commit 20dbfb99386a8802c8e30117191d93ec158cccd8
author Gilles Peskine <Gilles.Peskine@arm.com> Fri Oct 04 12:15:55 2019 +0200
committer Simon Butcher <simon.butcher@arm.com> Fri Mar 13 15:25:39 2020 +0000
tree 7a6b222596b26c4ed62d3e693ffa21d7a522674a
parent f0bf757f9cbfe719463e67b030442eea8299ee38

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.

library/ctr_drbg.c

diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index 9222f4e..4e47058 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -365,29 +365,22 @@
     return( ret );
 }
 
-/*
- * Non-public function wrapped by mbedtls_ctr_drbg_seed(). Necessary to allow
- * NIST tests to succeed (which require known length fixed entropy)
- */
 /* CTR_DRBG_Instantiate with derivation function (SP 800-90A §10.2.1.3.2)
- * mbedtls_ctr_drbg_seed_entropy_len(ctx, f_entropy, p_entropy,
- *                                   custom, len, entropy_len)
+ * mbedtls_ctr_drbg_seed(ctx, f_entropy, p_entropy, custom, len)
  * implements
  * CTR_DRBG_Instantiate(entropy_input, nonce, personalization_string,
  *                      security_strength) -> initial_working_state
  * with inputs
  *   custom[:len] = nonce || personalization_string
- * where entropy_input comes from f_entropy for entropy_len bytes
+ * where entropy_input comes from f_entropy for ctx->entropy_len bytes
  * and with outputs
  *   ctx = initial_working_state
  */
-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];
@@ -399,7 +392,8 @@
     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;
 
     /*
@@ -417,14 +411,15 @@
     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 ) );
 }
 
 /* CTR_DRBG_Generate with derivation function (SP 800-90A §10.2.1.5.2)
@@ -678,8 +673,11 @@
         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 ) );
@@ -699,8 +697,11 @@
     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 ) );