diff --git a/include/osmocom/crypt/auth.h b/include/osmocom/crypt/auth.h
index bed48e786..37b8a8ad0 100644
--- a/include/osmocom/crypt/auth.h
+++ b/include/osmocom/crypt/auth.h
@@ -91,7 +91,7 @@ int osmo_auth_register(struct osmo_auth_impl *impl);
 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);
 
diff --git a/src/gsm/a5.c b/src/gsm/a5.c
index f1fd697ee..dbba0f208 100644
--- a/src/gsm/a5.c
+++ b/src/gsm/a5.c
@@ -40,6 +40,7 @@
 
 #include <osmocom/gsm/a5.h>
 #include <osmocom/gsm/kasumi.h>
+#include <osmocom/crypt/auth.h>
 
 /* Somme OS (like Nuttx) don't have ENOTSUP */
 #ifndef ENOTSUP
@@ -95,8 +96,7 @@ void
 _a5_3(const uint8_t *key, uint32_t fn, ubit_t *dl, ubit_t *ul, bool fn_correct)
 {
        uint8_t ck[16];
-       memcpy(ck, key, 8);
-       memcpy(ck + 8, key, 8);
+       osmo_c4(ck, key);
        /* internal function require 128 bit key so we expand by concatenating supplied 64 bit key */
        _a5_4(ck, fn, dl, ul, fn_correct);
 }
diff --git a/src/gsm/auth_core.c b/src/gsm/auth_core.c
index f1ec2ed4f..3aac0694c 100644
--- a/src/gsm/auth_core.c
+++ b/src/gsm/auth_core.c
@@ -106,7 +106,7 @@ static inline void c5_function(uint8_t *ik, const uint8_t *kc)
 }
 
 /* C4 function to derive UMTS CK from GSM Kc */
-static inline void c4_function(uint8_t *ck, const uint8_t *kc)
+void osmo_c4(uint8_t *ck, const uint8_t *kc)
 {
 	memcpy(ck, kc, 8);
 	memcpy(ck+8, kc, 8);
@@ -125,7 +125,7 @@ int osmo_auth_3g_from_2g(struct osmo_auth_vector *vec)
 	if ((vec->auth_types & OSMO_AUTH_TYPE_GSM) &&
 	    !(vec->auth_types & OSMO_AUTH_TYPE_UMTS)) {
 		c5_function(vec->ik, vec->kc);
-		c4_function(vec->ck, vec->kc);
+		osmo_c4(vec->ck, vec->kc);
 		/* We cannot actually set OSMO_AUTH_TYPE_UMTS as we have no
 		 * AUTN and no RES, and thus can only perform GSM
 		 * authentication with this tuple.