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Add more variation to random delay countermeasure

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Add more variation to the random delay function by xor:ing two
variables. It is not enough to increment just a counter to create a
delay as it will be visible as uniform delay that can be easily
removed from the trace by analysis.
This commit is contained in:
Arto Kinnunen 2019-12-30 12:55:30 +02:00
parent 0490485be5
commit dbf2b43ceb
1 changed files with 12 additions and 3 deletions
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15
library/platform_util.c
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@ -171,19 +171,28 @@ int mbedtls_platform_random_delay( size_t max_rand )
(void) max_rand; (void) max_rand;
return -1; return -1;
#else #else
size_t random_number; size_t rn_1, rn_2, rn_3;
volatile size_t i = 0; volatile size_t i = 0;
uint8_t shift;
if( max_rand == 0 || max_rand > INT_MAX ) if( max_rand == 0 || max_rand > INT_MAX )
{ {
return( -1 ); return( -1 );
} }
random_number = mbedtls_platform_random_in_range( max_rand ); rn_1 = mbedtls_platform_random_in_range( max_rand );
rn_2 = mbedtls_platform_random_in_range( 0xffffffff ) + 1;
rn_3 = mbedtls_platform_random_in_range( 0xffffffff ) + 1;
do do
{ {
i++; i++;
} while( i < random_number ); shift = rn_2 & 0x07;
if ( i % 2 )
rn_2 = (uint32_t)( rn_2 >> shift | rn_2 << ( 32 - shift ) );
else
rn_3 = (uint32_t)( rn_3 << shift | rn_3 >> ( 32 - shift ) );
rn_2 ^= rn_3;
} while( i < rn_1 || rn_2 == 0 || rn_3 == 0 );
return( (int)i ); return( (int)i );
#endif /* !MBEDTLS_ENTROPY_HARDWARE_ALT */ #endif /* !MBEDTLS_ENTROPY_HARDWARE_ALT */