Rail Fence Cipher Encryption

encrypt.cpp

#include <iostream>
#include <string>
#include <stdexcept>

std::string encrypt(std::string const& plaintext) {
    if (plaintext.length() <= 4)
        throw std::runtime_error("Invalid string length.");

    std::string first;
    std::string second;

    for(std::size_t i = 0; i < plaintext.length(); ++i) {
        if (i % 2 == 0)
            first += plaintext[i];
        else
            second += plaintext[i];
    }
    return first + second;
}

int main() {
    std::string plaintext;
    std::cout << "Enter four or more chars:\n>";
    std::getline(std::cin, plaintext);
    std::cout << '\n';
    std::cout << "Ciphertext: " << encrypt(plaintext) << std::endl;
}