PayWithAmazon\IpnHandler::verifySignatureIsCorrectFromCertificate PHP Method

    public function verifySignatureIsCorrectFromCertificate($signature)
    {
        $certKey = openssl_get_publickey($this->certificate);
        if ($certKey === False) {
            throw new \Exception("Unable to extract public key from cert");
        }
        try {
            $certInfo = openssl_x509_parse($this->certificate, true);
            $certSubject = $certInfo["subject"];
            if (is_null($certSubject)) {
                throw new \Exception("Error with certificate - subject cannot be found");
            }
        } catch (\Exception $ex) {
            throw new \Exception("Unable to verify certificate - error with the certificate subject", null, $ex);
        }
        if (strcmp($certSubject["CN"], $this->expectedCnName)) {
            throw new \Exception("Unable to verify certificate issued by Amazon - error with certificate subject");
        }
        $result = -1;
        try {
            $result = openssl_verify($this->signatureFields, $signature, $certKey, OPENSSL_ALGO_SHA1);
        } catch (\Exception $ex) {
            throw new \Exception("Unable to verify signature - error with the verification algorithm", null, $ex);
        }
        return $result > 0;
    }