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Implement use of internal DRBG for ecp_mul()

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The case of MBEDTLS_ECP_RESTARTABLE isn't handled correctly yet: in that case
the DRBG instance should persist when resuming the operation. This will be
addressed in the next commit.

When both CTR_DRBG and HMAC_DRBG are available, CTR_DRBG is preferred since
both are suitable but CTR_DRBG tends to be faster and I needed a tie-breaker.

There are currently three possible cases to test:

- NO_INTERNAL_RNG is set -> tested in test_ecp_no_internal_rng
- it's unset and CTR_DRBG is available -> tested in the default config
- it's unset and CTR_DRBG is disabled -> tested in
  test_ecp_internal_rng_no_ctr_drbg

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard 2020-05-22 12:12:36 +02:00
parent 23983f30de
commit fb11d252b2
2 changed files with 139 additions and 0 deletions
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2
include/mbedtls/md.h
View file

@ -98,6 +98,8 @@ typedef struct mbedtls_md_context_t
* \brief This function returns the list of digests supported by the
* generic digest module.
*
* \note The list starts with the strongest available hashes.
*
* \return A statically allocated array of digests. Each element
* in the returned list is an integer belonging to the
* message-digest enumeration #mbedtls_md_type_t.

137
library/ecp.c
View file

@ -104,6 +104,16 @@
#include "mbedtls/ecp_internal.h"
#if !defined(MBEDTLS_ECP_NO_INTERNAL_RNG)
#if defined(MBEDTLS_CTR_DRBG_C)
#include "mbedtls/ctr_drbg.h"
#elif defined(MBEDTLS_HMAC_DRBG_C)
#include "mbedtls/hmac_drbg.h"
#else
#error "Invalid configuration detected. Include check_config.h to ensure that the configuration is valid."
#endif
#endif /* MBEDTLS_ECP_NO_INTERNAL_RNG */
#if ( defined(__ARMCC_VERSION) || defined(_MSC_VER) ) && \
!defined(inline) && !defined(__cplusplus)
#define inline __inline
@ -117,6 +127,113 @@
static unsigned long add_count, dbl_count, mul_count;
#endif
#if !defined(MBEDTLS_ECP_NO_INTERNAL_RNG)
/*
* Currently ecp_mul() takes a RNG function as an argument, used for
* side-channel protection, but it can be NULL. The initial reasonning was
* that people will pass non-NULL RNG when they care about side-channels, but
* unfortunately we have some APIs that call ecp_mul() with a NULL RNG, with
* no opportunity for the user to do anything about it.
*
* The obvious strategies for addressing that include:
* - change those APIs so that they take RNG arguments;
* - require a global RNG to be available to all crypto modules.
*
* Unfortunately those would break compatibility. So what we do instead is
* have our own internal DRBG instance, seeded from the secret scalar.
*
* The following is a light-weight abstraction layer for doing that with
* CTR_DRBG or HMAC_DRBG.
*/
#if defined(MBEDTLS_CTR_DRBG_C)
/* DRBG context type */
typedef mbedtls_ctr_drbg_context ecp_drbg_context;
/* DRBG context init */
static inline void ecp_drbg_init( ecp_drbg_context *ctx )
{
mbedtls_ctr_drbg_init( ctx );
}
/* DRBG context free */
static inline void ecp_drbg_free( ecp_drbg_context *ctx )
{
mbedtls_ctr_drbg_free( ctx );
}
/* DRBG function */
static inline int ecp_drbg_random( void *p_rng,
unsigned char *output, size_t output_len )
{
return( mbedtls_ctr_drbg_random( p_rng, output, output_len ) );
}
/*
* Since CTR_DRBG doesn't have a seed_buf() function the way HMAC_DRBG does,
* we need to pass an entropy function when seeding. So we use a dummy
* function for that, and pass the actual entropy as customisation string.
* (During seeding of CTR_DRBG the entropy input and customisation string are
* concatenated before being used to update the secret state.)
*/
static int ecp_ctr_drbg_null_entropy(void *ctx, unsigned char *out, size_t len)
{
(void) ctx;
memset( out, 0, len );
return( 0 );
}
/* DRBG context seeding */
static int ecp_drbg_seed( ecp_drbg_context *ctx, const mbedtls_mpi *secret )
{
const unsigned char *secret_p = (const unsigned char *) secret->p;
const size_t secret_size = secret->n * sizeof( mbedtls_mpi_uint );
return( mbedtls_ctr_drbg_seed( ctx, ecp_ctr_drbg_null_entropy, NULL,
secret_p, secret_size ) );
}
#elif defined(MBEDTLS_HMAC_DRBG_C)
/* DRBG context type */
typedef mbedtls_hmac_drbg_context ecp_drbg_context;
/* DRBG context init */
static inline void ecp_drbg_init( ecp_drbg_context *ctx )
{
mbedtls_hmac_drbg_init( ctx );
}
/* DRBG context free */
static inline void ecp_drbg_free( ecp_drbg_context *ctx )
{
mbedtls_hmac_drbg_free( ctx );
}
/* DRBG function */
static inline int ecp_drbg_random( void *p_rng,
unsigned char *output, size_t output_len )
{
return( mbedtls_hmac_drbg_random( p_rng, output, output_len ) );
}
/* DRBG context seeding */
static int ecp_drbg_seed( ecp_drbg_context *ctx, const mbedtls_mpi *secret )
{
const unsigned char *secret_p = (const unsigned char *) secret->p;
const size_t secret_size = secret->n * sizeof( mbedtls_mpi_uint );
/* The list starts with strong hashes */
const mbedtls_md_type_t md_type = mbedtls_md_list()[0];
const mbedtls_md_info_t *md_info = mbedtls_md_info_from_type( md_type );
return( mbedtls_hmac_drbg_seed_buf( ctx, md_info, secret_p, secret_size ) );
}
#else
#error "Invalid configuration detected. Include check_config.h to ensure that the configuration is valid."
#endif /* DRBG modules */
#endif /* MBEDTLS_ECP_NO_INTERNAL_RNG */
#if defined(MBEDTLS_ECP_RESTARTABLE)
/*
* Maximum number of "basic operations" to be done in a row.
@ -2363,12 +2480,19 @@ int mbedtls_ecp_mul_restartable( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
int ret = MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
#if defined(MBEDTLS_ECP_INTERNAL_ALT)
char is_grp_capable = 0;
#endif
#if !defined(MBEDTLS_ECP_NO_INTERNAL_RNG)
ecp_drbg_context drbg_ctx;
#endif
ECP_VALIDATE_RET( grp != NULL );
ECP_VALIDATE_RET( R != NULL );
ECP_VALIDATE_RET( m != NULL );
ECP_VALIDATE_RET( P != NULL );
#if !defined(MBEDTLS_ECP_NO_INTERNAL_RNG)
ecp_drbg_init( &drbg_ctx );
#endif /* !MBEDTLS_ECP_NO_INTERNAL_RNG */
#if defined(MBEDTLS_ECP_RESTARTABLE)
/* reset ops count for this call if top-level */
if( rs_ctx != NULL && rs_ctx->depth++ == 0 )
@ -2380,6 +2504,15 @@ int mbedtls_ecp_mul_restartable( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
MBEDTLS_MPI_CHK( mbedtls_internal_ecp_init( grp ) );
#endif /* MBEDTLS_ECP_INTERNAL_ALT */
#if !defined(MBEDTLS_ECP_NO_INTERNAL_RNG)
if( f_rng == NULL )
{
MBEDTLS_MPI_CHK( ecp_drbg_seed( &drbg_ctx, m ) );
f_rng = &ecp_drbg_random;
p_rng = &drbg_ctx;
}
#endif /* !MBEDTLS_ECP_NO_INTERNAL_RNG */
#if defined(MBEDTLS_ECP_RESTARTABLE)
/* skip argument check when restarting */
if( rs_ctx == NULL || rs_ctx->rsm == NULL )
@ -2410,6 +2543,10 @@ cleanup:
mbedtls_internal_ecp_free( grp );
#endif /* MBEDTLS_ECP_INTERNAL_ALT */
#if !defined(MBEDTLS_ECP_NO_INTERNAL_RNG)
ecp_drbg_free( &drbg_ctx );
#endif
#if defined(MBEDTLS_ECP_RESTARTABLE)
if( rs_ctx != NULL )
rs_ctx->depth--;