feat: add PKCE to 3 Legged OAuth exchange

README.md

@@ -97,6 +97,45 @@ get('/oauth2callback') do
 end
 ```
 
+### Example (Web with PKCE)
+
+Proof Key for Code Exchange (PKCE) is an [RFC](https://www.rfc-editor.org/rfc/rfc7636) that aims to prevent malicious operating system processes from hijacking an OAUTH 2.0 exchange. PKCE mitigates the above vulnerability by including `code_challenge` and `code_challenge_method` parameters in the Authorization Request and a `code_verifier` parameter in the Access Token Request.
+
+```ruby
+require 'googleauth'
+require 'googleauth/web_user_authorizer'
+require 'googleauth/stores/redis_token_store'
+require 'redis'
+
+client_id = Google::Auth::ClientId.from_file('/path/to/client_secrets.json')
+scope = ['https://www.googleapis.com/auth/drive']
+token_store = Google::Auth::Stores::RedisTokenStore.new(redis: Redis.new)
+authorizer = Google::Auth::WebUserAuthorizer.new(
+  client_id, scope, token_store, '/oauth2callback')
+
+
+get('/authorize') do
+  # NOTE: Assumes the user is already authenticated to the app
+  user_id = request.session['user_id']
+  # User needs to take care of generating the code_verifier and storing it in
+  # the session.
+  request.session['code_verifier'] ||= Google::Auth::WebUserAuthorizer.generate_code_verifier
+  authorizer.code_verifier = request.session['code_verifier']
+  credentials = authorizer.get_credentials(user_id, request)
+  if credentials.nil?
+    redirect authorizer.get_authorization_url(login_hint: user_id, request: request)
+  end
+  # Credentials are valid, can call APIs
+  # ...
+end
+
+get('/oauth2callback') do
+  target_url = Google::Auth::WebUserAuthorizer.handle_auth_callback_deferred(
+    request)
+  redirect target_url
+end
+```
+
 ### Example (Command Line) [Deprecated]
 
 The Google Auth OOB flow has been discontiued on January 31, 2023. The OOB flow is a legacy flow that is no longer considered secure. To continue using Google Auth, please migrate your applications to a more secure flow. For more information on how to do this, please refer to this [OOB Migration](https://developers.google.com/identity/protocols/oauth2/resources/oob-migration) guide.

lib/googleauth/user_authorizer.rb

@@ -16,6 +16,7 @@
 require "multi_json"
 require "googleauth/signet"
 require "googleauth/user_refresh"
+require "securerandom"
 
 module Google
   module Auth
@@ -57,14 +58,19 @@ class UserAuthorizer
       # @param [String] callback_uri
       #  URL (either absolute or relative) of the auth callback.
       #  Defaults to '/oauth2callback'
-      def initialize client_id, scope, token_store, callback_uri = nil
+      # @param [String] code_verifier
+      #  Random string of 43-128 chars used to verify the key exchange using
+      #  PKCE.
+      def initialize client_id, scope, token_store,
+                     callback_uri = nil, code_verifier: nil
         raise NIL_CLIENT_ID_ERROR if client_id.nil?
         raise NIL_SCOPE_ERROR if scope.nil?
 
         @client_id = client_id
         @scope = Array(scope)
         @token_store = token_store
         @callback_uri = callback_uri || "/oauth2callback"
+        @code_verifier = code_verifier
       end
 
       # Build the URL for requesting authorization.
@@ -86,6 +92,18 @@ def initialize client_id, scope, token_store, callback_uri = nil
       #  Authorization url
       def get_authorization_url options = {}
         scope = options[:scope] || @scope
+
+        options[:additional_parameters] ||= {}
+
+        if @code_verifier
+          options[:additional_parameters].merge!(
+            {
+              code_challenge: generate_code_challenge(@code_verifier),
+              code_challenge_method: code_challenge_method
+            }
+          )
+        end
+
         credentials = UserRefreshCredentials.new(
           client_id:     @client_id.id,
           client_secret: @client_id.secret,
@@ -157,6 +175,8 @@ def get_credentials_from_code options = {}
         code = options[:code]
         scope = options[:scope] || @scope
         base_url = options[:base_url]
+        options[:additional_parameters] ||= {}
+        options[:additional_parameters].merge!({ code_verifier: @code_verifier })
         credentials = UserRefreshCredentials.new(
           client_id:             @client_id.id,
           client_secret:         @client_id.secret,
@@ -228,6 +248,23 @@ def store_credentials user_id, credentials
         credentials
       end
 
+      # The code verifier for PKCE for OAuth 2.0. When set, the
+      # authorization URI will contain the Code Challenge and Code
+      # Challenge Method querystring parameters, and the token URI will
+      # contain the Code Verifier parameter.
+      #
+      # @param [String|nil] new_code_erifier
+      def code_verifier= new_code_verifier
+        @code_verifier = new_code_verifier
+      end
+
+      # Generate the code verifier needed to be sent while fetching
+      # authorization URL.
+      def self.generate_code_verifier
+        random_number = rand 32..96
+        SecureRandom.alphanumeric random_number
+      end
+
       private
 
       # @private Fetch stored token with given user_id
@@ -272,6 +309,15 @@ def redirect_uri_for base_url
       def uri_is_postmessage? uri
         uri.to_s.casecmp("postmessage").zero?
       end
+
+      def generate_code_challenge code_verifier
+        digest = Digest::SHA256.digest code_verifier
+        Base64.urlsafe_encode64 digest, padding: false
+      end
+
+      def code_challenge_method
+        "S256"
+      end
     end
   end
 end

lib/googleauth/web_user_authorizer.rb

@@ -96,8 +96,12 @@ def self.handle_auth_callback_deferred request
       # @param [String] callback_uri
       #  URL (either absolute or relative) of the auth callback. Defaults
       #  to '/oauth2callback'
-      def initialize client_id, scope, token_store, callback_uri = nil
-        super client_id, scope, token_store, callback_uri
+      # @param [String] code_verifier
+      #  Random string of 43-128 chars used to verify the key exchange using
+      #  PKCE.
+      def initialize client_id, scope, token_store,
+                     callback_uri = nil, code_verifier: nil
+        super client_id, scope, token_store, callback_uri, code_verifier: code_verifier
       end
 
       # Handle the result of the oauth callback. Exchanges the authorization

spec/googleauth/user_authorizer_spec.rb

@@ -98,6 +98,27 @@
     end
   end
 
+  context "when generating authorization URLs and code_verifier is manually passed" do
+    let(:code_verifier) { "IeJRY4uem0581Lcw6CiZ3fNwngg" }
+    let :authorizer do
+      Google::Auth::UserAuthorizer.new(client_id,
+                                       scope,
+                                       token_store,
+                                       callback_uri,
+                                       code_verifier: code_verifier)
+    end
+    let :uri do
+      authorizer.get_authorization_url
+    end
+
+    it_behaves_like "valid authorization url"
+
+    it "should include code_challenge and code_challenge_method" do
+      expect(URI(uri).query).to match(/code_challenge=/)
+      expect(URI(uri).query).to match(/code_challenge_method=S256/)
+    end
+  end
+
   context "when generating authorization URLs with user ID & state" do
     let :uri do
       authorizer.get_authorization_url login_hint: "user1", state: "mystate"

spec/googleauth/web_user_authorizer_spec.rb

@@ -88,6 +88,13 @@
       )
       expect(url).to match(/login_hint=user@example.com/)
     end
+
+    it "should include code_challenge and code_challenge_method" do
+      authorizer.code_verifier = Google::Auth::WebUserAuthorizer.generate_code_verifier
+      url = authorizer.get_authorization_url(request: request)
+      expect(url).to match(/code_challenge=/)
+      expect(url).to match(/code_challenge_method=S256/)
+    end
   end
 
   shared_examples "handles callback" do