Move mbedtls_mpi_random to the bignum module
Since mbedtls_mpi_random() is not specific to ECC code, move it from
the ECP module to the bignum module.
This increases the code size in builds without short Weierstrass
curves (including builds without ECC at all) that do not optimize out
unused functions.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_mpi.function b/tests/suites/test_suite_mpi.function
index c5bb5a6..8cf53b3 100644
--- a/tests/suites/test_suite_mpi.function
+++ b/tests/suites/test_suite_mpi.function
@@ -64,6 +64,40 @@
return( 0 );
}
+/* Test whether bytes represents (in big-endian base 256) a number B that
+ * is "significantly" above a power of 2, which is defined as follows.
+ * Let n be the integer such that 2^n <= B < 2^{n+1}. B is significantly
+ * above a power of 2 if (B - 2^n) / 2^n is not negligible. "Negligible"
+ * is defined as having a negligible chance that if you draw an integer
+ * in the range [1, B-1] K times, the number will always be less than 2^n,
+ * where K is the iteration count passed to genkey_sw_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;
+ while( len > 0 && p[0] == 0 )
+ {
+ ++p;
+ --len;
+ }
+ if( len == 0 )
+ return( 0 );
+ else if( len == 1 )
+ x = p[0];
+ else
+ x = ( p[0] << 8 ) | p[1];
+
+ if( x <= 4 )
+ return( 0 );
+
+ while( ( x & 0x8000 ) == 0 )
+ x <<= 1;
+ x &= 0x7fff;
+ return( x >= 0x1000 );
+}
+
/* END_HEADER */
/* BEGIN_DEPENDENCIES
@@ -1396,6 +1430,113 @@
}
/* END_CASE */
+/* BEGIN_CASE */
+void mpi_random_many( int min, data_t *bound_bytes, 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.
+ */
+
+ mbedtls_mpi upper_bound;
+ size_t n_bits;
+ mbedtls_mpi result;
+ 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;
+
+ mbedtls_mpi_init( &upper_bound );
+ mbedtls_mpi_init( &result );
+
+ TEST_EQUAL( 0, mbedtls_mpi_read_binary( &upper_bound,
+ bound_bytes->x, bound_bytes->len ) );
+ n_bits = mbedtls_mpi_bitlen( &upper_bound );
+ /* 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 = bound_bytes->x[bound_bytes->len - 1];
+ }
+ 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_random( &result, min, &upper_bound,
+ mbedtls_test_rnd_std_rand, NULL ) );
+
+ TEST_ASSERT( mbedtls_mpi_cmp_mpi( &result, &upper_bound ) < 0 );
+ TEST_ASSERT( mbedtls_mpi_cmp_int( &result, min ) >= 0 );
+ if( full_stats )
+ {
+ uint8_t value;
+ TEST_EQUAL( 0, mbedtls_mpi_write_binary( &result, &value, 1 ) );
+ TEST_ASSERT( value < stats_len );
+ ++stats[value];
+ }
+ else
+ {
+ for( b = 0; b < n_bits; b++ )
+ stats[b] += mbedtls_mpi_get_bit( &result, b );
+ }
+ }
+
+ if( full_stats )
+ {
+ for( b = 1; 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
+ {
+ 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( b != n_bits - 1 ||
+ is_significantly_above_a_power_of_2( bound_bytes ) )
+ {
+ TEST_ASSERT( stats[b] > 0 );
+ }
+ TEST_ASSERT( stats[b] < (size_t) iterations );
+ }
+ }
+
+exit:
+ mbedtls_mpi_free( &upper_bound );
+ mbedtls_mpi_free( &result );
+ mbedtls_free( stats );
+}
+/* END_CASE */
+
/* BEGIN_CASE depends_on:MBEDTLS_SELF_TEST */
void mpi_selftest( )
{