Шифр Гронсфельда

#include <iostream>
#include <string>
#include <Windows.h>
 
using namespace std;
 
inline constexpr int C2I(char ch) {
    return ch & 0xff;
}
 
// только большие латинские буквы
//const int FirstSymbol = C2I('A');
//const int AlphabetLength = C2I('Z') - FirstSymbol + 1;
 
// все печатные символы: ' '..'я'
const int FirstSymbol = C2I(' ');
const int AlphabetLength = C2I('\xff') - FirstSymbol + 1;
 
string enGronsfeld(const string& str, const string& key) {
    string out;
    out.reserve(str.length());
    for (size_t i = 0; i < str.length(); ++i) {
        out += (char)(((C2I(str[i]) - FirstSymbol) + (key[i % key.length()] - '0')) % AlphabetLength + FirstSymbol);
    }
    return out;
}
 
string deGronsfeld(const string& str, const string& key) {
    string out;
    out.reserve(str.length());
    for (size_t i = 0; i < str.length(); ++i) {
        int tmp = (C2I(str[i]) - FirstSymbol) - (key[i % key.length()] - '0');
        out += (char)((tmp < 0 ? tmp + AlphabetLength : tmp) % AlphabetLength + FirstSymbol);
    }
    return out;
}
 
int main()
{
    SetConsoleCP(1251);
    SetConsoleOutputCP(1251);
 
    string orig = "GRONSFELD";
    string key = "2015";
    cout
        << "text:      [" << orig << "]\n"
        << "key:       [" << key << "]\n" << endl;
    string encrypted = enGronsfeld(orig, key);
    cout 
        << "encrypted: ["<< encrypted << "]" << endl;
    string decrypted = deGronsfeld(encrypted, key);
    cout
        << "decrypted: [" << decrypted << "]\n" << endl;
 
    do {
        cout << "Enter string: ";
        getline(cin, orig);
        if (!orig.empty()) {
            cout << "Enter key: ";
            cin >> key;
            cin.ignore();
 
            encrypted = enGronsfeld(orig, key);
            cout
                << "encrypted: [" << encrypted << "]" << endl;
            decrypted = deGronsfeld(encrypted, key);
            cout
                << "decrypted: [" << decrypted << "]" << endl;
            cout << (orig == decrypted ? "Ok\n" : "Error ((\n") << endl;
        }
    } while (!orig.empty());
 
    return EXIT_SUCCESS;
}