diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 3c1430ce9..e14fc74d7 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -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
diff --git a/include/mbedtls/ecdsa.h b/include/mbedtls/ecdsa.h
index ad5118814..b009e7345 100644
--- a/include/mbedtls/ecdsa.h
+++ b/include/mbedtls/ecdsa.h
@@ -176,6 +176,12 @@ 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.
@@ -190,6 +196,19 @@ int mbedtls_ecdsa_sign( mbedtls_ecp_group *grp, mbedtls_mpi *r, mbedtls_mpi *s,
* (SECG): SEC1 Elliptic Curve Cryptography, 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.
@@ -214,7 +233,55 @@ int mbedtls_ecdsa_sign( mbedtls_ecp_group *grp, mbedtls_mpi *r, mbedtls_mpi *s,
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_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.
+ *
+ * For more information, see RFC-6979: Deterministic
+ * Usage of the Digital Signature Algorithm (DSA) and Elliptic
+ * Curve Digital Signature Algorithm (ECDSA).
+ *
+ * \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 Standards for Efficient Cryptography Group
+ * (SECG): SEC1 Elliptic Curve Cryptography, section
+ * 4.1.3, step 5.
+ *
+ * \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.
+ * \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_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 );
#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.
*
diff --git a/library/ecdsa.c b/library/ecdsa.c
index 5c3038048..bda9262c9 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -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
- ret = ecdsa_sign_restartable( grp, r, s, d, buf, blen,
- mbedtls_hmac_drbg_random, p_rng, rs_ctx );
+ if( f_rng_blind != NULL )
+ ret = ecdsa_sign_restartable( grp, r, s, d, buf, blen,
+ 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:
@@ -479,11 +548,14 @@ 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,
- mbedtls_md_type_t md_alg )
+
+#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 );
ECDSA_VALIDATE_RET( r != 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 */
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index ef2d50e62..a80f13de3 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -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 */
diff --git a/tests/suites/test_suite_ecdsa.function b/tests/suites/test_suite_ecdsa.function
index 22d92b6df..ab3db3adf 100644
--- a/tests/suites/test_suite_ecdsa.function
+++ b/tests/suites/test_suite_ecdsa.function
@@ -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,
- buf, sizeof( buf ),
- valid_md ) );
+ mbedtls_ecdsa_sign_det_ext( NULL, &m, &m, &m,
+ buf, sizeof( buf ),
+ valid_md,
+ rnd_std_rand, NULL ) );
TEST_INVALID_PARAM_RET( MBEDTLS_ERR_ECP_BAD_INPUT_DATA,
- mbedtls_ecdsa_sign_det( &grp, NULL, &m, &m,
- buf, sizeof( buf ),
- valid_md ) );
+ mbedtls_ecdsa_sign_det_ext( &grp, NULL, &m, &m,
+ buf, sizeof( buf ),
+ valid_md,
+ rnd_std_rand, NULL ) );
TEST_INVALID_PARAM_RET( MBEDTLS_ERR_ECP_BAD_INPUT_DATA,
- mbedtls_ecdsa_sign_det( &grp, &m, NULL, &m,
- buf, sizeof( buf ),
- valid_md ) );
+ mbedtls_ecdsa_sign_det_ext( &grp, &m, NULL, &m,
+ buf, sizeof( buf ),
+ valid_md,
+ rnd_std_rand, NULL ) );
TEST_INVALID_PARAM_RET( MBEDTLS_ERR_ECP_BAD_INPUT_DATA,
- mbedtls_ecdsa_sign_det( &grp, &m, &m, NULL,
- buf, sizeof( buf ),
- valid_md ) );
+ mbedtls_ecdsa_sign_det_ext( &grp, &m, &m, NULL,
+ buf, sizeof( buf ),
+ valid_md,
+ rnd_std_rand, NULL ) );
TEST_INVALID_PARAM_RET( MBEDTLS_ERR_ECP_BAD_INPUT_DATA,
- mbedtls_ecdsa_sign_det( &grp, &m, &m, &m,
- NULL, sizeof( buf ),
- valid_md ) );
+ mbedtls_ecdsa_sign_det_ext( &grp, &m, &m, &m,
+ NULL, sizeof( buf ),
+ 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 );