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TUCoPS :: Crypto :: a5attack.txt

An attack on A5




Newsgroups: alt.phreaking
Date: 28 Jan 2003 23:52:42 GMT
Subject: Re: A5

>sorry to bring this up agian but If you manage to get the A5 chip
>could you listen to people cell phone calls case it can decode and its
>only 40 bit encypted which is weak. Have i got one in my phone can i
>take it out i wana do some cool stuff with it got any good sites for
>this kinda stuff?
>

At botom of my post is reputed to be the sourcecode for A5 implementation. I'm
not sure if its correct (got it a few years back but haven't looked at it - one
of those keep meaning to type of things- so don't assume that it is true :) )

As for your question on A5 chip (remember there are 3 strengths to A5) - no
this would not do . and yes every GSM phone has a A5 crypto in it. .. remember
that the A5 implementation in your phone knows the secret - it knows the key
and thats how it can encrypt/decrypt. to attack it you have to derive the key.

There are attacks on A5 however..
(ti2server.fernuni-hagen.de/~jkeller/apc2001-final.pdf  might be sort of thing
you want to look at)

Trom 
(code below)

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);

}





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