Cryptographic Failures in Chi with Firestore

A Cryptographic Failure occurs when an application fails to properly protect data in transit or at rest, for example by using weak algorithms, storing keys in source code, or transmitting sensitive information without encryption. In a Chi application that uses Cloud Firestore as the backend, this risk is amplified by common patterns such as storing sensitive document fields in plaintext, relying solely on Firestore security rules for confidentiality, or using deterministic encryption that enables frequency analysis.

Firestore provides server-side encryption at rest by default, but client-side protections are the developer’s responsibility. A typical vulnerability chain in Chi might look like this: the client reads a Firestore document containing an encrypted sensitive field; the decryption key is hard‑coded or derived from a low‑entropy secret; and the same key is used across users or sessions. This mirrors findings often seen in Data Exposure checks and can lead to mass data disclosure if the key is compromised.

Chi’s typical web stack—HTTP endpoints, JSON payloads, and Firestore listeners—can inadvertently leak cryptographic material. For instance, if an API route in Chi returns a Firestore document with a field such as ssn or api_key without applying envelope encryption or authenticated encryption with associated data (AEAD), an attacker who exploits an Input Validation weakness (e.g., NoSQL injection) can exfiltrate sensitive records. Similarly, using weak or custom cipher implementations, reusing nonces with AES‑GCM, or storing keys in browser local storage increases the likelihood of successful cryptanalytic attacks, including known plaintext attacks against predictable initialization vectors.

Real-world examples include scenarios where developers store Firestore document IDs or tokens in URLs or logs, making them enumerable and subject to leakage. Another common pattern is using Firestore client SDKs with overly permissive rules while assuming encryption at the client layer, which does not exist. These mistakes map directly to OWASP API Top 10 A02:2023 — Cryptographic Failures and amplify Data Exposure findings reported by middleBrick scans. An attacker could exploit these to recover authentication tokens, payment details, or personal health information, leading to compliance violations under GDPR, HIPAA, or PCI‑DSS.

To mitigate cryptographic failures in Chi with Firestore, apply defense-in-depth: encrypt sensitive fields before they leave the client, use envelope encryption with a KMS-backed data key, and avoid hardcoding secrets. Below are concrete, idiomatic examples using Node.js with Chi and the Firebase Admin SDK.

1. Client‑side encryption before writing to Firestore

Encrypt sensitive values in the browser or Node.js client using AES‑GCM with a key derived via PBKDF2. Never send plaintext secrets to Firestore.

import { createCipheriv, randomBytes, pbkdf2Sync } from 'node:crypto';

function encryptField(plaintext, password, salt) {
  const keyMaterial = pbkdf2Sync(password, salt, 100000, 32, 'sha256');
  const iv = randomBytes(12);
  const cipher = createCipheriv('aes-256-gcm', keyMaterial, iv);
  let encrypted = cipher.update(plaintext, 'utf8');
  encrypted = Buffer.concat([encrypted, cipher.final(), cipher.getAuthTag()]);
  return { iv: iv.toString('base64'), encryptedData: encrypted.toString('base64') };
}

// Example usage within a Chi handler
import { Router } from 'https://esm.sh/@hono/hono';
import { add, getFirestore, doc, setDoc } from 'https://esm.sh/firebase-admin';
const router = new Router();

router.post('/save', async (c) => {
  const body = await c.req.json();
  const password = body.password; // ideally from a secure session or WebCrypto
  const salt = randomBytes(16);
  const { iv, encryptedData } = encryptField(body.ssn, password, salt);
  const db = getFirestore();
  await setDoc(doc(db, 'users', body.uid), {
    ssnEncrypted: encryptedData,
    ssnIv: iv,
    ssnSalt: salt.toString('base64'),
    updatedAt: new Date()
  });
  return c.json({ ok: true });
});

2. Envelope encryption with Google Cloud KMS

For production, use envelope encryption: generate a data key from Cloud KMS, encrypt the field with the data key, and store the encrypted data key alongside the ciphertext. This limits exposure of the KMS key and allows fine‑grained access control.

import { KeyManagementServiceClient } from '@google-cloud/kms';
import { createCipheriv, randomBytes } from 'node:crypto';

const kmsClient = new KeyManagementServiceClient();
const keyName = kmsClient.cryptoKeyPath('my-project', 'global', 'my-keyring', 'my-key');

async function encryptWithKms(plaintext) {
  const [result] = await kmsClient.encrypt({ name: keyName, plaintext });
  return result.ciphertext;
}

async function envelopeEncrypt(plaintext) {
  const [keyResp] = await kmsClient.generateRandomBytes({ length: 32 });
  const dataKey = keyResp.randomBytes;
  const iv = randomBytes(12);
  const cipher = createCipheriv('aes-256-gcm', dataKey, iv);
  let encrypted = cipher.update(plaintext, 'utf8');
  encrypted = Buffer.concat([encrypted, cipher.final(), cipher.getAuthTag()]);
  const [encryptedKeyResp] = await kmsClient.encrypt({ name: keyName, plaintext: dataKey });
  return {
    ciphertext: encrypted.toString('base64'),
    encryptedKey: encryptedKeyResp.ciphertext.toString('base64'),
    iv: iv.toString('base64')
  };
}

// In a Chi handler
router.post('/save-sensitive', async (c) => {
  const body = await c.req.json();
  const { ciphertext, encryptedKey, iv } = await envelopeEncrypt(body.creditCard);
  const db = getFirestore();
  await setDoc(doc(db, 'payments', body.id), {
    creditCardCiphertext: ciphertext,
    creditCardKey: encryptedKey,
    iv,
    kind: 'envelope'
  });
  return c.json({ ok: true });
});

3. Secure reading and decryption

When reading, decrypt only after verifying integrity. Avoid using the same key for many documents; prefer per‑document random keys wrapped by KMS.

import { createDecipheriv, createHash } from 'node:crypto';

async function decryptField(encryptedData, iv, password, salt) {
  const keyMaterial = pbkdf2Sync(password, salt, 100000, 32, 'sha256');
  const decipher = createDecipheriv('aes-256-gcm', keyMaterial, Buffer.from(iv, 'base64'));
  decipher.setAuthTag(Buffer.from(encryptedData.authTag, 'base64'));
  let decrypted = decipher.update(Buffer.from(encryptedData, 'base64'));
  decrypted = Buffer.concat([decrypted, decipher.final()]);
  return decrypted.toString();
}

// Chi route example
router.get('/profile/:uid', async (c) => {
  const db = getFirestore();
  const snap = await getDoc(doc(db, 'users', c.req.param('uid')));
  const data = snap.data();
  // In practice, retrieve password from session, not from request
  const password = c.env.USER_PASSWORD; // stored securely, e.g., via KMS or runtime secrets
  const ssn = await decryptField(
    { encryptedData: data.ssnEncrypted, authTag: data.ssnAuthTag },
    data.ssnIv,
    password,
    Buffer.from(data.ssnSalt, 'base64')
  );
  return c.json({ uid: c.req.param('uid'), ssn });
});

4. Firestore rules as a secondary layer, not encryption

Security rules enforce access but do not protect data at rest or in transit. Always encrypt sensitive fields before they reach Firestore. Use rules to enforce principle of least privilege, but never rely on them to provide confidentiality.

Finally, prefer using the official Firebase client SDKs with App Check and strict rules for request validation to reduce injection risks that could bypass application‑level cryptography. middleBrick scans can help identify missing encryption and insecure key handling in Chi + Firestore deployments, guiding focused remediation.