Merge remote-tracking branch 'upstream-restricted/pr/556' into mbedtls-2.16-restricted
diff --git a/ChangeLog b/ChangeLog
index 5e89480..2a87665 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,16 @@
* Fix a missing error detection in ECJPAKE. This could have caused a
predictable shared secret if a hardware accelerator failed and the other
side of the key exchange had a similar bug.
+ * The deterministic ECDSA calculation reused the scheme's HMAC-DRBG to
+ implement blinding. Because of this for the same key and message the same
+ blinding value was generated. This reduced the effectiveness of the
+ countermeasure and leaked information about the private key through side
+ channels. Reported by Jack Lloyd.
+
+API Changes
+ * The new function mbedtls_ecdsa_sign_det_ext() is similar to
+ mbedtls_ecdsa_sign_det() but allows passing an external RNG for the
+ purpose of blinding.
Bugfix
* Fix to allow building test suites with any warning that detects unused
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 6f523be..5df962e 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -440,6 +440,16 @@
* 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 f8b2850..932acc6 100644
--- a/include/mbedtls/ecdsa.h
+++ b/include/mbedtls/ecdsa.h
@@ -175,6 +175,19 @@
* (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.
@@ -200,6 +213,52 @@
mbedtls_mpi *s, const mbedtls_mpi *d,
const unsigned char *buf, size_t blen,
mbedtls_md_type_t md_alg );
+/**
+ * \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.
+ *
+ * \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 */
/**
diff --git a/library/ecdsa.c b/library/ecdsa.c
index dc19384..c5b8df9 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -254,6 +254,8 @@
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 @@
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 @@
* 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
@@ -392,8 +397,9 @@
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 */
@@ -405,6 +411,8 @@
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;
@@ -451,8 +459,70 @@
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:
@@ -465,11 +535,12 @@
}
/*
- * 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 )
+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 );
@@ -477,7 +548,27 @@
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 ) );
+}
+
+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 */
@@ -656,11 +747,9 @@
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;
@@ -668,8 +757,10 @@
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/tests/suites/test_suite_ecdsa.function b/tests/suites/test_suite_ecdsa.function
index 22d92b6..0e7283b 100644
--- a/tests/suites/test_suite_ecdsa.function
+++ b/tests/suites/test_suite_ecdsa.function
@@ -74,6 +74,31 @@
mbedtls_ecdsa_sign_det( &grp, &m, &m, &m,
NULL, sizeof( buf ),
valid_md ) );
+ TEST_INVALID_PARAM_RET( MBEDTLS_ERR_ECP_BAD_INPUT_DATA,
+ 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_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_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_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_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,
@@ -330,6 +355,16 @@
TEST_ASSERT( mbedtls_mpi_cmp_mpi( &r, &r_check ) == 0 );
TEST_ASSERT( mbedtls_mpi_cmp_mpi( &s, &s_check ) == 0 );
+ mbedtls_mpi_free( &r ); mbedtls_mpi_free( &s );
+ mbedtls_mpi_init( &r ); mbedtls_mpi_init( &s );
+
+ 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 );
exit:
mbedtls_ecp_group_free( &grp );
mbedtls_mpi_free( &d ); mbedtls_mpi_free( &r ); mbedtls_mpi_free( &s );