Using Signed URLs

Overview

Cloud CDN signed URLs enable you to serve responses from Google Cloud Platform's globally distributed caches, even when you need requests to be authorized.

Signed URLs are a mechanism to give a client temporary access to a private resource without requiring additional authorization. To achieve this, selected elements of a request are hashed and cryptographically signed using a strongly random key that you generate. When a request uses the signed URL that you provided, the request is considered authorized to receive the requested content. When Cloud CDN receives a request with a bad signature for an enabled service, the request is rejected and never goes to your backend for handling.

Generally, a signed URL can be used by anyone who has it. However, a signed URL is usually only intended to be used by the client to which the URL was given. To mitigate the risk of the URL being used by a different client, signed URLs expire at a time chosen by you. To minimize the risk of a signed URL being shared, set it to expire as soon as possible.

How URLs are signed

Before you can sign URLs, you create one or more cryptographic keys on a backend service, backend bucket, or both. Then you sign and cryptographically hash a URL using the gcloud command line tool or your own code.

Distributing signed URLs

When signed URL handling is enabled on a backend, Cloud CDN gives special handling to requests with signed URLs. Specifically, requests with a Signature query parameter are considered signed. When such a request is received, Cloud CDN verifies the following:

  1. The HTTP method is GET or HEAD.
  2. The Expires parameter is set to a future time.
  3. The request's signature matches the signature computed using the named key.

If any of these checks fails, a 403 Forbidden response is served. Otherwise, the request is either proxied to the backend or served from the cache. All valid signed requests for a particular base url (the part before the KeyName parameter) will share the same cache entry. Responses to signed and unsigned requests do not share cache entries. Responses are cached and served until the expiration time you set.

Content that requires signed requests is often marked as uncacheable using the Cache-Control header. To make such objects compatible with Cloud CDN without requiring backend changes, Cloud CDN overrides the Cache-Control header when responding to requests that have valid signed URLs. Cloud CDN treats the content as cacheable, and uses the max-age parameter set in your Cloud CDN configuration. The response served still has the Cache-Control headers that the backend generated.

Configuring signed URLs

Enabling signed URLs in your Cloud CDN installation requires several steps, which are described below.

Before you begin

Before you use signed URLs, ensure that Cloud CDN is enabled. For instructions, see Google Cloud CDN Documentation. You can configure signed URLs on a backend before enabling Cloud CDN, but there will be no effect until Cloud CDN is enabled.

If necessary, update to the latest version of Cloud SDK:

gcloud components update

Creating keys

You enable support for Cloud CDN signed URLs by creating one or more keys on a Cloud CDN-enabled backend service , backend bucket, or both.

For each backend service or backend bucket, you can create and delete signed URL keys as your security needs dictate. Each backend can have up to three (3) keys configured at a time. We suggest periodically rotating your keys by deleting the oldest, then adding a new key and using that when signing URLs. You can use the same key name in multiple backend services and backend buckets, because each set of keys is independent of the others. Key names can be up to 63 characters. Use the characters A-Z, a-z, 0-9, _ (underscore), and - (hyphen) to name your keys.

When you create keys, be sure to keep them secure, because anyone who has one of your keys can create signed URLs that Cloud CDN accepts until the key is deleted from Cloud CDN. The keys are stored on the computer where you generate the signed URLs. Cloud CDN also stores the keys to verify request signatures.

To keep the keys secret, the key values are not included in responses to any API requests. If you lose a key, you must create a new one.

Console


To create keys using the Console:

  1. Go to the Cloud CDN page in the Google Cloud Platform Console.
    Go to the Cloud CDN page
  2. Click Add New CDN Distribution, then Get started.
  3. After you select an origin and configure cache rules, under Signed URL, select On. and click Add.
  4. On the Configurations page, click Add signing key.
  5. Under Name, give the new signing key a name.
  6. Under Key creation method, elect Automatically generate or Let me enter.
  7. If you are entering your own key, type the key into the text field.
  8. Click Done.
  9. Under Cache entry maximum age, provide a value, then select a Unit of time from the drop-down list. You can choose among second, minute, hour, and day. The maximum amount of time is three (3) days.
  10. Click Save.
  11. Click Update

gcloud


The gcloud command line tool reads signed URL keys from a local file that you specify. The key file must be created by generating strongly random 128 bits, encoding them with base64, then replacing the character + with - and replacing the character / with _. For more information, see RFC 4648. It is vital that the key is strongly random. On a UNIX-like system, you can generate a strongly random key and store it in the key file with the following command:

head -c 16 /dev/urandom | base64 | tr +/ -_ > [KEY_FILE_NAME]

To add the key to a backend service or backend bucket:

gcloud compute backend-services \
   add-signed-url-key [BACKEND_NAME] \
   --key-name [KEY_NAME] \
   --key-file [KEY_FILE_NAME]
 

gcloud compute backend-buckets \
  add-signed-url-key [BACKEND_NAME] \
  --key-name [KEY_NAME] \
  --key-file [KEY_FILE_NAME]

Configuring Google Cloud Storage permissions

If you use Google Cloud Storage and you have restricted who can read the objects, you must give Cloud CDN permission to read the objects by adding the Cloud CDN service account to Cloud Storage's ACLs. Use the following command, where [PROJECT_NUM] is your project number and [BUCKET] is your storage bucket. For more information on project number, read Locate the project ID and project number in the Cloud Platform Console Help documentation.

The command must be run after you add at least one signed URL key to a backend bucket in your project. Otherwise, you will see an error and the command will fail.

gsutil iam ch \
  serviceAccount:service-[PROJECT_NUM]@cloud-cdn-fill.iam.gserviceaccount.com:objectViewer \
  gs://[BUCKET]

Configuring Google Compute Engine VM instances

Your VMs should validate the signatures on every signed request they serve, and should be prepared to accept or reject unsigned requests. Cloud CDN does not block requests without a Signature query parameter, and your project may be configured to allow requests that bypass the backend service configured above.

Signing URLs

The last step is to sign URLs and distribute them. You can sign URLs with the gcloud compute sign-url command or with code you write yourself, following the examples below. If you need a large number of signed URLs, custom code provides better performance.

Creating signed URLs

Use these instructions to create signed URLs.

Console


You cannot create signed URLs using the Console. You can use the gcloud command-line tool or write custom code, following the examples below.

gcloud


The gcloud command line tool includes a command for signing URLs. The command implements the algorithm described in the section on writing your own code.

gcloud compute sign-url \
  --key-name [KEY_NAME] \
  --key-file [KEY_FILE_NAME] \
  --expires-in [TIME_UNTIL_EXPIRATION] \
  [--validate] \
  "[URL]"

This command reads and decodes the base64url encoded key value from [KEY_FILE_NAME], then outputs a signed URL that you can use for GET or HEAD requests for the given URL.

For example:

gcloud compute sign-url \
  --key-name my-test-key \
  --expires-in 30m \
  --key-file sign-url-key-file \
  "https://example.com"

The URL must be a valid URL that has a path component. For example, http://example.com is invalid, but https://example.com/ and https://example.com/whatever are both valid URLs.

If the optional --validate flag is given, this command sends a HEAD request with the resulting URL, and prints out the HTTP response code. If the signed URL is correct, the response code is the same as the result code sent by your backend. If the response code isn't the same, recheck [KEY_NAME] and the contents of the specified file, and make sure that the value of [TIME_UNTIL_EXPIRATION] is at least several seconds. If --validate is not given, neither the inputs nor the generated URL are verified.

If the --validate flag is not given, the generated signed URL is not verified.

Programmatically creating signed URLs

The code samples below demonstrate how to programmatically create signed URLs.

Go

// SignURL creates a signed URL for an endpoint on Cloud CDN. url must start
// with "https://" and should not have the "Expires", "KeyName", or "Signature"
// query parameters. key should be in raw form (not base64url-encoded) which is
// 16-bytes long. keyName should be added to the backend service or bucket.
func SignURL(url, keyName string, key []byte, expiration time.Time) string {
	sep := "?"
	if strings.Contains(url, "?") {
		sep = "&"
	}
	url += sep
	url += fmt.Sprintf("Expires=%d", expiration.Unix())
	url += fmt.Sprintf("&KeyName=%s", keyName)

	mac := hmac.New(sha1.New, key)
	mac.Write([]byte(url))
	sig := base64.URLEncoding.EncodeToString(mac.Sum(nil))
	url += fmt.Sprintf("&Signature=%s", sig)
	return url
}

// readKeyFile reads the base64url-encoded key file and decodes it.
func readKeyFile(path string) ([]byte, error) {
	b, err := ioutil.ReadFile(path)
	if err != nil {
		return nil, fmt.Errorf("failed to read key file: %+v", err)
	}
	d := make([]byte, base64.URLEncoding.DecodedLen(len(b)))
	n, err := base64.URLEncoding.Decode(d, b)
	if err != nil {
		return nil, fmt.Errorf("failed to base64url decode: %+v", err)
	}
	return d[:n], nil
}

func main() {
	key, err := readKeyFile("/path/to/key")
	if err != nil {
		log.Fatal(err)
	}
	url := SignURL("https://example.com", "MY-KEY", key, time.Now().Add(time.Hour*24))
	fmt.Println(url)
}

Ruby

def signed_url url:, key_name:, key:, expiration:
  # url        = "URL of the endpoint served by Cloud CDN"
  # key_name   = "Name of the signing key added to the Google Cloud Storage bucket or service"
  # key        = "Signing key as urlsafe base64 encoded string"
  # expiration = Ruby Time object with expiration time

  require "base64"
  require "openssl"
  require "time"

  # Decode the URL safe base64 encode key
  decoded_key = Base64.urlsafe_decode64 key

  # Get UTC time in seconds
  expiration_utc = expiration.utc.to_i

  # Determine which separator makes sense given a URL
  separator = "?"
  separator = "&" if url.include? '?'

  # Concatenate url with expected query parameters Expires and KeyName
  url = "#{url}#{separator}Expires=#{expiration_utc}&KeyName=#{key_name}"

  # Sign the url using the key and url safe base64 encode the signature
  signature         = OpenSSL::HMAC.digest "SHA1", decoded_key, url
  encoded_signature = Base64.urlsafe_encode64 signature

  # Concatenate the URL and encoded signature
  signed_url = "#{url}&Signature=#{encoded_signature}"
end

.NET

/// <summary>
/// Creates signed URL for Google Cloud SDN
/// More details about order of operations is here: 
/// <see cref="https://cloud-dot-devsite.googleplex.com/cdn/docs/signed-urls#creatingkeys"/>
/// </summary>
/// <param name="url">The Url to sign. This URL can't include Expires and KeyName query parameters in it</param>
/// <param name="keyName">The name of the key used to sign the URL</param>
/// <param name="encodedKey">The key used to sign the Url</param>
/// <param name="expirationTime">Expiration time of the signature</param>
/// <returns>Signed Url that is valid until {expirationTime}</returns>
public static string CreateSignedUrl(string url, string keyName, string encodedKey, DateTime expirationTime)
{
    var builder = new UriBuilder(url);

    long unixTimestampExpiration = ToUnixTime(expirationTime);

    char queryParam = string.IsNullOrEmpty(builder.Query) ? '?' : '&';
    builder.Query += $"{queryParam}Expires={unixTimestampExpiration}&KeyName={keyName}".ToString();

    // Key is passed as base64url encoded
    byte[] decodedKey = Base64UrlDecode(encodedKey);

    // Computes HMAC SHA-1 hash of the URL using the key
    byte[] hash = ComputeHash(decodedKey, builder.Uri.AbsoluteUri);
    string encodedHash = Base64UrlEncode(hash);

    builder.Query += $"&Signature={encodedHash}";
    return builder.Uri.AbsoluteUri;
}

private static long ToUnixTime(DateTime date)
{
    var epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
    return Convert.ToInt64((date - epoch).TotalSeconds);
}

private static byte[] Base64UrlDecode(string arg)
{
    string s = arg;
    s = s.Replace('-', '+'); // 62nd char of encoding
    s = s.Replace('_', '/'); // 63rd char of encoding

    return Convert.FromBase64String(s); // Standard base64 decoder
}

private static string Base64UrlEncode(byte[] inputBytes)
{
    var output = Convert.ToBase64String(inputBytes);

    output = output.Replace('+', '-')      // 62nd char of encoding
                   .Replace('/', '_');     // 63rd char of encoding

    return output;
}

private static byte[] ComputeHash(byte[] secretKey, string signatureString)
{
    var enc = Encoding.ASCII;
    using (HMACSHA1 hmac = new HMACSHA1(secretKey))
    {
        hmac.Initialize();

        byte[] buffer = enc.GetBytes(signatureString);

        return hmac.ComputeHash(buffer);
    }
}

Java

/**
 * Creates a signed URL for a Cloud CDN endpoint with the given key
 * URL must start with http:// or https://, and must contain a forward
 * slash (/) after the hostname.
 * @param url the Cloud CDN endpoint to sign
 * @param key url signing key uploaded to the backend service/bucket, as a 16-byte array
 * @param keyName the name of the signing key added to the back end bucket or service
 * @param expirationTime the date that the signed URL expires
 * @return a properly formatted signed URL
 * @throws InvalidKeyException when there is an error generating the signature for the input key
 * @throws NoSuchAlgorithmException when HmacSHA1 algorithm is not available in the environment
 */
public static String signUrl(String url,
                             byte[] key,
                             String keyName,
                             Date expirationTime)
        throws InvalidKeyException, NoSuchAlgorithmException {

  final long unixTime = expirationTime.getTime() / 1000;

  String urlToSign = url
                      + (url.contains("?") ? "&" : "?")
                      + "Expires=" + unixTime
                      + "&KeyName=" + keyName;

  String encoded = SignedUrls.getSignature(key, urlToSign);
  return urlToSign + "&Signature=" + encoded;
}

public static String getSignature(byte[] privateKey, String input)
    throws InvalidKeyException, NoSuchAlgorithmException {

  final String algorithm = "HmacSHA1";
  final int offset = 0;
  Key key = new SecretKeySpec(privateKey, offset, privateKey.length, algorithm);
  Mac mac = Mac.getInstance(algorithm);
  mac.init(key);
  return  Base64.getUrlEncoder().encodeToString(mac.doFinal(input.getBytes()));
}

Python

def sign_url(url, key_name, base64_key, expiration_time):
    """Gets the Signed URL string for the specified URL and configuration.

    Args:
        url: URL to sign as a string.
        key_name: name of the signing key as a string.
        base64_key: signing key as a base64 encoded string.
        expiration_time: expiration time as a UTC datetime object.

    Returns:
        Returns the Signed URL appended with the query parameters based on the
        specified configuration.
    """
    stripped_url = url.strip()
    parsed_url = urllib.parse.urlsplit(stripped_url)
    query_params = urllib.parse.parse_qs(
        parsed_url.query, keep_blank_values=True)
    epoch = datetime.datetime.utcfromtimestamp(0)
    expiration_timestamp = int((expiration_time - epoch).total_seconds())
    decoded_key = base64.urlsafe_b64decode(base64_key)

    url_pattern = u'{url}{separator}Expires={expires}&KeyName={key_name}'

    url_to_sign = url_pattern.format(
            url=stripped_url,
            separator='&' if query_params else '?',
            expires=expiration_timestamp,
            key_name=key_name)

    digest = hmac.new(
        decoded_key, url_to_sign.encode('utf-8'), hashlib.sha1).digest()
    signature = base64.urlsafe_b64encode(digest).decode('utf-8')

    signed_url = u'{url}&Signature={signature}'.format(
            url=url_to_sign, signature=signature)

    print(signed_url)

Algorithm for signing URLs

When you write your own code to generate signed URLs, your goal is to create URLs with the following format:

https://example.com/foo?Expires=[EXPIRATION]&KeyName=[KEY_NAME]&Signature=[SIGNATURE]

All URL parameters are case sensitive and must be in the order above.

  1. Ensure that the URL for signing does not have the Expires, KeyName, or Signature query parameters.

  2. Determine when the URL expires and add an Expires query parameter with the desired expiration time in UTC time (the number of seconds since 1970-01-01 00:00:00 UTC). To maximize security, set the value to the shortest time period possible for your use case. The longer a signed URL is valid, the bigger the risk that the user you give it to shares it with others, accidentally or otherwise.

  3. Set the key name. The URL must be signed with a signed URL key of the backend service or backend bucket that will serve the URL. It is best to use the most recently added signed URL key for key rotation. Add the signed URL key to the URL by appending &KeyName=[KEY_NAME], where [KEY_NAME] is the name of the chosen key created in Create keys.

  4. Sign the URL. Create the signed URL by following these steps. Make sure that the query parameters are in the order shown immediately before Step 1, and make sure that nothing in the signed URL changes case.

    a. Hash the entire URL (including http:// or https:// at the beginning and the `&KeyName... at the end) with HMAC-SHA1, using the secret key that corresponds to the key name chosen above. Use the raw 16-byte secret key, not the base64url encoded key. Decode it if needed.

    b. Base64url encode the result.

    c. Append &Signature= to the URL, followed by the encoded signature.

Distributing and using signed URLs

The URL returned from the gcloud command line tool or produced by your custom code can be distributed according to your needs. We recommend signing only HTTPS URLs, because HTTPS provides a secure transport that will prevent the Signature component of the signed URL from being intercepted. Similarly, you should distribute the signed URLs over secure transport protocols such as TLS/HTTPS.

Optionally customizing the maximum cache time

As described above, Cloud CDN caches signed URL responses regardless of the backend’s Cache-Control header. The maximum time they can be cached without revalidation is set by the signed URL cache maximum age flag, which defaults to 1 hour and can be modified as shown here.

To set the maximum cache time for a backend service or backend bucket:

gcloud compute backend-services update [BACKEND_NAME]
  --signed-url-cache-max-age [max-age]
gcloud compute backend-buckets update [BACKEND_NAME]
  --signed-url-cache-max-age [max-age]

Additional gcloud commands

For other operations, use the following commands.

Listing key names

To list the keys on a backend service or backend bucket, use these commands.

gcloud compute backend-services describe [BACKEND_NAME]
gcloud compute backend-buckets describe [BACKEND_NAME]

Deleting keys

When URLs signed by a particular key should no longer be honored, delete that key from the backend service or backend bucket.

gcloud compute backend-services \
   delete-signed-url-key [BACKEND_NAME] --key-name [KEY_NAME]
gcloud compute backend-buckets \
   delete-signed-url-key [BACKEND_NAME] --key-name [KEY_NAME]
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