diff --git a/ForumApp.markdown b/ForumApp.markdown
index 2865c63..582eeeb 100644
--- a/ForumApp.markdown
+++ b/ForumApp.markdown
@@ -5,21 +5,42 @@ ForumApp is an application that uses [BSP](BSP) to synchronise invitation-only f
 ForumApp uses a channel with a random identifier for forum invitations:   
 `667e 2acb da80 2e61 3168 98f2 8ec5 8102 fcf2 3d10 04ee 3086 f5e9 94d8 30f5 b773`
 
-Each forum has its own channel with a hash identifier. The application data structure for generating the identifier is a [BDF](BDF) list with two elements: `forum_name` (string) and `salt` (raw, exactly HASH_LEN bytes). The random salt prevents collisions between forums with the same name. The application identifer is random:   
+Each forum has its own channel with a hash identifier. The application data structure for generating the identifier is a [BDF](BDF) list with two elements: `name` (string) and `salt` (raw, exactly HASH_LEN bytes). The random salt prevents collisions between forums with the same name. The application identifer is random:   
 `859a 7be5 0dca 035b 64bd 6902 fb79 7097 795a f837 abbf 8c16 d750 b3c2 ccc1 86ea`
 
 ### Message types
 
-**0: INVITATION** - The content is a BDF list with two elements: `forum` (list) and `message` (string, may be null). `forum` is a list with two elements, `forum_name` and `salt`, as described above. `message` is an optional note from the inviter to the invitee.
+**0: INVITATION** - The content is a BDF list with two elements: `forum` (list) and `message` (string, may be null). `forum` is a list with two elements, `name` and `salt`, as described above. `message` is an optional note from the inviter to the invitee.
 
 **1: RESPONSE** - The content is a BDF list with three elements: `invitation_id` (raw, exactly HASH_LEN bytes), `decision` (boolean), and `message` (string, may be null). `invitation_id` is the message identifier of the INVITATION to which this is a response. `decision` indicates whether the invitee wishes to join the forum. `message` is an optional note from the invitee to the inviter.
 
-**2: POST**
+**2: POST** - The content is a BDF list with two elements: `author` (list, may be null), and `content` (list). If `author` is null, the post is anonymous. Otherwise `author` is a list with two elements: `identity` (list), and `signature` (raw). `identity` is a list with two elements: `name` (string), and `public_key` (raw). `signature` is calculated over `content`.
 
-**3: ATTACHMENT**
+`content` is a list with three elements: `parent_id` (raw, exactly HASH_LEN bytes, may be null), `body` (string), and `attachments` (dictionary, may be null). `parent_id` is the message identifier of a POST to which this is a response. The key for each attachment is its name, and the value is a list with two elements: `mime_type` (string), and `message_id` (raw, exactly HASH_LEN bytes).
+
+**3: ATTACHMENT** - The content is raw data.
 
 ### Validation
 
+* An INVITATION or RESPONSE must belong to the invitation channel.
+* A POST or ATTACHMENT must belong to a forum 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.
+* A POST is valid if it is well-formed, its parent (if any) is a valid POST, and it is either anonymous or carries a valid signature.
+* An ATTACHMENT is valid if it is referenced by a valid POST.
+
 ### Storage policy
 
+* All valid local messages are stored.
+* All valid remote messages are stored.
+* An ATTACHMENT that is not referenced by a valid POST may be garbage-collected.
+
 ### Sharing policy
+
+For the invitation channel:
+* Each valid local message is shared with one remote device.
+* Remote messages are not shared with other remote devices.
+
+For forum channels:
+* Valid local messages are shared with all remote devices that belong to the forum.
+* Valid remote messages are shared with all remote devices that belong to the forum.