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+/*
+ * 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.
+ */
+
+/*! \addtogroup a5
+ * @{
+ */
+
+/*! \file gsm/a5.c
+ * \brief Osmocom GSM A5 ciphering algorithm implementation
+ */
+
+#include <string.h>
+
+#include <osmocom/gsm/a5.h>
+
+/*! \brief Main method to generate a A5/x cipher stream
+ * \param[in] n Which A5/x method to use
+ * \param[in] key 8 byte array for the key (as received from the SIM)
+ * \param[in] fn Frame number
+ * \param[out] dl Pointer to array of ubits to return Downlink cipher stream
+ * \param[out] ul Pointer to array of ubits to return Uplink cipher stream
+ *
+ * Currently A5/[0-2] are supported.
+ * Either (or both) of dl/ul can be NULL if not needed.
+ */
+void
+osmo_a5(int n, const 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^18 + x^17 + x^14 + 1 */
+#define A5_R2_TAPS 0x300000 /* x^22 + x^21 + 1 */
+#define A5_R3_TAPS 0x700080 /* x^23 + x^22 + x^21 + x^8 + 1 */
+#define A5_R4_TAPS 0x010800 /* x^17 + x^12 + 1 */
+
+/*! \brief Computes parity of a 32-bit word
+ * \param[in] x 32 bit word
+ * \return Parity bit (xor of all bits) as 0 or 1
+ */
+static inline uint32_t
+_a5_12_parity(uint32_t x)
+{
+ x ^= x >> 16;
+ x ^= x >> 8;
+ x ^= x >> 4;
+ x &= 0xf;
+ return (0x6996 >> x) & 1;
+}
+
+/*! \brief Compute majority bit from 3 taps
+ * \param[in] v1 LFSR state ANDed with tap-bit
+ * \param[in] v2 LFSR state ANDed with tap-bit
+ * \param[in] v3 LFSR state ANDed with tap-bit
+ * \return The majority bit (0 or 1)
+ */
+static inline uint32_t
+_a5_12_majority(uint32_t v1, uint32_t v2, uint32_t v3)
+{
+ return (!!v1 + !!v2 + !!v3) >= 2;
+}
+
+/*! \brief Compute the next LFSR state
+ * \param[in] r Current state
+ * \param[in] mask LFSR mask
+ * \param[in] taps LFSR taps
+ * \return Next state
+ */
+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
+
+/*! \brief GSM A5/1 Clocking function
+ * \param[in] r Register state
+ * \param[in] force Non-zero value disable conditional clocking
+ */
+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);
+}
+
+/*! \brief GSM A5/1 Output function
+ * \param[in] r Register state
+ * \return The A5/1 output function bit
+ */
+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));
+}
+
+/*! \brief Generate a GSM A5/1 cipher stream
+ * \param[in] key 8 byte array for the key (as received from the SIM)
+ * \param[in] fn Frame number
+ * \param[out] dl Pointer to array of ubits to return Downlink cipher stream
+ * \param[out] ul Pointer to array of ubits to return Uplink cipher stream
+ *
+ * Either (or both) of dl/ul can be NULL if not needed.
+ */
+void
+osmo_a5_1(const 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
+
+/*! \brief GSM A5/2 Clocking function
+ * \param[in] r Register state
+ * \param[in] force Non-zero value disable conditional clocking
+ */
+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);
+}
+
+/*! \brief GSM A5/2 Output function
+ * \param[in] r Register state
+ * \return The A5/2 output function bit
+ */
+static inline uint8_t
+_a5_2_get_output(uint32_t r[])
+{
+ uint8_t b;
+
+ b = (r[0] >> (A5_R1_LEN-1)) ^
+ (r[1] >> (A5_R2_LEN-1)) ^
+ (r[2] >> (A5_R3_LEN-1)) ^
+ _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 b;
+}
+
+/*! \brief Generate a GSM A5/1 cipher stream
+ * \param[in] key 8 byte array for the key (as received from the SIM)
+ * \param[in] fn Frame number
+ * \param[out] dl Pointer to array of ubits to return Downlink cipher stream
+ * \param[out] ul Pointer to array of ubits to return Uplink cipher stream
+ *
+ * Either (or both) of dl/ul can be NULL if not needed.
+ */
+void
+osmo_a5_2(const 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;
+ 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<99; i++)
+ {
+ _a5_2_clock(r, 0);
+ }
+
+ /* Output */
+ for (i=0; i<114; i++) {
+ _a5_2_clock(r, 0);
+ if (dl)
+ dl[i] = _a5_2_get_output(r);
+ }
+
+ for (i=0; i<114; i++) {
+ _a5_2_clock(r, 0);
+ if (ul)
+ ul[i] = _a5_2_get_output(r);
+ }
+}
+
+/*! @} */
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