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+MessagingClient is a [BSP client](BSP) that synchronises private messages between pairs of devices.
+
+### Channel identifiers
+
+MessagingClient has a single channel with the following random identifier:   
+`6bcd c006 c091 0b0f 44e4 0644 c3b3 1f1a 8bf9 a6d6 021d 40d2 19c8 6b73 1b90 3070`
+
+### Message types
+
+**0: PRIVATE_MESSAGE** - The content is a [BDF](BDF) list with two elements: `body` (string) and `attachments` (dictionary, may be null). Each key in `attachments` is the name of an attachment, and the value is a list with two elements: `mime_type` (string) and `message_id` (raw).
+
+**1: ATTACHMENT** - The content is raw data.
+
+### Validity policy
+
+* A private message is valid if it is well-formed.
+* An attachment is always valid.
+
+Note that a private message can be validated before its attachments have been received, and an attachment can be validated before it has been completely received.
+
+### Storage policy
+
+* All local messages are stored.
+* All remote messages are stored.
+
+### Sharing policy
+
+* Each local message is shared with one remote peer.
+* Each remote message is shared with the remote peer that created it.