The messaging client is a BSP client that synchronises private messages between pairs of devices.
The client's identifier is org.briarproject.briar.messaging. The major version is 0.
The client uses a separate BSP group for communicating with each contact. The group descriptor is a BDF list containing the unique IDs of the contacts' identities, sorted in ascending order as byte strings.
PRIVATE_MESSAGE - The message body is a BDF list with one element: text (string).