diff --git a/protos/rendezvous.proto b/protos/rendezvous.proto
index fe0ef637a..f126714a8 100644
--- a/protos/rendezvous.proto
+++ b/protos/rendezvous.proto
@@ -238,6 +238,9 @@ message KeyExchange {
   // From the server: the newest key exchange version it speaks. From the client: the version it
   // picked, the highest both speak. Absent, i.e. 0, is the original scheme.
   uint32 version = 2;
+  // Server signature of "rdkx-ver" || on-wire ephemeral public key || little-endian uint32 version.
+  // Bit 255 of the signed X25519 public key requires this field; legacy keys leave it clear.
+  bytes signed_version = 3;
 }
 
 message HealthCheck {