feat(crypto): boot-generated key + AES-256-GCM sealed values

server/src/lib/crypto.ts

@@ -0,0 +1,31 @@
+import { createCipheriv, createDecipheriv, randomBytes } from "node:crypto";
+import { existsSync, mkdirSync, readFileSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+
+export function loadOrCreateKey(dataDir: string): Buffer {
+  mkdirSync(dataDir, { recursive: true });
+  const path = join(dataDir, "secret.key");
+  if (!existsSync(path)) writeFileSync(path, randomBytes(32), { mode: 0o600 });
+  const key = readFileSync(path);
+  if (key.length !== 32) throw new Error(`secret.key must be 32 bytes, got ${key.length}`);
+  return key;
+}
+
+export function encrypt(key: Buffer, plaintext: string): string {
+  const iv = randomBytes(12);
+  const cipher = createCipheriv("aes-256-gcm", key, iv);
+  const ct = Buffer.concat([cipher.update(plaintext, "utf8"), cipher.final()]);
+  return "enc:" + Buffer.concat([iv, cipher.getAuthTag(), ct]).toString("base64");
+}
+
+export function decrypt(key: Buffer, sealed: string): string {
+  if (!sealed.startsWith("enc:")) throw new Error("not an encrypted value");
+  const buf = Buffer.from(sealed.slice(4), "base64");
+  const decipher = createDecipheriv("aes-256-gcm", key, buf.subarray(0, 12));
+  decipher.setAuthTag(buf.subarray(12, 28));
+  return Buffer.concat([decipher.update(buf.subarray(28)), decipher.final()]).toString("utf8");
+}
+
+export function mask(secret: string): string {
+  return secret.length >= 5 ? "…" + secret.slice(-4) : "…";
+}