diff --git a/openbsc/include/openbsc/gprs_sgsn.h b/openbsc/include/openbsc/gprs_sgsn.h
index 0e574d8..9604084 100644
--- a/openbsc/include/openbsc/gprs_sgsn.h
+++ b/openbsc/include/openbsc/gprs_sgsn.h
@@ -158,6 +158,8 @@
 	/* Iu: CK, IK, KSI */
 	/* CKSN */
 	enum gprs_ciph_algo	ciph_algo;
+	/* Auth & Ciphering Request reference from 3GPP TS 24.008 § 10.5.5.19: */
+	uint8_t ac_ref_nr_used;
 
 	struct {
 		uint8_t	len;
diff --git a/openbsc/src/gprs/gprs_gmm.c b/openbsc/src/gprs/gprs_gmm.c
index 5db69dd..7f10b07 100644
--- a/openbsc/src/gprs/gprs_gmm.c
+++ b/openbsc/src/gprs/gprs_gmm.c
@@ -31,6 +31,8 @@
 #include <arpa/inet.h>
 #include <netdb.h>
 
+#include <openssl/rand.h>
+
 #include <openbsc/db.h>
 #include <osmocom/core/msgb.h>
 #include <osmocom/gsm/tlv.h>
@@ -417,16 +419,16 @@
 }
 
 /* Section 9.4.9: Authentication and Ciphering Request */
-static int gsm48_tx_gmm_auth_ciph_req(struct sgsn_mm_ctx *mm, uint8_t *rand,
+static int gsm48_tx_gmm_auth_ciph_req(struct sgsn_mm_ctx *mm, uint8_t *rnd,
 				      uint8_t key_seq, uint8_t algo)
 {
 	struct msgb *msg = gsm48_msgb_alloc_name("GSM 04.08 AUTH CIPH REQ");
 	struct gsm48_hdr *gh;
 	struct gsm48_auth_ciph_req *acreq;
-	uint8_t *m_rand, *m_cksn;
+	uint8_t *m_rand, *m_cksn, rbyte;
 
 	LOGMMCTXP(LOGL_INFO, mm, "<- GPRS AUTH AND CIPHERING REQ (rand = %s)\n",
-		osmo_hexdump(rand, 16));
+		osmo_hexdump(rnd, 16));
 
 	mmctx2msgid(msg, mm);
 
@@ -438,13 +440,20 @@
 	acreq->ciph_alg = algo & 0xf;
 	acreq->imeisv_req = 0x1;
 	acreq->force_stby = 0x0;
-	acreq->ac_ref_nr = 0x0;	/* FIXME: increment this? */
+	/* 3GPP TS 24.008 § 10.5.5.19: */
+	if (RAND_bytes(&rbyte, 1) != 1) {
+		LOGP(DMM, LOGL_NOTICE, "RAND_bytes failed for A&C ref, falling "
+		     "back to rand()\n");
+		acreq->ac_ref_nr = rand();
+	} else
+		acreq->ac_ref_nr = rbyte;
+	mm->ac_ref_nr_used = acreq->ac_ref_nr;
 
 	/* Only if authentication is requested we need to set RAND + CKSN */
-	if (rand) {
+	if (rnd) {
 		m_rand = msgb_put(msg, 16+1);
 		m_rand[0] = GSM48_IE_GMM_AUTH_RAND;
-		memcpy(m_rand+1, rand, 16);
+		memcpy(m_rand + 1, rnd, 16);
 
 		m_cksn = msgb_put(msg, 1);
 		m_cksn[0] = (GSM48_IE_GMM_CIPH_CKSN << 4) | (key_seq & 0x07);
@@ -490,14 +499,19 @@
 		return 0;
 	}
 
+	if (acr->ac_ref_nr != ctx->ac_ref_nr_used) {
+		LOGMMCTXP(LOGL_NOTICE, ctx, "Reference mismatch for Auth & Ciph"
+			  " Response: %u received, %u expected\n",
+			  acr->ac_ref_nr, ctx->ac_ref_nr_used);
+		return 0;
+	}
+
 	/* Stop T3360 */
 	mmctx_timer_stop(ctx, 3360);
 
 	tlv_parse(&tp, &gsm48_gmm_att_tlvdef, acr->data,
 			(msg->data + msg->len) - acr->data, 0, 0);
 
-	/* FIXME: compare ac_ref? */
-
 	if (!TLVP_PRESENT(&tp, GSM48_IE_GMM_AUTH_SRES) ||
 	    !TLVP_PRESENT(&tp, GSM48_IE_GMM_IMEISV)) {
 		/* TODO: missing mandatory IE, return STATUS or REJ? */
diff --git a/openbsc/src/gprs/sgsn_main.c b/openbsc/src/gprs/sgsn_main.c
index c852840..4475136 100644
--- a/openbsc/src/gprs/sgsn_main.c
+++ b/openbsc/src/gprs/sgsn_main.c
@@ -21,6 +21,7 @@
 
 #include <unistd.h>
 #include <stdio.h>
+#include <time.h>
 #include <stdlib.h>
 #include <string.h>
 #include <getopt.h>
@@ -306,6 +307,7 @@
 	struct gsm_network dummy_network;
 	int rc;
 
+	srand(time(NULL));
 	tall_bsc_ctx = talloc_named_const(NULL, 0, "osmo_sgsn");
 	tall_msgb_ctx = talloc_named_const(tall_bsc_ctx, 0, "msgb");
 
diff --git a/openbsc/tests/sgsn/Makefile.am b/openbsc/tests/sgsn/Makefile.am
index e3cec50..b6036c7 100644
--- a/openbsc/tests/sgsn/Makefile.am
+++ b/openbsc/tests/sgsn/Makefile.am
@@ -7,6 +7,7 @@
 
 sgsn_test_SOURCES = sgsn_test.c
 sgsn_test_LDFLAGS = \
+	-Wl,--wrap=RAND_bytes \
 	-Wl,--wrap=sgsn_update_subscriber_data \
 	-Wl,--wrap=gprs_subscr_request_update_location \
 	-Wl,--wrap=gprs_subscr_request_auth_info \
diff --git a/openbsc/tests/sgsn/sgsn_test.c b/openbsc/tests/sgsn/sgsn_test.c
index d568807..e8e05f6 100644
--- a/openbsc/tests/sgsn/sgsn_test.c
+++ b/openbsc/tests/sgsn/sgsn_test.c
@@ -101,6 +101,25 @@
 	return 0;
 }
 
+/* override, requires '-Wl,--wrap=RAND_bytes' */
+int __real_RAND_bytes(unsigned char *buf, int num);
+int mock_RAND_bytes(unsigned char *buf, int num);
+int (*RAND_bytes_cb)(unsigned char *, int) =
+  &mock_RAND_bytes;
+
+int __wrap_RAND_bytes(unsigned char *buf, int num)
+{
+	return (*RAND_bytes_cb)(buf, num);
+}
+/* make results of A&C ref predictable */
+int mock_RAND_bytes(unsigned char *buf, int num)
+{
+	if (num > 1)
+		return __real_RAND_bytes(buf, num);
+	buf[0] = 0;
+	return 1;
+}
+
 /* override, requires '-Wl,--wrap=sgsn_update_subscriber_data' */
 void __real_sgsn_update_subscriber_data(struct sgsn_mm_ctx *);
 void (*update_subscriber_data_cb)(struct sgsn_mm_ctx *) =