AWS SNSメッセージの署名検証処理を確認したい

<?php

$json = file_get_contents('php://input');
$message = json_decode($json, true);

$cert = file_get_contents($message['SigningCertURL']);
$pubkey = openssl_pkey_get_public($cert);

// 検証対象文字列を構築（Amazon公式仕様に準拠）
function build_string_to_sign($message) {
    $fields = [];
    if (isset($message['Message'])) $fields[] = "Message\n{$message['Message']}";
    if (isset($message['MessageId'])) $fields[] = "MessageId\n{$message['MessageId']}";
    if (isset($message['Subject'])) $fields[] = "Subject\n{$message['Subject']}";
    if (isset($message['Timestamp'])) $fields[] = "Timestamp\n{$message['Timestamp']}";
    if (isset($message['TopicArn'])) $fields[] = "TopicArn\n{$message['TopicArn']}";
    if (isset($message['Type'])) $fields[] = "Type\n{$message['Type']}";
    
    return implode("\n", $fields) . "\n";
}

// 受信側の署名
$data = build_string_to_sign($message);
// 送信側の署名
$signature = base64_decode($message['Signature']);
$is_signature_valid = openssl_verify($data, $signature, $pubkey, OPENSSL_ALGO_SHA1);

// 検証結果の確認
if ($is_signature_valid !== 1) {
    file_put_contents('log.txt', $message['Message']."失敗！！");
    http_response_code(403);
    exit("Invalid signature");
} else {
    file_put_contents('log.txt', $message['Message']."成功！！");
    http_response_code(200);
    echo "OK";
}