public static function validate(string $key) : bool
    {
        $le = strlen($key);
        if ($le < static::VALID_LENGTH_MIN || $le > static::VALID_LENGTH_MAX) {
            return false;
        }
        if ($key{0} === '-') {
            return false;
        }
        // Faster than a regular expression.
        // Prefix letter to make key without any alphanum pass.
        return !!ctype_alnum('A' . str_replace(static::VALID_NON_ALPHANUM, '', $key));
    }