OAuth2 - PKCE

* Add a `pkce` option to the oauth2 strategy that defaults
  to `false`.
* When the option is true, the client will authorize with the
  provider using PKCE (proof key for code exchange) [1]. This
  enhances the security footprint of the interaction and is now
  recommended by the IETF for all OAuth2 code grant interactions.
* At a high level, PKCE works as follows:
  1. Generate a new random code verifier string value with a
     minimum length of 43 characters and a maximum length of
     128 characters.
  2. Take the SHA256 hash value of the code verifier string and
     perform a URL-safe Base64 encode of the result as defined
     in [2].
  3. Pass `code_challenge={Base64(SHA256(code_verifier)}`
     and `code_challenge_method=S256` query parameters with
     the client OAuth2 authorize request.
  4. In the callback_phase, pass the `code_verifier` in plaintext
     to the provider as a query parameter to the OAuth2 token
     endpoint. This provides strong guarantees to the OAuth provider
     that the client is the same entity that requested authorization.

[1]: https://tools.ietf.org/html/rfc7636
[2]: https://tools.ietf.org/html/rfc7636#appendix-A

lib/omniauth/strategies/oauth2.rb

@@ -29,6 +29,7 @@ def self.inherited(subclass)
       option :token_options, []
       option :auth_token_params, {}
       option :provider_ignores_state, false
+      option :pkce, false
 
       attr_accessor :access_token
 
@@ -49,18 +50,33 @@ def request_phase
       end
 
       def authorize_params
+        verifier = SecureRandom.hex(64)
+
+        if options.pkce
+          # NOTE: see https://tools.ietf.org/html/rfc7636#appendix-A
+          challenge = Base64
+                 .urlsafe_encode64(Digest::SHA2.digest(verifier))
+                 .split("=")
+                 .first
+          options.authorize_params[:code_challenge] = challenge
+          options.authorize_params[:code_challenge_method] = "S256"
+        end
+
         options.authorize_params[:state] = SecureRandom.hex(24)
         params = options.authorize_params.merge(options_for("authorize"))
+
         if OmniAuth.config.test_mode
           @env ||= {}
           @env["rack.session"] ||= {}
         end
+
+        session["omniauth.pkce.verifier"] = verifier if options.pkce
         session["omniauth.state"] = params[:state]
         params
       end
 
       def token_params
-        options.token_params.merge(options_for("token"))
+        options.token_params.merge(options_for("token")).merge(pkce_token_params)
       end
 
       def callback_phase # rubocop:disable AbcSize, CyclomaticComplexity, MethodLength, PerceivedComplexity
@@ -84,17 +100,21 @@ def callback_phase # rubocop:disable AbcSize, CyclomaticComplexity, MethodLength
 
     protected
 
+      def pkce_token_params
+        return {} unless options.pkce
+
+        { code_verifier: session.delete("omniauth.pkce.verifier") }
+      end
+
       def build_access_token
         verifier = request.params["code"]
         client.auth_code.get_token(verifier, {:redirect_uri => callback_url}.merge(token_params.to_hash(:symbolize_keys => true)), deep_symbolize(options.auth_token_params))
       end
 
       def deep_symbolize(options)
-        hash = {}
-        options.each do |key, value|
+        options.each_with_object({}) do |(key, value), hash|
           hash[key.to_sym] = value.is_a?(Hash) ? deep_symbolize(value) : value
         end
-        hash
       end
 
       def options_for(option)

spec/omniauth/strategies/oauth2_spec.rb

@@ -66,6 +66,13 @@ def app
       expect(instance.authorize_params.keys).to eq(["state"])
       expect(instance.session["omniauth.state"]).to eq("qux")
     end
+
+    it "includes PKCE parameters if enabled" do
+      instance = subject.new("abc", "def", pkce: true)
+      expect(instance.authorize_params[:code_challenge]).to be_a(String)
+      expect(instance.authorize_params[:code_challenge_method]).to eq("S256")
+      expect(instance.session["omniauth.pkce.verifier"]).to be_a(String)
+    end
   end
 
   describe "#token_params" do
@@ -80,6 +87,13 @@ def app
       instance = subject.new("abc", "def", :token_options => %i[scope foo], :scope => "bar", :foo => "baz")
       expect(instance.token_params).to eq("scope" => "bar", "foo" => "baz")
     end
+
+    it "includes the PKCE code_verifier if enabled" do
+      instance = subject.new("abc", "def", pkce: true)
+      # setup session
+      instance.authorize_params
+      expect(instance.token_params[:code_verifier]).to be_a(String)
+    end
   end
 
   describe "#callback_phase" do