Client-Side Field Level Encryption with mongocxx

Client-Side Field Level Encryption

New in MongoDB 4.2 Client-Side Field Level Encryption (CSFLE) allows administrators and developers to encrypt specific data fields in addition to other MongoDB encryption features.

With CSFLE, developers can encrypt fields client side without any server-side configuration or directives. Client-Side Field Level Encryption supports workloads where applications must guarantee that unauthorized parties, including server administrators, cannot read the encrypted data.

For an overview of CSFLE, please read the official MongoDB documentation in the manual.



Client-Side Field Level Encryption relies on a C library called libmongocrypt to do the heavy lifting encryption work. This dependency is managed by the C driver. As long as the C driver installation is 1.16.0 or higher, and has been compiled with Client-Side Field Level Encryption support, this dependency should be managed internally. See the C driver’s Using Client-Side Field Level Encryption for more information.


Automatic CSFLE relies upon a new binary called mongocryptd running as a daemon while the driver operates. This binary is only available with MongoDB Enterprise.

mongocryptd can either be started separately from the driver, or left to spawn automatically when encryption is used.

To run mongocryptd separately, pass the mongocryptdBypassSpawn flag to the client’s auto encryption options:

auto mongocryptd_options = make_document(kvp("mongocryptdBypassSpawn", true));

options::auto_encryption auto_encrypt_opts{};

If the mongocryptd binary is on the current path, the driver will be able to spawn it without any custom flags. However, if the mongocryptd binary is on a different path, set the path with the mongocryptdSpawnPath option:

auto mongocryptd_options = make_document(kvp("mongocryptdSpawnPath", "path/to/mongocryptd"));

options::auto_encryption auto_encrypt_opts{};


Automatic Client-Side Field Level Encryption

Automatic Client-Side Field Level Encryption is enabled by creating a mongocxx::client with the auto_encryption_opts option set to an instance of ‘mongocxx::options::auto_encryption. The following examples show how to set up automatic client-side field level encryption using the mongocxx::client_encryption` class to create a new encryption data key.

Automatic client-side field level encryption requires MongoDB 4.2 enterprise or a MongoDB 4.2 Atlas cluster. The community version of the server supports automatic decryption as well as explicit client-side field level encryption.

Providing Local Automatic Encryption Rules

The following example shows how to specify automatic encryption rules via the schema_map option. The automatic encryption rules are expressed using a strict subset of the JSON Schema syntax.

Supplying a schema_map provides more security than relying on JSON Schemas obtained from the server. It protects against a malicious server advertising a false JSON Schema, which could trick the client into sending unencrypted data that should be encrypted.

JSON Schemas supplied in the schema_map only apply to configuring automatic client-side field level encryption. Other validation rules in the JSON schema will not be enforced by the driver and will result in an error.

// The schema map has the following form:
//   {
//      "test.coll" : {
//         "bsonType" : "object",
//         "properties" : {
//            "encryptedFieldName" : {
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//               "encrypt" : {
//               "encryptField" : {
>>>>>>> CXX-1855 add tutorial and example for client-side encryption
//                  "keyId" : [ <datakey as UUID> ],
//                  "bsonType" : "string",
//                  "algorithm" : <algorithm>
//               }
//            }
//         }
//      }
//   }

Please see examples/mongocxx/automatic_client_side_field_level_encryption.cpp for a full example of how to set a json schema for automatic encryption.

Server-Side Field Level Encryption Enforcement

The MongoDB 4.2 server supports using schema validation to enforce encryption of specific fields in a collection. This schema validation will prevent an application from inserting unencrypted values for any fields marked with the "encrypt" JSON schema keyword.

It is possible to set up automatic client-side field level encryption using the mongocxx::client_encryption to create a new encryption data key and create a collection with the Automatic Encryption JSON Schema Syntax.

// Please see the linked example below for full json_schema construction.
bsoncxx::document::value json_schema{};

// Create the collection with the encryption JSON Schema.
auto cmd = document{} << "create"
                      << "coll"
                      << "validator" << open_document
		      << "$jsonSchema" << json_schema.view()
                      << close_document << finalize;


Please see examples/mongocxx/server_side_field_level_encryption_enforcement.cpp for a full example of setting encryption enforcement on the server.

Explicit Encryption

Explicit encryption is a MongoDB community feature and does not use the mongocryptd process. Explicit encryption is provided by the mongocxx::client_encryption class.

// Explicitly encrypt a BSON value.
auto to_encrypt = bsoncxx::types::bson_value::make_value("secret message");
auto encrypted_message = client_encryption.encrypt(to_encrypt, encrypt_opts);

// Explicitly decrypt a BSON value.
auto decrypted_message = client_encryption.decrypt(encrypted_message);

Please see examples/mongocxx/explicit_encryption.cpp for a full example of using explicit encryption and decryption.

Explicit Encryption with Automatic Decryption

Although automatic encryption requires MongoDB 4.2 enterprise or a MongoDB 4.2 Atlas cluster, automatic decryption is supported for all users. To configure automatic decryption without automatic encryption, set bypass_auto_encryption=True in the options::auto_encryption class.

options::auto_encryption auto_encrypt_opts{};
// Please see full example for complete options construction.

// Create a client with automatic decryption enabled, but automatic encryption bypassed.
options::client client_opts{};
class client client_encrypted {uri{}, std::move(client_opts)};

Please see examples/mongocxx/explicit_encryption_auto_decryption.cpp for an example of using explicit encryption with automatic decryption.

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CXX-1855 add tutorial and example for client-side encryption