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133 lines
4.1 KiB
PHP
133 lines
4.1 KiB
PHP
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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 $tolerance maximum difference allowed between the header's
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* timestamp and the current time
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* @throws SignatureVerification if the verification fails.
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*/
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public static function verifyHeader($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) && ((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 = array();
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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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