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Add a safer deterministic ECDSA function

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`mbedtls_ecdsa_sign_det` reuses the internal HMAC-DRBG instance to
implement blinding. The advantage of this is that the algorithm is
deterministic too, not just the resulting signature. The drawback is
that the blinding is always the same for the same key and message.
This diminishes the efficiency of blinding and leaks information about
the private key.

A function that takes external randomness fixes this weakness.
This commit is contained in:
Janos Follath 2019-01-04 14:32:30 +00:00
parent 799cd57c72
commit f1713e96c9
2 changed files with 115 additions and 15 deletions
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59
include/mbedtls/ecdsa.h
View File

@ -175,6 +175,19 @@ int mbedtls_ecdsa_sign( mbedtls_ecp_group *grp, mbedtls_mpi *r, mbedtls_mpi *s,
* (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 @@ 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 */
/**

65
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
@ -392,8 +397,9 @@ 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 */
@ -405,6 +411,8 @@ 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;
@ -451,7 +459,19 @@ 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,
f_rng_blind, p_rng_blind, rs_ctx );
else
/*
* Use the same RNG for both blinding and ephemeral key generation.
* Since the RNG output is always the same for the same key and message,
* this limits the efficiency of blinding and leaks information through
* side channels.
*/
ret = ecdsa_sign_restartable( grp, r, s, d, buf, blen,
mbedtls_hmac_drbg_random, p_rng,
mbedtls_hmac_drbg_random, p_rng, rs_ctx );
#endif /* MBEDTLS_ECDSA_SIGN_ALT */
@ -465,10 +485,11 @@ 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,
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 );
@ -477,7 +498,27 @@ 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 ) );
}
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 +697,9 @@ 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;
@ -668,8 +707,10 @@ 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 */