CTR_DRBG entropy usage: test the exact amount of consumed entropy

Author: gilles-peskine-arm

tests/suites/test_suite_ctr_drbg.function

@@ -197,7 +197,7 @@ void ctr_drbg_entropy_usage(  )
     unsigned char entropy[1024];
     mbedtls_ctr_drbg_context ctx;
     size_t i, reps = 10;
-    size_t last_idx;
+    size_t expected_idx = 0;
 
     mbedtls_ctr_drbg_init( &ctx );
     test_offset_idx = 0;
@@ -207,20 +207,19 @@ void ctr_drbg_entropy_usage(  )
     memset( add, 0, sizeof( add ) );
 
     /* Init must use entropy */
-    last_idx = test_offset_idx;
     TEST_ASSERT( mbedtls_ctr_drbg_seed( &ctx, mbedtls_test_entropy_func, entropy, NULL, 0 ) == 0 );
-    TEST_ASSERT( last_idx < test_offset_idx );
+    expected_idx += MBEDTLS_CTR_DRBG_ENTROPY_LEN;
+    TEST_EQUAL( test_offset_idx, expected_idx );
 
     /* By default, PR is off and reseed_interval is large,
      * so the next few calls should not use entropy */
-    last_idx = test_offset_idx;
     for( i = 0; i < reps; i++ )
     {
         TEST_ASSERT( mbedtls_ctr_drbg_random( &ctx, out, sizeof( out ) - 4 ) == 0 );
         TEST_ASSERT( mbedtls_ctr_drbg_random_with_add( &ctx, out, sizeof( out ) - 4,
                                                 add, sizeof( add ) ) == 0 );
     }
-    TEST_ASSERT( last_idx == test_offset_idx );
+    TEST_EQUAL( test_offset_idx, expected_idx );
 
     /* While at it, make sure we didn't write past the requested length */
     TEST_ASSERT( out[sizeof( out ) - 4] == 0 );
@@ -232,17 +231,17 @@ void ctr_drbg_entropy_usage(  )
      * so the next call should reseed */
     mbedtls_ctr_drbg_set_reseed_interval( &ctx, 2 * reps );
     TEST_ASSERT( mbedtls_ctr_drbg_random( &ctx, out, sizeof( out ) ) == 0 );
-    TEST_ASSERT( last_idx < test_offset_idx );
+    expected_idx += MBEDTLS_CTR_DRBG_ENTROPY_LEN;
+    TEST_EQUAL( test_offset_idx, expected_idx );
 
     /* The new few calls should not reseed */
-    last_idx = test_offset_idx;
     for( i = 0; i < reps / 2; i++ )
     {
         TEST_ASSERT( mbedtls_ctr_drbg_random( &ctx, out, sizeof( out ) ) == 0 );
         TEST_ASSERT( mbedtls_ctr_drbg_random_with_add( &ctx, out, sizeof( out ) ,
                                                 add, sizeof( add ) ) == 0 );
     }
-    TEST_ASSERT( last_idx == test_offset_idx );
+    TEST_EQUAL( test_offset_idx, expected_idx );
 
     /* Call update with too much data (sizeof entropy > MAX(_SEED)_INPUT).
      * Make sure it's detected as an error and doesn't cause memory
@@ -253,18 +252,19 @@ void ctr_drbg_entropy_usage(  )
     /* Now enable PR, so the next few calls should all reseed */
     mbedtls_ctr_drbg_set_prediction_resistance( &ctx, MBEDTLS_CTR_DRBG_PR_ON );
     TEST_ASSERT( mbedtls_ctr_drbg_random( &ctx, out, sizeof( out ) ) == 0 );
-    TEST_ASSERT( last_idx < test_offset_idx );
+    expected_idx += MBEDTLS_CTR_DRBG_ENTROPY_LEN;
+    TEST_EQUAL( test_offset_idx, expected_idx );
 
     /* Finally, check setting entropy_len */
     mbedtls_ctr_drbg_set_entropy_len( &ctx, 42 );
-    last_idx = test_offset_idx;
     TEST_ASSERT( mbedtls_ctr_drbg_random( &ctx, out, sizeof( out ) ) == 0 );
-    TEST_ASSERT( test_offset_idx - last_idx == 42 );
+    expected_idx += 42;
+    TEST_EQUAL( test_offset_idx, expected_idx );
 
     mbedtls_ctr_drbg_set_entropy_len( &ctx, 13 );
-    last_idx = test_offset_idx;
     TEST_ASSERT( mbedtls_ctr_drbg_random( &ctx, out, sizeof( out ) ) == 0 );
-    TEST_ASSERT( test_offset_idx - last_idx == 13 );
+    expected_idx += 13;
+    TEST_EQUAL( test_offset_idx, expected_idx );
 
 exit:
     mbedtls_ctr_drbg_free( &ctx );