diff --git a/src/gsm/auth_core.c b/src/gsm/auth_core.c
index 2b6f35a..ad5967d 100644
--- a/src/gsm/auth_core.c
+++ b/src/gsm/auth_core.c
@@ -363,4 +363,33 @@
 		kc[i] = ck[i] ^ ck[i + 8] ^ ik[i] ^ ik[i + 8];
 }
 
+/*! Derive GSM SRES from UMTS [X]RES (auth function c2 from 3GPP TS 33.103 Section 6.8.1.2
+ *  \param[out] sres GSM SRES value, 4 byte target buffer
+ *  \param[in] res UMTS XRES, 4..16 bytes input buffer
+ *  \param[in] res_len length of res parameter (in bytes)
+ *  \param[in] sres_deriv_func SRES derivation function (1 or 2, see 3GPP TS 55.205 Section 4
+ */
+void osmo_auth_c2(uint8_t sres[4], const uint8_t *res, size_t res_len, uint8_t sres_deriv_func)
+{
+	uint8_t xres[16];
+
+	OSMO_ASSERT(sres_deriv_func == 1 || sres_deriv_func == 2);
+	OSMO_ASSERT(res_len <= sizeof(xres));
+
+	memcpy(xres, res, res_len);
+
+	/* zero-pad the end, if XRES is < 16 bytes */
+	if (res_len < sizeof(xres))
+		memset(xres+res_len, 0, sizeof(xres)-res_len);
+
+	if (sres_deriv_func == 1) {
+		/* SRES derivation function #1 */
+		for (unsigned int i = 0; i < 4; i++)
+			sres[i] = xres[i] ^ xres[4+i] ^ xres[8+i] ^ xres[12+i];
+	} else {
+		/* SRES derivation function #2 */
+		memcpy(sres, xres, 4);
+	}
+}
+
 /*! @} */
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index e03e014..0e45dc1 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -585,6 +585,7 @@
 osmo_auth_load;
 osmo_auth_register;
 osmo_auth_supported;
+osmo_auth_c2;
 osmo_auth_c3;
 osmo_sub_auth_type_names;
 
diff --git a/src/gsm/milenage/milenage.c b/src/gsm/milenage/milenage.c
index 3c14ab9..ba9ab33 100644
--- a/src/gsm/milenage/milenage.c
+++ b/src/gsm/milenage/milenage.c
@@ -244,19 +244,13 @@
 int gsm_milenage(const u8 *opc, const u8 *k, const u8 *_rand, u8 *sres, u8 *kc)
 {
 	u8 res[8], ck[16], ik[16];
-	int i;
 
 	if (milenage_f2345(opc, k, _rand, res, ck, ik, NULL, NULL))
 		return -1;
 
 	osmo_auth_c3(kc, ck, ik);
+	osmo_auth_c2(sres, res, sizeof(res), 1);
 
-#ifdef GSM_MILENAGE_ALT_SRES
-	os_memcpy(sres, res, 4);
-#else /* GSM_MILENAGE_ALT_SRES */
-	for (i = 0; i < 4; i++)
-		sres[i] = res[i] ^ res[i + 4];
-#endif /* GSM_MILENAGE_ALT_SRES */
 	return 0;
 }