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TUCoPS :: Crypto
:: a5attack.txtAn 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); }