Merge pull request #3537 from piotr-now/platform_random
Expanded the random number generator in the `platform_util.c` file
diff --git a/include/mbedtls/platform_util.h b/include/mbedtls/platform_util.h
index 81d0f0f..8d00eba 100644
--- a/include/mbedtls/platform_util.h
+++ b/include/mbedtls/platform_util.h
@@ -167,11 +167,10 @@
/**
* \brief Secure memset
*
- * This is a constant-time version of memset(). If
- * MBEDTLS_ENTROPY_HARDWARE_ALT is defined, the buffer is
- * initialised with random data and the order is also
- * randomised using the hardware RNG in order to further harden
- * against side-channel attacks.
+ * This is a constant-time version of memset(). The buffer is
+ * initialised with random data and the order is also randomised
+ * using the RNG in order to further harden against side-channel
+ * attacks.
*
* \param ptr Buffer to be set.
* \param value Value to be used when setting the buffer.
@@ -184,11 +183,10 @@
/**
* \brief Secure memcpy
*
- * This is a constant-time version of memcpy(). If
- * MBEDTLS_ENTROPY_HARDWARE_ALT is defined, the buffer is
- * initialised with random data and the order is also
- * randomised using the hardware RNG in order to further harden
- * against side-channel attacks.
+ * This is a constant-time version of memcpy(). The buffer is
+ * initialised with random data and the order is also randomised
+ * using the RNG in order to further harden against side-channel
+ * attacks.
*
* \param dst Destination buffer where the data is being copied to.
* \param src Source buffer where the data is being copied from.
@@ -218,9 +216,8 @@
* \brief Secure check if the buffers have the same data.
*
* This is a constant-time version of memcmp(), but without checking
- * if the bytes are greater or lower. If MBEDTLS_ENTROPY_HARDWARE_ALT
- * is defined, the order is also randomised using the hardware RNG in
- * order to further harden against side-channel attacks.
+ * if the bytes are greater or lower. The order is also randomised
+ * using the RNG in order to further harden against side-channel attacks.
*
* \param buf1 First buffer to compare.
* \param buf2 Second buffer to compare against.
@@ -234,11 +231,6 @@
/**
* \brief RNG-function for getting a random 32-bit integer.
*
- *
- * \note Currently the function is dependent of hardware providing an
- * rng with MBEDTLS_ENTROPY_HARDWARE_ALT. By default, 0 is
- * returned.
- *
* \return The generated random number.
*/
uint32_t mbedtls_platform_random_uint32( void );
@@ -253,10 +245,6 @@
* cryptographically secure RNG, but provide an RNG for utility
* functions.
*
- * \note Currently the function is dependent of hardware providing an
- * rng with MBEDTLS_ENTROPY_HARDWARE_ALT. By default, 0 is
- * returned.
- *
* \note If the given range is [0, 0), 0 is returned.
*
* \param num Max-value for the generated random number, exclusive.
@@ -264,7 +252,7 @@
*
* \return The generated random number.
*/
-uint32_t mbedtls_platform_random_in_range( size_t num );
+uint32_t mbedtls_platform_random_in_range( uint32_t num );
/**
* \brief Random delay function.
@@ -275,8 +263,9 @@
* Duration of the delay is random as number of variable increments
* is randomized.
*
- * \note Currently the function is dependent of hardware providing an
- * rng with MBEDTLS_ENTROPY_HARDWARE_ALT.
+ * \note This function works only if the MBEDTLS_FI_COUNTERMEASURES flag
+ * is defined in the configuration. Otherwise, the function does
+ * nothing.
*/
void mbedtls_platform_random_delay( void );
diff --git a/library/platform_util.c b/library/platform_util.c
index 24c7b41..5e938f9 100644
--- a/library/platform_util.c
+++ b/library/platform_util.c
@@ -51,9 +51,20 @@
#include <stddef.h>
#include <string.h>
-/* Max number of loops for mbedtls_platform_random_delay */
+/* Max number of loops for mbedtls_platform_random_delay. */
#define MAX_RAND_DELAY 100
+/* Parameters for the linear congruential generator used as a non-cryptographic
+ * random number generator. The same parameters are used by e.g. ANSI C. */
+#define RAND_MULTIPLIER 1103515245
+#define RAND_INCREMENT 12345
+#define RAND_MODULUS 0x80000000
+
+/* The number of iterations after which the seed of the non-cryptographic
+ * random number generator will be changed. This is used only if the
+ * MBEDTLS_ENTROPY_HARDWARE_ALT option is enabled. */
+#define RAND_SEED_LIFE 10000
+
#if !defined(MBEDTLS_PLATFORM_ZEROIZE_ALT)
/*
* This implementation should never be optimized out by the compiler
@@ -96,7 +107,7 @@
void *mbedtls_platform_memset( void *ptr, int value, size_t num )
{
/* Randomize start offset. */
- size_t start_offset = (size_t) mbedtls_platform_random_in_range( num );
+ size_t start_offset = (size_t) mbedtls_platform_random_in_range( (uint32_t) num );
/* Randomize data */
uint32_t data = mbedtls_platform_random_in_range( 256 );
@@ -113,7 +124,7 @@
void *mbedtls_platform_memcpy( void *dst, const void *src, size_t num )
{
/* Randomize start offset. */
- size_t start_offset = (size_t) mbedtls_platform_random_in_range( num );
+ size_t start_offset = (size_t) mbedtls_platform_random_in_range( (uint32_t) num );
/* Randomize initial data to prevent leakage while copying */
uint32_t data = mbedtls_platform_random_in_range( 256 );
@@ -152,7 +163,7 @@
/* Start from a random location and check the correct number of iterations */
size_t i, flow_counter = 0;
- size_t start_offset = (size_t) mbedtls_platform_random_in_range( num );
+ size_t start_offset = (size_t) mbedtls_platform_random_in_range( (uint32_t) num );
for( i = start_offset; i < num; i++ )
{
@@ -172,51 +183,80 @@
return( (int) diff | (int) ( flow_counter ^ num ) );
}
-uint32_t mbedtls_platform_random_uint32( )
+/* This function implements a non-cryptographic random number generator based
+ * on the linear congruential generator algorithm. Additionally, if the
+ * MBEDTLS_ENTROPY_HARDWARE_ALT flag is defined, the seed is set at the first
+ * call of this function with using a hardware random number generator and
+ * changed every RAND_SEED_LIFE number of iterations.
+ *
+ * The value of the returned number is in the range [0; 0xffff].
+ *
+ * Note: The range of values with a 16-bit precision is related to the modulo
+ * parameter of the generator and the fact that the function does not return the
+ * full value of the internal state of the generator.
+ */
+static uint32_t mbedtls_platform_random_uint16( void )
{
-#if !defined(MBEDTLS_ENTROPY_HARDWARE_ALT)
- return 0;
-#else
- uint32_t result = 0;
- size_t olen = 0;
+ /* Set random_state - the first random value should not be zero. */
+ static uint32_t random_state = RAND_INCREMENT;
- mbedtls_hardware_poll( NULL, (unsigned char *) &result, sizeof( result ),
- &olen );
- return( result );
-#endif
+#if defined(MBEDTLS_ENTROPY_HARDWARE_ALT)
+
+ static uint32_t random_seed_life = 0;
+
+ if( 0 < random_seed_life )
+ {
+ --random_seed_life;
+ }
+ else
+ {
+ size_t olen = 0;
+ uint32_t hw_random;
+ mbedtls_hardware_poll( NULL,
+ (unsigned char *) &hw_random, sizeof( hw_random ),
+ &olen );
+ if( olen == sizeof( hw_random ) )
+ {
+ random_state ^= hw_random;
+ random_seed_life = RAND_SEED_LIFE;
+ }
+ }
+
+#endif /* MBEDTLS_ENTROPY_HARDWARE_ALT */
+
+ random_state = ( ( random_state * RAND_MULTIPLIER ) + RAND_INCREMENT ) % RAND_MODULUS;
+
+ /* Do not return the entire random_state to hide generator predictability for
+ * the next iteration */
+ return( ( random_state >> 15 ) & 0xffff );
}
-uint32_t mbedtls_platform_random_in_range( size_t num )
+uint32_t mbedtls_platform_random_uint32( void )
{
-#if !defined(MBEDTLS_ENTROPY_HARDWARE_ALT)
- (void) num;
- return 0;
-#else
- uint32_t result = 0;
- size_t olen = 0;
+ return( ( mbedtls_platform_random_uint16() << 16 ) |
+ mbedtls_platform_random_uint16() );
+}
- mbedtls_hardware_poll( NULL, (unsigned char *) &result, sizeof( result ),
- &olen );
+uint32_t mbedtls_platform_random_in_range( uint32_t num )
+{
+ uint32_t result;
- if( num == 0 )
+ if( num <= 1 )
{
result = 0;
}
else
{
- result %= num;
+ result = mbedtls_platform_random_uint32() % num;
}
return( result );
-#endif
}
void mbedtls_platform_random_delay( void )
{
-#if !defined(MBEDTLS_ENTROPY_HARDWARE_ALT)
- return;
-#else
- size_t rn_1, rn_2, rn_3;
+#if defined(MBEDTLS_FI_COUNTERMEASURES)
+ uint32_t rn_1, rn_2, rn_3;
volatile size_t i = 0;
uint8_t shift;
@@ -229,17 +269,17 @@
i++;
/* Dummy calculations to increase the time between iterations and
* make side channel attack more difficult by reducing predictability
- * of its behaviour */
- shift = rn_2 & 0x07;
+ * of its behaviour. */
+ shift = ( rn_2 & 0x07 ) + 1;
if ( i % 2 )
- rn_2 = (uint32_t)( rn_2 >> shift | rn_2 << ( 32 - shift ) );
+ rn_2 = ( rn_2 >> shift ) | ( rn_2 << ( 32 - shift ) );
else
- rn_3 = (uint32_t)( rn_3 << shift | rn_3 >> ( 32 - shift ) );
+ rn_3 = ( rn_3 << shift ) | ( rn_3 >> ( 32 - shift ) );
rn_2 ^= rn_3;
} while( i < rn_1 || rn_2 == 0 || rn_3 == 0 );
+#endif /* MBEDTLS_FI_COUNTERMEASURES */
return;
-#endif /* !MBEDTLS_ENTROPY_HARDWARE_ALT */
}
#if defined(MBEDTLS_HAVE_TIME_DATE) && !defined(MBEDTLS_PLATFORM_GMTIME_R_ALT)