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CTR_DRBG: support set_entropy_len() before seed()

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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().

This removes the need for the test-only function
mbedtls_ctr_drbg_seed_entropy_len(). Just call
mbedtls_ctr_drbg_set_entropy_len() followed by
mbedtls_ctr_drbg_seed(), it works now.
This commit is contained in:
Gilles Peskine 2019-10-04 12:15:55 +02:00
parent 8bf5613336
commit 50ed86b6b9
3 changed files with 25 additions and 41 deletions
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16
include/mbedtls/ctr_drbg.h
View file

@ -214,11 +214,8 @@ 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.
@ -255,6 +252,8 @@ 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
@ -298,7 +297,7 @@ 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.
*
@ -499,11 +498,6 @@ int mbedtls_ctr_drbg_self_test( int verbose );
#endif /* MBEDTLS_SELF_TEST */
/* Internal functions (do not call directly) */
int mbedtls_ctr_drbg_seed_entropy_len( mbedtls_ctr_drbg_context *,
int (*)(void *, unsigned char *, size_t), void *,
const unsigned char *, size_t, size_t );
#ifdef __cplusplus
}
#endif

38
library/ctr_drbg.c
View file

@ -381,10 +381,6 @@ exit:
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)
@ -397,13 +393,11 @@ exit:
* and with outputs
* ctx = initial_working_state
*/
int mbedtls_ctr_drbg_seed_entropy_len(
mbedtls_ctr_drbg_context *ctx,
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,
size_t entropy_len )
size_t len )
{
int ret;
unsigned char key[MBEDTLS_CTR_DRBG_KEYSIZE];
@ -415,7 +409,8 @@ 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;
/*
@ -434,17 +429,6 @@ 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 )
{
return( mbedtls_ctr_drbg_seed_entropy_len( ctx, f_entropy, p_entropy,
custom, len,
MBEDTLS_CTR_DRBG_ENTROPY_LEN ) );
}
/* CTR_DRBG_Generate with derivation function (SP 800-90A §10.2.1.5.2)
* mbedtls_ctr_drbg_random_with_add(ctx, output, output_len, additional, add_len)
* implements
@ -708,8 +692,11 @@ 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 ) );
@ -729,8 +716,11 @@ 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 ) );

6
tests/suites/test_suite_ctr_drbg.function
View file

@ -44,11 +44,11 @@ static void ctr_drbg_validate_internal( int reseed_mode, data_t * nonce,
/* CTR_DRBG_Instantiate(entropy[:entropy->len], nonce, perso, <ignored>)
* where nonce||perso = nonce[nonce->len] */
TEST_ASSERT( mbedtls_ctr_drbg_seed_entropy_len(
mbedtls_ctr_drbg_set_entropy_len( &ctx, entropy_chunk_len );
TEST_ASSERT( mbedtls_ctr_drbg_seed(
&ctx,
mbedtls_test_entropy_func, entropy->x,
nonce->x, nonce->len,
entropy_chunk_len ) == 0 );
nonce->x, nonce->len ) == 0 );
if( reseed_mode == RESEED_ALWAYS )
mbedtls_ctr_drbg_set_prediction_resistance(
&ctx,