diff --git a/ui/vnc.c b/ui/vnc.c
index 94e4f19a60..1483958c45 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -2551,7 +2551,7 @@ static int protocol_client_auth_vnc(VncState *vs, uint8_t *data, size_t len)
         goto reject;
     }
 
-    if (qcrypto_cipher_decrypt(cipher,
+    if (qcrypto_cipher_encrypt(cipher,
                                vs->challenge,
                                response,
                                VNC_AUTH_CHALLENGE_SIZE,