feat(federation): seal federation peer client keys at rest (FED-M2-05) (#495)

packages/auth/src/index.ts

@@ -10,3 +10,4 @@ export {
   type SsoTeamSyncConfig,
   type SupportedSsoProviderId,
 } from './sso.js';
+export { seal, unseal } from './seal.js';

packages/auth/src/seal.ts

@@ -0,0 +1,52 @@
+import { createCipheriv, createDecipheriv, createHash, randomBytes } from 'node:crypto';
+
+const ALGORITHM = 'aes-256-gcm';
+const IV_LENGTH = 12; // 96-bit IV for GCM
+const TAG_LENGTH = 16; // 128-bit auth tag
+
+/**
+ * Derive a 32-byte AES-256 key from BETTER_AUTH_SECRET using SHA-256.
+ * Throws if BETTER_AUTH_SECRET is not set.
+ */
+function deriveKey(): Buffer {
+  const secret = process.env['BETTER_AUTH_SECRET'];
+  if (!secret) {
+    throw new Error('BETTER_AUTH_SECRET is not set — cannot derive encryption key');
+  }
+  return createHash('sha256').update(secret).digest();
+}
+
+/**
+ * Seal a plaintext string using AES-256-GCM.
+ * Output format: base64(IV || authTag || ciphertext)
+ */
+export function seal(plaintext: string): string {
+  const key = deriveKey();
+  const iv = randomBytes(IV_LENGTH);
+  const cipher = createCipheriv(ALGORITHM, key, iv);
+
+  const encrypted = Buffer.concat([cipher.update(plaintext, 'utf8'), cipher.final()]);
+  const authTag = cipher.getAuthTag();
+
+  const combined = Buffer.concat([iv, authTag, encrypted]);
+  return combined.toString('base64');
+}
+
+/**
+ * Unseal a value sealed by `seal()`.
+ * Throws on authentication failure (tampered data) or if BETTER_AUTH_SECRET is unset.
+ */
+export function unseal(encoded: string): string {
+  const key = deriveKey();
+  const combined = Buffer.from(encoded, 'base64');
+
+  const iv = combined.subarray(0, IV_LENGTH);
+  const authTag = combined.subarray(IV_LENGTH, IV_LENGTH + TAG_LENGTH);
+  const ciphertext = combined.subarray(IV_LENGTH + TAG_LENGTH);
+
+  const decipher = createDecipheriv(ALGORITHM, key, iv);
+  decipher.setAuthTag(authTag);
+
+  const decrypted = Buffer.concat([decipher.update(ciphertext), decipher.final()]);
+  return decrypted.toString('utf8');
+}