Client-Side Field Level Encryption
Learners will configure MongoDB's Client-Side Field Level Encryption to encrypt individual sensitive fields before they leave the application, keeping plaintext out of the server.
Client-Side Field Level Encryption is a free MongoDB Academy lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the MongoDB Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Field-Level Encryption?
Even with TLS and encryption at rest, the MongoDB server sees plaintext data once it is decrypted from disk. A compromised DBA account, a rogue cloud engineer with disk access, or a database backup leak could expose sensitive fields. Client-Side Field Level Encryption (CSFLE) solves this by encrypting individual sensitive fields — like SSNs, credit card numbers, or health data — inside the client driver, before the data ever reaches the server. The server only ever stores ciphertext.
How CSFLE Works at a High Level
CSFLE uses two layers of keys. The Customer Master Key (CMK) is stored in an external Key Management System (AWS KMS, Azure Key Vault, GCP KMS, or a local key). The CMK encrypts a Data Encryption Key (DEK), which is stored in a MongoDB collection called the Key Vault. The driver fetches and decrypts the DEK using the CMK at query time, then uses the DEK to encrypt/decrypt individual field values. The server never sees the CMK or the DEK in plaintext.
Two Modes: Automatic and Explicit
CSFLE offers two encryption modes. Automatic CSFLE (requires MongoDB Enterprise or Atlas) encrypts and decrypts fields transparently based on a JSON schema — your application code does not change. Explicit (Manual) CSFLE is available in the Community driver and requires the application to call encrypt/decrypt methods explicitly. Automatic is far more convenient for new projects; explicit gives maximum control over which fields are encrypted per operation.
Setting Up the Key Vault Collection
Before encrypting any data, create a Key Vault collection — a special MongoDB collection that stores Data Encryption Keys. The key vault is just a regular collection (e.g., encryption.__keyVault) but it must have a unique index on the keyAltNames field. DEKs are stored as BSON documents with the key material encrypted by your CMK — even the key vault only stores ciphertext.
const { MongoClient, ClientEncryption } = require('mongodb-client-encryption')
// Step 1: Create key vault collection with unique index
const client = new MongoClient('mongodb://localhost:27017')
await client.connect()
const keyVaultColl = client.db('encryption').collection('__keyVault')
await keyVaultColl.createIndex(
{ keyAltNames: 1 },
{ unique: true, partialFilterExpression: { keyAltNames: { $exists: true } } }
)Creating a Data Encryption Key
Use the ClientEncryption helper to create a DEK. The key is encrypted by your CMK (here a local master key for development) and stored in the key vault. In production, replace the local provider with aws, azure, or gcp and provide the KMS credentials. You can create multiple DEKs — for example, one per tenant in a multi-tenant application.
const crypto = require('crypto')
// 96-byte local master key (development only — use KMS in production)
const localMasterKey = crypto.randomBytes(96)
const encryption = new ClientEncryption(client, {
keyVaultNamespace: 'encryption.__keyVault',
kmsProviders: { local: { key: localMasterKey } }
})
// Create a DEK with an alias for easy reference
const dataKey = await encryption.createDataKey('local', {
keyAltNames: ['userSensitiveDataKey']
})
console.log('DEK id:', dataKey)Defining the Encrypted Fields Schema
For automatic CSFLE, define an encrypted fields map that tells the driver which fields to encrypt and with which algorithm. AEAD_AES_256_CBC_HMAC_SHA_512-Deterministic produces the same ciphertext for the same plaintext — enabling equality queries on encrypted fields. AEAD_AES_256_CBC_HMAC_SHA_512-Random produces different ciphertext each time — stronger but not queryable.
const encryptedFieldsMap = {
'myApp.users': {
fields: [
{
path: 'ssn',
bsonType: 'string',
// Deterministic: can query encrypted SSN with equality
algorithm: 'AEAD_AES_256_CBC_HMAC_SHA_512-Deterministic',
keyId: dataKey
},
{
path: 'creditCardNumber',
bsonType: 'string',
// Random: cannot query, but stronger encryption
algorithm: 'AEAD_AES_256_CBC_HMAC_SHA_512-Random',
keyId: dataKey
}
]
}
}Creating an Auto-CSFLE MongoClient
To enable automatic CSFLE, configure the MongoClient with the autoEncryption option, providing the key vault namespace, KMS credentials, and the encrypted fields map. The driver will automatically encrypt matching fields on insert/update and decrypt them on read. No changes to your application queries are required.
const secureClient = new MongoClient('mongodb://localhost:27017', {
autoEncryption: {
keyVaultNamespace: 'encryption.__keyVault',
kmsProviders: { local: { key: localMasterKey } },
encryptedFieldsMap: encryptedFieldsMap
}
})
await secureClient.connect()
const users = secureClient.db('myApp').collection('users')
// SSN and creditCardNumber are auto-encrypted on insert
await users.insertOne({
name: 'Alice',
ssn: '123-45-6789', // encrypted transparently
creditCardNumber: '4111-1111-1111-1111' // encrypted transparently
})Querying Encrypted Fields
With deterministic encryption, you can perform equality queries on encrypted fields — the driver encrypts the query value with the same DEK before sending it to the server, so the server compares ciphertexts. With random encryption, equality queries are not possible because the same plaintext produces different ciphertexts each time. Range and regex queries are not supported on encrypted fields in CSFLE.
// Query an encrypted SSN field (deterministic encryption)
// The driver auto-encrypts '123-45-6789' before sending the query
const user = await users.findOne({ ssn: '123-45-6789' })
// The result has SSN decrypted automatically by the driver:
console.log(user.ssn) // '123-45-6789' (decrypted)
// A client WITHOUT the key sees ciphertext:
// user.ssn = Binary(Buffer.from('...'), 6) // encrypted blobExplicit Encryption With the Driver API
Explicit CSFLE gives you per-operation control. Call encryption.encrypt() before inserting and encryption.decrypt() after reading. This works in Community edition drivers without requiring the automatic CSFLE shared library. It is more verbose but gives complete flexibility — you can encrypt different fields in different documents with different DEKs.
// Explicit encryption
const encryptedSsn = await encryption.encrypt('123-45-6789', {
algorithm: 'AEAD_AES_256_CBC_HMAC_SHA_512-Deterministic',
keyAltName: 'userSensitiveDataKey'
})
await users.insertOne({
name: 'Bob',
ssn: encryptedSsn // manually encrypted Binary value
})
// Explicit decryption
const doc = await users.findOne({ name: 'Bob' })
const decryptedSsn = await encryption.decrypt(doc.ssn)
console.log(decryptedSsn) // '123-45-6789'Key Rotation for Field-Level Encryption
Rotate DEKs periodically to limit the exposure window if a key is compromised. Key rotation in CSFLE involves creating a new DEK, re-encrypting all documents that use the old DEK (field by field), and then deleting the old DEK from the key vault. This process can be done as a background migration script without downtime. Rotating CMKs in KMS (wrapping the DEK) does not require touching the encrypted documents at all.
CSFLE Limitations and Considerations
CSFLE has important limitations to plan for: no server-side operations on encrypted fields (aggregation, sorting, and range queries on encrypted fields are not supported, except equality on deterministic fields); schema changes require DEK re-use or re-encryption; automatic CSFLE requires MongoDB Enterprise or Atlas; and performance overhead from encryption/decryption in the driver adds latency. Design your data model to minimise which fields need encryption.
Quick Check
Test your understanding of MongoDB & NoSQL Databases concepts from this lesson.
Lesson Recap
In this lesson you learned: CSFLE encrypts sensitive fields inside the driver before data reaches the server, so even MongoDB itself only sees ciphertext, deterministic encryption enables equality queries while random encryption provides stronger security without queryability, and the two-tier key model (CMK in KMS wrapping DEK in key vault) keeps encryption keys outside MongoDB. Next up we explore MongoDB schema design patterns.
Frequently asked questions
Is the “Client-Side Field Level Encryption” lesson free?
Yes — the full text of “Client-Side Field Level Encryption” is free to read here on the web, and the MongoDB Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the MongoDB Academy course, upgrade to CoddyKit PRO.
What will I learn in “Client-Side Field Level Encryption”?
Learners will configure MongoDB's Client-Side Field Level Encryption to encrypt individual sensitive fields before they leave the application, keeping plaintext out of the server. You practise MongoDB Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start MongoDB Academy?
No prior experience is required. MongoDB Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Client-Side Field Level Encryption” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this MongoDB Academy lesson?
Yes. Every MongoDB Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Authentication Mechanisms: SCRAM and x.509
- Role-Based Access Control: Built-In and Custom Roles
- Encryption at Rest and TLS in Transit
- Client-Side Field Level Encryption