在PHP中为URL生成随机字符

Question

I have answered with my own implementation (below), where I'd appreciate if you could check the maths and logic, but I realise there are other possibilities as well.

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I'm trying to generate 32 random characters to be used in a registration URL.

The new account is partially created by a member of staff (setting the name/email), and a plain text email is sent to the new user so they can confirm their email address and set their password.

Trying to keep to [A-Za-z0-9] characters, I believe this creates a base 62 system, taking just under 6 bits to store... which is just over 190 bits of entropy? or 190.53428193238?

As this is a security feature, I don't believe uniqid() alone is a good idea, as this is based on the current microtime.

And I don't believe using encryption or hashing of the users ID or email address is a good solution either (collisions, low entropy, and presumably secured by a single key).

Answer 1

This works, with the PHP 7.0 random_bytes() function:

<?php

function random_key($length, $safe = false) {

    if ($safe !== false) {
        $bad_words = array_map('trim', file('/path/to/bad-words.txt', FILE_IGNORE_NEW_LINES));
    } else {
        $bad_words = NULL;
    }

    $j = 0;

    do {

        $bytes = (ceil($length / 4) * 3); // Must be divisible by 3, otherwise base64 encoding introduces padding characters, and the last character biases towards "0 4 8 A E I M Q U Y c g k o s w".
        $bytes = ($bytes * 2); // Get even more, because some characters will be dropped.

        $key = random_bytes($bytes);
        $key = base64_encode($key);
        $key = str_replace(array('0', 'O', 'I', 'l', '/', '+'), '', $key); // Make URL safe (base58), and drop similar looking characters (no substitutions, as we don't want to bias certain characters)
        $key = substr($key, 0, $length);

        if (preg_match('/[^a-zA-Z0-9]/', $key)) {
            exit_with_error('Invalid characters detected in key "' . $key . '"');
        }

        $valid = (strlen($key) == $length);

        if ($bad_words) {
            foreach ($bad_words as $bad_word) {
                if (stripos($key, $bad_word) !== false) {
                    $valid = false;
                    break;
                }
            }
        }

        if ($valid) {
            return $key;
        }

    } while ($j++ < 10);

    exit_with_error('Cannot generate a safe key after 10 attempts.');

}

?>

This code shows how the base64_encode() function can biases to certain characters:

<?php

$characters = [];

for ($k = 0; $k < 500000; $k++) {

    $key = base64_encode(random_bytes(32)); // 32 bytes results in "=" padding; try changing to 30 to fix.

    foreach (str_split($key) as $c) {
        if (!isset($characters[$c])) {
            $characters[$c] = 0;
        }
        $characters[$c]++;
    }

}

$characters = array_filter($characters, function($value) {
        return ($value > 343750); // ((((33/3)*4)*500000)/64) = 343750, everything else is about ~327000
    });

ksort($characters, SORT_STRING);

print_r($characters);

?>