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+BlogClient is a [BSP client](BSP) that synchronises blog posts among groups of devices. Any subscriber to a blog can invite new subscribers, but only the creator of the blog can write posts.
+
+### Channel identifiers
+
+All blogs share a single invitation channel with the following random identifier:   
+`4f39 df93 e933 5027 9909 82bf 442f 7b40 75d3 b7f6 6139 34dd c6d1 b539 7a2a 65a3`
+
+Each blog has its own channel with a [hash identifier](BSP#channel-identifiers). The client data structure for generating the identifier is a [BDF](BDF) list with two elements: `name` (string) and `public_key` (raw).
+
+The client identifer is random:   
+`dafb e56f 0c89 7136 5cea 4bb5 f08e c9a6 1d68 6e05 8b94 3997 b6ff 259b a423 f613`
+
+### Message types
+
+**0: INVITATION** - The content is a BDF list with three elements: `blog` (list) and `note` (string, may be null). `blog` is a list with two elements, `name` and `public_key`, as described above. `note` is an optional note from the inviter to the invitee.
+
+**1: RESPONSE** - The content is a BDF list with three elements: `invitation_id` (raw), `decision` (boolean), and `note` (string, may be null). `invitation_id` is the identifier of an invitation created by the opposite peer. `decision` indicates whether the invitee wishes to subscribe. `note` is an optional note from the invitee to the inviter.
+
+**2: DEPARTURE** - The content is a BDF list with one element: `invitation_id` (raw), which is the identifier of an invitation created by either peer.
+
+**3: POST** - The content is a BDF list with two elements: `signature` (raw) and `content` (list). `signature` is calculated over `content`.
+
+`content` is a list with three elements: `parent_id` (raw, may be null), `body` (string), and `attachments` (dictionary, may be null). If `parent_id` is not null, it is the identifier of a post to which this is a follow-up. 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).
+
+**4: ATTACHMENT** - The content is raw data.
+
+### Validity policy
+
+* An invitation or response must belong to the invitation channel.
+* A post or attachment must belong to a blog channel.
+* An invitation is valid if it is well-formed.
+* A response is valid if it is well-formed and it references a valid invitation created by the opposite peer.
+* A departure is valid if it is well-formed and it references a valid invitation created by either peer.
+* A post is valid if it is well-formed, its parent (if any) is a valid post, and it carries a valid signature over its content.
+* An attachment is always valid.
+
+Note that a post can be validated before its attachments have been received, and an attachment can be validated before it has been completely received.
+
+### Storage policy
+
+For the invitation channel:
+* All local messages are stored.
+* All remote messages are stored.
+
+For forum channels:
+* Local messages are stored while the local per subscribes to the blog.
+* Remote messages are stored while the local peer subscribes to the blog.
+
+### Sharing policy
+
+For the invitation channel:
+* Each local message is shared with one remote peer.
+* Each remote message is shared with the remote peer that created it.
+
+For forum channels:
+* Local messages are shared with all remote peers that subscribe to the blog.
+* Remote messages are shared with all remote peers that subscribe to the blog.