CTR_DRBG: support set_entropy_len() before seed()

mbedtls_ctr_drbg_seed() always set the entropy length to the default,
so a call to mbedtls_ctr_drbg_set_entropy_len() before seed() had no
effect. Change this to the more intuitive behavior that
set_entropy_len() sets the entropy length and seed() respects that and
only uses the default entropy length if there was no call to
set_entropy_len().

The former test-only function mbedtls_ctr_drbg_seed_entropy_len() is
no longer used, but keep it for strict ABI compatibility.

Author: gilles-peskine-arm

include/mbedtls/ctr_drbg.h

@@ -190,11 +190,8 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
  *   with mbedtls_entropy_init() (which registers the platform's default
  *   entropy sources).
  *
- * \p f_entropy is always called with a buffer size equal to the entropy
- * length. The entropy length is initially #MBEDTLS_CTR_DRBG_ENTROPY_LEN
- * and this value is always used for the initial seeding. You can change
- * the entropy length for subsequent seeding by calling
- * mbedtls_ctr_drbg_set_entropy_len() after this function.
+ * The entropy length is #MBEDTLS_CTR_DRBG_ENTROPY_LEN by default.
+ * You can override it by calling mbedtls_ctr_drbg_set_entropy_len().
  *
  * You can provide a personalization string in addition to the
  * entropy source, to make this instantiation as unique as possible.
@@ -230,6 +227,8 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
  * \param f_entropy     The entropy callback, taking as arguments the
  *                      \p p_entropy context, the buffer to fill, and the
  *                      length of the buffer.
+ *                      \p f_entropy is always called with a buffer size
+ *                      equal to the entropy length.
  * \param p_entropy     The entropy context to pass to \p f_entropy.
  * \param custom        The personalization string.
  *                      This can be \c NULL, in which case the personalization
@@ -273,7 +272,7 @@ void mbedtls_ctr_drbg_set_prediction_resistance( mbedtls_ctr_drbg_context *ctx,
 
 /**
  * \brief               This function sets the amount of entropy grabbed on each
- *                      subsequent reseed.
+ *                      seed or reseed.
  *
  * The default value is #MBEDTLS_CTR_DRBG_ENTROPY_LEN.
  *

library/ctr_drbg.c

@@ -336,17 +336,11 @@ int mbedtls_ctr_drbg_reseed( mbedtls_ctr_drbg_context *ctx,
     return( ret );
 }
 
-/*
- * Non-public function wrapped by mbedtls_ctr_drbg_seed(). Necessary to allow
- * NIST tests to succeed (which require known length fixed entropy)
- */
-int mbedtls_ctr_drbg_seed_entropy_len(
-                   mbedtls_ctr_drbg_context *ctx,
-                   int (*f_entropy)(void *, unsigned char *, size_t),
-                   void *p_entropy,
-                   const unsigned char *custom,
-                   size_t len,
-                   size_t entropy_len )
+int mbedtls_ctr_drbg_seed( mbedtls_ctr_drbg_context *ctx,
+                           int (*f_entropy)(void *, unsigned char *, size_t),
+                           void *p_entropy,
+                           const unsigned char *custom,
+                           size_t len )
 {
     int ret;
     unsigned char key[MBEDTLS_CTR_DRBG_KEYSIZE];
@@ -358,7 +352,8 @@ int mbedtls_ctr_drbg_seed_entropy_len(
     ctx->f_entropy = f_entropy;
     ctx->p_entropy = p_entropy;
 
-    ctx->entropy_len = entropy_len;
+    if( ctx->entropy_len == 0 )
+        ctx->entropy_len = MBEDTLS_CTR_DRBG_ENTROPY_LEN;
     ctx->reseed_interval = MBEDTLS_CTR_DRBG_RESEED_INTERVAL;
 
     /*
@@ -376,14 +371,15 @@ int mbedtls_ctr_drbg_seed_entropy_len(
     return( 0 );
 }
 
-int mbedtls_ctr_drbg_seed( mbedtls_ctr_drbg_context *ctx,
-                   int (*f_entropy)(void *, unsigned char *, size_t),
-                   void *p_entropy,
-                   const unsigned char *custom,
-                   size_t len )
+/* Backward compatibility wrapper */
+int mbedtls_ctr_drbg_seed_entropy_len(
+    mbedtls_ctr_drbg_context *ctx,
+    int (*f_entropy)(void *, unsigned char *, size_t), void *p_entropy,
+    const unsigned char *custom, size_t len,
+    size_t entropy_len )
 {
-    return( mbedtls_ctr_drbg_seed_entropy_len( ctx, f_entropy, p_entropy, custom, len,
-                                       MBEDTLS_CTR_DRBG_ENTROPY_LEN ) );
+    mbedtls_ctr_drbg_set_entropy_len( ctx, entropy_len );
+    return( mbedtls_ctr_drbg_seed( ctx, f_entropy, p_entropy, custom, len ) );
 }
 
 int mbedtls_ctr_drbg_random_with_add( void *p_rng,
@@ -617,8 +613,11 @@ int mbedtls_ctr_drbg_self_test( int verbose )
         mbedtls_printf( "  CTR_DRBG (PR = TRUE) : " );
 
     test_offset = 0;
-    CHK( mbedtls_ctr_drbg_seed_entropy_len( &ctx, ctr_drbg_self_test_entropy,
-                                (void *) entropy_source_pr, nonce_pers_pr, 16, 32 ) );
+    mbedtls_ctr_drbg_set_entropy_len( &ctx, 32 );
+    CHK( mbedtls_ctr_drbg_seed( &ctx,
+                                ctr_drbg_self_test_entropy,
+                                (void *) entropy_source_pr,
+                                nonce_pers_pr, 16 ) );
     mbedtls_ctr_drbg_set_prediction_resistance( &ctx, MBEDTLS_CTR_DRBG_PR_ON );
     CHK( mbedtls_ctr_drbg_random( &ctx, buf, MBEDTLS_CTR_DRBG_BLOCKSIZE ) );
     CHK( mbedtls_ctr_drbg_random( &ctx, buf, MBEDTLS_CTR_DRBG_BLOCKSIZE ) );
@@ -638,8 +637,11 @@ int mbedtls_ctr_drbg_self_test( int verbose )
     mbedtls_ctr_drbg_init( &ctx );
 
     test_offset = 0;
-    CHK( mbedtls_ctr_drbg_seed_entropy_len( &ctx, ctr_drbg_self_test_entropy,
-                            (void *) entropy_source_nopr, nonce_pers_nopr, 16, 32 ) );
+    mbedtls_ctr_drbg_set_entropy_len( &ctx, 32 );
+    CHK( mbedtls_ctr_drbg_seed( &ctx,
+                                ctr_drbg_self_test_entropy,
+                                (void *) entropy_source_nopr,
+                                nonce_pers_nopr, 16 ) );
     CHK( mbedtls_ctr_drbg_random( &ctx, buf, 16 ) );
     CHK( mbedtls_ctr_drbg_reseed( &ctx, NULL, 0 ) );
     CHK( mbedtls_ctr_drbg_random( &ctx, buf, 16 ) );

tests/suites/test_suite_ctr_drbg.function

@@ -72,7 +72,10 @@ void ctr_drbg_validate_pr( char *add_init_string, char *entropy_string,
     add2_len = unhexify( add2, add2_string );
 
     test_offset_idx = 0;
-    TEST_ASSERT( mbedtls_ctr_drbg_seed_entropy_len( &ctx, mbedtls_test_entropy_func, entropy, add_init, add_init_len, 32 ) == 0 );
+    mbedtls_ctr_drbg_set_entropy_len( &ctx, 32 );
+    TEST_ASSERT( mbedtls_ctr_drbg_seed( &ctx,
+                                        mbedtls_test_entropy_func, entropy,
+                                        add_init, add_init_len ) == 0 );
     mbedtls_ctr_drbg_set_prediction_resistance( &ctx, MBEDTLS_CTR_DRBG_PR_ON );
 
     TEST_ASSERT( mbedtls_ctr_drbg_random_with_add( &ctx, buf, 16, add1, add1_len ) == 0 );
@@ -110,7 +113,10 @@ void ctr_drbg_validate_nopr( char *add_init_string, char *entropy_string,
     add2_len = unhexify( add2, add2_string );
 
     test_offset_idx = 0;
-    TEST_ASSERT( mbedtls_ctr_drbg_seed_entropy_len( &ctx, mbedtls_test_entropy_func, entropy, add_init, add_init_len, 32 ) == 0 );
+    mbedtls_ctr_drbg_set_entropy_len( &ctx, 32 );
+    TEST_ASSERT( mbedtls_ctr_drbg_seed( &ctx,
+                                        mbedtls_test_entropy_func, entropy,
+                                        add_init, add_init_len ) == 0 );
 
     TEST_ASSERT( mbedtls_ctr_drbg_random_with_add( &ctx, buf, 16, add1, add1_len ) == 0 );
     TEST_ASSERT( mbedtls_ctr_drbg_reseed( &ctx, add_reseed, add_reseed_len ) == 0 );