docs: sending message diagram

.gitignore

@@ -13,3 +13,5 @@
 
 # Dependency directories (remove the comment below to include it)
 # vendor/
+
+.DS_Store

diagrams/message.mmd

@@ -0,0 +1,12 @@
+sequenceDiagram
+  participant Alice
+  participant App A
+  participant Server A
+  participant Server B
+  participant App B
+  participant Bob
+  Alice ->> App A: Alice writes the message to Bob in the app
+  App A ->> Server A: the message (including ID & timestamp) is encrypted by <Alice-Bob public key> and sent to all Alice's servers<br>{from: <Bob-Alice key hash>, to: <Alice-Bob key hash>, message: <encrypted with Alice-Bob public key>}
+  Server A ->> Server B: Alice's servers use <Alice-Bob key hash> to locate Bob's servers<br>and forwards encrypted message to all Bob's servers (in case they are different from Alice's servers
+  Server B ->> App B: Bob's servers send the message to Bob's app
+  App B ->> Bob: Bob's app uses <Bob-Alice key hash> to identify Alice<br>and decrypts the message with <Bob-Alice private key> to show it

readme.md

@@ -71,4 +71,10 @@ Client apps should provide the following:
   - optionally define the server(s) they trust and will connect to, including the servers that require server key to access.
   - optionally require that connected users connect to one of the servers they trust when sending messages to the user profile.
   - store history of all conversations encrypted using user client app password (or some other device specific encryption mechanism).
-  
+
+
+## Sequence diagrams
+
+### Sending message
+
+![Sending message](/diagrams/message.svg)