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__ */
diff --git a/src/gsm/Makefile.am b/src/gsm/Makefile.am
index a8c2e56..57eb5ca 100644
--- a/src/gsm/Makefile.am
+++ b/src/gsm/Makefile.am
@@ -7,7 +7,7 @@
 
 lib_LTLIBRARIES = libosmogsm.la
 
-libosmogsm_la_SOURCES = rxlev_stat.c tlv_parser.c comp128.c gsm_utils.c \
+libosmogsm_la_SOURCES = a5.c rxlev_stat.c tlv_parser.c comp128.c gsm_utils.c \
                         rsl.c gsm48.c gsm48_ie.c gsm0808.c \
 			gprs_cipher_core.c gsm0480.c
 libosmogsm_la_LIBADD = $(top_builddir)/src/libosmocore.la
diff --git a/src/gsm/a5.c b/src/gsm/a5.c
new file mode 100644
index 0000000..d3f2b2c
--- /dev/null
+++ b/src/gsm/a5.c
@@ -0,0 +1,311 @@
+/*
+ * a5.c
+ *
+ * Full reimplementation of A5/1,2 (split and threadsafe)
+ *
+ * The logic behind the algorithm is taken from "A pedagogical implementation
+ * of the GSM A5/1 and A5/2 "voice privacy" encryption algorithms." by
+ * Marc Briceno, Ian Goldberg, and David Wagner.
+ *
+ * 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.
+ */
+
+#include <string.h>
+
+#include <osmocom/gsm/a5.h>
+
+void
+osmo_a5(int n, uint8_t *key, uint32_t fn, ubit_t *dl, ubit_t *ul)
+{
+	switch (n)
+	{
+	case 0:
+		if (dl)
+			memset(dl, 0x00, 114);
+		if (ul)
+			memset(ul, 0x00, 114);
+		break;
+
+	case 1:
+		osmo_a5_1(key, fn, dl, ul);
+		break;
+
+	case 2:
+		osmo_a5_2(key, fn, dl, ul);
+		break;
+
+	default:
+		/* a5/[3..7] not supported here/yet */
+		break;
+	}
+}
+
+
+/* ------------------------------------------------------------------------ */
+/* A5/1&2 common stuff                                                                     */
+/* ------------------------------------------------------------------------ */
+
+#define A5_R1_LEN	19
+#define A5_R2_LEN	22
+#define A5_R3_LEN	23
+#define A5_R4_LEN	17	/* A5/2 only */
+
+#define A5_R1_MASK	((1<<A5_R1_LEN)-1)
+#define A5_R2_MASK	((1<<A5_R2_LEN)-1)
+#define A5_R3_MASK	((1<<A5_R3_LEN)-1)
+#define A5_R4_MASK	((1<<A5_R4_LEN)-1)
+
+#define A5_R1_TAPS	0x072000 /* x^19 + x^5 + x^2 + x + 1 */
+#define A5_R2_TAPS	0x300000 /* x^22 + x + 1 */
+#define A5_R3_TAPS	0x700080 /* x^23 + x^15 + x^2 + x + 1 */
+#define A5_R4_TAPS	0x010800 /* x^17 + x^5 + 1 */
+
+static inline uint32_t
+_a5_12_parity(uint32_t x)
+{
+	x ^= x >> 16;
+	x ^= x >> 8;
+	x ^= x >> 4;
+	x ^= x >> 2;
+	x ^= x >> 1;
+	return x & 1;
+}
+
+static inline uint32_t
+_a5_12_majority(uint32_t v1, uint32_t v2, uint32_t v3)
+{
+	return (!!v1 + !!v2 + !!v3) >= 2;
+}
+
+static inline uint32_t
+_a5_12_clock(uint32_t r, uint32_t mask, uint32_t taps)
+{
+	return ((r << 1) & mask) | _a5_12_parity(r & taps);
+}
+
+
+/* ------------------------------------------------------------------------ */
+/* A5/1                                                                     */
+/* ------------------------------------------------------------------------ */
+
+#define A51_R1_CLKBIT	0x000100
+#define A51_R2_CLKBIT	0x000400
+#define A51_R3_CLKBIT	0x000400
+
+static inline void
+_a5_1_clock(uint32_t r[], int force)
+{
+	int cb[3], maj;
+
+	cb[0] = !!(r[0] & A51_R1_CLKBIT);
+	cb[1] = !!(r[1] & A51_R2_CLKBIT);
+	cb[2] = !!(r[2] & A51_R3_CLKBIT);
+
+	maj = _a5_12_majority(cb[0], cb[1], cb[2]);
+
+	if (force || (maj == cb[0]))
+		r[0] = _a5_12_clock(r[0], A5_R1_MASK, A5_R1_TAPS);
+
+	if (force || (maj == cb[1]))
+		r[1] = _a5_12_clock(r[1], A5_R2_MASK, A5_R2_TAPS);
+
+	if (force || (maj == cb[2]))
+		r[2] = _a5_12_clock(r[2], A5_R3_MASK, A5_R3_TAPS);
+}
+
+static inline uint8_t
+_a5_1_get_output(uint32_t r[])
+{
+	return	(r[0] >> (A5_R1_LEN-1)) ^
+		(r[1] >> (A5_R2_LEN-1)) ^
+		(r[2] >> (A5_R3_LEN-1));
+}
+
+void
+osmo_a5_1(uint8_t *key, uint32_t fn, ubit_t *dl, ubit_t *ul)
+{
+	uint32_t r[3] = {0, 0, 0};
+	uint32_t fn_count;
+	uint32_t b;
+	int i;
+
+	/* Key load */
+	for (i=0; i<64; i++)
+	{
+		b = ( key[7 - (i>>3)] >> (i&7) ) & 1;
+
+		_a5_1_clock(r, 1);
+
+		r[0] ^= b;
+		r[1] ^= b;
+		r[2] ^= b;
+	}
+
+	/* Frame count load */
+	fn_count = osmo_a5_fn_count(fn);
+
+	for (i=0; i<22; i++)
+	{
+		b = (fn_count >> i) & 1;
+
+		_a5_1_clock(r, 1);
+
+		r[0] ^= b;
+		r[1] ^= b;
+		r[2] ^= b;
+	}
+
+	/* Mix */
+	for (i=0; i<100; i++)
+	{
+		_a5_1_clock(r, 0);
+	}
+
+	/* Output */
+	for (i=0; i<114; i++) {
+		_a5_1_clock(r, 0);
+		if (dl)
+			dl[i] = _a5_1_get_output(r);
+	}
+
+	for (i=0; i<114; i++) {
+		_a5_1_clock(r, 0);
+		if (ul)
+			ul[i] = _a5_1_get_output(r);
+	}
+}
+
+
+/* ------------------------------------------------------------------------ */
+/* A5/2                                                                     */
+/* ------------------------------------------------------------------------ */
+
+#define A52_R4_CLKBIT0	0x000400
+#define A52_R4_CLKBIT1	0x000008
+#define A52_R4_CLKBIT2	0x000080
+
+static inline void
+_a5_2_clock(uint32_t r[], int force)
+{
+	int cb[3], maj;
+
+	cb[0] = !!(r[3] & A52_R4_CLKBIT0);
+	cb[1] = !!(r[3] & A52_R4_CLKBIT1);
+	cb[2] = !!(r[3] & A52_R4_CLKBIT2);
+
+	maj = (cb[0] + cb[1] + cb[2]) >= 2;
+
+	if (force || (maj == cb[0]))
+		r[0] = _a5_12_clock(r[0], A5_R1_MASK, A5_R1_TAPS);
+
+	if (force || (maj == cb[1]))
+		r[1] = _a5_12_clock(r[1], A5_R2_MASK, A5_R2_TAPS);
+
+	if (force || (maj == cb[2]))
+		r[2] = _a5_12_clock(r[2], A5_R3_MASK, A5_R3_TAPS);
+
+	r[3] = _a5_12_clock(r[3], A5_R4_MASK, A5_R4_TAPS);
+}
+
+static inline uint8_t
+_a5_2_get_output(uint32_t r[], uint8_t *db)
+{
+	uint8_t cb, tb;
+
+	tb =	(r[0] >> (A5_R1_LEN-1)) ^
+		(r[1] >> (A5_R2_LEN-1)) ^
+		(r[2] >> (A5_R3_LEN-1));
+
+	cb = *db;
+
+	*db = (	tb ^
+		_a5_12_majority( r[0] & 0x08000, ~r[0] & 0x04000,  r[0] & 0x1000) ^
+		_a5_12_majority(~r[1] & 0x10000,  r[1] & 0x02000,  r[1] & 0x0200) ^
+		_a5_12_majority( r[2] & 0x40000,  r[2] & 0x10000, ~r[2] & 0x2000)
+	);
+
+	return cb;
+}
+
+void
+osmo_a5_2(uint8_t *key, uint32_t fn, ubit_t *dl, ubit_t *ul)
+{
+	uint32_t r[4] = {0, 0, 0, 0};
+	uint32_t fn_count;
+	uint32_t b;
+	uint8_t db = 0, o;
+	int i;
+
+	/* Key load */
+	for (i=0; i<64; i++)
+	{
+		b = ( key[7 - (i>>3)] >> (i&7) ) & 1;
+
+		_a5_2_clock(r, 1);
+
+		r[0] ^= b;
+		r[1] ^= b;
+		r[2] ^= b;
+		r[3] ^= b;
+	}
+
+	/* Frame count load */
+	fn_count = osmo_a5_fn_count(fn);
+
+	for (i=0; i<22; i++)
+	{
+		b = (fn_count >> i) & 1;
+
+		_a5_2_clock(r, 1);
+
+		r[0] ^= b;
+		r[1] ^= b;
+		r[2] ^= b;
+		r[3] ^= b;
+	}
+
+	r[0] |= 1 << 15;
+	r[1] |= 1 << 16;
+	r[2] |= 1 << 18;
+	r[3] |= 1 << 10;
+
+	/* Mix */
+	for (i=0; i<100; i++)
+	{
+		_a5_2_clock(r, 0);
+	}
+
+	_a5_2_get_output(r, &db);
+
+
+	/* Output */
+	for (i=0; i<114; i++) {
+		_a5_2_clock(r, 0);
+		o = _a5_2_get_output(r, &db);
+		if (dl)
+			dl[i] = o;
+	}
+
+	for (i=0; i<114; i++) {
+		_a5_2_clock(r, 0);
+		o = _a5_2_get_output(r, &db);
+		if (ul)
+			ul[i] = o;
+	}
+}