diff --git a/openbsc/src/gprs/oap_messages.c b/openbsc/src/gprs/oap_messages.c
new file mode 100644
index 0000000..eb52053
--- /dev/null
+++ b/openbsc/src/gprs/oap_messages.c
@@ -0,0 +1,178 @@
+/* Osmocom Authentication Protocol message encoder/decoder */
+
+/* (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 <openbsc/oap_messages.h>
+
+#include <openbsc/debug.h>
+#include <openbsc/gprs_utils.h>
+#include <openbsc/utils.h>
+
+#include <osmocom/gsm/tlv.h>
+#include <osmocom/core/msgb.h>
+
+#include <stdint.h>
+
+
+int oap_decode(const uint8_t *const_data, size_t data_len,
+	       struct oap_message *oap_msg)
+{
+	int rc;
+	uint8_t tag;
+	/* the shift/match functions expect non-const pointers, but we'll
+	 * either copy the data or cast pointers back to const before returning
+	 * them
+	 */
+	uint8_t *data = (uint8_t *)const_data;
+	uint8_t *value;
+	size_t value_len;
+
+	memset(oap_msg, 0, sizeof(*oap_msg));
+
+	/* message type */
+	rc = gprs_shift_v_fixed(&data, &data_len, 1, &value);
+	if (rc < 0)
+		return -GMM_CAUSE_INV_MAND_INFO;
+	oap_msg->message_type = decode_big_endian(value, 1);
+
+	/* specific parts */
+	while (data_len > 0) {
+		enum oap_iei iei;
+
+		rc = gprs_shift_tlv(&data, &data_len, &tag, &value, &value_len);
+		if (rc < 0)
+			return -GMM_CAUSE_PROTO_ERR_UNSPEC;
+
+		iei = tag;
+
+		switch (iei) {
+		case OAP_CLIENT_ID_IE:
+			if (value_len != 2) {
+				LOGP(DGPRS, LOGL_NOTICE,
+				     "OAP IE type client ID (%d) should be 2 octets, but has %d\n",
+				     (int)iei, (int)value_len);
+				return -GMM_CAUSE_PROTO_ERR_UNSPEC;
+			}
+
+			oap_msg->client_id = decode_big_endian(value, value_len);
+
+			if (oap_msg->client_id == 0) {
+				LOGP(DGPRS, LOGL_NOTICE,
+				     "OAP IE type client ID (%d): client ID must be nonzero.\n",
+				     (int)iei);
+				return -GMM_CAUSE_PROTO_ERR_UNSPEC;
+			}
+			break;
+
+		case OAP_AUTN_IE:
+			if (value_len != sizeof(oap_msg->autn)) {
+				LOGP(DGPRS, LOGL_NOTICE,
+				     "OAP IE type AUTN (%d) should be %d octets, but has %d\n",
+				     (int)iei, (int)sizeof(oap_msg->autn), (int)value_len);
+				return -GMM_CAUSE_PROTO_ERR_UNSPEC;
+			}
+			memcpy(oap_msg->autn, value, value_len);
+			oap_msg->autn_present = value_len;
+			break;
+
+		case OAP_RAND_IE:
+			if (value_len != sizeof(oap_msg->rand)) {
+				LOGP(DGPRS, LOGL_NOTICE,
+				     "OAP IE type RAND (%d) should be %d octets, but has %d\n",
+				     (int)iei, (int)sizeof(oap_msg->rand), (int)value_len);
+				return -GMM_CAUSE_PROTO_ERR_UNSPEC;
+			}
+			memcpy(oap_msg->rand, value, value_len);
+			oap_msg->rand_present = value_len;
+			break;
+
+		case OAP_XRES_IE:
+			if (value_len != sizeof(oap_msg->xres)) {
+				LOGP(DGPRS, LOGL_NOTICE,
+				     "OAP IE type XRES (%d) should be %d octets, but has %d\n",
+				     (int)iei, (int)sizeof(oap_msg->xres), (int)value_len);
+				return -GMM_CAUSE_PROTO_ERR_UNSPEC;
+			}
+			memcpy(oap_msg->xres, value, value_len);
+			oap_msg->xres_present = value_len;
+			break;
+
+		case OAP_AUTS_IE:
+			if (value_len != sizeof(oap_msg->auts)) {
+				LOGP(DGPRS, LOGL_NOTICE,
+				     "OAP IE type AUTS (%d) should be %d octets, but has %d\n",
+				     (int)iei, (int)sizeof(oap_msg->auts), (int)value_len);
+				return -GMM_CAUSE_PROTO_ERR_UNSPEC;
+			}
+			memcpy(oap_msg->auts, value, value_len);
+			oap_msg->auts_present = value_len;
+			break;
+
+		case OAP_CAUSE_IE:
+			if (value_len > 1) {
+				LOGP(DGPRS, LOGL_ERROR,
+				     "OAP cause may not exceed one octet, is %d", (int)value_len);
+				return -GMM_CAUSE_PROTO_ERR_UNSPEC;
+			}
+			oap_msg->cause = *value;
+			break;
+
+		default:
+			LOGP(DGPRS, LOGL_NOTICE,
+			     "OAP IE type %d unknown\n", iei);
+			continue;
+		}
+	}
+
+	return 0;
+}
+
+void oap_encode(struct msgb *msg, const struct oap_message *oap_msg)
+{
+	uint8_t u8;
+
+	/* generic part */
+	OSMO_ASSERT(oap_msg->message_type);
+	msgb_v_put(msg, (uint8_t)oap_msg->message_type);
+
+	/* specific parts */
+	if ((u8 = oap_msg->cause))
+		msgb_tlv_put(msg, OAP_CAUSE_IE, sizeof(u8), &u8);
+
+	if (oap_msg->client_id > 0)
+		msgb_tlv_put(msg, OAP_CLIENT_ID_IE, sizeof(oap_msg->client_id),
+			     encode_big_endian(oap_msg->client_id, sizeof(oap_msg->client_id)));
+
+	if (oap_msg->rand_present)
+		msgb_tlv_put(msg, OAP_RAND_IE, sizeof(oap_msg->rand), oap_msg->rand);
+
+	if (oap_msg->autn_present)
+		msgb_tlv_put(msg, OAP_AUTN_IE, sizeof(oap_msg->autn), oap_msg->autn);
+
+	if (oap_msg->auts_present)
+		msgb_tlv_put(msg, OAP_AUTS_IE, sizeof(oap_msg->auts), oap_msg->auts);
+
+	if (oap_msg->xres_present)
+		msgb_tlv_put(msg, OAP_XRES_IE, sizeof(oap_msg->xres), oap_msg->xres);
+
+	msg->l2h = msg->data;
+}
+