<?php

const ALPHABET = ['A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z','0','1','2','3','4','5','6','7','8','9'];
const DEBUG = false;

function vernam_crypt($string, $key = "", $decrypt = false){
        $string_len = mb_strlen($string, 'utf-8');
        $result = "";
        if(empty($key)){
            for($j = 0; $j < 8; $j++){
                $key .= ALPHABET[rand(0, count(ALPHABET)-1)];
            }
            echo 'new key: '.$key.'<br/>';
        }

        $key_len = strlen($key);
        for($i = 0; $i < $string_len; $i+=$key_len){
            for($j = 0; $j < $key_len; $j++){
                if(empty($string[$i+$j])) break;
                if(DEBUG) echo '/'.($i+$j).'=['.$string[$i+$j].']/';
                //$result .= $string[$i+$j] == " " ? " " : chr(ord($string[$i+$j])^ord($key[$j])); //Нестабилен, если прописная заглавная буква ключа совпадает с прописной строки и наоборот.
                if(!$decrypt){
                    $result .= $string[$i+$j] == " " ? " " : chr(ord($string[$i+$j]) + (ord($key[$j]) % 26));
                } else {
                    $result .= $string[$i+$j] == " " ? " " : chr(ord($string[$i+$j]) + ( (26-ord($key[$j]))  % 26));
                }
            }
        }

        echo ($decrypt ? 'decrypted: ' : 'encrypted: ').$result;
        return [$key, $result];
}

$res = vernam_crypt('It was a wonderful world with nothing!');
echo '<br/>-----------------<br/>';
vernam_crypt($res[1], $res[0], true); //I am grut