Signing in users with Facebook on iOS

You can let your users authenticate with Identity Platform using their Facebook accounts by integrating Facebook Login or Facebook Limited Login into your app. This page shows you how to use Identity Platform to add Sign in with Facebook to your iOS app.

Before you begin

  1. Add Firebase to your iOS project.

  2. Include the following pods in your Podfile:

    pod 'Firebase/Auth'
    
  3. Go to the Identity Providers page in the Cloud Console.

    Go to the Identity Providers page

  4. Click Add A Provider.

  5. Select Facebook from the list.

  6. Enter your Facebook App ID and App Secret. If you don't already have an ID and secret, you can obtain one from the Facebook for Developers page.

  7. Configure the URI listed under Configure Facebook as a valid OAuth redirect URI for your Facebook app.

  8. Register your app's domains by clicking Add Domain under Authorized Domains. For development purposes, localhost is already enabled by default.

  9. Under Configure your application, click Setup Details. Copy the snippet into your app's code to initialize the Identity Platform Client SDK.

  10. Click Save.

Implement Facebook Login

To use "classic" Facebook Login, complete the following steps. Alternatively, you can use Facebook Limited Login, as shown in the next section.

  1. Integrate Facebook Login into your app by following the developer's documentation. When you initialize the FBSDKLoginButton object, set a delegate to receive login and logout events. For example:

    Swift

    let loginButton = FBSDKLoginButton()
    loginButton.delegate = self
    

    Objective-C

    FBSDKLoginButton *loginButton = [[FBSDKLoginButton alloc] init];
    loginButton.delegate = self;
    
    In your delegate, implement didCompleteWithResult:error:.

    Swift

    func loginButton(_ loginButton: FBSDKLoginButton!, didCompleteWith result: FBSDKLoginManagerLoginResult!, error: Error!) {
      if let error = error {
        print(error.localizedDescription)
        return
      }
      // ...
    }
    

    Objective-C

    - (void)loginButton:(FBSDKLoginButton *)loginButton
        didCompleteWithResult:(FBSDKLoginManagerLoginResult *)result
                        error:(NSError *)error {
      if (error == nil) {
        // ...
      } else {
        NSLog(error.localizedDescription);
      }
    }
    
  2. Import the Firebase module in your UIApplicationDelegate:

    Swift

    import Firebase

    Objective-C

    @import Firebase;
  3. Configure a FirebaseApp shared instance, typically in your app's application:didFinishLaunchingWithOptions: method:

    Swift

    // Use Firebase library to configure APIs
    FirebaseApp.configure()

    Objective-C

    // Use Firebase library to configure APIs
    [FIRApp configure];
  4. After a user successfully signs in, in your implementation of didCompleteWithResult:error:, get an access token for the signed-in user and exchange it for an Identity Platform credential:

    Swift

    let credential = FacebookAuthProvider
      .credential(withAccessToken: AccessToken.current!.tokenString)
    

    Objective-C

    FIRAuthCredential *credential = [FIRFacebookAuthProvider
        credentialWithAccessToken:[FBSDKAccessToken currentAccessToken].tokenString];
    

Implement Facebook Limited Login

To use Facebook Limited Login instead of "classic" Facebook Login, complete the following steps.

  1. Integrate Facebook Limited Login into your app by following the developer's documentation.
  2. For every sign-in request, generate a unique random string—a "nonce"—which you will use to make sure the ID token you get was granted specifically in response to your app's authentication request. This step is important to prevent replay attacks. You can generate a cryptographically secure nonce on iOS with SecRandomCopyBytes(_:_:_), as in the following example:

    Swift

    // Adapted from https://auth0.com/docs/api-auth/tutorials/nonce#generate-a-cryptographically-random-nonce
    private func randomNonceString(length: Int = 32) -> String {
      precondition(length > 0)
      let charset: [Character] =
        Array("0123456789ABCDEFGHIJKLMNOPQRSTUVXYZabcdefghijklmnopqrstuvwxyz-._")
      var result = ""
      var remainingLength = length
    
      while remainingLength > 0 {
        let randoms: [UInt8] = (0 ..< 16).map { _ in
          var random: UInt8 = 0
          let errorCode = SecRandomCopyBytes(kSecRandomDefault, 1, &random)
          if errorCode != errSecSuccess {
            fatalError(
              "Unable to generate nonce. SecRandomCopyBytes failed with OSStatus \(errorCode)"
            )
          }
          return random
        }
    
        randoms.forEach { random in
          if remainingLength == 0 {
            return
          }
    
          if random < charset.count {
            result.append(charset[Int(random)])
            remainingLength -= 1
          }
        }
      }
    
      return result
    }
    
            

    Objective-C

    // Adapted from https://auth0.com/docs/api-auth/tutorials/nonce#generate-a-cryptographically-random-nonce
    - (NSString *)randomNonce:(NSInteger)length {
      NSAssert(length > 0, @"Expected nonce to have positive length");
      NSString *characterSet = @"0123456789ABCDEFGHIJKLMNOPQRSTUVXYZabcdefghijklmnopqrstuvwxyz-._";
      NSMutableString *result = [NSMutableString string];
      NSInteger remainingLength = length;
    
      while (remainingLength > 0) {
        NSMutableArray *randoms = [NSMutableArray arrayWithCapacity:16];
        for (NSInteger i = 0; i < 16; i++) {
          uint8_t random = 0;
          int errorCode = SecRandomCopyBytes(kSecRandomDefault, 1, &random);
          NSAssert(errorCode == errSecSuccess, @"Unable to generate nonce: OSStatus %i", errorCode);
    
          [randoms addObject:@(random)];
        }
    
        for (NSNumber *random in randoms) {
          if (remainingLength == 0) {
            break;
          }
    
          if (random.unsignedIntValue < characterSet.length) {
            unichar character = [characterSet characterAtIndex:random.unsignedIntValue];
            [result appendFormat:@"%C", character];
            remainingLength--;
          }
        }
      }
    
      return [result copy];
    }
            
    You will send the SHA-256 hash of the nonce with your sign-in request, which Facebook will pass unchanged in the response. Identity Platform validates the response by hashing the original nonce and comparing it to the value passed by Facebook.

    Swift

    @available(iOS 13, *)
    private func sha256(_ input: String) -> String {
      let inputData = Data(input.utf8)
      let hashedData = SHA256.hash(data: inputData)
      let hashString = hashedData.compactMap {
        String(format: "%02x", $0)
      }.joined()
    
      return hashString
    }
    
            

    Objective-C

    - (NSString *)stringBySha256HashingString:(NSString *)input {
      const char *string = [input UTF8String];
      unsigned char result[CC_SHA256_DIGEST_LENGTH];
      CC_SHA256(string, (CC_LONG)strlen(string), result);
    
      NSMutableString *hashed = [NSMutableString stringWithCapacity:CC_SHA256_DIGEST_LENGTH * 2];
      for (NSInteger i = 0; i < CC_SHA256_DIGEST_LENGTH; i++) {
        [hashed appendFormat:@"%02x", result[i]];
      }
      return hashed;
    }
            
  3. When you set up the FBSDKLoginButton, set a delegate to receive login and logout events, set the tracking mode to FBSDKLoginTrackingLimited, and attach a nonce. For example:

    Swift

    func setupLoginButton() {
        let nonce = randomNonceString()
        currentNonce = nonce
        loginButton.delegate = self
        loginButton.loginTracking = .limited
        loginButton.nonce = sha256(nonce)
    }
            

    Objective-C

    - (void)setupLoginButton {
      NSString *nonce = [self randomNonce:32];
      self.currentNonce = nonce;
      self.loginButton.delegate = self;
      self.loginButton.loginTracking = FBSDKLoginTrackingLimited
      self.loginButton.nonce = [self stringBySha256HashingString:nonce];
    }
            
    In your delegate, implement didCompleteWithResult:error:.

    Swift

    func loginButton(_ loginButton: FBSDKLoginButton!, didCompleteWith result: FBSDKLoginManagerLoginResult!, error: Error!) {
      if let error = error {
        print(error.localizedDescription)
        return
      }
      // ...
    }
            

    Objective-C

    - (void)loginButton:(FBSDKLoginButton *)loginButton
        didCompleteWithResult:(FBSDKLoginManagerLoginResult *)result
                        error:(NSError *)error {
      if (error == nil) {
        // ...
      } else {
        NSLog(error.localizedDescription);
      }
    }
            
  4. Import the Firebase module in your UIApplicationDelegate:

    Swift

    import Firebase

    Objective-C

    @import Firebase;
  5. Configure a FirebaseApp shared instance, typically in your app's application:didFinishLaunchingWithOptions: method:

    Swift

    // Use Firebase library to configure APIs
    FirebaseApp.configure()

    Objective-C

    // Use Firebase library to configure APIs
    [FIRApp configure];
  6. After a user successfully signs in, in your implementation of didCompleteWithResult:error:, use the ID token from Facebook's response with the unhashed nonce to get an Identity Platform credential:

    Swift

    // Initialize an Identity Platform credential.
    let idTokenString = AuthenticationToken.current?.tokenString
    let nonce = currentNonce
    let credential = OAuthProvider.credential(withProviderID: "facebook.com",
                                              IDToken: idTokenString,
                                              rawNonce: nonce)
            

    Objective-C

    // Initialize an Identity Platform credential.
    NSString *idTokenString = FBSDKAuthenticationToken.currentAuthenticationToken.tokenString;
    NSString *rawNonce = self.currentNonce;
    FIROAuthCredential *credential = [FIROAuthProvider credentialWithProviderID:@"facebook.com"
                                                                        IDToken:idTokenString
                                                                       rawNonce:rawNonce];
            

Authenticate with Identity Platform

Finally, authenticate with Identity Platform using the Identity Platform credential:

Swift

Auth.auth().signIn(with: credential) { authResult, error in
    if let error = error {
      let authError = error as NSError
      if isMFAEnabled, authError.code == AuthErrorCode.secondFactorRequired.rawValue {
        // The user is a multi-factor user. Second factor challenge is required.
        let resolver = authError
          .userInfo[AuthErrorUserInfoMultiFactorResolverKey] as! MultiFactorResolver
        var displayNameString = ""
        for tmpFactorInfo in resolver.hints {
          displayNameString += tmpFactorInfo.displayName ?? ""
          displayNameString += " "
        }
        self.showTextInputPrompt(
          withMessage: "Select factor to sign in\n\(displayNameString)",
          completionBlock: { userPressedOK, displayName in
            var selectedHint: PhoneMultiFactorInfo?
            for tmpFactorInfo in resolver.hints {
              if displayName == tmpFactorInfo.displayName {
                selectedHint = tmpFactorInfo as? PhoneMultiFactorInfo
              }
            }
            PhoneAuthProvider.provider()
              .verifyPhoneNumber(with: selectedHint!, uiDelegate: nil,
                                 multiFactorSession: resolver
                                   .session) { verificationID, error in
                if error != nil {
                  print(
                    "Multi factor start sign in failed. Error: \(error.debugDescription)"
                  )
                } else {
                  self.showTextInputPrompt(
                    withMessage: "Verification code for \(selectedHint?.displayName ?? "")",
                    completionBlock: { userPressedOK, verificationCode in
                      let credential: PhoneAuthCredential? = PhoneAuthProvider.provider()
                        .credential(withVerificationID: verificationID!,
                                    verificationCode: verificationCode!)
                      let assertion: MultiFactorAssertion? = PhoneMultiFactorGenerator
                        .assertion(with: credential!)
                      resolver.resolveSignIn(with: assertion!) { authResult, error in
                        if error != nil {
                          print(
                            "Multi factor finanlize sign in failed. Error: \(error.debugDescription)"
                          )
                        } else {
                          self.navigationController?.popViewController(animated: true)
                        }
                      }
                    }
                  )
                }
              }
          }
        )
      } else {
        self.showMessagePrompt(error.localizedDescription)
        return
      }
      // ...
      return
    }
    // User is signed in
    // ...
}
    

Objective-C

[[FIRAuth auth] signInWithCredential:credential
                          completion:^(FIRAuthDataResult * _Nullable authResult,
                                       NSError * _Nullable error) {
    if (isMFAEnabled && error && error.code == FIRAuthErrorCodeSecondFactorRequired) {
      FIRMultiFactorResolver *resolver = error.userInfo[FIRAuthErrorUserInfoMultiFactorResolverKey];
      NSMutableString *displayNameString = [NSMutableString string];
      for (FIRMultiFactorInfo *tmpFactorInfo in resolver.hints) {
        [displayNameString appendString:tmpFactorInfo.displayName];
        [displayNameString appendString:@" "];
      }
      [self showTextInputPromptWithMessage:[NSString stringWithFormat:@"Select factor to sign in\n%@", displayNameString]
                           completionBlock:^(BOOL userPressedOK, NSString *_Nullable displayName) {
       FIRPhoneMultiFactorInfo* selectedHint;
       for (FIRMultiFactorInfo *tmpFactorInfo in resolver.hints) {
         if ([displayName isEqualToString:tmpFactorInfo.displayName]) {
           selectedHint = (FIRPhoneMultiFactorInfo *)tmpFactorInfo;
         }
       }
       [FIRPhoneAuthProvider.provider
        verifyPhoneNumberWithMultiFactorInfo:selectedHint
        UIDelegate:nil
        multiFactorSession:resolver.session
        completion:^(NSString * _Nullable verificationID, NSError * _Nullable error) {
          if (error) {
            [self showMessagePrompt:error.localizedDescription];
          } else {
            [self showTextInputPromptWithMessage:[NSString stringWithFormat:@"Verification code for %@", selectedHint.displayName]
                                 completionBlock:^(BOOL userPressedOK, NSString *_Nullable verificationCode) {
             FIRPhoneAuthCredential *credential =
                 [[FIRPhoneAuthProvider provider] credentialWithVerificationID:verificationID
                                                              verificationCode:verificationCode];
             FIRMultiFactorAssertion *assertion = [FIRPhoneMultiFactorGenerator assertionWithCredential:credential];
             [resolver resolveSignInWithAssertion:assertion completion:^(FIRAuthDataResult * _Nullable authResult, NSError * _Nullable error) {
               if (error) {
                 [self showMessagePrompt:error.localizedDescription];
               } else {
                 NSLog(@"Multi factor finanlize sign in succeeded.");
               }
             }];
           }];
          }
        }];
     }];
    }
  else if (error) {
    // ...
    return;
  }
  // User successfully signed in. Get user data from the FIRUser object
  if (authResult == nil) { return; }
  FIRUser *user = authResult.user;
  // ...
}];
    

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