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+MessagingApp is an application that uses [BSP] to synchronise private messages.
+
+### Message types
+
+MessagingApp uses two message types:
+
+**0: PRIVATE_MESSAGE** - The content is a [BDF] list with two elements: message body (string) and attachments (dictionary, may be null). Each attachment's key is its name, and the value is a list with two elements: MIME type (string), and message identifier (raw, exactly HASH_LEN bytes).
+
+**1: ATTACHMENT** - The content is raw data.
+
+### Validation
+
+* A message of type PRIVATE_MESSAGE is valid if it is well-formed as described above
+* A message of type ATTACHMENT is always valid
+
+### Storage policy
+
+* All local messages are stored
+* All remote messages are stored
+
+### Sharing policy
+
+* Each local message is shared with one remote device
+* Remote messages are not shared with other remote devices