The driver depends on libmongocrypt to create encrypted key material so it can insert into the key vault. The driver spec "Driver: Key Vault" describes the document that gets created. libmongocrypt is used to generate the "keyMaterial" field's value.

To do so, libmongocrypt should provide API that does the following:

• creates a data key (64 byte random string)
• asks the driver to encrypt the data key material with KMS by returning a mongocrypt_key_decryptor_t (being added in CDRIVER-2949)
• returns the final encrypted data key