diff --git a/include/osmocom/gsm/Makefile.am b/include/osmocom/gsm/Makefile.am
index 8685fc9..a39d2d0 100644
--- a/include/osmocom/gsm/Makefile.am
+++ b/include/osmocom/gsm/Makefile.am
@@ -1,4 +1,4 @@
-osmogsm_HEADERS = comp128.h gsm0808.h gsm48_ie.h mncc.h rxlev_stat.h \
+osmogsm_HEADERS = a5.h comp128.h gsm0808.h gsm48_ie.h mncc.h rxlev_stat.h \
 		  gsm0480.h gsm48.h gsm_utils.h rsl.h tlv.h
 
 SUBDIRS = protocol
diff --git a/include/osmocom/gsm/a5.h b/include/osmocom/gsm/a5.h
new file mode 100644
index 0000000..2c630e5
--- /dev/null
+++ b/include/osmocom/gsm/a5.h
@@ -0,0 +1,49 @@
+/*
+ * a5.h
+ *
+ * Copyright (C) 2011  Sylvain Munaut <tnt@246tNt.com>
+ *
+ * All Rights Reserved
+ *
+ * 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, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef __OSMO_A5_H__
+#define __OSMO_A5_H__
+
+#include <stdint.h>
+
+#include <osmocom/core/bits.h>
+
+static inline uint32_t
+osmo_a5_fn_count(uint32_t fn)
+{
+	int t1 = fn / (26 * 51);
+	int t2 = fn % 26;
+	int t3 = fn % 51;
+	return (t1 << 11) | (t3 << 5) | t2;
+}
+
+	/* Notes:
+	 *  - key must be 8 bytes long (or NULL for A5/0)
+	 *  - the dl and ul pointer must be either NULL or 114 bits long
+	 *  - fn is the _real_ GSM frame number.
+	 *    (converted internally to fn_count)
+	 */
+void osmo_a5(int n, uint8_t *key, uint32_t fn, ubit_t *dl, ubit_t *ul);
+void osmo_a5_1(uint8_t *key, uint32_t fn, ubit_t *dl, ubit_t *ul);
+void osmo_a5_2(uint8_t *key, uint32_t fn, ubit_t *dl, ubit_t *ul);
+
+#endif /* __OSMO_A5_H__ */