/*
* 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 <errno.h>
#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
* \returns 0 for success, -ENOTSUP for invalid cipher selection.
*
* Currently A5/[0-2] are supported.
* Either (or both) of dl/ul can be NULL if not needed.
*/
int
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 */
return -ENOTSUP;
}
return 0;
}
/* ------------------------------------------------------------------------ */
/* 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);
}
}
/*! @} */