captcha: don't do the encryption thing, just use a ttl cache

src/DittoWallet.ts

@@ -39,11 +39,6 @@ export class DittoWallet {
     return this.deriveKey(Conf.wallet.dbKeyPath);
   }
 
-  /** Captcha encryption key for encrypting answer data in AES-GCM. */
-  static get captchaKey(): Uint8Array {
-    return this.deriveKey(Conf.wallet.captchaKeyPath);
-  }
-
   /** VAPID secret key, used for web push notifications. ES256. */
   static get vapidKey(): Uint8Array {
     return this.deriveKey(Conf.wallet.vapidKeyPath);

src/app.ts

@@ -279,7 +279,7 @@ app.put('/api/v1/admin/ditto/relays', requireRole('admin'), adminSetRelaysContro
 app.post('/api/v1/ditto/names', requireSigner, nameRequestController);
 app.get('/api/v1/ditto/names', requireSigner, nameRequestsController);
 
-app.get('/api/v1/pleroma/captcha', captchaController);
+app.get('/api/v1/ditto/captcha', captchaController);
 
 app.get('/api/v1/ditto/zap_splits', getZapSplitsController);
 app.get('/api/v1/ditto/:id{[0-9a-f]{64}}/zap_splits', statusZapSplitsController);

src/config.ts

@@ -99,6 +99,10 @@ class Conf {
       },
     },
   };
+  /** Time-to-live for captchas in milliseconds. */
+  static get captchaTTL(): number {
+    return Number(Deno.env.get('CAPTCHA_TTL') || 5 * 60 * 1000);
+  }
   /**
    * BIP-32 derivation paths for different crypto use-cases.
    * The `DITTO_NSEC` is used as the seed.
@@ -109,10 +113,6 @@ class Conf {
     get dbKeyPath(): string {
       return Deno.env.get('WALLET_DB_KEY_PATH') || "m/0'/1'";
     },
-    /** Private key for AES-GCM encryption of captcha answer data. */
-    get captchaKeyPath(): string {
-      return Deno.env.get('WALLET_CAPTCHA_KEY_PATH') || "m/0'/2'";
-    },
     /** VAPID private key path. */
     get vapidKeyPath(): string {
       return Deno.env.get('WALLET_VAPID_KEY_PATH') || "m/0'/3'";

src/controllers/api/captcha.ts

@@ -1,10 +1,17 @@
 import { createCanvas, loadImage } from '@gfx/canvas-wasm';
-import { encodeBase64 } from '@std/encoding/base64';
+import TTLCache from '@isaacs/ttlcache';
 
 import { AppController } from '@/app.ts';
-import { DittoWallet } from '@/DittoWallet.ts';
+import { Conf } from '@/config.ts';
-import { aesEncrypt } from '@/utils/aes.ts';
 
+interface Point {
+  x: number;
+  y: number;
+}
+
+const captchas = new TTLCache<string, Point>();
+
+/** Puzzle captcha controller. */
 export const captchaController: AppController = async (c) => {
   const { puzzle, piece, solution } = await generateCaptcha(
     await Deno.readFile(new URL('../../../captcha/tj-holowaychuk.jpg', import.meta.url)),
@@ -18,28 +25,23 @@ export const captchaController: AppController = async (c) => {
     },
   );
 
-  const answerData = {
+  const id = crypto.randomUUID();
-    solution,
+  const now = new Date();
-    created_at: new Date().toISOString(),
+  const ttl = Conf.captchaTTL;
-  };
 
-  const encoded = new TextEncoder().encode(JSON.stringify(answerData));
+  captchas.set(id, solution, { ttl });
-  const encrypted = await aesEncrypt(DittoWallet.captchaKey, encoded);
 
   return c.json({
     type: 'puzzle',
-    token: crypto.randomUUID(), // Useless, but Pleroma does it.
+    id,
     puzzle: puzzle.toDataURL(),
     piece: piece.toDataURL(),
-    answer_data: encodeBase64(encrypted),
+    created_at: now.toISOString(),
+    expires_at: new Date(now.getTime() + ttl).toISOString(),
   });
 };
 
-interface Point {
+/** Generate a puzzle captcha, returning canvases for the board and piece. */
-  x: number;
-  y: number;
-}
-
 async function generateCaptcha(
   from: Uint8Array,
   mask: Uint8Array,