diff --git a/specs/phase0/p2p-interface.md b/specs/phase0/p2p-interface.md
index c28deb2ee..2ad95f54b 100644
--- a/specs/phase0/p2p-interface.md
+++ b/specs/phase0/p2p-interface.md
@@ -243,10 +243,10 @@ Each gossipsub [message](https://github.com/libp2p/go-libp2p-pubsub/blob/master/
 Clients MUST reject (fail validation) messages that are over this size limit.
 Likewise, clients MUST NOT emit or propagate messages larger than this limit.
 
-The `message-id` of a gossipsub message MUST be:
+The `message-id` of a gossipsub message MUST be the first 8 bytes of the SHA-256 hash of the message data, i.e.:
 
 ```python
-   message-id: SHA256(message.data)
+   message-id: (SHA256(message.data))[0..8]
 ```
 
 The payload is carried in the `data` field of a gossipsub message, and varies depending on the topic: