diff --git a/include/osmocom/crypt/auth.h b/include/osmocom/crypt/auth.h
index e544126b9..c653b6161 100644
--- a/include/osmocom/crypt/auth.h
+++ b/include/osmocom/crypt/auth.h
@@ -42,7 +42,7 @@ struct osmo_sub_auth_data {
 	union {
 		struct {
 			uint8_t opc[16]; /*!< operator invariant value */
-			uint8_t k[16];	/*!< secret key of the subscriber */
+			uint8_t k[OSMO_A5_MAX_KEY_LEN_BYTES];	/*!< secret key of the subscriber */
 			uint8_t amf[2];
 			uint64_t sqn;	/*!< sequence number (in: prev sqn; out: used sqn) */
 			int opc_is_op;	/*!< is the OPC field OPC (0) or OP (1) ? */
@@ -60,8 +60,8 @@ struct osmo_sub_auth_data {
 struct osmo_auth_vector {
 	uint8_t rand[16];	/*!< random challenge */
 	uint8_t autn[16];	/*!< authentication nonce */
-	uint8_t ck[16];		/*!< ciphering key */
-	uint8_t ik[16];		/*!< integrity key */
+	uint8_t ck[OSMO_A5_MAX_KEY_LEN_BYTES];		/*!< ciphering key */
+	uint8_t ik[OSMO_A5_MAX_KEY_LEN_BYTES];		/*!< integrity key */
 	uint8_t res[16];	/*!< authentication result */
 	uint8_t res_len;	/*!< length (in bytes) of res */
 	uint8_t kc[8];		/*!< Kc for GSM encryption (A5) */
diff --git a/src/gsm/auth_core.c b/src/gsm/auth_core.c
index f171ed49f..9e750a01d 100644
--- a/src/gsm/auth_core.c
+++ b/src/gsm/auth_core.c
@@ -98,7 +98,7 @@ int osmo_auth_supported(enum osmo_auth_algo algo)
 	return 0;
 }
 
-/* C5 function to derive UMTS IK from GSM Kc */
+/* 3GPP TS 33.102 §6.8.2.3 C5 function to derive UMTS IK from GSM Kc */
 static inline void c5_function(uint8_t *ik, const uint8_t *kc)
 {
 	unsigned int i;
@@ -110,7 +110,7 @@ static inline void c5_function(uint8_t *ik, const uint8_t *kc)
 		ik[i] = ik[i-12];
 }
 
-/* C4 function to derive UMTS CK from GSM Kc */
+/* 3GPP TS 33.102 §6.8.2.3 C4 function to derive UMTS CK from GSM Kc */
 void osmo_c4(uint8_t *ck, const uint8_t *kc)
 {
 	memcpy(ck, kc, 8);
diff --git a/tests/auth/milenage_test.c b/tests/auth/milenage_test.c
index 2bd3cf203..8b058e565 100644
--- a/tests/auth/milenage_test.c
+++ b/tests/auth/milenage_test.c
@@ -23,7 +23,10 @@ static void dump_auth_vec(struct osmo_auth_vector *vec)
 
 	if (vec->auth_types & OSMO_AUTH_TYPE_GSM) {
 		printf("SRES:\t%s\n", osmo_hexdump(vec->sres, sizeof(vec->sres)));
-		printf("Kc:\t%s\n", osmo_hexdump(vec->kc, sizeof(vec->kc)));
+		/* According to 3GPP TS 55.205 Sec. 4 the GSM-MILENAGE output is limited to 64 bits.
+		   According to 3GPP TS 33.102 Annex. B5 in UMTS security context Kc can be 128 bits.
+		   Here we test the former, so make sure we only print interesting Kc bits. */
+		printf("Kc:\t%s\n", osmo_hexdump(vec->kc, OSMO_A5_MAX_KEY_LEN_BYTES/2));
 	}
 }