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<?php |
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|
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/** |
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* Helper functions for security-related things. |
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*/ |
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class Dropbox_Security |
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{ |
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/** |
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* A string equality function that compares strings in a way that isn't suceptible to timing |
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* attacks. An attacker can figure out the length of the string, but not the string's value. |
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* |
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* Use this when comparing two strings where: |
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* - one string could be influenced by an attacker |
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* - the other string contains data an attacker shouldn't know |
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* |
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* @param string $a |
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* @param string $b |
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* |
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* @return bool |
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*/ |
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public static function stringEquals($a, $b) |
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{ |
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// Be strict with arguments. PHP's liberal types could get us pwned. |
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if (func_num_args() !== 2) { |
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throw new InvalidArgumentException("Expecting 2 args, got ".func_num_args()."."); |
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} |
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Dropbox_Checker::argString("a", $a); |
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Dropbox_Checker::argString("b", $b); |
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|
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if (strlen($a) !== strlen($b)) { |
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return false; |
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} |
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$result = 0; |
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for ($i = 0; $i < strlen($a); $i++) { |
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$result |= ord($a[$i]) ^ ord($b[$i]); |
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} |
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|
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return $result === 0; |
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} |
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|
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/** |
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* Returns cryptographically strong secure random bytes (as a PHP string). |
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* |
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* @param int $numBytes |
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* The number of bytes of random data to return. |
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* |
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* @return string |
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*/ |
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public static function getRandomBytes($numBytes) |
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{ |
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Dropbox_Checker::argIntPositive("numBytes", $numBytes); |
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|
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// openssl_random_pseudo_bytes had some issues prior to PHP 5.3.4 |
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if (function_exists('openssl_random_pseudo_bytes') |
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&& version_compare(PHP_VERSION, '5.3.4') >= 0 |
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) { |
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$s = openssl_random_pseudo_bytes($numBytes, $isCryptoStrong); |
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if ($isCryptoStrong) { |
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return $s; |
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} |
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} |
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|
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if (function_exists('mcrypt_create_iv')) { |
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return mcrypt_create_iv($numBytes); |
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} |
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|
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// Hopefully the above two options cover all our users. But if not, there are |
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// other platform-specific options we could add. |
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throw new Exception("no suitable random number source available"); |
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} |
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} |
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|
