diff --git a/cmd/anchor/main.go b/cmd/anchor/main.go
index 89dbfa4..9136d2a 100644
--- a/cmd/anchor/main.go
+++ b/cmd/anchor/main.go
@@ -155,8 +155,8 @@ func (a *anchor) handleWS(w http.ResponseWriter, r *http.Request) {
 	ctx, cancel := context.WithCancel(r.Context())
 	defer cancel()
 
-	// Send a challenge nonce immediately.
-	nonce := make([]byte, 16)
+	// Send a challenge nonce immediately. §5.1 requires 32 bytes.
+	nonce := make([]byte, 32)
 	rand.Read(nonce)
 	nonceHex := hex.EncodeToString(nonce)
 	if err := wsjson.Write(ctx, conn, proto.AnchorMessage{