diff --git a/src/gsm/oap_client.c b/src/gsm/oap_client.c
new file mode 100644
index 0000000..2227a3c
--- /dev/null
+++ b/src/gsm/oap_client.c
@@ -0,0 +1,280 @@
+/* Osmocom Authentication Protocol API */
+
+/* (C) 2015 by Sysmocom s.f.m.c. GmbH
+ * All Rights Reserved
+ *
+ * Author: Neels Hofmeyr
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include <string.h>
+#include <errno.h>
+
+#include <osmocom/core/utils.h>
+#include <osmocom/core/logging.h>
+#include <osmocom/crypt/auth.h>
+#include <osmocom/gsm/oap.h>
+
+#include <osmocom/gsm/oap_client.h>
+
+int oap_client_init(struct oap_client_config *config,
+		    struct oap_client_state *state)
+{
+	OSMO_ASSERT(state->state == OAP_UNINITIALIZED);
+
+	if (!config)
+		goto disable;
+
+	if (config->client_id == 0)
+		goto disable;
+
+	if (config->secret_k_present == 0) {
+		LOGP(DLOAP, LOGL_NOTICE, "OAP: client ID set, but secret K missing.\n");
+		goto disable;
+	}
+
+	if (config->secret_opc_present == 0) {
+		LOGP(DLOAP, LOGL_NOTICE, "OAP: client ID set, but secret OPC missing.\n");
+		goto disable;
+	}
+
+	state->client_id = config->client_id;
+	memcpy(state->secret_k, config->secret_k, sizeof(state->secret_k));
+	memcpy(state->secret_opc, config->secret_opc, sizeof(state->secret_opc));
+	state->state = OAP_INITIALIZED;
+	return 0;
+
+disable:
+	state->state = OAP_DISABLED;
+	return 0;
+}
+
+/* From the given state and received RAND and AUTN octets, validate the
+ * server's authenticity and formulate the matching milenage reply octets in
+ * *tx_xres. The state is not modified.
+ * On success, and if tx_res is not NULL, exactly 8 octets will be written to
+ * *tx_res. If not NULL, tx_res must point at allocated memory of at least 8
+ * octets. The caller will want to send XRES back to the server in a challenge
+ * response message and update the state.
+ * Return 0 on success; -1 if OAP is disabled; -2 if rx_random and rx_autn fail
+ * the authentication check; -3 for any other errors. */
+static int oap_evaluate_challenge(const struct oap_client_state *state,
+				  const uint8_t *rx_random,
+				  const uint8_t *rx_autn,
+				  uint8_t *tx_xres)
+{
+	struct osmo_auth_vector vec;
+
+	struct osmo_sub_auth_data auth = {
+		.type		= OSMO_AUTH_TYPE_UMTS,
+		.algo		= OSMO_AUTH_ALG_MILENAGE,
+	};
+
+	osmo_static_assert(sizeof(((struct osmo_sub_auth_data*)0)->u.umts.k)
+			   == sizeof(state->secret_k), _secret_k_size_match);
+	osmo_static_assert(sizeof(((struct osmo_sub_auth_data*)0)->u.umts.opc)
+			   == sizeof(state->secret_opc), _secret_opc_size_match);
+
+	switch (state->state) {
+	case OAP_UNINITIALIZED:
+	case OAP_DISABLED:
+		return -1;
+	default:
+		break;
+	}
+
+	memcpy(auth.u.umts.k, state->secret_k, sizeof(auth.u.umts.k));
+	memcpy(auth.u.umts.opc, state->secret_opc, sizeof(auth.u.umts.opc));
+	memset(auth.u.umts.amf, '\0', sizeof(auth.u.umts.amf));
+	auth.u.umts.sqn = 41; /* TODO use incrementing sequence nr */
+
+	memset(&vec, 0, sizeof(vec));
+	osmo_auth_gen_vec(&vec, &auth, rx_random);
+
+	if (vec.res_len != 8) {
+		LOGP(DLOAP, LOGL_ERROR, "OAP: Expected XRES to be 8 octets, got %d\n",
+		     vec.res_len);
+		return -3;
+	}
+
+	if (osmo_constant_time_cmp(vec.autn, rx_autn, sizeof(vec.autn)) != 0) {
+		LOGP(DLOAP, LOGL_ERROR, "OAP: AUTN mismatch!\n");
+		LOGP(DLOAP, LOGL_INFO, "OAP: AUTN from server: %s\n",
+		     osmo_hexdump_nospc(rx_autn, sizeof(vec.autn)));
+		LOGP(DLOAP, LOGL_INFO, "OAP: AUTN expected:    %s\n",
+		     osmo_hexdump_nospc(vec.autn, sizeof(vec.autn)));
+		return -2;
+	}
+
+	if (tx_xres != NULL)
+		memcpy(tx_xres, vec.res, 8);
+	return 0;
+}
+
+struct msgb *oap_client_encoded(const struct osmo_oap_message *oap_msg)
+{
+	struct msgb *msg = msgb_alloc_headroom(1000, 64, __func__);
+	OSMO_ASSERT(msg);
+	osmo_oap_encode(msg, oap_msg);
+	return msg;
+}
+
+/* Create a new msgb containing an OAP registration message.
+ * On error, return NULL. */
+static struct msgb* oap_msg_register(uint16_t client_id)
+{
+	struct osmo_oap_message oap_msg = {0};
+
+	if (client_id < 1) {
+		LOGP(DLOAP, LOGL_ERROR, "OAP: Invalid client ID: %d\n", client_id);
+		return NULL;
+	}
+
+	oap_msg.message_type = OAP_MSGT_REGISTER_REQUEST;
+	oap_msg.client_id = client_id;
+	return oap_client_encoded(&oap_msg);
+}
+
+int oap_client_register(struct oap_client_state *state, struct msgb **msg_tx)
+{
+	*msg_tx = oap_msg_register(state->client_id);
+	if (!(*msg_tx))
+		return -1;
+
+	state->state = OAP_REQUESTED_CHALLENGE;
+	return 0;
+}
+
+/* Create a new msgb containing an OAP challenge response message.
+ * xres must point at 8 octets to return as challenge response.
+ * On error, return NULL. */
+static struct msgb* oap_msg_challenge_response(uint8_t *xres)
+{
+	struct osmo_oap_message oap_reply = {0};
+
+	oap_reply.message_type = OAP_MSGT_CHALLENGE_RESULT;
+	memcpy(oap_reply.xres, xres, sizeof(oap_reply.xres));
+	oap_reply.xres_present = 1;
+	return oap_client_encoded(&oap_reply);
+}
+
+static int handle_challenge(struct oap_client_state *state,
+			    struct osmo_oap_message *oap_rx,
+			    struct msgb **msg_tx)
+{
+	int rc;
+	uint8_t xres[8];
+
+	if (!(oap_rx->rand_present && oap_rx->autn_present)) {
+		LOGP(DLOAP, LOGL_ERROR,
+		     "OAP challenge incomplete (rand_present: %d, autn_present: %d)\n",
+		     oap_rx->rand_present, oap_rx->autn_present);
+		rc = -2;
+		goto failure;
+	}
+
+	rc = oap_evaluate_challenge(state,
+				    oap_rx->rand,
+				    oap_rx->autn,
+				    xres);
+	if (rc < 0)
+		goto failure;
+
+	*msg_tx = oap_msg_challenge_response(xres);
+	if ((*msg_tx) == NULL) {
+		rc = -1;
+		goto failure;
+	}
+
+	state->state = OAP_SENT_CHALLENGE_RESULT;
+	return 0;
+
+failure:
+	OSMO_ASSERT(rc < 0);
+	state->state = OAP_INITIALIZED;
+	return rc;
+}
+
+int oap_client_handle(struct oap_client_state *state,
+		      const struct msgb *msg_rx, struct msgb **msg_tx)
+{
+	uint8_t *data = msgb_l2(msg_rx);
+	size_t data_len = msgb_l2len(msg_rx);
+	struct osmo_oap_message oap_msg = {0};
+	int rc = 0;
+
+	*msg_tx = NULL;
+
+	OSMO_ASSERT(data);
+
+	rc = osmo_oap_decode(&oap_msg, data, data_len);
+	if (rc < 0) {
+		LOGP(DLOAP, LOGL_ERROR,
+		     "Decoding OAP message failed with error '%s' (%d)\n",
+		     get_value_string(gsm48_gmm_cause_names, -rc), -rc);
+		return -10;
+	}
+
+	switch (state->state) {
+	case OAP_UNINITIALIZED:
+		LOGP(DLOAP, LOGL_ERROR,
+		     "Received OAP message %d, but the OAP client is"
+		     " not initialized\n", oap_msg.message_type);
+		return -ENOTCONN;
+	case OAP_DISABLED:
+		LOGP(DLOAP, LOGL_ERROR,
+		     "Received OAP message %d, but the OAP client is"
+		     " disabled\n", oap_msg.message_type);
+		return -ENOTCONN;
+	default:
+		break;
+	}
+
+	switch (oap_msg.message_type) {
+	case OAP_MSGT_CHALLENGE_REQUEST:
+		return handle_challenge(state, &oap_msg, msg_tx);
+
+	case OAP_MSGT_REGISTER_RESULT:
+		/* successfully registered */
+		state->state = OAP_REGISTERED;
+		break;
+
+	case OAP_MSGT_REGISTER_ERROR:
+		LOGP(DLOAP, LOGL_ERROR,
+		     "OAP registration failed\n");
+		state->state = OAP_INITIALIZED;
+		if (state->registration_failures < 3) {
+			state->registration_failures++;
+			return oap_client_register(state, msg_tx);
+		}
+		return -11;
+
+	case OAP_MSGT_REGISTER_REQUEST:
+	case OAP_MSGT_CHALLENGE_RESULT:
+		LOGP(DLOAP, LOGL_ERROR,
+		     "Received invalid OAP message type for OAP client side: %d\n",
+		     (int)oap_msg.message_type);
+		return -12;
+
+	default:
+		LOGP(DLOAP, LOGL_ERROR,
+		     "Unknown OAP message type: %d\n",
+		     (int)oap_msg.message_type);
+		return -13;
+	}
+
+	return 0;
+}