Change challange expiration based on purpose

app/javascript/lib/action_pack/passkey.js

@@ -63,7 +63,7 @@ class PasskeyButton extends HTMLElement {
       if (!passkeysAvailable()) throw new Error("Passkeys are not supported by this browser")
       if (!options) throw new Error("Missing passkey options")
 
-      await refreshChallenge(options, this.challengeUrl)
+      await refreshChallenge(options, this.challengeUrl, this.purpose)
       const passkey = await this.perform(options)
 
       this.button.dispatchEvent(new CustomEvent("passkey:success", { bubbles: true }))
@@ -93,6 +93,8 @@ class PasskeyButton extends HTMLElement {
 }
 
 class PasskeyRegistrationButton extends PasskeyButton {
+  get purpose() { return "registration" }
+
   async perform(options) {
     return await register(options)
   }
@@ -103,6 +105,8 @@ class PasskeyRegistrationButton extends PasskeyButton {
 }
 
 class PasskeySignInButton extends PasskeyButton {
+  get purpose() { return "authentication" }
+
   connectedCallback() {
     super.connectedCallback()
     if (this.mediation === "conditional") this.#attemptConditionalMediation()
@@ -127,7 +131,7 @@ class PasskeySignInButton extends PasskeyButton {
       this.form.dispatchEvent(new CustomEvent("passkey:start", { bubbles: true }))
 
       try {
-        await refreshChallenge(options, this.challengeUrl)
+        await refreshChallenge(options, this.challengeUrl, this.purpose)
         const passkey = await this.perform(options, { mediation: this.mediation })
 
         this.form.dispatchEvent(new CustomEvent("passkey:success", { bubbles: true }))
@@ -157,17 +161,21 @@ function passkeysAvailable() {
   return !!window.PublicKeyCredential
 }
 
-async function refreshChallenge(options, challengeUrl) {
+async function refreshChallenge(options, challengeUrl, purpose) {
   if (!challengeUrl) throw new Error("Missing passkey challenge URL")
   const token = document.querySelector('meta[name="csrf-token"]')?.content
 
+  const body = new URLSearchParams()
+  if (purpose) body.append("purpose", purpose)
+
   const response = await fetch(challengeUrl, {
     method: "POST",
     credentials: "same-origin",
     headers: {
       "X-CSRF-Token": token,
       "Accept": "application/json"
-    }
+    },
+    body
   })
 
   if (!response.ok) throw new Error("Failed to refresh challenge")

lib/action_pack/passkey/challenges_controller.rb

@@ -18,7 +18,9 @@
 class ActionPack::Passkey::ChallengesController < ActionController::Base
   include ActionPack::Passkey::Request
 
-  # Generates a fresh challenge and returns it as JSON.
+  # Generates a fresh challenge and returns it as JSON. Accepts an optional
+  # +purpose+ parameter ("registration" or "authentication") to select the
+  # appropriate challenge expiration. Defaults to "authentication".
   def create
     render json: { challenge: create_passkey_challenge }
   end
@@ -26,7 +28,17 @@ class ActionPack::Passkey::ChallengesController < ActionController::Base
   private
     def create_passkey_challenge
       ActionPack::WebAuthn::PublicKeyCredential::Options.new(
-        challenge_expiration: Rails.configuration.action_pack.web_authn.request_challenge_expiration
+        challenge_expiration: challenge_expiration
       ).challenge
     end
+
+    def challenge_expiration
+      config = Rails.configuration.action_pack.web_authn
+
+      if params[:purpose] == "registration"
+        config.creation_challenge_expiration
+      else
+        config.request_challenge_expiration
+      end
+    end
 end

test/controllers/my/passkey_challenges_controller_test.rb

@@ -21,4 +21,38 @@ class My::PasskeyChallengesControllerTest < ActionDispatch::IntegrationTest
       assert_not_equal first_challenge, second_challenge
     end
   end
+
+  test "uses registration challenge expiration for registration purpose" do
+    untenanted do
+      post my_passkey_challenge_url, params: { purpose: "registration" }
+
+      assert_response :success
+
+      challenge = response.parsed_body["challenge"]
+      signed_message = Base64.urlsafe_decode64(challenge)
+
+      travel Rails.configuration.action_pack.web_authn.creation_challenge_expiration - 1.second
+      assert ActionPack::WebAuthn.challenge_verifier.verified(signed_message)
+
+      travel 2.seconds
+      assert_nil ActionPack::WebAuthn.challenge_verifier.verified(signed_message)
+    end
+  end
+
+  test "uses authentication challenge expiration by default" do
+    untenanted do
+      post my_passkey_challenge_url
+
+      assert_response :success
+
+      challenge = response.parsed_body["challenge"]
+      signed_message = Base64.urlsafe_decode64(challenge)
+
+      travel Rails.configuration.action_pack.web_authn.request_challenge_expiration - 1.second
+      assert ActionPack::WebAuthn.challenge_verifier.verified(signed_message)
+
+      travel 2.seconds
+      assert_nil ActionPack::WebAuthn.challenge_verifier.verified(signed_message)
+    end
+  end
 end