How Can I Generate Secure Alphanumeric Strings for Verification Links in PHP?

Generate Unique Alphanumeric Strings for Verification Links

Creating a random, unique string that contains both letters and numbers is a secure method for verification links in account creation processes. Here are two PHP functions that can be used for this purpose:

PHP 7 (Recommended)

The PHP 7 standard library includes the random_bytes() function, which generates cryptographically secure pseudo-random bytes.

$bytes = random_bytes(20);
echo bin2hex($bytes);

PHP 5 (Outdated)

For PHP 5, it's recommended to use openssl_random_pseudo_bytes() for generating secure tokens:

echo bin2hex(openssl_random_pseudo_bytes(20));

Additional Considerations

To enhance security and limit token guessing, it's advisable to create a custom function that meets specific requirements. Here's a comprehensive implementation that addresses those requirements:

function crypto_rand_secure($min, $max) {
    // Calculate and filter random bits
    $bytes = (int) (($max - $min) / 8) + 1;
    $bits = (int) log($max - $min, 2) + 1;
    $filter = (int) (1 << $bits) - 1;

    do {
        $rnd = hexdec(bin2hex(openssl_random_pseudo_bytes($bytes)));
        $rnd &= $filter;
    } while ($rnd > $range);

    return $min + $rnd;
}

function getToken($length) {
    // Define character set and calculate maximum length
    $codeAlphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
    $max = strlen($codeAlphabet); // edited

    // Generate token of specified length
    $token = "";
    for ($i = 0; $i < $length; $i++) {
        $token .= $codeAlphabet[crypto_rand_secure(0, $max-1)];
    }

    return $token;
}

This function provides a secure alternative to rand() and mt_rand, ensuring that generated tokens are difficult to guess and satisfy the desired criteria.

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