Merge remote-tracking branch 'upstream-restricted/pr/549' into mbedtls-2.7-restricted
diff --git a/ChangeLog b/ChangeLog
index 677632b..ea652ee 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,18 @@
* 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.
+ * The new function mbedtls_ecp_gen_privkey() allows to generate a private
+ key without generating the public part of the pair.
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 75d5eb1..fa88ebf 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -333,6 +333,16 @@
* 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 cfd1370..08811a3 100644
--- a/include/mbedtls/ecdsa.h
+++ b/include/mbedtls/ecdsa.h
@@ -107,6 +107,20 @@
* Usage of the Digital Signature Algorithm (DSA) and Elliptic
* Curve Digital Signature Algorithm (ECDSA)</em>.
*
+ *
+ * \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.)
+ *
* \param grp The ECP group.
* \param r The first output integer.
* \param s The second output integer.
@@ -127,9 +141,56 @@
*
* \see ecp.h
*/
-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 );
+/**
+ * \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/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index 6c43c00..691415e 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -602,6 +602,22 @@
int mbedtls_ecp_check_privkey( const mbedtls_ecp_group *grp, const mbedtls_mpi *d );
/**
+ * \brief Generate a private key
+ *
+ * \param grp ECP group
+ * \param d Destination MPI (secret part)
+ * \param f_rng RNG function
+ * \param p_rng RNG parameter
+ *
+ * \return 0 if successful,
+ * or a MBEDTLS_ERR_ECP_XXX or MBEDTLS_MPI_XXX error code
+ */
+int mbedtls_ecp_gen_privkey( const mbedtls_ecp_group *grp,
+ mbedtls_mpi *d,
+ int (*f_rng)(void *, unsigned char *, size_t),
+ void *p_rng );
+
+/**
* \brief Generate a keypair with configurable base point
*
* \param grp ECP group
diff --git a/library/ecdsa.c b/library/ecdsa.c
index ab75620..c635a50 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -70,9 +70,14 @@
* Compute ECDSA signature of a hashed message (SEC1 4.1.3)
* Obviously, compared to SEC1 4.1.3, we skip step 4 (hash message)
*/
-int mbedtls_ecdsa_sign( 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 )
+static int ecdsa_sign_internal( 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 )
{
int ret, key_tries, sign_tries, blind_tries;
mbedtls_ecp_point R;
@@ -99,7 +104,10 @@
key_tries = 0;
do
{
- MBEDTLS_MPI_CHK( mbedtls_ecp_gen_keypair( grp, &k, &R, f_rng, p_rng ) );
+ MBEDTLS_MPI_CHK( mbedtls_ecp_gen_privkey( grp, &k, f_rng, p_rng ) );
+
+ MBEDTLS_MPI_CHK( mbedtls_ecp_mul( grp, &R, &k, &grp->G,
+ f_rng_blind, p_rng_blind ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( r, &R.X, &grp->N ) );
if( key_tries++ > 10 )
@@ -118,15 +126,20 @@
/*
* Generate a random value to blind inv_mod in next step,
* avoiding a potential timing leak.
+ *
+ * This loop does the same job as mbedtls_ecp_gen_privkey() and it is
+ * replaced by a call to it in the mainline. This change is not
+ * necessary to backport the fix separating the blinding and ephemeral
+ * key generating RNGs, therefore the original code is kept.
*/
blind_tries = 0;
do
{
size_t n_size = ( grp->nbits + 7 ) / 8;
- MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &t, n_size, f_rng, p_rng ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &t, n_size, f_rng_blind,
+ p_rng_blind ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( &t, 8 * n_size - grp->nbits ) );
- /* See mbedtls_ecp_gen_keypair() */
if( ++blind_tries > 30 )
return( MBEDTLS_ERR_ECP_RANDOM_FAILED );
}
@@ -158,15 +171,27 @@
return( ret );
}
+
+int mbedtls_ecdsa_sign( 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 )
+{
+ /* Use the same RNG for both blinding and ephemeral key generation */
+ return( ecdsa_sign_internal( grp, r, s, d, buf, blen, f_rng, p_rng,
+ f_rng, p_rng ) );
+}
#endif /* MBEDTLS_ECDSA_SIGN_ALT */
#if defined(MBEDTLS_ECDSA_DETERMINISTIC)
-/*
- * Deterministic signature wrapper
- */
-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 )
+static int ecdsa_sign_det_internal( 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 )
{
int ret;
mbedtls_hmac_drbg_context rng_ctx;
@@ -174,12 +199,16 @@
size_t grp_len = ( grp->nbits + 7 ) / 8;
const mbedtls_md_info_t *md_info;
mbedtls_mpi h;
+ /* Variables for deterministic blinding fallback */
+ const char* blind_label = "BLINDING CONTEXT";
+ mbedtls_hmac_drbg_context rng_ctx_blind;
if( ( md_info = mbedtls_md_info_from_type( md_alg ) ) == NULL )
return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );
mbedtls_mpi_init( &h );
mbedtls_hmac_drbg_init( &rng_ctx );
+ mbedtls_hmac_drbg_init( &rng_ctx_blind );
/* Use private key and message hash (reduced) to initialize HMAC_DRBG */
MBEDTLS_MPI_CHK( mbedtls_mpi_write_binary( d, data, grp_len ) );
@@ -187,15 +216,71 @@
MBEDTLS_MPI_CHK( mbedtls_mpi_write_binary( &h, data + grp_len, grp_len ) );
mbedtls_hmac_drbg_seed_buf( &rng_ctx, md_info, data, 2 * grp_len );
- ret = mbedtls_ecdsa_sign( grp, r, s, d, buf, blen,
- mbedtls_hmac_drbg_random, &rng_ctx );
+ if( f_rng_blind != NULL )
+ ret = ecdsa_sign_internal( grp, r, s, d, buf, blen,
+ mbedtls_hmac_drbg_random, &rng_ctx,
+ f_rng_blind, p_rng_blind );
+ else
+ {
+ /*
+ * 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.
+ */
+
+ mbedtls_hmac_drbg_seed_buf( &rng_ctx_blind, md_info,
+ data, 2 * grp_len );
+ ret = mbedtls_hmac_drbg_update_ret( &rng_ctx_blind,
+ (const unsigned char*) blind_label,
+ strlen( blind_label ) );
+ if( ret != 0 )
+ goto cleanup;
+
+ /*
+ * 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_internal( grp, r, s, d, buf, blen,
+ mbedtls_hmac_drbg_random, &rng_ctx,
+ mbedtls_hmac_drbg_random, &rng_ctx_blind );
+
+ }
cleanup:
mbedtls_hmac_drbg_free( &rng_ctx );
+ mbedtls_hmac_drbg_free( &rng_ctx_blind );
mbedtls_mpi_free( &h );
return( ret );
}
+
+/*
+ * 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 )
+{
+ return( ecdsa_sign_det_internal( grp, r, s, d, buf, blen, md_alg,
+ 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 )
+{
+ return( ecdsa_sign_det_internal( grp, r, s, d, buf, blen, md_alg,
+ f_rng_blind, p_rng_blind ) );
+}
#endif /* MBEDTLS_ECDSA_DETERMINISTIC */
#if !defined(MBEDTLS_ECDSA_VERIFY_ALT)
@@ -326,17 +411,15 @@
mbedtls_mpi_init( &s );
#if defined(MBEDTLS_ECDSA_DETERMINISTIC)
- (void) f_rng;
- (void) p_rng;
-
- MBEDTLS_MPI_CHK( mbedtls_ecdsa_sign_det( &ctx->grp, &r, &s, &ctx->d,
- hash, hlen, md_alg ) );
+ MBEDTLS_MPI_CHK( ecdsa_sign_det_internal( &ctx->grp, &r, &s, &ctx->d,
+ hash, hlen, md_alg,
+ f_rng, p_rng ) );
#else
(void) md_alg;
MBEDTLS_MPI_CHK( mbedtls_ecdsa_sign( &ctx->grp, &r, &s, &ctx->d,
hash, hlen, f_rng, p_rng ) );
-#endif
+#endif /* MBEDTLS_ECDSA_DETERMINISTIC */
MBEDTLS_MPI_CHK( ecdsa_signature_to_asn1( &r, &s, sig, slen ) );
diff --git a/library/ecp.c b/library/ecp.c
index 75233f8..cb8e947 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -1918,15 +1918,14 @@
}
/*
- * Generate a keypair with configurable base point
+ * Generate a private key
*/
-int mbedtls_ecp_gen_keypair_base( mbedtls_ecp_group *grp,
- const mbedtls_ecp_point *G,
- mbedtls_mpi *d, mbedtls_ecp_point *Q,
+int mbedtls_ecp_gen_privkey( const mbedtls_ecp_group *grp,
+ mbedtls_mpi *d,
int (*f_rng)(void *, unsigned char *, size_t),
void *p_rng )
{
- int ret;
+ int ret = MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
size_t n_size = ( grp->nbits + 7 ) / 8;
#if defined(ECP_MONTGOMERY)
@@ -1951,8 +1950,8 @@
MBEDTLS_MPI_CHK( mbedtls_mpi_set_bit( d, 1, 0 ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_set_bit( d, 2, 0 ) );
}
- else
#endif /* ECP_MONTGOMERY */
+
#if defined(ECP_SHORTWEIERSTRASS)
if( ecp_get_type( grp ) == ECP_TYPE_SHORT_WEIERSTRASS )
{
@@ -1986,15 +1985,28 @@
while( mbedtls_mpi_cmp_int( d, 1 ) < 0 ||
mbedtls_mpi_cmp_mpi( d, &grp->N ) >= 0 );
}
- else
#endif /* ECP_SHORTWEIERSTRASS */
- return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );
cleanup:
- if( ret != 0 )
- return( ret );
+ return( ret );
+}
- return( mbedtls_ecp_mul( grp, Q, d, G, f_rng, p_rng ) );
+/*
+ * Generate a keypair with configurable base point
+ */
+int mbedtls_ecp_gen_keypair_base( mbedtls_ecp_group *grp,
+ const mbedtls_ecp_point *G,
+ mbedtls_mpi *d, mbedtls_ecp_point *Q,
+ int (*f_rng)(void *, unsigned char *, size_t),
+ void *p_rng )
+{
+ int ret;
+
+ MBEDTLS_MPI_CHK( mbedtls_ecp_gen_privkey( grp, d, f_rng, p_rng ) );
+ MBEDTLS_MPI_CHK( mbedtls_ecp_mul( grp, Q, d, G, f_rng, p_rng ) );
+
+cleanup:
+ return( ret );
}
/*
diff --git a/tests/suites/test_suite_ecdsa.function b/tests/suites/test_suite_ecdsa.function
index 9b1315f..74fc49b 100644
--- a/tests/suites/test_suite_ecdsa.function
+++ b/tests/suites/test_suite_ecdsa.function
@@ -129,6 +129,16 @@
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 );