depau/mfoc-always-skip-probes
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Simplify PRNG validation

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This commit is contained in:
Philippe Teuwen 2017-02-17 16:14:01 +01:00
parent 34d42e5e47
commit 9d9f01fba4
3 changed files with 13 additions and 50 deletions
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6
src/crapto1.c
View file

@ -374,6 +374,12 @@ int nonce_distance(uint32_t from, uint32_t to)
}
return (65535 + dist[to >> 16] - dist[from >> 16]) % 65535;
}
bool validate_prng_nonce(uint32_t nonce)
{
// init prng table:
nonce_distance(nonce, nonce);
return ((65535 - dist[nonce >> 16] + dist[nonce & 0xffff]) % 65535) == 16;
}
static uint32_t fastfwd[2][8] = {

2
src/crapto1.h
View file

@ -20,6 +20,7 @@
#ifndef CRAPTO1_INCLUDED
#define CRAPTO1_INCLUDED
#include <stdint.h>
#include <stdbool.h>
#ifdef __cplusplus
extern "C" {
#endif
@ -39,6 +40,7 @@ extern "C" {
void lfsr_rollback(struct Crypto1State *s, uint32_t in, int fb);
uint32_t lfsr_rollback_word(struct Crypto1State *s, uint32_t in, int fb);
int nonce_distance(uint32_t from, uint32_t to);
bool validate_prng_nonce(uint32_t nonce);
#define FOREACH_VALID_NONCE(N, FILTER, FSIZE)\
uint32_t __n = 0,__M = 0, N = 0;\
int __i;\

55
src/mfoc.c
View file

@ -72,41 +72,6 @@ uint32_t unknownSector = 0;
char unknownKeyLetter = 'A';
uint32_t unexpected_random = 0;
// Determine the distance between two nonces.
// Assume that the difference is small, but we don't know which is first.
// Therefore try in alternating directions.
int32_t dist_nt(uint32_t nt1, uint32_t nt2) {
if (nt1 == nt2) return 0;
uint16_t i;
uint32_t nttmp1 = nt1;
uint32_t nttmp2 = nt2;
for (i = 1; i < (32768/8); ++i) {
nttmp1 = prng_successor(nttmp1, 1); if (nttmp1 == nt2) return i;
nttmp2 = prng_successor(nttmp2, 1); if (nttmp2 == nt1) return -i;
nttmp1 = prng_successor(nttmp1, 1); if (nttmp1 == nt2) return i+1;
nttmp2 = prng_successor(nttmp2, 1); if (nttmp2 == nt1) return -(i+1);
nttmp1 = prng_successor(nttmp1, 1); if (nttmp1 == nt2) return i+2;
nttmp2 = prng_successor(nttmp2, 1); if (nttmp2 == nt1) return -(i+2);
nttmp1 = prng_successor(nttmp1, 1); if (nttmp1 == nt2) return i+3;
nttmp2 = prng_successor(nttmp2, 1); if (nttmp2 == nt1) return -(i+3);
nttmp1 = prng_successor(nttmp1, 1); if (nttmp1 == nt2) return i+4;
nttmp2 = prng_successor(nttmp2, 1); if (nttmp2 == nt1) return -(i+4);
nttmp1 = prng_successor(nttmp1, 1); if (nttmp1 == nt2) return i+5;
nttmp2 = prng_successor(nttmp2, 1); if (nttmp2 == nt1) return -(i+5);
nttmp1 = prng_successor(nttmp1, 1); if (nttmp1 == nt2) return i+6;
nttmp2 = prng_successor(nttmp2, 1); if (nttmp2 == nt1) return -(i+6);
nttmp1 = prng_successor(nttmp1, 1); if (nttmp1 == nt2) return i+7;
nttmp2 = prng_successor(nttmp2, 1); if (nttmp2 == nt1) return -(i+7);
}
// either nt1 or nt2 are invalid nonces
return(-99999);
}
int main(int argc, char *const argv[])
{
int ch, i, k, n, j, m;
@ -1093,23 +1058,13 @@ int mf_enhanced_auth(int e_sector, int a_sector, mftag t, mfreader r, denonce *d
}
NtLast = bytes_to_num(Rx, 4) ^ crypto1_word(pcs, bytes_to_num(Rx, 4) ^ t.authuid, 1);
// Make sure the card is using the known PRNG
if (! validate_prng_nonce(NtLast)) {
printf("Card is not vulnerable to nested attack\n");
return -99999;
}
// Save the determined nonces distance
d->distances[m] = nonce_distance(Nt, NtLast);
int checkForValidPRNG = dist_nt(Nt, NtLast);
//printf("NT distance: %d\n", checkForValidPRNG);
// if no distance between, then we are in sync.
if (checkForValidPRNG == 0) {
printf("NT Distance is zero..........\n");
} else {
if (checkForValidPRNG == -99999) { // invalid nonce received
++unexpected_random;
if (unexpected_random > 4) {
printf("PRNG is not vulnerable to nested attack\n");
return -99999;
}
}
}
// Again, prepare and send {At}
for (i = 0; i < 4; i++) {