Backport 2.7: Allow xxx_drbg_set_entropy_len before xxx_drbg_seed

ChangeLog

@@ -2,6 +2,12 @@ mbed TLS ChangeLog (Sorted per branch, date)
 
 = mbed TLS 2.7.x branch released xxxx-xx-xx
 
+Bugfix
+   * Support mbedtls_hmac_drbg_set_entropy_len() and
+     mbedtls_ctr_drbg_set_entropy_len() before the DRBG is seeded. Before,
+     the initial seeding always reset the entropy length to the compile-time
+     default.
+
 Changes
    * Add unit tests for AES-GCM when called through mbedtls_cipher_auth_xxx()
      from the cipher abstraction layer. Fixes #2198.

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.
@@ -227,9 +224,18 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
 #endif
 /**
  * \param ctx           The CTR_DRBG context to seed.
+ *                      It must have been initialized with
+ *                      mbedtls_ctr_drbg_init().
+ *                      After a successful call to mbedtls_ctr_drbg_seed(),
+ *                      you may not call mbedtls_ctr_drbg_seed() again on
+ *                      the same context unless you call
+ *                      mbedtls_ctr_drbg_free() and mbedtls_ctr_drbg_init()
+ *                      again first.
  * \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
@@ -273,15 +279,10 @@ 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.
  *
- * \note                mbedtls_ctr_drbg_seed() always sets the entropy length
- *                      to #MBEDTLS_CTR_DRBG_ENTROPY_LEN, so this function
- *                      only has an effect when it is called after
- *                      mbedtls_ctr_drbg_seed().
- *
  * \note                The security strength of CTR_DRBG is bounded by the
  *                      entropy length. Thus \p len must be at least
  *                      32 (in bytes) to achieve a 256-bit strength.

include/mbedtls/hmac_drbg.h

@@ -139,13 +139,11 @@ void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx );
  *                      Note that SHA-256 is just as efficient as SHA-224.
  *                      The security strength can be reduced if a smaller
  *                      entropy length is set with
- *                      mbedtls_hmac_drbg_set_entropy_len() afterwards.
+ *                      mbedtls_hmac_drbg_set_entropy_len().
  *
- * \note                The entropy length for the initial seeding is
- *                      the security strength (converted from bits to bytes).
- *                      You can set a different entropy length for subsequent
- *                      seeding by calling mbedtls_hmac_drbg_set_entropy_len()
- *                      after this function.
+ * \note                The default entropy length is the security strength
+ *                      (converted from bits to bytes). You can override
+ *                      it by calling mbedtls_hmac_drbg_set_entropy_len().
  *
  * \note                During the initial seeding, this function calls
  *                      the entropy source to obtain a nonce
@@ -224,14 +222,9 @@ void mbedtls_hmac_drbg_set_prediction_resistance( mbedtls_hmac_drbg_context *ctx
 
 /**
  * \brief               This function sets the amount of entropy grabbed on each
- *                      reseed.
+ *                      seed or reseed.
  *
- * The default value is set by mbedtls_hmac_drbg_seed().
- *
- * \note                mbedtls_hmac_drbg_seed() always sets the entropy length
- *                      to the default value based on the chosen MD algorithm,
- *                      so this function only has an effect if it is called
- *                      after mbedtls_hmac_drbg_seed().
+ * See the documentation of mbedtls_hmac_drbg_seed() for the default value.
  *
  * \param ctx           The HMAC_DRBG context.
  * \param len           The amount of entropy to grab, in bytes.

library/ctr_drbg.c

@@ -66,56 +66,6 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx )
 #endif
 }
 
-/*
- * 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 ret;
-    unsigned char key[MBEDTLS_CTR_DRBG_KEYSIZE];
-
-    memset( key, 0, MBEDTLS_CTR_DRBG_KEYSIZE );
-
-    mbedtls_aes_init( &ctx->aes_ctx );
-
-    ctx->f_entropy = f_entropy;
-    ctx->p_entropy = p_entropy;
-
-    ctx->entropy_len = entropy_len;
-    ctx->reseed_interval = MBEDTLS_CTR_DRBG_RESEED_INTERVAL;
-
-    /*
-     * Initialize with an empty key
-     */
-    if( ( ret = mbedtls_aes_setkey_enc( &ctx->aes_ctx, key, MBEDTLS_CTR_DRBG_KEYBITS ) ) != 0 )
-    {
-        return( ret );
-    }
-
-    if( ( ret = mbedtls_ctr_drbg_reseed( ctx, custom, len ) ) != 0 )
-    {
-        return( ret );
-    }
-    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 )
-{
-    return( mbedtls_ctr_drbg_seed_entropy_len( ctx, f_entropy, p_entropy, custom, len,
-                                       MBEDTLS_CTR_DRBG_ENTROPY_LEN ) );
-}
-
 void mbedtls_ctr_drbg_free( mbedtls_ctr_drbg_context *ctx )
 {
     if( ctx == NULL )
@@ -386,6 +336,52 @@ int mbedtls_ctr_drbg_reseed( mbedtls_ctr_drbg_context *ctx,
     return( ret );
 }
 
+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];
+
+    memset( key, 0, MBEDTLS_CTR_DRBG_KEYSIZE );
+
+    mbedtls_aes_init( &ctx->aes_ctx );
+
+    ctx->f_entropy = f_entropy;
+    ctx->p_entropy = p_entropy;
+
+    if( ctx->entropy_len == 0 )
+        ctx->entropy_len = MBEDTLS_CTR_DRBG_ENTROPY_LEN;
+    ctx->reseed_interval = MBEDTLS_CTR_DRBG_RESEED_INTERVAL;
+
+    /*
+     * Initialize with an empty key
+     */
+    if( ( ret = mbedtls_aes_setkey_enc( &ctx->aes_ctx, key, MBEDTLS_CTR_DRBG_KEYBITS ) ) != 0 )
+    {
+        return( ret );
+    }
+
+    if( ( ret = mbedtls_ctr_drbg_reseed( ctx, custom, len ) ) != 0 )
+    {
+        return( ret );
+    }
+    return( 0 );
+}
+
+/* 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 )
+{
+    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,
                               unsigned char *output, size_t output_len,
                               const unsigned char *additional, size_t add_len )
@@ -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 ) );

library/hmac_drbg.c

@@ -275,16 +275,19 @@ int mbedtls_hmac_drbg_seed( mbedtls_hmac_drbg_context *ctx,
 
     ctx->reseed_interval = MBEDTLS_HMAC_DRBG_RESEED_INTERVAL;
 
-    /*
-     * See SP800-57 5.6.1 (p. 65-66) for the security strength provided by
-     * each hash function, then according to SP800-90A rev1 10.1 table 2,
-     * min_entropy_len (in bits) is security_strength.
-     *
-     * (This also matches the sizes used in the NIST test vectors.)
-     */
-    ctx->entropy_len = md_size <= 20 ? 16 : /* 160-bits hash -> 128 bits */
-                       md_size <= 28 ? 24 : /* 224-bits hash -> 192 bits */
-                       32;  /* better (256+) -> 256 bits */
+    if( ctx->entropy_len == 0 )
+    {
+        /*
+         * See SP800-57 5.6.1 (p. 65-66) for the security strength provided by
+         * each hash function, then according to SP800-90A rev1 10.1 table 2,
+         * min_entropy_len (in bits) is security_strength.
+         *
+         * (This also matches the sizes used in the NIST test vectors.)
+         */
+        ctx->entropy_len = md_size <= 20 ? 16 : /* 160-bits hash -> 128 bits */
+                           md_size <= 28 ? 24 : /* 224-bits hash -> 192 bits */
+                           32;  /* better (256+) -> 256 bits */
+    }
 
     if( ( ret = hmac_drbg_reseed_core( ctx, custom, len,
                                        1 /* add nonce */ ) ) != 0 )
@@ -305,7 +308,7 @@ void mbedtls_hmac_drbg_set_prediction_resistance( mbedtls_hmac_drbg_context *ctx
 }
 
 /*
- * Set entropy length grabbed for reseeds
+ * Set entropy length grabbed for seeding
  */
 void mbedtls_hmac_drbg_set_entropy_len( mbedtls_hmac_drbg_context *ctx, size_t len )
 {

programs/test/benchmark.c

@@ -560,13 +560,14 @@ int main( int argc, char *argv[] )
         mbedtls_ctr_drbg_context ctr_drbg;
 
         mbedtls_ctr_drbg_init( &ctr_drbg );
-
         if( mbedtls_ctr_drbg_seed( &ctr_drbg, myrand, NULL, NULL, 0 ) != 0 )
             mbedtls_exit(1);
         TIME_AND_TSC( "CTR_DRBG (NOPR)",
                 if( mbedtls_ctr_drbg_random( &ctr_drbg, buf, BUFSIZE ) != 0 )
                 mbedtls_exit(1) );
+        mbedtls_ctr_drbg_free( &ctr_drbg );
 
+        mbedtls_ctr_drbg_init( &ctr_drbg );
         if( mbedtls_ctr_drbg_seed( &ctr_drbg, myrand, NULL, NULL, 0 ) != 0 )
             mbedtls_exit(1);
         mbedtls_ctr_drbg_set_prediction_resistance( &ctr_drbg, MBEDTLS_CTR_DRBG_PR_ON );

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 );