    cout << "Encrypted text: " << enc_text << endl;
    return 0;
}

int main()
{
    string encryption_key, user_input;

    cout << "Enter the encryption key: ";
    getline (cin, encryption_key);
    check_enc_key(encryption_key);

    cout << "Enter the text to be encrypted: ";
    getline (cin, user_input);

    encrypt(encryption_key, user_input);

    return 0;
}