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<?php |
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namespace Stripe; |
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abstract class WebhookSignature |
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{ |
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const EXPECTED_SCHEME = "v1"; |
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/** |
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* Verifies the signature header sent by Stripe. Throws a |
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* SignatureVerification exception if the verification fails for any |
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* reason. |
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* |
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* @param string $payload the payload sent by Stripe. |
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* @param string $header the contents of the signature header sent by |
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* Stripe. |
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* @param string $secret secret used to generate the signature. |
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* @param int $tolerance maximum difference allowed between the header's |
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* timestamp and the current time |
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* @throws \Stripe\Error\SignatureVerification if the verification fails. |
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* @return bool |
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*/ |
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public static function verifyHeader($payload, $header, $secret, $tolerance = null) |
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{ |
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// Extract timestamp and signatures from header |
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$timestamp = self::getTimestamp($header); |
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$signatures = self::getSignatures($header, self::EXPECTED_SCHEME); |
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if ($timestamp == -1) { |
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throw new Error\SignatureVerification( |
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"Unable to extract timestamp and signatures from header", |
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$header, |
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$payload |
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); |
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} |
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if (empty($signatures)) { |
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throw new Error\SignatureVerification( |
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"No signatures found with expected scheme", |
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$header, |
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$payload |
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); |
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} |
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// Check if expected signature is found in list of signatures from |
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// header |
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$signedPayload = "$timestamp.$payload"; |
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$expectedSignature = self::computeSignature($signedPayload, $secret); |
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$signatureFound = false; |
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foreach ($signatures as $signature) { |
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if (Util\Util::secureCompare($expectedSignature, $signature)) { |
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$signatureFound = true; |
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break; |
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} |
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} |
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if (!$signatureFound) { |
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throw new Error\SignatureVerification( |
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"No signatures found matching the expected signature for payload", |
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$header, |
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$payload |
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); |
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} |
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// Check if timestamp is within tolerance |
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if (($tolerance > 0) && (abs(time() - $timestamp) > $tolerance)) { |
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throw new Error\SignatureVerification( |
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"Timestamp outside the tolerance zone", |
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$header, |
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$payload |
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); |
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} |
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return true; |
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} |
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/** |
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* Extracts the timestamp in a signature header. |
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* |
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* @param string $header the signature header |
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* @return int the timestamp contained in the header, or -1 if no valid |
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* timestamp is found |
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*/ |
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private static function getTimestamp($header) |
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{ |
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$items = explode(",", $header); |
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foreach ($items as $item) { |
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$itemParts = explode("=", $item, 2); |
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if ($itemParts[0] == "t") { |
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if (!is_numeric($itemParts[1])) { |
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return -1; |
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} |
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return intval($itemParts[1]); |
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} |
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} |
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return -1; |
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} |
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/** |
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* Extracts the signatures matching a given scheme in a signature header. |
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* |
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* @param string $header the signature header |
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* @param string $scheme the signature scheme to look for. |
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* @return array the list of signatures matching the provided scheme. |
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*/ |
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private static function getSignatures($header, $scheme) |
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{ |
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$signatures = []; |
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$items = explode(",", $header); |
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foreach ($items as $item) { |
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$itemParts = explode("=", $item, 2); |
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if ($itemParts[0] == $scheme) { |
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array_push($signatures, $itemParts[1]); |
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} |
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} |
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return $signatures; |
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} |
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/** |
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* Computes the signature for a given payload and secret. |
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* |
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* The current scheme used by Stripe ("v1") is HMAC/SHA-256. |
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* |
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* @param string $payload the payload to sign. |
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* @param string $secret the secret used to generate the signature. |
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* @return string the signature as a string. |
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*/ |
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private static function computeSignature($payload, $secret) |
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{ |
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return hash_hmac("sha256", $payload, $secret); |
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} |
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} |
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