The GSM encryption algorithm, A5, is not much good. Its effective key length 
is at most five bytes; and anyone with the time and energy to look for faster 
attacks can find source code for it at the bottom of this post.

The politics of all this is bizarre. Readers may recall that there was a fuss 
last year about whether GSM phones could be exported to the Middle East; the 
official line then was that A5 was too good for the likes of Saddam Hussein.

However, a couple of weeks ago, they switched from saying that A5 was too 
strong to disclose, to saying that it was too weak to disclose! The 
government line now pleads that discussing it might harm export sales. 

Maybe all the fuss was just a ploy to get Saddam to buy A5 chips on the black 
market; but Occam's razor suggests that we are really seeing the results of 
the usual blundering, infighting and incompetence of bloated government 
departments. 

Indeed, my spies inform me that there was a terrific row between the NATO 
signals agencies in the mid 1980's over whether GSM encryption should be 
strong or not. The Germans said it should be, as they shared a long border 
with the Evil Empire; but the other countries didn't feel this way. and the 
algorithm as now fielded is a French design.

A5 is a stream cipher, and the keystream is the xor of three clock controlled
registers. The clock control of each register is that register's own middle 
bit, xor'ed with a threshold function of the middle bits of all three 
registers (ie if two or more of the middle bits are 1, then invert each of 
these bits; otherwise just use them as they are). The register lengths are 
19, 22 and 23, and all the feedback polynomials are sparse.

Readers will note that there is a trivial 2^40 attack (guess the contents of
registers 1 and 2, work out register 3 from the keystream, and then step on 
to check whether the guess was right). 2^40 trial encryptions could take 
weeks on a workstation, but the low gate count of the algorithm means that 
a Xilinx chip can easily be programmed to do keysearch, and an A5 cracker 
might have a few dozen of these running at maybe 2 keys per microsecond each. 
Of course, if all you want to do is break the Royal Family's keys for sale to 
News International, then software would do fine.

It is thus clear that A5 should be free of all export controls, just like 
CDMF and the 40-bit versions of RC2 and RC4.

Indeed, there seems to be an even faster attack. As the clock control is 
stop-go rather than 1-2, one would expect some kind of correlation attack to 
be possible, and on June 3rd, Dr Simon Shepherd of Bradford University was 
due to present an attack on A5 to an IEE colloquium in London. However, his 
talk was spiked at the last minute by GCHQ, and all we know about his attack 
is:

(a) that sparse matrix techniques are used to reconstruct the initial state
    (this was published as a `trailer' in the April 93 `Mobile Europe');

(b) that he used some of the tricks from my paper `Solving a class of stream 
    ciphers' (Cryptologia XIV no 3 [July 90] pp 285 - 288) and from the 
    follow-up paper `Divide and conquer attacks on certain classes of stream 
    ciphers' by Ed Dawson and Andy Clark (Cryptologia XVIII no 1 [Jan 94] 
    pp 25 - 40) (he mentioned this to me on the phone).

I believe that we have to stand up for academic freedom, and I hope that 
placing A5 in the public domain will lead to the embargo on Simon's paper 
being lifted.


Ross Anderson


APPENDIX - AN IMPLEMENTATION OF A5

The documentation we have, which arrived anonymously in two brown envelopes, 
is incomplete; we do not know the feedback taps of registers 2 and 3, but we 
do know from the chip's gate count that they have at most 6 feedback taps 
between them.

The following implementation of A5 is due to Mike Roe <mrr@cl.cam.ac.uk>, and
all comments and queries should be sent to him.



/*
 * In writing this program, I've had to guess a few pices of information:
 *
 * 1. Which bits of the key are loaded into which bits of the shift register
 * 2. Which order the frame sequence number is shifted into the SR (MSB
 *    first or LSB first)
 * 3. The position of the feedback taps on R2 and R3 (R1 is known).
 * 4. The position of the clock control taps. These are on the `middle' one, 
 *    I've assumed to be 9 on R1, 11 on R2, 11 on R3.
 */

/*
 * Look at the `middle' stage of each of the 3 shift registers.
 * Either 0, 1, 2 or 3 of these 3 taps will be set high.
 * If 0 or 1 or one of them are high, return true. This will cause each of 
 * the middle taps to be inverted before being used as a clock control. In all
 * cases either 2 or 3 of the clock enable lines will be active. Thus, at least
 * two shift registers change on every clock-tick and the system never becomes
 * stuck.
 */

static int threshold(r1, r2, r3)
unsigned int r1;
unsigned int r2;
unsigned int r3;
{
int total;

  total = (((r1 >>  9) & 0x1) == 1) +
          (((r2 >> 11) & 0x1) == 1) +
          (((r3 >> 11) & 0x1) == 1);

  if (total > 1)
    return (0);
  else
    return (1);
}

unsigned long clock_r1(ctl, r1)
int ctl;
unsigned long r1;
{
unsigned long feedback;

 /*
  * Primitive polynomial x**19 + x**5 + x**2 + x + 1
  */

  ctl ^= ((r1 >> 9) & 0x1);
  if (ctl)
  {
    feedback = (r1 >> 18) ^ (r1 >> 17) ^ (r1 >> 16) ^ (r1 >> 13);
    r1 = (r1 << 1) & 0x7ffff;
    if (feedback & 0x01)
      r1 ^= 0x01;
  }
  return (r1);
}

unsigned long clock_r2(ctl, r2)
int ctl;
unsigned long r2;
{
unsigned long feedback;

  
 /*
  * Primitive polynomial x**22 + x**9 + x**5 + x + 1
  */   

  ctl ^= ((r2 >> 11) & 0x1);
  if (ctl)
  {
    feedback = (r2 >> 21) ^ (r2 >> 20) ^ (r2 >> 16) ^ (r2 >> 12);
    r2 = (r2 << 1) & 0x3fffff;
    if (feedback & 0x01)
      r2 ^= 0x01;
  }
  return (r2);
}

unsigned long clock_r3(ctl, r3)
int ctl;
unsigned long r3;
{
unsigned long feedback;

 /*
  * Primitive polynomial x**23 + x**5 + x**4 + x + 1
  */

  ctl ^= ((r3 >> 11) & 0x1);
  if (ctl)
  {
    feedback = (r3 >> 22) ^ (r3 >> 21) ^ (r3 >> 18) ^ (r3 >> 17);
    r3 = (r3 << 1) & 0x7fffff;
    if (feedback & 0x01)
      r3 ^= 0x01;
  }
  return (r3);
}

int keystream(key, frame, alice, bob)
unsigned char *key;   /* 64 bit session key              */
unsigned long frame;  /* 22 bit frame sequence number    */
unsigned char *alice; /* 114 bit Alice to Bob key stream */
unsigned char *bob;   /* 114 bit Bob to Alice key stream */
{
unsigned long r1;   /* 19 bit shift register */
unsigned long r2;   /* 22 bit shift register */
unsigned long r3;   /* 23 bit shift register */
int i;              /* counter for loops     */
int clock_ctl;      /* xored with clock enable on each shift register */
unsigned char *ptr; /* current position in keystream */
unsigned char byte; /* byte of keystream being assembled */
unsigned int bits;  /* number of bits of keystream in byte */
unsigned int bit;   /* bit output from keystream generator */

  /* Initialise shift registers from session key */

  r1 = (key[0] | (key[1] << 8) | (key[2] << 16) ) & 0x7ffff;
  r2 = ((key[2] >> 3) | (key[3] << 5) | (key[4] << 13) | (key[5] << 21)) & 0x3fffff;
  r3 = ((key[5] >> 1) | (key[6] << 7) | (key[7] << 15) ) & 0x7fffff;


  /* Merge frame sequence number into shift register state, by xor'ing it
   * into the feedback path
   */

  for (i=0;i<22;i++)
  {
    clock_ctl = threshold(r1, r2, r2);
    r1 = clock_r1(clock_ctl, r1);
    r2 = clock_r2(clock_ctl, r2);
    r3 = clock_r3(clock_ctl, r3);
    if (frame & 1)
    {
      r1 ^= 1;
      r2 ^= 1;
      r3 ^= 1;
    }
    frame = frame >> 1;
  }

  /* Run shift registers for 100 clock ticks to allow frame number to
   * be diffused into all the bits of the shift registers
   */

  for (i=0;i<100;i++)
  {
    clock_ctl = threshold(r1, r2, r2);
    r1 = clock_r1(clock_ctl, r1);
    r2 = clock_r2(clock_ctl, r2);
    r3 = clock_r3(clock_ctl, r3);
  }

  /* Produce 114 bits of Alice->Bob key stream */

  ptr = alice;
  bits = 0;
  byte = 0;
  for (i=0;i<114;i++)
  {
    clock_ctl = threshold(r1, r2, r2);
    r1 = clock_r1(clock_ctl, r1);
    r2 = clock_r2(clock_ctl, r2);
    r3 = clock_r3(clock_ctl, r3);

    bit = ((r1 >> 18) ^ (r2 >> 21) ^ (r3 >> 22)) & 0x01;
    byte = (byte << 1) | bit;
    bits++;
    if (bits == 8)
    {
      *ptr = byte;
      ptr++;
      bits = 0;
      byte = 0;
    }
  }
  if (bits)
    *ptr = byte;

  /* Run shift registers for another 100 bits to hide relationship between
   * Alice->Bob key stream and Bob->Alice key stream.
   */

  for (i=0;i<100;i++)
  {
    clock_ctl = threshold(r1, r2, r2);
    r1 = clock_r1(clock_ctl, r1);
    r2 = clock_r2(clock_ctl, r2);
    r3 = clock_r3(clock_ctl, r3);
  }

  /* Produce 114 bits of Bob->Alice key stream */

  ptr = bob;
  bits = 0;
  byte = 0;
  for (i=0;i<114;i++)
  {
    clock_ctl = threshold(r1, r2, r2);
    r1 = clock_r1(clock_ctl, r1);
    r2 = clock_r2(clock_ctl, r2);
    r3 = clock_r3(clock_ctl, r3);

    bit = ((r1 >> 18) ^ (r2 >> 21) ^ (r3 >> 22)) & 0x01;
    byte = (byte << 1) | bit;
    bits++;
    if (bits == 8)
    {
      *ptr = byte;
      ptr++;
      bits = 0;
      byte = 0;
    }
  }
  if (bits)
    *ptr = byte;
 
  return (0);

}