Object Storage Service (OSS) SDK for iOS provides two authentication modes to ensure the data security of mobile devices: Security Token Service (STS) authentication mode and self-signed mode.


When you use the STS authentication mode or self-signed mode, ensure that the callback function that you implement returns results for Token and Signature. If you must obtain the token and signature from the app server by sending a request in the callback function, we recommend that you call the synchronous API operations included in the network library. The callback function is run in the child thread of the request generated by the SDK and does not block the main thread.

STS authentication mode

Note To use the STS authentication mode, you must first activate Alibaba Cloud Resource Access Management (RAM).

You can use Alibaba Cloud STS to authorize temporary access to OSS. STS is a web service that provides temporary access tokens for users. You can use STS to grant an access credential that has a custom validity period and custom permissions for a third-party application or a Resource Access Management (RAM) user managed by you. For more information about STS, see What is STS?.

STS has the following benefits:

  • You need only to generate an access token and send the access token to a third-party application, instead of exposing your AccessKey pair to the third-party application. You can customize the access permissions and validity period of this token.
  • The access token automatically expires after the validity period. Therefore, you do not need to manually revoke the permissions of an access token.
Note For more information about how to configure STS, see Use a temporary access credential provided by STS to access OSS in OSS Developer Guide. You can call the AssumeRole operation or use STS SDKs for various programming languages to obtain a temporary access credential. For more information, see STS SDK overview. The temporary access credential contains a security token and a temporary AccessKey pair. The AccessKey pair consists of an AccessKey ID and an AccessKey secret. The minimum validity period of a temporary access credential is 900 seconds. The maximum validity period of a temporary access credential is the maximum session duration specified for the current role. For more information, see Specify the maximum session duration for a RAM role.
  • Configure an STS token

    You can obtain an STS token in the app by a method, such as sending a request to the app server, and use the STS token to initialize the SDK. If you use this method, you must pay attention to the validity period of the STS token. When the STS token is about to expire, you must update the new STS token for the SDK.

    The following code provides an example on how to use an STS token to initialize the SDK:
    Note To use OSSAuthCredentialProvider to initialize OSS SDK for iOS, see Initialization.
    id<OSSCredentialProvider> credential = [[OSSStsTokenCredentialProvider alloc] initWithAccessKeyId:@"<StsToken.AccessKeyId>" secretKeyId:@"<StsToken.SecretKeyId>" securityToken:@"<StsToken.SecurityToken>"];
    client = [[OSSClient alloc] initWithEndpoint:endpoint credentialProvider:credential];

    When the STS token is about to expire, you can create a new OSSClient or update CredentialProvider by using the following method:

    id<OSSCredentialProvider> credential = [[OSSStsTokenCredentialProvider alloc] initWithAccessKeyId:@"<StsToken.AccessKeyId>" secretKeyId:@"<StsToken.SecretKeyId>" securityToken:@"<StsToken.SecurityToken>"];
    client = [[OSSClient alloc] initWithEndpoint:endpoint credentialProvider:credential];
  • Obtain an STS token by implementing callback

    If you want the SDK to automatically update the STS token, you must implement callback in your app. The app uses the callback to obtain a federation token (STS token) and returns the token to the SDK. The SDK uses the STS token to generate signatures. When the STS token needs to be updated, the SDK calls the callback to obtain a new token.

    id<OSSCredentialProvider> credential = [[OSSFederationCredentialProvider alloc] initWithFederationTokenGetter:^OSSFederationToken * {
        // Implement a method to obtain a FederationToken and return it as an OSSFederationToken object. 
        // If the FederationToken is not obtained, nil is returned. 
          OSSFederationToken * token;
        // Obtain a FederationToken from your server. 
        return token;
    client = [[OSSClient alloc] initWithEndpoint:endpoint credentialProvider:credential];
    Note Additionally, if you obtain all fields required to generate a token in other ways, you can also return these fields by using the callback. In this case, you must manually update the token, and then reconfigure OSSCredentialProvider of the OSSClient instance.


    The URL of the server from which you obtain the token is http://localhost:8080/distribute-token.json. If you access this URL, the following similar data is returned:

        "StatusCode": 200,

    The following code provides an example on how to implement OSSFederationCredentialProvider:

    id<OSSCredentialProvider> credential2 = [[OSSFederationCredentialProvider alloc] initWithFederationTokenGetter:^OSSFederationToken * {
        // Create a request to access your service server. 
        NSURL * url = [NSURL URLWithString:@"http://localhost:8080/distribute-token.json"];
        // Use a request to set the parameters required by your server. 
        NSURLRequest * request = [NSURLRequest requestWithURL:url];
        OSSTaskCompletionSource * tcs = [OSSTaskCompletionSource taskCompletionSource];
        NSURLSession * session = [NSURLSession sharedSession];
        // Send the request. 
        NSURLSessionTask * sessionTask = [session dataTaskWithRequest:request
                                                    completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
                                                        if (error) {
                                                            [tcs setError:error];
                                                        [tcs setResult:data];
        [sessionTask resume];
        // Wait until the response to the request is returned. 
        [tcs.task waitUntilFinished];
        // Parse the returned results. 
        if (tcs.task.error) {
            NSLog(@"get token error: %@", tcs.task.error);
            return nil;
        } else {
            // The returned data is in the JSON format. Parse the data to obtain the values of the fields of the token. 
            NSDictionary * object = [NSJSONSerialization JSONObjectWithData:tcs.task.result
            OSSFederationToken * token = [OSSFederationToken new];
            token.tAccessKey = [object objectForKey:@"AccessKeyId"];
            token.tSecretKey = [object objectForKey:@"AccessKeySecret"];
            token.tToken = [object objectForKey:@"SecurityToken"];
            token.expirationTimeInGMTFormat = [object objectForKey:@"Expiration"];
            NSLog(@"get token: %@", token);
            return token;

Self-signed mode

You can perform the following operations to save the AccessKey ID and AccessKey secret on your own server:
  1. Obtain the string-to-sign from the client and send the string to your own server.
    1. Use the signContent method of OSSCustomSignerCredentialProvider provided by OSS SDK for iOS to obtain the string-to-sign when you create the request.
    2. Send the string-to-sign to your own server.
  2. Sign the string on your own server and return the signed string to the client.
    1. Use the specified signature algorithm to sign the string. For more information about the signature algorithm, see Include signatures in the Authorization header.

      The signature is in the following format: signature = "OSS " + AccessKeyId + ":" + base64(hmac-sha1(AccessKeySecret, content)), in which content is the string that is concatenated based on the request parameters. Sample code:

      id<OSSCredentialProvider> credential = [[OSSCustomSignerCredentialProvider alloc] initWithImplementedSigner:^NSString *(NSString *contentToSign, NSError *__autoreleasing *error) {
          // Use the specified signature algorithm to sign a string, concatenate your AccessKey ID to the signed string, and then return the final string. 
          // Send the signed string to your own server and return the signature. 
          // If the string fails to be signed, the server returns nil with an error message. 
      NSString *signature = [OSSUtil calBase64Sha1WithData:contentToSign withSecret:@"<your accessKeySecret>"]; // In this example, the string is signed on the client by using the tool provided by OSS SDK for iOS. We recommend that you sign the string on your business server. 
          if (signature != nil) {
              *error = nil;
          } else {
              *error = [NSError errorWithDomain:@"<your domain>" code:-1001 userInfo:@"<your error info>"];
              return nil;
          return [NSString stringWithFormat:@"OSS %@:%@", @"<your accessKeyId>", signature];
      client = [[OSSClient alloc] initWithEndpoint:endpoint credentialProvider:credential];
    2. Return the signed string to the client.
  3. Send the signed string on the client to the OSS server for authentication.