MongoDB’s new field-level encryption allows websites to solve this issue quickly as it does not involve performing cryptographic operations or storing encryption keys on the server. Field Level Encryption is a transparent client-side mechanism that relies on the MongoDB client library to act as a driver and handles encryption and decryption operations on the users’ devices to protect specific fields in a document against everyone except the user. FLE is only compatible with the AWS Key Management Service (KMS) service to make it easier for users to create and manage their keys.
Source: https://thehackernews.com/2019/06/mongodb-fle-data-encryption.html