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The guide to integrating libmongocrypt

libmongocrypt is a C library meant to assist drivers in supporting client side encryption. libmongocrypt acts as a state machine and the driver is responsible for I/O between mongod, mongocryptd, and KMS.

There are two major parts to integrating libmongocrypt into your driver:

  • Writing a language-specific binding to libmongocrypt
  • Using the binding in your driver to support client side encryption

Design Rationale

Simple interface

The library interface is intended to be used with multiple languages.

The API tries to be minimal. Most structs are opaque. Global initialization is lazy.

Much of the API passes and returns BSON since all drivers can produce and parse BSON.

No I/O

libmongocrypt deliberately does not do I/O to avoid poor behavior with some language runtimes. Example: in Go a blocking C call may block an OS thread, rather than a goroutine.

Part 1: Writing a Language-Specific Binding

The binding is the glue between your driver's native language and libmongocrypt.

The binding uses the native language's foreign function interface to C. For example, Java can accomplish this with JNA, CPython with extensions, Node.js with add-ons, etc.

The libmongocrypt library files (.so/.dll) are pre-built on its Evergreen project. Click the variant's "built-and-test-and-upload" tasks to download the attached files.

libmongocrypt describes all API that needs to be called from your driver in the main public header mongocrypt.h.

There are many types and functions in mongocrypt.h to bind. Consider as a first step binding to only mongocrypt_version. Once you have that working, proceed to write bindings for the remaining API. Here are a few things to keep in mind:

  • "ctx" is short for context, and is a generic term indicating that the object stores state.
  • By C convention, functions are named like: mongocrypt_<type>_<method>. For example mongocrypt_ctx_id can be thought of as a class method "id" on the class "ctx".
  • mongocrypt_binary_t is a non-owning view of data. Calling mongocrypt_binary_destroy frees the view, but does nothing to the underlying data. When a mongocrypt_binary_t is returned (e.g. mongocrypt_ctx_mongo_op), the lifetime of the data is tied to the type that returned it (so the data returned will be freed when the mongocrypt_ctx_t) is freed.

Once you have full bindings for the API, it's time to do a sanity check. The crux of libmongocrypt's API is the state machine represented by mongocrypt_ctx_t. This state machine is exercised in the example-state-machine executable included with libmongocrypt. It uses mock responses from mongod, mongocryptd, and KMS. Reimplement the state machine loop (_run_state_machine) in example-state-machine with your binding.

To debug, configure with the cmake option -DENABLE_TRACE=ON, and set the environment variable MONGOCRYPT_TRACE=ON to log the arguments to mongocrypt functions. Note, this is insecure and should only be used for debugging.

Seek help in the slack channel #drivers-fle.

Part 2: Integrate into Driver

After you have a binding, integrate libmongocrypt in your driver to support client side encryption.

See the driver spec for a reference of the user-facing API. libmongocrypt is needed for:

  • Automatic encryption/decryption
  • Explicit encryption/decryption
  • KeyVault (explicit encryption/decryption + createDataKey)

It is recommended to start by integrating libmongocrypt to support automatic encryption/decryption. Then reuse the implementation to implement the KeyVault.

A MongoClient enabled with client side encryption MUST have one shared mongocrypt_t handle (important because keys + JSON Schemas are cached in this handle). Each KeyVault also has its own mongocrypt_t.

Any encryption or decryption operation is done by creating a mongocrypt_ctx_t and initializing it for the appropriate operation. mongocrypt_ctx_t is a state machine, and each state requires the driver to perform some action. This may be performing I/O on one of the following:

  • the encrypted MongoClient to which the operation is occurring (for auto encrypt).
  • the key vault MongoClient (which may be the same as the encrypted MongoClient).
  • KMS (via a TLS socket).
  • the MongoClient to the local mongocryptd process.

Initializing

There are five different types of mongocrypt_ctx_t's, distinguished by how they are initialized:

  • auto encrypt (mongocrypt_ctx_encrypt_init)
  • auto decrypt (mongocrypt_ctx_decrypt_init)
  • explicit encrypt (mongocrypt_ctx_explicit_encrypt_init)
  • explicit decrypt (mongocrypt_ctx_explicit_decrypt_init)
  • create data key (mongocrypt_ctx_datakey_init)

State Machine

Below is a list of the various states a mongocrypt ctx can be in. For each state, there is a description of what the driver is expected to do to advance the state machine. Not all states will be entered for all types of contexts. But one state machine runner can be used for all types of contexts.

State: MONGOCRYPT_CTX_ERROR

Driver needs to...

Throw an exception based on the status from mongocrypt_ctx_status.

Applies to...

All contexts.

State: MONGOCRYPT_CTX_NEED_MONGO_COLLINFO

libmongocrypt needs...

A result from a listCollections cursor.

Driver needs to...

  1. Run listCollections on the encrypted MongoClient with the filter provided by mongocrypt_ctx_mongo_op
  2. Return the first result (if any) with mongocrypt_ctx_mongo_feed or proceed to the next step if nothing was returned.
  3. Call mongocrypt_ctx_mongo_done

Applies to...

auto encrypt

State: MONGOCRYPT_CTX_NEED_MONGO_COLLINFO_WITH_DB

libmongocrypt needs...

Results from a listCollections cursor from a specified database.

Driver needs to...

  1. Run listCollections on the encrypted MongoClient with the filter provided by mongocrypt_ctx_mongo_op on the database provided by mongocrypt_ctx_mongo_db.
  2. Return the first result (if any) with mongocrypt_ctx_mongo_feed or proceed to the next step if nothing was returned.
  3. Call mongocrypt_ctx_mongo_done

Applies to...

A context initialized with mongocrypt_ctx_encrypt_init for automatic encryption. This state is only entered when mongocrypt_setopt_use_need_mongo_collinfo_with_db_state is called to opt-in.

State: MONGOCRYPT_CTX_NEED_MONGO_MARKINGS

libmongocrypt needs...

A reply from mongocryptd indicating which values in a command need to be encrypted.

Driver needs to...

  1. Use db.runCommand to run the command provided by mongocrypt_ctx_mongo_op on the MongoClient connected to mongocryptd.
  2. Feed the reply back with mongocrypt_ctx_mongo_feed.
  3. Call mongocrypt_ctx_mongo_done.

Applies to...

auto encrypt

State: MONGOCRYPT_CTX_NEED_MONGO_KEYS

libmongocrypt needs...

Documents from the key vault collection.

Driver needs to...

  1. Use MongoCollection.find on the MongoClient connected to the key vault client (which may be the same as the encrypted client). Use the filter provided by mongocrypt_ctx_mongo_op.
  2. Feed all resulting documents back (if any) with repeated calls to mongocrypt_ctx_mongo_feed.
  3. Call mongocrypt_ctx_mongo_done.

Applies to...

All contexts except for create data key.

State: MONGOCRYPT_CTX_NEED_KMS

libmongocrypt needs...

The responses from one or more messages to KMS.

Ensure mongocrypt_setopt_retry_kms is called on the mongocrypt_t to enable retry.

Driver needs to...

  1. For each context returned by mongocrypt_ctx_next_kms_ctx:

    a. Delay the message by the time in microseconds indicated by mongocrypt_kms_ctx_usleep if returned value is greater than 0.

    b. Create/reuse a TLS socket connected to the endpoint indicated by mongocrypt_kms_ctx_endpoint. The endpoint string is a host name with a port number separated by a colon. E.g. "kms.us-east-1.amazonaws.com:443". A port number will always be included. Drivers may assume the host name is not an IP address or IP literal.

    c. Write the message from mongocrypt_kms_ctx_message to the > socket.

    d. Feed the reply back with mongocrypt_kms_ctx_feed. Repeat > until mongocrypt_kms_ctx_bytes_needed returns 0.

    If any step encounters a network error, call mongocrypt_kms_ctx_fail. If mongocrypt_kms_ctx_fail returns true, continue to the next KMS context. If mongocrypt_kms_ctx_fail returns false, abort and report an error. Consider wrapping the error reported in mongocrypt_kms_ctx_status to include the last network error.

  2. When done feeding all replies, call mongocrypt_ctx_kms_done.

Note, the driver MAY fan out KMS requests in parallel. More KMS requests may be added when processing responses to retry.

Applies to...

All contexts.

State: MONGOCRYPT_CTX_NEED_KMS_CREDENTIALS

MONGOCRYPT_CTX_NEED_KMS_CREDENTIALS was added in libmongocrypt 1.4.0 as part of MONGOCRYPT-382.

MONGOCRYPT_CTX_NEED_KMS_CREDENTIALS can only be entered if mongocrypt_setopt_use_need_kms_credentials_state is called. This prevents breaking drivers that do not handle the MONGOCRYPT_CTX_NEED_KMS_CREDENTIALS state.

If a KMS provider is configured with an empty document (e.g. { "aws": {} }), the MONGOCRYPT_CTX_NEED_KMS_CREDENTIALS is entered before KMS requests are made.

libmongocrypt needs...

Credentials for one or more KMS providers.

Driver needs to...

Fetch credentials for supported KMS providers. See the Client Side Encryption specification for details.

Pass credentials to libmongocrypt using mongocrypt_ctx_provide_kms_providers.

Applies to...

All contexts.

State: MONGOCRYPT_CTX_READY

Driver needs to...

Call mongocrypt_ctx_finalize to perform the encryption/decryption and get the final result.

Applies to...

All contexts except for create data key.

State: MONGOCRYPT_CTX_DONE

Driver needs to...

Exit the state machine loop.

Applies to...

All contexts.

Seek help in the slack channel #drivers-fle.