php - 署名付き URL での CloudFront "MalformedPolicy" エラー

Question

PHP を使用してカスタム ポリシーで署名付きの CloudFront URL を作成する必要がありますが、何をしても私のポリシーは明らかに「不正な形式」です。関数で生成されたポリシーの例を次に示します。

{"Statement":{"Resource":"https://d15xojelh58w5d.cloudfront.net/memo/kwz/cvyhkfdqn5oz0z1dz5at4z4s1jsn.kwz","Condition":{"DateLessThan":{"AWS:EpochTime":1490463203},"IpAddress":{"AWS:SourceIp":"1.2.3.4/32"}}}}

生成された URL:

https://d15xojelh58w5d.cloudfront.net/memo/kwz/cvyhkfdqn5oz0z1dz5at4z4s1jsn.kwz?Policy=eyJTdGF0ZW1lbnQiOnsiUmVzb3VyY2UiOiJodHRwczovL2QxNXhvamVsaDU4dzVkLmNsb3VkZnJvbnQubmV0L21lbW8va3d6L2N2eWhrZmRxbjVvejB6MWR6NWF0NHo0czFqc24ua3d6IiwiQ29uZGl0aW9uIjp7IkRhdGVMZXNzVGhhbiI6eyJBV1M6RXBvY2hUaW1lIjoxNDkwNDYzMjAzfSwiSXBBZGRyZXNzIjp7IkFXUzpTb3VyY2VJcCI6IjEuMi4zLjQvMzIifX19fQ__&Signature=MmBPtpipFLuNwaPliGLJajG4gJ7INwD0ptFdxPFYQP9CT-luq6W0SrAs9O9CqbJPHoukXwDzG~c88Rr5I2I9KP5QwD8MHpogGh~3SM3gBYm8ao0Zm7a5C9tWnBVRCtzuGrCrFstK-qLswWmqo6tNiOynSuFpvm9uDe3C8oWE2RzSZavEXoL35D3F8y98NeM0aOJe37EeSpdz3lrZZxei2TugoO-OmnApXa2YYJR2HiQ2l0t8paxcb3xyhCK1c1AR51uOpWLm63k~d0eNZJGo3x0Y6bx0GBqafdvV6jiUv6PbhiMC1ZcTxGnZhLmsz3~ONsEvaR1jyyOPt6y9Nos8yA__&Key-Pair-Id=APKAJ6RV6ACUX5M5IAOQ

コード：

function cloudfront_sign($url, $expiry = null, $ipLock = true) {
    $policy = array(
        'Statement' => array(
            'Resource' => $url,
            'Condition' => array(),
        ),
    );

    if(!$expiry || $expiry <= time()) $expiry = 2147483647; // CloudFront *requires* an expiry date, so set to 03:14:07 UTC on Tuesday, 19 January 2038 if one is not provided
    $policy['Statement']['Condition']['DateLessThan'] = array('AWS:EpochTime' => $expiry);
    if($ipLock) $policy['Statement']['Condition']['IpAddress'] = array('AWS:SourceIp' => $_SERVER['REMOTE_ADDR'].'/32');

    $signer = new Aws\CloudFront\UrlSigner($_config['keyID'], $_config['keyPath']);

    $jsonPolicy = json_encode($policy, JSON_UNESCAPED_SLASHES);

    $url = $signer->getSignedUrl($url, null, $jsonPolicy);

    return $url;
}
$url = kaeru_cloudfront_sign('https://d15xojelh58w5d.cloudfront.net/memo/kwz/cvyhkfdqn5oz0z1dz5at4z4s1jsn.kwz', 1490463203);