diff --git a/include/osmocom/gsm/oap.h b/include/osmocom/gsm/oap.h
new file mode 100644
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--- /dev/null
+++ b/include/osmocom/gsm/oap.h
@@ -0,0 +1,72 @@
+/* Osmocom Authentication Protocol message encoder/decoder */
+
+/* (C) 2015-2016 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 General Public License as published by
+ * the Free Software Foundation; either version 2 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#pragma once
+
+#include <stdint.h>
+#include <osmocom/core/msgb.h>
+#include <osmocom/gsm/protocol/gsm_04_08_gprs.h>
+
+/*! \brief Information Element Identifiers for OAP IEs.
+ * They match osmo_gsup_iei (so far). */
+enum osmo_oap_iei {
+	OAP_CAUSE_IE			= 0x02,
+	OAP_RAND_IE			= 0x20,
+	OAP_AUTN_IE			= 0x23,
+	OAP_XRES_IE			= 0x24,
+	OAP_AUTS_IE			= 0x25,
+	OAP_CLIENT_ID_IE		= 0x30,
+};
+
+/*! \brief OAP message types */
+enum osmo_oap_message_type {
+	OAP_MSGT_REGISTER_REQUEST	= 0b00000100,
+	OAP_MSGT_REGISTER_ERROR		= 0b00000101,
+	OAP_MSGT_REGISTER_RESULT	= 0b00000110,
+
+	OAP_MSGT_CHALLENGE_REQUEST	= 0b00001000,
+	OAP_MSGT_CHALLENGE_ERROR	= 0b00001001,
+	OAP_MSGT_CHALLENGE_RESULT	= 0b00001010,
+
+	OAP_MSGT_SYNC_REQUEST		= 0b00001100,
+	OAP_MSGT_SYNC_ERROR		= 0b00001101,
+	OAP_MSGT_SYNC_RESULT		= 0b00001110,
+};
+
+/*! \brief Parsed/decoded OAP protocol message */
+struct osmo_oap_message {
+	enum osmo_oap_message_type message_type;
+	enum gsm48_gmm_cause cause;
+	uint16_t client_id;
+	int rand_present;
+	uint8_t rand[16];
+	int autn_present;
+	uint8_t autn[16];
+	int xres_present;
+	uint8_t xres[8];
+	int auts_present;
+	uint8_t auts[16];
+};
+
+int osmo_oap_decode(struct osmo_oap_message *oap_msg, const uint8_t *data,
+		    size_t data_len);
+void osmo_oap_encode(struct msgb *msg, const struct osmo_oap_message *oap_msg);