tests/suites/test_suite_bignum_random.function

/* BEGIN_HEADER */
/* Dedicated test suite for mbedtls_mpi_core_random() and the upper-layer
 * functions. Due to the complexity of how these functions are tested,
 * we test all the layers in a single test suite, unlike the way other
 * functions are tested with each layer in its own test suite.
 */

#include "mbedtls/bignum.h"
#include "mbedtls/entropy.h"
#include "bignum_core.h"
#include "constant_time_internal.h"

/* This test suite only manipulates non-negative bignums. */
static int sign_is_valid( const mbedtls_mpi *X )
{
    return( X->s == 1 );
}

/* Test whether bytes represents (in big-endian base 256) a number b that
 * is significantly above a power of 2. That is, b must not have a long run
 * of unset bits after the most significant bit.
 *
 * Let n be the bit-size of b, i.e. the integer such that 2^n <= b < 2^{n+1}.
 * This function returns 1 if, when drawing a number between 0 and b,
 * the probability that this number is at least 2^n is not negligible.
 * This probability is (b - 2^n) / b and this function checks that this
 * number is above some threshold A. The threshold value is heuristic and
 * based on the needs of mpi_random_many().
 */
static int is_significantly_above_a_power_of_2( data_t *bytes )
{
    const uint8_t *p = bytes->x;
    size_t len = bytes->len;
    unsigned x;

    /* Skip leading null bytes */
    while( len > 0 && p[0] == 0 )
    {
        ++p;
        --len;
    }
    /* 0 is not significantly above a power of 2 */
    if( len == 0 )
        return( 0 );
    /* Extract the (up to) 2 most significant bytes */
    if( len == 1 )
        x = p[0];
    else
        x = ( p[0] << 8 ) | p[1];

    /* Shift the most significant bit of x to position 8 and mask it out */
    while( ( x & 0xfe00 ) != 0 )
        x >>= 1;
    x &= 0x00ff;

    /* At this point, x = floor((b - 2^n) / 2^(n-8)). b is significantly above
     * a power of 2 iff x is significantly above 0 compared to 2^8.
     * Testing x >= 2^4 amounts to picking A = 1/16 in the function
     * description above. */
    return( x >= 0x10 );
}

/* END_HEADER */

/* BEGIN_DEPENDENCIES
 * depends_on:MBEDTLS_BIGNUM_C
 * END_DEPENDENCIES
 */

/* BEGIN_CASE */
void mpi_core_random_basic( int min, char *bound_bytes, int expected_ret )
{
    /* Same RNG as in mpi_random_values */
    mbedtls_test_rnd_pseudo_info rnd = {
        {'T', 'h', 'i', 's', ' ', 'i', ',', 'a',
         's', 'e', 'e', 'd', '!', 0},
        0, 0};
    size_t limbs;
    mbedtls_mpi_uint *lower_bound = NULL;
    mbedtls_mpi_uint *upper_bound = NULL;
    mbedtls_mpi_uint *result = NULL;

    TEST_EQUAL( 0, mbedtls_test_read_mpi_core( &upper_bound, &limbs,
                                               bound_bytes ) );
    ASSERT_ALLOC( lower_bound, limbs * ciL );
    lower_bound[0] = min;
    ASSERT_ALLOC( result, limbs * ciL );

    TEST_EQUAL( expected_ret,
                mbedtls_mpi_core_random( result, min, upper_bound, limbs,
                                         mbedtls_test_rnd_pseudo_rand, &rnd ) );

    if( expected_ret == 0 )
    {
        TEST_EQUAL( 0, mbedtls_mpi_core_lt_ct( result, lower_bound, limbs ) );
        TEST_EQUAL( 1, mbedtls_mpi_core_lt_ct( result, upper_bound, limbs ) );
    }

exit:
    mbedtls_free( lower_bound );
    mbedtls_free( upper_bound );
    mbedtls_free( result );
}
/* END_CASE */

/* BEGIN_CASE */
void mpi_random_values( int min, char *max_hex )
{
    /* Same RNG as in mpi_core_random_basic */
    mbedtls_test_rnd_pseudo_info rnd_core = {
        {'T', 'h', 'i', 's', ' ', 'i', ',', 'a',
         's', 'e', 'e', 'd', '!', 0},
        0, 0};
    mbedtls_test_rnd_pseudo_info rnd_legacy;
    memcpy( &rnd_legacy, &rnd_core, sizeof( rnd_core ) );
    mbedtls_mpi max_legacy;
    mbedtls_mpi_init( &max_legacy );
    mbedtls_mpi_uint *R_core = NULL;
    mbedtls_mpi R_legacy;
    mbedtls_mpi_init( &R_legacy );

    TEST_EQUAL( 0, mbedtls_test_read_mpi( &max_legacy, max_hex ) );
    size_t limbs = max_legacy.n;
    ASSERT_ALLOC( R_core, limbs * ciL );

    /* Call the legacy function and the core function with the same random
     * stream. */
    int core_ret = mbedtls_mpi_core_random( R_core, min, max_legacy.p, limbs,
                                            mbedtls_test_rnd_pseudo_rand,
                                            &rnd_core );
    int legacy_ret = mbedtls_mpi_random( &R_legacy, min, &max_legacy,
                                         mbedtls_test_rnd_pseudo_rand,
                                         &rnd_legacy );

    /* They must return the same status, and, on success, output the
     * same number, with the same limb count. */
    TEST_EQUAL( core_ret, legacy_ret );
    if( core_ret == 0 )
    {
        ASSERT_COMPARE( R_core, limbs * ciL,
                        R_legacy.p, R_legacy.n * ciL );
    }

    /* Also check that they have consumed the RNG in the same way. */
    /* This may theoretically fail on rare platforms with padding in
     * the structure! If this is a problem in practice, change to a
     * field-by-field comparison. */
    ASSERT_COMPARE( &rnd_core, sizeof( rnd_core ),
                    &rnd_legacy, sizeof( rnd_legacy ) );

exit:
    mbedtls_mpi_free( &max_legacy );
    mbedtls_free( R_core );
    mbedtls_mpi_free( &R_legacy );
}
/* END_CASE */

/* BEGIN_CASE */
void mpi_random_many( int min, char *bound_hex, int iterations )
{
    /* Generate numbers in the range 1..bound-1. Do it iterations times.
     * This function assumes that the value of bound is at least 2 and
     * that iterations is large enough that a one-in-2^iterations chance
     * effectively never occurs.
     */

    data_t bound_bytes = {NULL, 0};
    mbedtls_mpi_uint *upper_bound = NULL;
    size_t limbs;
    size_t n_bits;
    mbedtls_mpi_uint *result = NULL;
    size_t b;
    /* If upper_bound is small, stats[b] is the number of times the value b
     * has been generated. Otherwise stats[b] is the number of times a
     * value with bit b set has been generated. */
    size_t *stats = NULL;
    size_t stats_len;
    int full_stats;
    size_t i;

    TEST_EQUAL( 0, mbedtls_test_read_mpi_core( &upper_bound, &limbs,
                                               bound_hex ) );
    ASSERT_ALLOC( result, limbs * ciL );

    n_bits = mbedtls_mpi_core_bitlen( upper_bound, limbs );
    /* Consider a bound "small" if it's less than 2^5. This value is chosen
     * to be small enough that the probability of missing one value is
     * negligible given the number of iterations. It must be less than
     * 256 because some of the code below assumes that "small" values
     * fit in a byte. */
    if( n_bits <= 5 )
    {
        full_stats = 1;
        stats_len = (uint8_t) upper_bound[0];
    }
    else
    {
        full_stats = 0;
        stats_len = n_bits;
    }
    ASSERT_ALLOC( stats, stats_len );

    for( i = 0; i < (size_t) iterations; i++ )
    {
        mbedtls_test_set_step( i );
        TEST_EQUAL( 0, mbedtls_mpi_core_random( result,
                                                min, upper_bound, limbs,
                                                mbedtls_test_rnd_std_rand, NULL ) );

        /* Temporarily use a legacy MPI for analysis, because the
         * necessary auxiliary functions don't exist yet in core. */
        mbedtls_mpi B = {1, limbs, upper_bound};
        mbedtls_mpi R = {1, limbs, result};

        TEST_ASSERT( mbedtls_mpi_cmp_mpi( &R, &B ) < 0 );
        TEST_ASSERT( mbedtls_mpi_cmp_int( &R, min ) >= 0 );
        if( full_stats )
        {
            uint8_t value;
            TEST_EQUAL( 0, mbedtls_mpi_write_binary( &R, &value, 1 ) );
            TEST_ASSERT( value < stats_len );
            ++stats[value];
        }
        else
        {
            for( b = 0; b < n_bits; b++ )
                stats[b] += mbedtls_mpi_get_bit( &R, b );
        }
    }

    if( full_stats )
    {
        for( b = min; b < stats_len; b++ )
        {
            mbedtls_test_set_step( 1000000 + b );
            /* Assert that each value has been reached at least once.
             * This is almost guaranteed if the iteration count is large
             * enough. This is a very crude way of checking the distribution.
             */
            TEST_ASSERT( stats[b] > 0 );
        }
    }
    else
    {
        bound_bytes.len = limbs * sizeof( mbedtls_mpi_uint );
        ASSERT_ALLOC( bound_bytes.x, bound_bytes.len );
        mbedtls_mpi_core_write_be( upper_bound, limbs,
                                   bound_bytes.x, bound_bytes.len );
        int statistically_safe_all_the_way =
            is_significantly_above_a_power_of_2( &bound_bytes );
        for( b = 0; b < n_bits; b++ )
        {
            mbedtls_test_set_step( 1000000 + b );
            /* Assert that each bit has been set in at least one result and
             * clear in at least one result. Provided that iterations is not
             * too small, it would be extremely unlikely for this not to be
             * the case if the results are uniformly distributed.
             *
             * As an exception, the top bit may legitimately never be set
             * if bound is a power of 2 or only slightly above.
             */
            if( statistically_safe_all_the_way || b != n_bits - 1 )
            {
                TEST_ASSERT( stats[b] > 0 );
            }
            TEST_ASSERT( stats[b] < (size_t) iterations );
        }
    }

exit:
    mbedtls_free( bound_bytes.x );
    mbedtls_free( upper_bound );
    mbedtls_free( result );
    mbedtls_free( stats );
}
/* END_CASE */

/* BEGIN_CASE */
void mpi_random_sizes( int min, data_t *bound_bytes, int nlimbs, int before )
{
    mbedtls_mpi upper_bound;
    mbedtls_mpi result;

    mbedtls_mpi_init( &upper_bound );
    mbedtls_mpi_init( &result );

    if( before != 0 )
    {
        /* Set result to sign(before) * 2^(|before|-1) */
        TEST_ASSERT( mbedtls_mpi_lset( &result, before > 0 ? 1 : -1 ) == 0 );
        if( before < 0 )
            before = - before;
        TEST_ASSERT( mbedtls_mpi_shift_l( &result, before - 1 ) == 0 );
    }

    TEST_EQUAL( 0, mbedtls_mpi_grow( &result, nlimbs ) );
    TEST_EQUAL( 0, mbedtls_mpi_read_binary( &upper_bound,
                                            bound_bytes->x, bound_bytes->len ) );
    TEST_EQUAL( 0, mbedtls_mpi_random( &result, min, &upper_bound,
                                       mbedtls_test_rnd_std_rand, NULL ) );
    TEST_ASSERT( sign_is_valid( &result ) );
    TEST_ASSERT( mbedtls_mpi_cmp_mpi( &result, &upper_bound ) < 0 );
    TEST_ASSERT( mbedtls_mpi_cmp_int( &result, min ) >= 0 );

exit:
    mbedtls_mpi_free( &upper_bound );
    mbedtls_mpi_free( &result );
}
/* END_CASE */

/* BEGIN_CASE */
void mpi_random_fail( int min, data_t *bound_bytes, int expected_ret )
{
    mbedtls_mpi upper_bound;
    mbedtls_mpi result;
    int actual_ret;

    mbedtls_mpi_init( &upper_bound );
    mbedtls_mpi_init( &result );

    TEST_EQUAL( 0, mbedtls_mpi_read_binary( &upper_bound,
                                            bound_bytes->x, bound_bytes->len ) );
    actual_ret = mbedtls_mpi_random( &result, min, &upper_bound,
                                     mbedtls_test_rnd_std_rand, NULL );
    TEST_EQUAL( expected_ret, actual_ret );

exit:
    mbedtls_mpi_free( &upper_bound );
    mbedtls_mpi_free( &result );
}
/* END_CASE */