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Remove potential timing leak in ecdsa_sign()

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Manuel Pégourié-Gonnard 2014-03-31 11:55:42 +02:00
parent 6b0d268bc9
commit dd75c3183b
3 changed files with 32 additions and 7 deletions
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4
ChangeLog
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@ -2,6 +2,10 @@ PolarSSL ChangeLog (Sorted per branch, date)
= PolarSSL 1.3 branch
Security
* Avoid potential timing leak in ecdsa_sign() by blinding modular division.
(Found by Watson Ladd.)
Bugfix
* The length of various ClientKeyExchange messages was not properly checked.
* Some example server programs were not sending the close_notify alert.

3
include/polarssl/ecp.h
View file

@ -118,6 +118,9 @@ ecp_point;
* short weierstrass, this subgroup is actually the whole curve, and its
* cardinal is denoted by N.
*
* In the case of Short Weierstrass curves, our code requires that N is an odd
* prime. (Use odd in ecp_mul() and prime in ecdsa_sign() for blinding.)
*
* In the case of Montgomery curves, we don't store A but (A + 2) / 4 which is
* the quantity actualy used in the formulas. Also, nbits is not the size of N
* but the required size for private keys.

32
library/ecdsa.c
View file

@ -99,17 +99,16 @@ int ecdsa_sign( ecp_group *grp, mpi *r, mpi *s,
const mpi *d, const unsigned char *buf, size_t blen,
int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
{
int ret, key_tries, sign_tries;
int ret, key_tries, sign_tries, blind_tries;
ecp_point R;
mpi k, e;
mpi k, e, t;
/* Fail cleanly on curves such as Curve25519 that can't be used for ECDSA */
if( grp->N.p == NULL )
return( POLARSSL_ERR_ECP_BAD_INPUT_DATA );
ecp_point_init( &R );
mpi_init( &k );
mpi_init( &e );
mpi_init( &k ); mpi_init( &e ); mpi_init( &t );
sign_tries = 0;
do
@ -138,10 +137,30 @@ int ecdsa_sign( ecp_group *grp, mpi *r, mpi *s,
MPI_CHK( derive_mpi( grp, &e, buf, blen ) );
/*
* Step 6: compute s = (e + r * d) / k mod n
* Generate a random value to blind inv_mod in next step,
* avoiding a potential timing leak.
*/
blind_tries = 0;
do
{
size_t n_size = (grp->nbits + 7) / 8;
MPI_CHK( mpi_fill_random( &t, n_size, f_rng, p_rng ) );
MPI_CHK( mpi_shift_r( &t, 8 * n_size - grp->nbits ) );
/* See ecp_gen_keypair() */
if( ++blind_tries > 30 )
return( POLARSSL_ERR_ECP_RANDOM_FAILED );
}
while( mpi_cmp_int( &t, 1 ) < 0 ||
mpi_cmp_mpi( &t, &grp->N ) >= 0 );
/*
* Step 6: compute s = (e + r * d) / k = t (e + rd) / (kt) mod n
*/
MPI_CHK( mpi_mul_mpi( s, r, d ) );
MPI_CHK( mpi_add_mpi( &e, &e, s ) );
MPI_CHK( mpi_mul_mpi( &e, &e, &t ) );
MPI_CHK( mpi_mul_mpi( &k, &k, &t ) );
MPI_CHK( mpi_inv_mod( s, &k, &grp->N ) );
MPI_CHK( mpi_mul_mpi( s, s, &e ) );
MPI_CHK( mpi_mod_mpi( s, s, &grp->N ) );
@ -156,8 +175,7 @@ int ecdsa_sign( ecp_group *grp, mpi *r, mpi *s,
cleanup:
ecp_point_free( &R );
mpi_free( &k );
mpi_free( &e );
mpi_free( &k ); mpi_free( &e ); mpi_free( &t );
return( ret );
}