Implement Passkey authentication

app/javascript/controllers/passkey_controller.js

@@ -1,36 +1,92 @@
 import { Controller } from "@hotwired/stimulus"
 
 export default class extends Controller {
+  static values = { publicKey: Object, url: String, csrfToken: String }
+
+  #abortController
+
   connect() {
-    if (window.PublicKeyCredential?.isConditionalMediationAvailable) {
-      this.attemptConditionalMediation()
-    }
+    this.#attemptConditionalMediation()
   }
 
   disconnect() {
-    this.abortController?.abort()
+    this.#abortController?.abort()
   }
 
-  async attemptConditionalMediation() {
-    if (!await PublicKeyCredential.isConditionalMediationAvailable()) return
+  async #attemptConditionalMediation() {
+    if (!await PublicKeyCredential?.isConditionalMediationAvailable?.()) return
 
-    this.abortController = new AbortController()
+    this.#abortController = new AbortController()
 
     try {
       const credential = await navigator.credentials.get({
-        publicKey: {
-          challenge: crypto.getRandomValues(new Uint8Array(32)),
-          rpId: window.location.hostname
-        },
+        publicKey: this.#prepareOptions(this.publicKeyValue),
         mediation: "conditional",
-        signal: this.abortController.signal
+        signal: this.#abortController.signal
       })
 
-      console.log("Passkey selected:", credential)
+      this.#submitAssertion(credential)
     } catch (error) {
       if (error.name !== "AbortError") {
         console.error("Passkey error:", error)
       }
     }
   }
+
+  #submitAssertion(credential) {
+    const form = document.createElement("form")
+    form.method = "POST"
+    form.action = this.urlValue
+    form.style.display = "none"
+
+    const fields = {
+      authenticity_token: this.csrfTokenValue,
+      credential_id: credential.id,
+      "response[client_data_json]": new TextDecoder().decode(credential.response.clientDataJSON),
+      "response[authenticator_data]": this.#bufferToBase64url(credential.response.authenticatorData),
+      "response[signature]": this.#bufferToBase64url(credential.response.signature)
+    }
+
+    for (const [name, value] of Object.entries(fields)) {
+      const input = document.createElement("input")
+      input.type = "hidden"
+      input.name = name
+      input.value = value
+      form.appendChild(input)
+    }
+
+    document.body.appendChild(form)
+    form.submit()
+  }
+
+  #prepareOptions(options) {
+    const prepared = {
+      ...options,
+      challenge: this.#base64urlToBuffer(options.challenge)
+    }
+
+    if (options.allowCredentials?.length) {
+      prepared.allowCredentials = options.allowCredentials.map(cred => ({
+        ...cred,
+        id: this.#base64urlToBuffer(cred.id)
+      }))
+    } else {
+      delete prepared.allowCredentials
+    }
+
+    return prepared
+  }
+
+  #base64urlToBuffer(base64url) {
+    const base64 = base64url.replace(/-/g, "+").replace(/_/g, "/")
+    const padding = "=".repeat((4 - base64.length % 4) % 4)
+    const binary = atob(base64 + padding)
+    return Uint8Array.from(binary, c => c.charCodeAt(0)).buffer
+  }
+
+  #bufferToBase64url(buffer) {
+    const bytes = new Uint8Array(buffer)
+    const binary = String.fromCharCode(...bytes)
+    return btoa(binary).replace(/\+/g, "-").replace(/\//g, "_").replace(/=+$/, "")
+  }
 }