Simple Cipher/Deipher Message [C++]

/*
 *  Program to accept secret message and encrypt and decrypt it
 */

#include<iostream>
#include<string>

using namespace std;

// function to find the position of each letter in the key
size_t find_pos(const char ch, const string str)
{
    size_t pos{}, rpos{str.length()-1};     // rpos is reverse position to search the number from the end of the string simultaneously
    while(pos <= rpos)
    {
        if(str[pos]==ch)
            return pos;
        else if(str[rpos]==ch)
            return rpos;
        pos++, rpos--;
    }

    return string::npos;
}

// Function to encrypt the plain-text
string encrypt(const string &txt, const string &alpha, const string &key)
{
    string ciphered;
    size_t key_pos;

    for(size_t i{}; i<txt.length(); i++)
    {
        key_pos = find_pos(txt[i], alpha);
        if(key_pos!=string::npos)
            ciphered += key[key_pos];
        else
            ciphered += txt[i];
    }

    return ciphered;
}

// Function to decrypt the cipher
string decrypt(const string &c_txt, const string &alpha, const string &key)
{
    string deciphered;
    size_t alpha_pos;

    for(size_t i{}; i<c_txt.length(); i++)
    {
        alpha_pos = find_pos(c_txt[i], key);
        if(alpha_pos!=string::npos)
            deciphered += alpha[alpha_pos];
        else
            deciphered += c_txt[i];
    }

    return deciphered;
}

// Function to print menu and prompt user to choose
int menu()
{
    int choice;

    cout << "========================" << endl;
    cout << "  ENCYPTION/DECRYPTION" << endl;
    cout << "========================" << endl;
    cout << "    Choose an option " << endl;
    cout << "1. Enter a Message." << endl;
    cout << "2. Encrypt a Message." << endl;
    cout << "3. Decrypt a Message." << endl;
    cout << "4. Exit." << endl;
    cout << "========================" << endl;
    cout << "Enter : ";

    while(!(cin>>choice))
    {
        cout << "\nWrong choice!\nPlease Enter Again : ";
        cin.clear();
        cin.ignore(' ', '\n');
    }

    return choice;
}

int main()
{
    const string alphabets {"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"};
    const string key {"QWERTYUIOPasdfghjklZXCVBNMqwertyuiopASDFGHJKLzxcvbnm"};
    string message, ciphered_txt, deciphered_txt;
    int choice;

    do
    {
        choice = menu();
        switch(choice)
        {
            case 1:
                char ch;
                if(message.length())        // Condition to check if there is already a message
                {
                    cout << "\nYou've already entered a Message.";
                    cout << "\nIf you re-enter, previous Message will be over-written";
                    cout << "\nContinue Overwriting? (Y/N): ";
                    cin >> ch;

                    while(ch!='n'&&ch!='N'&&ch!='y'&&ch!='Y')   // condition to check wrong choice
                    {
                        cout <<"\nWrong choice entered\n";
                        cout <<"Enter again : ";
                        cin >> ch;
                    }

                    if(ch=='n' || ch=='N')
                    {
                        cout << endl;
                        break;
                    }

                }

                cin.clear();
                cin.ignore(' ', '\n');

                cout << "\nEnter your Mssage\n";
                getline(cin, message);
                cout << "Your Message has been saved!\n\n";

                break;

            case 2:
                if(message.length())        // if there is a message
                {
                    ciphered_txt = encrypt(message, alphabets, key);
                    cout << "\n-------------------------------\n";
                    cout << "Your Message has been Encrypted!\n\n";
                    cout << " Original Message:\n" << "\"" << message << "\"\n\n";
                    cout << " Encrypted Message:\n" << "\"" << ciphered_txt << "\"";
                    cout << "\n-------------------------------\n\n";
                }
                else
                    cout << "\nThere is no message to Encrypt!\n\n";
                break;

            case 3:
                if(ciphered_txt.length())       // if there is a ciphered text
                {
                    deciphered_txt = decrypt(ciphered_txt, alphabets, key);
                    cout << "\n-------------------------------\n";
                    cout << "Your Message has been Decrypted!\n\n";
                    cout << " Encrypted Message:\n" << "\"" << ciphered_txt << "\"\n\n";
                    cout << " Decrypted Message:\n" << "\"" << deciphered_txt << "\"";
                    cout << "\n-------------------------------\n";
                }
                else
                    cout << "\nThere is no Encrypted message to Decrypt!\n\n";
                break;

            case 4:
                cout << "\nThank You!\n";
                break;

            default:
                cout << "\nWrong choice! Please try again!\n";
        }
    }
    while(choice!=4);

    return 0;
}