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Merge pull request #276 from dgreen-arm/dev/dgreen-arm/iotcrypt-616-fix-ecdsa-rng

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Fix deterministic ECDSA RNG misuse
This commit is contained in:
Jaeden Amero 2019-09-05 15:22:38 +01:00 committed by GitHub
commit c7cde03fec
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GPG key ID: 4AEE18F83AFDEB23
5 changed files with 217 additions and 36 deletions
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10
include/mbedtls/config.h
View file

@ -441,6 +441,16 @@
* dependencies on them, and considering stronger message digests
* and ciphers instead.
*
* \warning If both MBEDTLS_ECDSA_SIGN_ALT and MBEDTLS_ECDSA_DETERMINISTIC are
* enabled, then the deterministic ECDH signature functions pass the
* the static HMAC-DRBG as RNG to mbedtls_ecdsa_sign(). Therefore
* alternative implementations should use the RNG only for generating
* the ephemeral key and nothing else. If this is not possible, then
* MBEDTLS_ECDSA_DETERMINISTIC should be disabled and an alternative
* implementation should be provided for mbedtls_ecdsa_sign_det_ext()
* (and for mbedtls_ecdsa_sign_det() too if backward compatibility is
* desirable).
*
*/
//#define MBEDTLS_MD2_PROCESS_ALT
//#define MBEDTLS_MD4_PROCESS_ALT

74
include/mbedtls/ecdsa.h
View file

@ -176,6 +176,67 @@ int mbedtls_ecdsa_sign( mbedtls_ecp_group *grp, mbedtls_mpi *r, mbedtls_mpi *s,
int (*f_rng)(void *, unsigned char *, size_t), void *p_rng );
#if defined(MBEDTLS_ECDSA_DETERMINISTIC)
#if ! defined(MBEDTLS_DEPRECATED_REMOVED)
#if defined(MBEDTLS_DEPRECATED_WARNING)
#define MBEDTLS_DEPRECATED __attribute__((deprecated))
#else
#define MBEDTLS_DEPRECATED
#endif
/**
* \brief This function computes the ECDSA signature of a
* previously-hashed message, deterministic version.
*
* For more information, see <em>RFC-6979: Deterministic
* Usage of the Digital Signature Algorithm (DSA) and Elliptic
* Curve Digital Signature Algorithm (ECDSA)</em>.
*
* \note If the bitlength of the message hash is larger than the
* bitlength of the group order, then the hash is truncated as
* defined in <em>Standards for Efficient Cryptography Group
* (SECG): SEC1 Elliptic Curve Cryptography</em>, section
* 4.1.3, step 5.
*
* \warning Since the output of the internal RNG is always the same for
* the same key and message, this limits the efficiency of
* blinding and leaks information through side channels. For
* secure behavior use mbedtls_ecdsa_sign_det_ext() instead.
*
* (Optimally the blinding is a random value that is different
* on every execution. In this case the blinding is still
* random from the attackers perspective, but is the same on
* each execution. This means that this blinding does not
* prevent attackers from recovering secrets by combining
* several measurement traces, but may prevent some attacks
* that exploit relationships between secret data.)
*
* \see ecp.h
*
* \param grp The context for the elliptic curve to use.
* This must be initialized and have group parameters
* set, for example through mbedtls_ecp_group_load().
* \param r The MPI context in which to store the first part
* the signature. This must be initialized.
* \param s The MPI context in which to store the second part
* the signature. This must be initialized.
* \param d The private signing key. This must be initialized
* and setup, for example through mbedtls_ecp_gen_privkey().
* \param buf The hashed content to be signed. This must be a readable
* buffer of length \p blen Bytes. It may be \c NULL if
* \p blen is zero.
* \param blen The length of \p buf in Bytes.
* \param md_alg The hash algorithm used to hash the original data.
*
* \return \c 0 on success.
* \return An \c MBEDTLS_ERR_ECP_XXX or \c MBEDTLS_MPI_XXX
* error code on failure.
*/
int mbedtls_ecdsa_sign_det( mbedtls_ecp_group *grp, mbedtls_mpi *r,
mbedtls_mpi *s, const mbedtls_mpi *d,
const unsigned char *buf, size_t blen,
mbedtls_md_type_t md_alg ) MBEDTLS_DEPRECATED;
#undef MBEDTLS_DEPRECATED
#endif /* MBEDTLS_DEPRECATED_REMOVED */
/**
* \brief This function computes the ECDSA signature of a
* previously-hashed message, deterministic version.
@ -206,15 +267,21 @@ int mbedtls_ecdsa_sign( mbedtls_ecp_group *grp, mbedtls_mpi *r, mbedtls_mpi *s,
* \p blen is zero.
* \param blen The length of \p buf in Bytes.
* \param md_alg The hash algorithm used to hash the original data.
* \param f_rng_blind The RNG function used for blinding. This must not be
* \c NULL.
* \param p_rng_blind The RNG context to be passed to \p f_rng. This may be
* \c NULL if \p f_rng doesn't need a context parameter.
*
* \return \c 0 on success.
* \return An \c MBEDTLS_ERR_ECP_XXX or \c MBEDTLS_MPI_XXX
* error code on failure.
*/
int mbedtls_ecdsa_sign_det( mbedtls_ecp_group *grp, mbedtls_mpi *r,
int mbedtls_ecdsa_sign_det_ext( mbedtls_ecp_group *grp, mbedtls_mpi *r,
mbedtls_mpi *s, const mbedtls_mpi *d,
const unsigned char *buf, size_t blen,
mbedtls_md_type_t md_alg );
mbedtls_md_type_t md_alg,
int (*f_rng_blind)(void *, unsigned char *, size_t),
void *p_rng_blind );
#endif /* MBEDTLS_ECDSA_DETERMINISTIC */
/**
@ -293,7 +360,8 @@ int mbedtls_ecdsa_verify( mbedtls_ecp_group *grp,
* the signature written. Must not be \c NULL.
* \param f_rng The RNG function. This must not be \c NULL if
* #MBEDTLS_ECDSA_DETERMINISTIC is unset. Otherwise,
* it is unused and may be set to \c NULL.
* it is used only for blinding and may be set to \c NULL, but
* doing so is DEPRECATED.
* \param p_rng The RNG context to be passed to \p f_rng. This may be
* \c NULL if \p f_rng is \c NULL or doesn't use a context.
*

119
library/ecdsa.c
View file

@ -254,6 +254,8 @@ static int ecdsa_sign_restartable( mbedtls_ecp_group *grp,
mbedtls_mpi *r, mbedtls_mpi *s,
const mbedtls_mpi *d, const unsigned char *buf, size_t blen,
int (*f_rng)(void *, unsigned char *, size_t), void *p_rng,
int (*f_rng_blind)(void *, unsigned char *, size_t),
void *p_rng_blind,
mbedtls_ecdsa_restart_ctx *rs_ctx )
{
int ret, key_tries, sign_tries;
@ -323,7 +325,9 @@ static int ecdsa_sign_restartable( mbedtls_ecp_group *grp,
mul:
#endif
MBEDTLS_MPI_CHK( mbedtls_ecp_mul_restartable( grp, &R, pk, &grp->G,
f_rng, p_rng, ECDSA_RS_ECP ) );
f_rng_blind,
p_rng_blind,
ECDSA_RS_ECP ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( pr, &R.X, &grp->N ) );
}
while( mbedtls_mpi_cmp_int( pr, 0 ) == 0 );
@ -349,7 +353,8 @@ modn:
* Generate a random value to blind inv_mod in next step,
* avoiding a potential timing leak.
*/
MBEDTLS_MPI_CHK( mbedtls_ecp_gen_privkey( grp, &t, f_rng, p_rng ) );
MBEDTLS_MPI_CHK( mbedtls_ecp_gen_privkey( grp, &t, f_rng_blind,
p_rng_blind ) );
/*
* Step 6: compute s = (e + r * d) / k = t (e + rd) / (kt) mod n
@ -406,8 +411,9 @@ int mbedtls_ecdsa_sign( mbedtls_ecp_group *grp, mbedtls_mpi *r, mbedtls_mpi *s,
ECDSA_VALIDATE_RET( f_rng != NULL );
ECDSA_VALIDATE_RET( buf != NULL || blen == 0 );
/* Use the same RNG for both blinding and ephemeral key generation */
return( ecdsa_sign_restartable( grp, r, s, d, buf, blen,
f_rng, p_rng, NULL ) );
f_rng, p_rng, f_rng, p_rng, NULL ) );
}
#endif /* !MBEDTLS_ECDSA_SIGN_ALT */
@ -419,6 +425,8 @@ static int ecdsa_sign_det_restartable( mbedtls_ecp_group *grp,
mbedtls_mpi *r, mbedtls_mpi *s,
const mbedtls_mpi *d, const unsigned char *buf, size_t blen,
mbedtls_md_type_t md_alg,
int (*f_rng_blind)(void *, unsigned char *, size_t),
void *p_rng_blind,
mbedtls_ecdsa_restart_ctx *rs_ctx )
{
int ret;
@ -465,8 +473,69 @@ sign:
ret = mbedtls_ecdsa_sign( grp, r, s, d, buf, blen,
mbedtls_hmac_drbg_random, p_rng );
#else
if( f_rng_blind != NULL )
ret = ecdsa_sign_restartable( grp, r, s, d, buf, blen,
mbedtls_hmac_drbg_random, p_rng, rs_ctx );
mbedtls_hmac_drbg_random, p_rng,
f_rng_blind, p_rng_blind, rs_ctx );
else
{
mbedtls_hmac_drbg_context *p_rng_blind_det;
#if !defined(MBEDTLS_ECP_RESTARTABLE)
/*
* To avoid reusing rng_ctx and risking incorrect behavior we seed a
* second HMAC-DRBG with the same seed. We also apply a label to avoid
* reusing the bits of the ephemeral key for blinding and eliminate the
* risk that they leak this way.
*/
const char* blind_label = "BLINDING CONTEXT";
mbedtls_hmac_drbg_context rng_ctx_blind;
mbedtls_hmac_drbg_init( &rng_ctx_blind );
p_rng_blind_det = &rng_ctx_blind;
mbedtls_hmac_drbg_seed_buf( p_rng_blind_det, md_info,
data, 2 * grp_len );
ret = mbedtls_hmac_drbg_update_ret( p_rng_blind_det,
(const unsigned char*) blind_label,
strlen( blind_label ) );
if( ret != 0 )
{
mbedtls_hmac_drbg_free( &rng_ctx_blind );
goto cleanup;
}
#else
/*
* In the case of restartable computations we would either need to store
* the second RNG in the restart context too or set it up at every
* restart. The first option would penalize the correct application of
* the function and the second would defeat the purpose of the
* restartable feature.
*
* Therefore in this case we reuse the original RNG. This comes with the
* price that the resulting signature might not be a valid deterministic
* ECDSA signature with a very low probability (same magnitude as
* successfully guessing the private key). However even then it is still
* a valid ECDSA signature.
*/
p_rng_blind_det = p_rng;
#endif /* MBEDTLS_ECP_RESTARTABLE */
/*
* Since the output of the RNGs is always the same for the same key and
* message, this limits the efficiency of blinding and leaks information
* through side channels. After mbedtls_ecdsa_sign_det() is removed NULL
* won't be a valid value for f_rng_blind anymore. Therefore it should
* be checked by the caller and this branch and check can be removed.
*/
ret = ecdsa_sign_restartable( grp, r, s, d, buf, blen,
mbedtls_hmac_drbg_random, p_rng,
mbedtls_hmac_drbg_random, p_rng_blind_det,
rs_ctx );
#if !defined(MBEDTLS_ECP_RESTARTABLE)
mbedtls_hmac_drbg_free( &rng_ctx_blind );
#endif
}
#endif /* MBEDTLS_ECDSA_SIGN_ALT */
cleanup:
@ -479,10 +548,13 @@ cleanup:
}
/*
* Deterministic signature wrapper
* Deterministic signature wrappers
*/
int mbedtls_ecdsa_sign_det( mbedtls_ecp_group *grp, mbedtls_mpi *r, mbedtls_mpi *s,
const mbedtls_mpi *d, const unsigned char *buf, size_t blen,
#if !defined(MBEDTLS_DEPRECATED_REMOVED)
int mbedtls_ecdsa_sign_det( mbedtls_ecp_group *grp, mbedtls_mpi *r,
mbedtls_mpi *s, const mbedtls_mpi *d,
const unsigned char *buf, size_t blen,
mbedtls_md_type_t md_alg )
{
ECDSA_VALIDATE_RET( grp != NULL );
@ -491,7 +563,28 @@ int mbedtls_ecdsa_sign_det( mbedtls_ecp_group *grp, mbedtls_mpi *r, mbedtls_mpi
ECDSA_VALIDATE_RET( d != NULL );
ECDSA_VALIDATE_RET( buf != NULL || blen == 0 );
return( ecdsa_sign_det_restartable( grp, r, s, d, buf, blen, md_alg, NULL ) );
return( ecdsa_sign_det_restartable( grp, r, s, d, buf, blen, md_alg,
NULL, NULL, NULL ) );
}
#endif /* MBEDTLS_DEPRECATED_REMOVED */
int mbedtls_ecdsa_sign_det_ext( mbedtls_ecp_group *grp, mbedtls_mpi *r,
mbedtls_mpi *s, const mbedtls_mpi *d,
const unsigned char *buf, size_t blen,
mbedtls_md_type_t md_alg,
int (*f_rng_blind)(void *, unsigned char *,
size_t),
void *p_rng_blind )
{
ECDSA_VALIDATE_RET( grp != NULL );
ECDSA_VALIDATE_RET( r != NULL );
ECDSA_VALIDATE_RET( s != NULL );
ECDSA_VALIDATE_RET( d != NULL );
ECDSA_VALIDATE_RET( buf != NULL || blen == 0 );
ECDSA_VALIDATE_RET( f_rng_blind != NULL );
return( ecdsa_sign_det_restartable( grp, r, s, d, buf, blen, md_alg,
f_rng_blind, p_rng_blind, NULL ) );
}
#endif /* MBEDTLS_ECDSA_DETERMINISTIC */
@ -670,11 +763,9 @@ int mbedtls_ecdsa_write_signature_restartable( mbedtls_ecdsa_context *ctx,
mbedtls_mpi_init( &s );
#if defined(MBEDTLS_ECDSA_DETERMINISTIC)
(void) f_rng;
(void) p_rng;
MBEDTLS_MPI_CHK( ecdsa_sign_det_restartable( &ctx->grp, &r, &s, &ctx->d,
hash, hlen, md_alg, rs_ctx ) );
hash, hlen, md_alg, f_rng,
p_rng, rs_ctx ) );
#else
(void) md_alg;
@ -682,8 +773,10 @@ int mbedtls_ecdsa_write_signature_restartable( mbedtls_ecdsa_context *ctx,
MBEDTLS_MPI_CHK( mbedtls_ecdsa_sign( &ctx->grp, &r, &s, &ctx->d,
hash, hlen, f_rng, p_rng ) );
#else
/* Use the same RNG for both blinding and ephemeral key generation */
MBEDTLS_MPI_CHK( ecdsa_sign_restartable( &ctx->grp, &r, &s, &ctx->d,
hash, hlen, f_rng, p_rng, rs_ctx ) );
hash, hlen, f_rng, p_rng, f_rng,
p_rng, rs_ctx ) );
#endif /* MBEDTLS_ECDSA_SIGN_ALT */
#endif /* MBEDTLS_ECDSA_DETERMINISTIC */

8
library/psa_crypto.c
View file

@ -3271,9 +3271,11 @@ static psa_status_t psa_ecdsa_sign( mbedtls_ecp_keypair *ecp,
psa_algorithm_t hash_alg = PSA_ALG_SIGN_GET_HASH( alg );
const mbedtls_md_info_t *md_info = mbedtls_md_info_from_psa( hash_alg );
mbedtls_md_type_t md_alg = mbedtls_md_get_type( md_info );
MBEDTLS_MPI_CHK( mbedtls_ecdsa_sign_det( &ecp->grp, &r, &s, &ecp->d,
hash, hash_length,
md_alg ) );
MBEDTLS_MPI_CHK( mbedtls_ecdsa_sign_det_ext( &ecp->grp, &r, &s,
&ecp->d, hash,
hash_length, md_alg,
mbedtls_ctr_drbg_random,
&global_data.ctr_drbg ) );
}
else
#endif /* MBEDTLS_ECDSA_DETERMINISTIC */

30
tests/suites/test_suite_ecdsa.function
View file

@ -55,25 +55,30 @@ void ecdsa_invalid_param( )
#if defined(MBEDTLS_ECDSA_DETERMINISTIC)
TEST_INVALID_PARAM_RET( MBEDTLS_ERR_ECP_BAD_INPUT_DATA,
mbedtls_ecdsa_sign_det( NULL, &m, &m, &m,
mbedtls_ecdsa_sign_det_ext( NULL, &m, &m, &m,
buf, sizeof( buf ),
valid_md ) );
valid_md,
rnd_std_rand, NULL ) );
TEST_INVALID_PARAM_RET( MBEDTLS_ERR_ECP_BAD_INPUT_DATA,
mbedtls_ecdsa_sign_det( &grp, NULL, &m, &m,
mbedtls_ecdsa_sign_det_ext( &grp, NULL, &m, &m,
buf, sizeof( buf ),
valid_md ) );
valid_md,
rnd_std_rand, NULL ) );
TEST_INVALID_PARAM_RET( MBEDTLS_ERR_ECP_BAD_INPUT_DATA,
mbedtls_ecdsa_sign_det( &grp, &m, NULL, &m,
mbedtls_ecdsa_sign_det_ext( &grp, &m, NULL, &m,
buf, sizeof( buf ),
valid_md ) );
valid_md,
rnd_std_rand, NULL ) );
TEST_INVALID_PARAM_RET( MBEDTLS_ERR_ECP_BAD_INPUT_DATA,
mbedtls_ecdsa_sign_det( &grp, &m, &m, NULL,
mbedtls_ecdsa_sign_det_ext( &grp, &m, &m, NULL,
buf, sizeof( buf ),
valid_md ) );
valid_md,
rnd_std_rand, NULL ) );
TEST_INVALID_PARAM_RET( MBEDTLS_ERR_ECP_BAD_INPUT_DATA,
mbedtls_ecdsa_sign_det( &grp, &m, &m, &m,
mbedtls_ecdsa_sign_det_ext( &grp, &m, &m, &m,
NULL, sizeof( buf ),
valid_md ) );
valid_md,
rnd_std_rand, NULL ) );
#endif /* MBEDTLS_ECDSA_DETERMINISTIC */
TEST_INVALID_PARAM_RET( MBEDTLS_ERR_ECP_BAD_INPUT_DATA,
@ -325,7 +330,10 @@ void ecdsa_det_test_vectors( int id, char * d_str, int md_alg, char * msg,
TEST_ASSERT( mbedtls_md( md_info, (const unsigned char *) msg,
strlen( msg ), hash ) == 0 );
TEST_ASSERT( mbedtls_ecdsa_sign_det( &grp, &r, &s, &d, hash, hlen, md_alg ) == 0 );
TEST_ASSERT(
mbedtls_ecdsa_sign_det_ext( &grp, &r, &s, &d, hash, hlen,
md_alg, rnd_std_rand, NULL )
== 0 );
TEST_ASSERT( mbedtls_mpi_cmp_mpi( &r, &r_check ) == 0 );
TEST_ASSERT( mbedtls_mpi_cmp_mpi( &s, &s_check ) == 0 );