wip: Add the basics for the Double Ratchet

protobuf/schema.proto

@@ -0,0 +1,20 @@
+// Taken from https://xmpp.org/extensions/xep-0384.html#protobuf-schema
+message OMEMOMessage {
+    required uint32 n          = 1;
+    required uint32 pn         = 2;
+    required bytes  dh_pub     = 3;
+    optional bytes  ciphertext = 4;
+}
+
+message OMEMOAuthenticatedMessage {
+    required bytes mac     = 1;
+    required bytes message = 2; // Byte-encoding of an OMEMOMessage
+}
+
+message OMEMOKeyExchange {
+    required uint32 pk_id  = 1;
+    required uint32 spk_id = 2;
+    required bytes  ik     = 3;
+    required bytes  ek     = 4;
+    required OMEMOAuthenticatedMessage message = 5;
+}