You can let your users authenticate with Firebase using OAuth providers like Yahoo by integrating generic OAuth Login into your app using the Firebase SDK to carry out the end-to-end sign-in flow. ## Before you begin To sign in users using Yahoo accounts, you must first enable Yahoo as a sign-in provider for your Firebase project: 1. [Add Firebase to your Apple project](https://firebase.google.com/docs/ios/setup.md.txt). 2. In the Firebase console, go to **Security** \> [**Authentication**](https://console.firebase.google.com/project/_/authentication/). 3. In the **Sign-in method** tab, enable the **Yahoo** sign-in provider. 4. Add the **Client ID** and **Client Secret** from that provider's developer console to the provider configuration: 1. To register a Yahoo OAuth client, follow the Yahoo developer documentation on [registering a web application with Yahoo](https://developer.yahoo.com/oauth2/guide/openid_connect/getting_started.html). Be sure to select the two OpenID Connect API permissions: `profile` and `email`. 2. When registering apps with these providers, be sure to register the `*.firebaseapp.com` domain for your project as the redirect domain for your app. 5. Click **Save**. ## Handle the sign-in flow with the Firebase SDK To handle the sign-in flow with the Firebase Apple platforms SDK, follow these steps: 1. Add custom URL schemes to your Xcode project: 1. Open your project configuration: double-click the project name in the left tree view. Select your app from the **TARGETS** section, then select the **Info** tab, and expand the **URL Types** section. 2. Click the **+** button, and add your Encoded App ID as a URL scheme. Leave the other fields blank. You can find your Encoded App ID in the Firebase console: go to the **Settings** \> [**General** tab](https://console.firebase.google.com/project/_/settings/general/), then scroll down to the **Your apps** section to view details about your iOS app. When completed, your config should look something similar to the following (but with your application-specific values): ![Screenshot of Xcode's custom URL scheme setup interface](https://firebase.google.com/static/docs/auth/images/app-id-url-scheme.png)
2. Create an instance of an **OAuthProvider** using the provider ID **yahoo.com**. #### Swift ```swift var provider = OAuthProvider(providerID: "yahoo.com") ``` #### Objective-C ```objective-c FIROAuthProvider *provider = [FIROAuthProvider providerWithProviderID:@"yahoo.com"]; ``` 3. **Optional**: Specify additional custom OAuth parameters that you want to send with the OAuth request. #### Swift ```swift provider.customParameters = [ "prompt": "login", "language": "fr" ] ``` #### Objective-C ```objective-c [provider setCustomParameters:@{@"prompt": @"login", @"language": @"fr"}]; ``` For the parameters Yahoo supports, see the [Yahoo OAuth documentation](https://developer.yahoo.com/oauth2/guide/openid_connect/getting_started.html). Note that you can't pass Firebase-required parameters with `setCustomParameters`. These parameters are **client_id** , **redirect_uri** , **response_type** , **scope** , and **state**. 4. **Optional** : Specify additional OAuth 2.0 scopes beyond `profile` and `email` that you want to request from the authentication provider. If your application requires access to private user data from Yahoo APIs, you'll need to request permissions to Yahoo APIs under **API Permissions** in the Yahoo developer console. Requested OAuth scopes must be exact matches to the preconfigured ones in the app's API permissions. For example if, read/write access is requested to user contacts and preconfigured in the app's API permissions, `sdct-w` has to be passed instead of the readonly OAuth scope `sdct-r`. Otherwise, the flow will fail and an error would be shown to the end user. #### Swift ```swift // Request access to Yahoo Mail API. // Request read/write access to user contacts. // This must be preconfigured in the app's API permissions. provider.scopes = ["mail-r", "sdct-w"] ``` #### Objective-C ```objective-c // Request access to Yahoo Mail API. // Request read/write access to user contacts. // This must be preconfigured in the app's API permissions. [provider setScopes:@[@"mail-r", @"sdct-w"]]; ``` To learn more, refer to the [Yahoo scopes documentation](https://developer.yahoo.com/oauth2/guide/yahoo_scopes/). 5. **Optional** : If you want to customize the way your app presents the `SFSafariViewController` or `UIWebView` when displaying the reCAPTCHA to the user, create a custom class that conforms to the `AuthUIDelegate` protocol, and pass it to `credentialWithUIDelegate`. 6. Authenticate with Firebase using the OAuth provider object. #### Swift ```swift provider.getCredentialWith(nil) { credential, error in if error != nil { // Handle error. } if credential != nil { Auth().signIn(with: credential) { authResult, error in if error != nil { // Handle error. } // User is signed in. // IdP data available in authResult.additionalUserInfo.profile. // Yahoo OAuth access token can also be retrieved by: // (authResult.credential as? OAuthCredential)?.accessToken // Yahoo OAuth ID token can be retrieved by calling: // (authResult.credential as? OAuthCredential)?.idToken } } } ``` #### Objective-C ```objective-c [provider getCredentialWithUIDelegate:nil completion:^(FIRAuthCredential *_Nullable credential, NSError *_Nullable error) { if (error) { // Handle error. } if (credential) { [[FIRAuth auth] signInWithCredential:credential completion:^(FIRAuthDataResult *_Nullable authResult, NSError *_Nullable error) { if (error) { // Handle error. } // User is signed in. // IdP data available in authResult.additionalUserInfo.profile. // Yahoo OAuth access token can also be retrieved by: // ((FIROAuthCredential *)authResult.credential).accessToken // Yahoo OAuth ID token can be retrieved by calling: // ((FIROAuthCredential *)authResult.credential).idToken }]; } }]; ``` Using the OAuth access token, you can call the [Yahoo API](https://developer.yahoo.com/oauth2/guide/apirequests/). For example, to get basic profile information, you can call the REST API, passing the access token in the `Authorization` header: ``` https://social.yahooapis.com/v1/user/YAHOO_USER_UID/profile?format=json ``` Where `YAHOO_USER_UID` is the Yahoo user's ID which can be retrieved from the `Auth.auth.currentUser.providerData[0].uid` field or from `authResult.additionalUserInfo.profile`. 7. While the preceding examples focus on sign-in flows, you can also link a Yahoo provider to an existing user using `linkWithPopup`. For example, you can link multiple providers to the same user, allowing them to sign in with either. #### Swift ```swift Auth().currentUser.link(withCredential: credential) { authResult, error in if error != nil { // Handle error. } // Yahoo credential is linked to the current user. // IdP data available in authResult.additionalUserInfo.profile. // Yahoo OAuth access token can also be retrieved by: // (authResult.credential as? OAuthCredential)?.accessToken // Yahoo OAuth ID token can be retrieved by calling: // (authResult.credential as? OAuthCredential)?.idToken } ``` #### Objective-C ```objective-c [[FIRAuth auth].currentUser linkWithCredential:credential completion:^(FIRAuthDataResult * _Nullable authResult, NSError * _Nullable error) { if (error) { // Handle error. } // Yahoo credential is linked to the current user. // IdP data available in authResult.additionalUserInfo.profile. // Yahoo OAuth access token is can also be retrieved by: // ((FIROAuthCredential *)authResult.credential).accessToken // Yahoo OAuth ID token can be retrieved by calling: // ((FIROAuthCredential *)authResult.credential).idToken }]; ``` 8. You can use the same pattern with `reauthenticateWithPopup` or `reauthenticateWithRedirect` to retrieve fresh credentials for sensitive operations that require recent login. #### Swift ```swift Auth().currentUser.reauthenticateWithCredential(withCredential: credential) { authResult, error in if error != nil { // Handle error. } // User is re-authenticated with fresh tokens minted and // should be able to perform sensitive operations like account // deletion and email or password update. // IdP data available in result.additionalUserInfo.profile. // Additional OAuth access token is can also be retrieved by: // (authResult.credential as? OAuthCredential)?.accessToken // Yahoo OAuth ID token can be retrieved by calling: // (authResult.credential as? OAuthCredential)?.idToken } ``` #### Objective-C ```objective-c [[FIRAuth auth].currentUser reauthenticateWithCredential:credential completion:^(FIRAuthDataResult * _Nullable authResult, NSError * _Nullable error) { if (error) { // Handle error. } // User is re-authenticated with fresh tokens minted and // should be able to perform sensitive operations like account // deletion and email or password update. // IdP data available in result.additionalUserInfo.profile. // Additional OAuth access token is can also be retrieved by: // ((FIROAuthCredential *)authResult.credential).accessToken // Yahoo OAuth ID token can be retrieved by calling: // ((FIROAuthCredential *)authResult.credential).idToken }]; ``` ## Handling account-exists-with-different-credential Errors If you enabled the **One account per email address** setting in the [Firebase console](https://console.firebase.google.com/), when a user tries to sign in a to a provider (such as Yahoo) with an email that already exists for another Firebase user's provider (such as Google), the error `FIRAuthErrorCodeAccountExistsWithDifferentCredential` is thrown along with a temporary `FIRAuthCredential` object (Yahoo credential). To complete the sign in to the intended provider, the user has to sign first to the existing provider (Google) and then link to the former `FIRAuthCredential` (Yahoo credential). This would look as illustrated below: #### Swift ```swift // Sign-in with an OAuth credential. provider.getCredentialWith(nil) { credential, error in // An account with the same email already exists. if (error as NSError?)?.code == AuthErrorCode.accountExistsWithDifferentCredential.rawValue { // Get pending credential and email of existing account. let existingAcctEmail = (error! as NSError).userInfo[AuthErrorUserInfoEmailKey] as! String let pendingCred = (error! as NSError).userInfo[AuthErrorUserInfoUpdatedCredentialKey] as! AuthCredential // Lookup existing account identifier by the email. Auth.auth().fetchProviders(forEmail:existingAcctEmail) { providers, error in // Existing email/password account. if (providers?.contains(EmailAuthProviderID))! { // Existing password account for email. Ask user to provide the password of the // existing account. // Sign in with existing account. Auth.auth().signIn(withEmail:existingAcctEmail, password:password) { user, error in // Successfully signed in. if user != nil { // Link pending credential to account. Auth.auth().currentUser?.linkAndRetrieveData(with: pendingCred) { result, error in // ... } } } } } return } // Other errors. if error != nil { // handle the error. return } // Sign in with the credential. if credential != nil { Auth.auth().signInAndRetrieveData(with: credential!) { result, error in if error != nil { // handle the error. return } } } } ``` #### Objective-C ```objective-c // Sign-in with an OAuth credential. [provider getCredentialWithUIDelegate:nil completion:^(FIRAuthCredential *_Nullable credential, NSError *_Nullable error) { // An account with the same email already exists. if (error.code == FIRAuthErrorCodeAccountExistsWithDifferentCredential) { // Get pending credential and email of existing account. NSString *existingAcctEmail = error.userInfo[FIRAuthErrorUserInfoEmailKey]; FIRAuthCredential *pendingCred = error.userInfo[FIRAuthErrorUserInfoUpdatedCredentialKey]; // Lookup existing account identifier by the email. [[FIRAuth auth] fetchProvidersForEmail:existingAcctEmail completion:^(NSArray *_Nullable providers, NSError *_Nullable error) { // Existing email/password account. if ( [providers containsObject:FIREmailAuthProviderID] ) { // Existing password account for email. Ask user to provide the password of the // existing account. // Sign in with existing account. [[FIRAuth auth] signInWithEmail:existingAcctEmail password:password completion:^(FIRUser *user, NSError *error) { // Successfully signed in. if (user) { // Link pending credential to account. [[FIRAuth auth].currentUser linkWithCredential:pendingCred completion:^(FIRUser *_Nullable user, NSError *_Nullable error) { // ... }]; } }]; } }]; return; } // Other errors. if (error) { // handle the error. return; } // Sign in with the credential. if (credential) { [[FIRAuth auth] signInAndRetrieveDataWithCredential:credential completion:^(FIRAuthDataResult *_Nullable authResult, NSError *_Nullable error) { if (error) { // handle the error. return; } }]; } }]; ``` ## Advanced: Handle the sign-in flow manually Unlike other OAuth providers supported by Firebase such as Google, Facebook, and Twitter, where sign-in can directly be achieved with OAuth access token based credentials, Firebase Auth does not support the same capability for providers such as Yahoo due to the inability of the Firebase Auth server to verify the audience of Yahoo OAuth access tokens. This is a critical security requirement and could expose applications and websites to replay attacks where a Yahoo OAuth access token obtained for one project (attacker) can be used to sign in to another project (victim). Instead, Firebase Auth offers the ability to handle the entire OAuth flow and the authorization code exchange using the OAuth client ID and secret configured in the Firebase Console. As the authorization code can only be used in conjunction with a specific client ID/secret, an authorization code obtained for one project cannot be used with another. If these providers are required to be used in unsupported environments, a third party OAuth library and [Firebase custom authentication](https://firebase.google.com/docs/auth/admin/create-custom-tokens.md.txt) would need to be used. The former is needed to authenticate with the provider and the latter to exchange the provider's credential for a custom token. ## Next steps After a user signs in for the first time, a new user account is created and linked to the credentials---that is, the user name and password, phone number, or auth provider information---the user signed in with. This new account is stored as part of your Firebase project, and can be used to identify a user across every app in your project, regardless of how the user signs in. - In your apps, you can get the user's basic profile information from the [`User`](https://firebase.google.com/docs/reference/ios/firebaseauth/interface_f_i_r_user) object. See [Manage Users](https://firebase.google.com/docs/auth/ios/manage-users.md.txt). - In your Firebase Realtime Database and Cloud Storage [Security Rules](https://firebase.google.com/docs/database/security/user-security), you can get the signed-in user's unique user ID from the `auth` variable, and use it to control what data a user can access. You can allow users to sign in to your app using multiple authentication providers by [linking auth provider credentials to an existing user account.](https://firebase.google.com/docs/auth/ios/account-linking) To sign out a user, call [`signOut:`](https://firebase.google.com/docs/reference/ios/firebaseauth/interface_f_i_r_auth#ab0d5111f05c3f1906243852cc8ef41b1). #### Swift ```swift let firebaseAuth = Auth.auth() do { try firebaseAuth.signOut() } catch let signOutError as NSError { print("Error signing out: %@", signOutError) } ``` #### Objective-C ```objective-c NSError *signOutError; BOOL status = [[FIRAuth auth] signOut:&signOutError]; if (!status) { NSLog(@"Error signing out: %@", signOutError); return; } ``` You may also want to add error handling code for the full range of authentication errors. See [Handle Errors](https://firebase.google.com/docs/auth/ios/errors.md.txt).