diff --git a/include/osmocom/crypt/auth.h b/include/osmocom/crypt/auth.h
index bed48e7..37b8a8a 100644
--- a/include/osmocom/crypt/auth.h
+++ b/include/osmocom/crypt/auth.h
@@ -91,7 +91,7 @@
 int osmo_auth_load(const char *path);
 
 int osmo_auth_supported(enum osmo_auth_algo algo);
-
+void osmo_c4(uint8_t *ck, const uint8_t *kc);
 const char *osmo_auth_alg_name(enum osmo_auth_algo alg);
 enum osmo_auth_algo osmo_auth_alg_parse(const char *name);