Alright, so here's the deal with field-level encryption in Firebase Firestore. It's not something that's built-in. Firestore security rules don't really cover field-level security. So, if you want to keep your sensitive data safe, you'll need to handle the encryption yourself, either on your server or through cloud functions. Encrypting data before storing it and decrypting it when you need it adds an extra layer of security. This way, even if someone gets their hands on your data, they won't be able to read it.

First things first, you need to set up a Firebase project. Then, get your Firestore Database and Firebase Authentication up and running. Firestore is where you'll store your data, and Firebase Authentication will handle user authentication. Simple, right?

Now, let's talk about Cloud Functions. These are pretty cool because they let you run backend code in response to HTTP requests or Firestore database triggers. The idea here is to create a Cloud Function that handles all your Firestore write/read operations. It'll encrypt sensitive fields before writing them and decrypt them after reading. Here's a little code snippet to show you how it works:

const functions = require('firebase-functions');
const crypto = require('crypto');
const admin = require('firebase-admin');
admin.initializeApp();

exports.addData = functions.https.onRequest((request, response) => {
  const rawData = request.body.data; // data to be encrypted and stored
  const cipher = crypto.createCipher('aes-256-cbc', 'encryptionKey');
  let encrypted = cipher.update(rawData, 'utf8', 'hex');
  encrypted += cipher.final('hex');
  const docRef = admin.firestore().collection('collection').doc('document');
  return docRef.set({data: encrypted}).then(() => {
    response.send('Data added.');
  });
});

Now, about that encryption key. You don't want to hardcode it in your source code like we did in the example above. That's a big no-no. Instead, you can store your encryption key in Firebase's environment configuration. It's a secure way to keep your keys safe. Here's how you do it:

firebase functions:config:set keys.encryption="myEncryptionKey"

And then, in your Cloud Functions code, you can get the encryption key like this:

const encryptionKey = functions.config().keys.encryption;

After setting this up, make sure all your read/write operations to Firestore go through these cloud functions. Don't let anyone access Firestore data directly.