diff --git a/BlogClient.markdown b/BlogClient.markdown
index 7a2f908..7c7eacd 100644
--- a/BlogClient.markdown
+++ b/BlogClient.markdown
@@ -20,7 +20,7 @@ The client identifer is random:
 
 **3: POST** - The content is a list with two elements: `content` (list) and `signature` (raw). `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).
 
-`signature` is a signature with `public_key`, calculated over a list with three elements: `blog`, `content`, and `timestamp` (int). `blog` and `content` are described above. `timestamp` is the timestamp from the [message header](BSP#message-format).
+`signature` is a signature with `public_key`, calculated over a list with four elements: `client_id` (raw), `blog` (list), `content` (list), and `timestamp` (int). `client_id` is the client identifier. `blog` and `content` are described above. `timestamp` is the timestamp from the [message header](BSP#message-format).
 
 **4: ATTACHMENT** - The content is raw data.