Dissect the PKCE Authorization Code Grant Circulate on iOS


Proof Key for Code Alternate (PKCE) is an addition to the OAuth authorization framework that protects the authorization circulation from safety assaults. As information homeowners undertake the protocol, it’s obligatory for functions utilizing their APIs to authorize entry utilizing this new protocol.

On this tutorial, you’ll construct an app referred to as MyGoogleInfo. The app authenticates customers with Google utilizing the OAuth authorization circulation with PKCE, and it makes use of the Google API to retrieve customers’ profile names and photos.

Right here’s what you’ll study:

  • The OAuth 2.0 authorization code grant circulation particulars and its vulnerability.
  • What the PKCE authorization circulation is and the way it strengthens the OAuth circulation.
  • Easy methods to configure entry to the Google API on the Google Cloud console.
  • Easy methods to implement the PKCE authorization circulation in Swift to authenticate the person.
  • Easy methods to use the offered token to entry the Google API.

When you’ve got ever puzzled how the authentication protocol works or for those who’re eager about utilizing an API from one of many distinguished suppliers in your subsequent mission, keep tuned. You’ll get all the small print on this article.

Getting Began

Obtain the starter mission by clicking the Obtain Supplies button on the prime or backside of the tutorial.

Open MyGoogleInfo.xcodeproj within the starter folder. Construct and run. The Login display will appear like this.

MyGoogleInfo Login screen.

The Login button doesn’t do something but. You’ll implement the PKCE circulation with the Google OAuth service in PKCEAuthenticationService.

As soon as that’s performed, when the person logs in, MyGoogleInfo presents the person’s profile data.

MyGoogleInfo Profile screen.

Introducing the OAuth 2.0 Authorization Framework

The OAuth 2 Authorization framework is the usual protocol used for shopper authentication. The principle thought behind OAuth authorization is the separation of roles. Particularly, the usual defines a protocol to permit information homeowners to delegate purchasers to entry their information, with out giving them their credentials.

Listed here are some phrases to know:

  • Useful resource Proprietor: That is the entity that owns the assets your app want to entry. Sometimes, that is you, holding your information.
  • Consumer: The applying that wishes to entry the info on the useful resource server, corresponding to MyGoogleInfo on this case.
  • Authorization server: The server in control of authenticating the person and issuing the tokens to the shopper.
  • Useful resource Server: The server internet hosting the info to entry. An entry token protects the entry to the useful resource server.

Authorization Code Grant Circulate

This diagram represents the OAuth 2.0 Authorization code grant circulation that cellular functions implement:

OAuth 2.0 Authorization Flow.

  1. The person begins the login circulation by tapping the MyGoogleInfo Login button.
  2. Consequently, the app asks the authorization server to determine the person and ask their consent to entry the info. The request features a client_id in order that the server can determine the app requesting the entry.
  3. So, the authorization server redirects the person to its login display (e.g. Google) and asks the person’s consent to offer the app entry to the API.
  4. The person logs in and approves the request.
  5. If the person approves the entry, the authorization server returns a grant code to the shopper.
  6. The shopper requests a token to the authorization server, passing its client_id and the acquired grant code.
  7. In response, the authorization server emits a token after verifying the client_id and the grant code.
  8. Lastly, the shopper accesses the info to the useful resource server, authenticating its requests with the token.

For all the small print on this circulation and the opposite ones outlined in the usual, seek the advice of the RFC 6749: The OAuth 2.0 Authorization Framework (RFC 6749).

Attacking the Authorization Code Grant Circulate

Though the authorization code grant circulation is the best way to go for cellular apps, it’s topic to shopper impersonation assaults. A malicious app can impersonate a respectable shopper and obtain a sound authentication token to entry the person information.

For the circulation diagram above, to obtain a token the attacker ought to know these two parameters:

  • The app’s client_id.
  • The code acquired within the callback URL from the authorization token.

Below sure circumstances, a malicious app can recuperate each. The app’s shopper ID is normally hardcoded, for instance, and an attacker might discover it by reverse-engineering the app. Or, by registering the malicious app as a respectable invoker of the callback URL, the attacker may sniff the callback URL.

As soon as the attacker is aware of the shopper ID and the grant code, they will request a token to the token endpoint. From that time ahead, they use the entry token to retrieve information illegally.

Introducing PKCE

Proof Key for Code Alternate (PKCE) is an addition to the Authorization Code Grant circulation to mitigate the assault depicted above. In observe, it provides a code to every request that’s dynamically generated by the shopper so an attacker can’t guess or intercept it.

The next diagram depicts how PKCE strengthens the Authorization Code Grant circulation in observe:

OAuth Authorization Flow with PKCE.

That’s to say, PKCE introduces the next adjustments with respect to the plain circulation:

  • [1] That is the place the login circulation begins.
  • [2] On every login request, the shopper generates a random code (code_verifier) and derives a code_challenge from it.
  • [3] When beginning the circulation, the shopper contains the code_challenge within the request to the authorization server. On receiving the authorization request, the authorization server saves this code for later verification.
  • [7] The shopper sends the code_verifier when requesting an entry token.
  • [8] Subsequently, the authorization server verifies that code_verifier matches code_challenge. If these two codes match, the server is aware of the shopper is legit and emits the token.

As regards to the earlier assault situation, even when the attacker can intercept the authorization grant code and the code code_challenge, it’s far more tough — if not unattainable — to intercept the code_verifier.

PKCE is safe, and it’s one of the best ways to implement OAuth authorization circulation in cellular apps.

You’ll find all of the PKCE particulars on the RFC 7636 – Proof Key for Code Alternate by OAuth Public Purchasers.

Now, you’ll have a look at the code verifier/problem technology and transmit the PKCE parameters with the HTTP requests.

Producing Code Verifier and Problem

The usual itself specifies generate the code_verifier and code_challenge.

Open PKCECodeGenerator.swift and substitute the physique of generateCodeVerifier() with:


// 1
var buffer = [UInt8](repeating: 0, depend: 32)
_ = SecRandomCopyBytes(kSecRandomDefault, buffer.depend, &buffer)
// 2
return Information(buffer).base64URLEncodedString()

This generates the code_verifier as follows:

  1. Get a 32-byte random sequence.
  2. Cross the 32 bytes sequence to base64 URL encoder to generate a 43 octet URL secure string.

Now, substitute the physique of generateCodeChallenge(codeVerifier:) with:


guard let information = codeVerifier.information(utilizing: .utf8) else { return nil }

let dataHash = SHA256.hash(information: information)
return Information(dataHash).base64URLEncodedString()

This derives the code_challenge because the SHA256 hash of the code verifier after which base64 URL encodes it.

Producing HTTP Requests

As well as, the usual specifies two completely different endpoints on the Authorization server for the 2 authorization phases.

Open PKCERequestBuilder.swift and observe the properties for every of those endpoints on the prime:

  • Authorization endpoint at /authorize is in control of emitting the authorization code grant.
  • Token endpoint at /token-generation, to emit and refresh tokens.

Based on the RFC, the shopper ought to talk with these two endpoints with two completely different HTTP request varieties:

  • Utilizing a GET with all of the required parameters handed as URL parameters, for the authorization endpoint.
  • Sending a POST with the parameters handed within the request’s physique, encoded as URL parameters, for the token endpoint.

PKCERequestBuilder already accommodates all the things that you must generate the 2 requests.

  • createAuthorizationRequestURL(codeChallenge:) generates a URL with the required parameters.
  • createTokenExchangeURLRequest(code:codeVerifier:) generates a URLRequest for the token alternate.

Making ready Server Facet (Google Cloud Platform)

Earlier than continuing with the shopper implementation, you must arrange the backend service.

This setup course of permits you to register your utility and its redirection URI used all through the authorization circulation and obtain the clientID.

On this particular instance, since Google already presents a service for person authentication with OAuth, you need to use their service.

The service setup course of consists of the next steps:

  • Creating a brand new mission.
  • Enabling the precise APIs your app intend to make use of.
  • Producing the authorization credentials for the app (the shopper ID).

You’ll want a Google account to register an app.

Making a New Challenge

First, open the Google API Console and click on Create Challenge.

Project creation screen.

In case you’ve beforehand created a mission, you may must click on the identify of the mission within the blue bar to convey up a dialog with a New Challenge button.

Screenshot showing how to create a new project when there is an existing one

You may be requested to enroll within the Google Cloud Developer program. In case you’re not already in, don’t fear — it’s so simple as accepting their phrases and situations.

Enter MyGoogleInfo within the mission’s identify. Then, Google assigns you a shopper ID that you simply’ll want when you generate the authorization requests from the app.

Click on CREATE.

Project screen.

Enabling the Required API

Now, it’s time to inform Google what sort of API your app will use.

Declaring the required APIs is twofold.

First, it impacts the sort of permission Google presents to the person throughout the authorization part.

And, extra essential, it permits Google to implement the info scope when your app requests information. Every token has a scope that defines which API the token grants entry to.

For example, within the case of MyGoogleInfo, that you must allow the Google Folks API to permit the app to question the person data.

From the mission web page, click on ENABLE APIS AND SERVICES.

Enable APIs screen.

Then, seek for Google Folks API and click on ENABLE.

Google People API screen.

Producing the Authorization Credentials

Lastly, that you must create the authorization credentials earlier than you need to use the API.

Click on Credentials within the sidebar.

In case you see a immediate to configure a consent display, choose exterior person kind and fill out the registration type for the required fields. Then, click on Credentials within the sidebar once more.

Click on CREATE CREDENTIALS, then select OAuth Consumer ID.

Add credentials menu.

These credentials allow you to specify which entry degree and to which API your customers’ tokens have entry.

Fill within the required fields as proven within the determine under.

Most significantly, the Bundle ID ought to have the identical worth because the one set in Xcode in your app. For instance, within the instance under, it’s com.alessandrodn.MyGoogleInfo. In your case, it’s your app bundle ID.

OAuth client ID screen.

Lastly, click on CREATE. You need to have an OAuth shopper definition for iOS as within the image under:

OAuth client screen.

Change REPLACE_WITH_CLIENTID_FROM_GOOGLE_APP within the definition under with the Consumer ID out of your Google app in PKCERequestBuilder.

PKCERequestBuilder for MyGoogleInfo.

It took some time to organize, however you’re now able to implement the PKCE shopper in Swift!

Implementing PKCE Consumer in Swift

In any case that principle, it’s now time to get your fingers soiled in Xcode :]

Authenticating the Person

First, implement the primary part of the authorization circulation, asking the authorization endpoint to confirm the person id.

Open PKCEAuthenticationService.swift. Add the next code to the top of startAuthentication():


// 1
let codeVerifier = PKCECodeGenerator.generateCodeVerifier()
guard
  let codeChallenge = PKCECodeGenerator.generateCodeChallenge(
    codeVerifier: codeVerifier
  ),
  // 2
  let authenticationURL = requestBuilder.createAuthorizationRequestURL(
    codeChallenge: codeChallenge
  )
else {
  print("[Error] Cannot construct authentication URL!")
  standing = .error(error: .internalError)
  return
}
print("[Debug] Authentication with: (authenticationURL.absoluteString)")
guard let bundleIdentifier = Bundle.essential.bundleIdentifier else {
  print("[Error] Bundle Identifier is nil!")
  standing = .error(error: .internalError)
  return
}
// 3
let session = ASWebAuthenticationSession(
  url: authenticationURL,
  callbackURLScheme: bundleIdentifier
) { callbackURL, error in
  // 4
  self.handleAuthenticationResponse(
    callbackURL: callbackURL,
    error: error,
    codeVerifier: codeVerifier
  )
}
// 5
session.presentationContextProvider = self
// 6
session.begin()

The code above implements the primary a part of the authorization circulation:

  1. Generates the code verifier and derives the code problem from it.
  2. Put together the authorization endpoint URL with all of the required parameters.
  3. Instantiate ASWebAuthenticationSession to carry out the authentication, passing authenticationURL generated earlier than.
  4. In its completion handler, ASWebAuthenticationSession returns the parameters acquired from the server as callbackURL.
  5. Inform the browser occasion that your class is its presentation context supplier. So, iOS instantiates the system browser window on prime of the app’s essential window.
  6. Lastly, begin the session.

ASWebAuthenticationSession offers you again an elective callback URL and an elective error.

For now, handleAuthenticationResponse(callbackURL:error:codeVerifier:) parses the error and prints the callback URL.

Construct and run. Faucet the Login button, and also you’ll see an alert saying MyGoogleInfo desires to make use of google.com to register.

Dialog asking permission to use google.com to sign in

Faucet Proceed and also you’ll see the Google login display.

Word the Google request to share the person’s profile data.

MyGoogleInfo login screen

Enter your credentials, authorize the app and test the logs.
Verify the app’s log for the callback URL returned from Google with the authorization response parameters.

Received callback URL log.

Parsing the Callback URL

To proceed with the authorization circulation, you now must do two issues.

First, in PKCEAuthenticationService.swift, add the operate getToken(code:codeVerifier:) as follows.


non-public func getToken(code: String, codeVerifier: String) async {
  guard let tokenURLRequest = requestBuilder.createTokenExchangeURLRequest(
    code: code,
    codeVerifier: codeVerifier
  ) else {
    print("[Error] Cannot construct token alternate URL!")
    standing = .error(error: .internalError)
    return
  }
  let tokenURLRequestBody = tokenURLRequest.httpBody ?? Information()
  print("[Debug] Get token parameters: (String(information: tokenURLRequestBody, encoding: .utf8) ?? "")")
  //TODO: make request
}

createTokenExchangeURLRequest() generates the HTTP request, given the grant code and code_verifier.

Word: The operate getToken(code:codeVerifier:) is async, because it’ll return instantly and full the community name within the background. Because you invoke it from a synchronous context, you employ a Process.

Then, substitute the implementation of handleAuthenticationResponse(callbackURL:error:codeVerifier:) with the next.


if let error = error {
  print("[Error] Authentication failed with: (error.localizedDescription)")
  standing = .error(error: .authenticationFailed)
  return
}
guard let code = extractCodeFromCallbackURL(callbackURL) else {
  standing = .error(error: .authenticationFailed)
  return
}
Process {
  await getToken(code: code, codeVerifier: codeVerifier)
}

The code above extracts the code parameter worth within the callback URL and passes it to getToken(code:codeVerifier:).

Construct and run, then log in along with your credentials. Confirm the log now accommodates the parameters for the credential alternate.

Get Token Parameters log.

Getting the Entry Token

Lastly, you’re able to get the token.

Change the //TODO: make request remark in getToken(code:codeVerifier:) with the next:


do {
  // 1
  let (information, response) = strive await URLSession.shared.information(for: tokenURLRequest)
  // 2
  guard let response = response as? HTTPURLResponse else {
    print("[Error] HTTP response parsing failed!")
    standing = .error(error: .tokenExchangeFailed)
    return
  }
  guard response.isOk else {
    let physique = String(information: information, encoding: .utf8) ?? "EMPTY"
    print("[Error] Get token failed with standing: (response.statusCode), physique: (physique)")
    standing = .error(error: .tokenExchangeFailed)
    return
  }
  print("[Debug] Get token response: (String(information: information, encoding: .utf8) ?? "EMPTY")")
  // 3
  let decoder = JSONDecoder()
  decoder.keyDecodingStrategy = .convertFromSnakeCase
  let token = strive decoder.decode(GoogleToken.self, from: information)
  // TODO: Retailer the token within the Keychain
  // 4
  standing = .authenticated(token: token)
} catch {
  print("[Error] Get token failed with: (error.localizedDescription)")
  standing = .error(error: .tokenExchangeFailed)
}

The operate getToken(code:codeVerifier:) performs the next actions:

  1. Use the tokenURLRequest to start out the token alternate session with the token endpoint. Because of this, it receives again a URLResponse and an elective Information.
  2. Parse the server response standing.
  3. If there are not any errors, decode the end result as a GoogleToken.
  4. Lastly, set the standing to authenticated, together with the entry token as a parameter.

Now you’re prepared to start out querying information. :]

When you get the token, you can begin utilizing it to entry the API.

The code in ViewModel listens to the authentication service standing and passes the token to the GoogleProfileInfoService. Then, the profile information service makes use of the token to entry your profile data.

Construct and run. Log in a single final time. Lastly, you may see your Google profile data.

MyGoogleInfo showing user profile.

You can too see within the logs the token response from the server:

Get token response log.

Storing the Token

Up to now, you didn’t save the entry token in persistent storage. In different phrases, each time the app begins, the person must log in once more.

To make the person expertise flawless, the app ought to do two extra issues.
First, it ought to save each the entry and the refresh tokens in persistent storage, as quickly as they’re acquired from the server.
Second, it ought to restore the token from the persistent storage when the app begins.

Because the tokens include credential entry, it’s best to keep away from UserDefaults and use the Apple keychain.

Refreshing the Token

The entry token and the refresh token have a restricted timeframe. In different phrases, they’ve a time expiration date enforced on the server.

As soon as the entry token expires, your API calls will fail with error 401. In these circumstances, that you must set off the token refresh circulation with the token endpoint. The HTTP request physique accommodates the shopper ID and the refresh token encoded as URL parameters.

For a reference, createRefreshTokenURLRequest(refreshToken:) within the last mission generates the URLRequest for the token refresh.

The place to Go From Right here?

Obtain the finished mission recordsdata by clicking the Obtain Supplies button on the prime or backside of the tutorial.

You dug into the small print of the OAuth Authorization circulation with PKCE, and now you’re able to implement the authentication service in your subsequent app. You can too experiment with issues like token administration and higher error dealing with.

For all the small print on retailer and retrieve the token within the keychain, try How To Safe iOS Person Information: Keychain Providers and Biometrics with SwiftUI.

Then again, if you wish to undertake one of many SDKs out there for OAuth, you now know the way PKCE works below the hood to completely management the bundle habits.

For reference, listed below are some third-party SDKs that implement OAuth with PKCE.

  • AppAuth is an open-source SDK from the OpenID consortium; it helps native apps (iOS and Android) and all the opposite Apple OSs and Native JS.
  • Auth0 presents an entire answer for OpenID authentication and, as part of it, they supply an SDK that helps each iOS and macOS.

We hope you loved this tutorial. When you’ve got any questions or feedback, please be a part of the discussion board dialogue under!

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