Make memory access pattern constant
diff --git a/include/polarssl/bignum.h b/include/polarssl/bignum.h
index 6e2afac..f225fc3 100644
--- a/include/polarssl/bignum.h
+++ b/include/polarssl/bignum.h
@@ -246,7 +246,8 @@
* if( assign ) mpi_copy( X, Y );
* except that it avoids leaking any information about whether
* the assignment was done or not (the above code may leak
- * information through branch prediction analysis).
+ * information through branch prediction and/or memory access
+ * patterns analysis).
*/
int mpi_safe_cond_assign( mpi *X, mpi *Y, unsigned char assign );
diff --git a/include/polarssl/ecp.h b/include/polarssl/ecp.h
index 81b789e..33c09fc 100644
--- a/include/polarssl/ecp.h
+++ b/include/polarssl/ecp.h
@@ -463,15 +463,15 @@
* or P is not a valid pubkey,
* POLARSSL_ERR_MPI_MALLOC_FAILED if memory allocation failed
*
- * \note In order to prevent simple timing attacks, this function
- * executes a constant number of operations (that is, point
- * doubling and addition of distinct points) for random m in
- * the allowed range.
+ * \note In order to prevent timing attacks, this function
+ * executes the exact same sequence of (base field)
+ * operations for any valid m. It avoids any if-branch or
+ * array index depending on the value of m.
*
* \note If f_rng is not NULL, it is used to randomize intermediate
- * results in order to prevent potential attacks targetting
- * these results. It is recommended to always provide a
- * non-NULL f_rng (the overhead is negligible).
+ * results in order to prevent potential timing attacks
+ * targetting these results. It is recommended to always
+ * provide a non-NULL f_rng (the overhead is negligible).
*/
int ecp_mul( ecp_group *grp, ecp_point *R,
const mpi *m, const ecp_point *P,
diff --git a/library/ecp.c b/library/ecp.c
index 9f1d3ce..756beba 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -1385,14 +1385,23 @@
* Select precomputed point: R = sign(i) * T[ abs(i) / 2 ]
*/
static int ecp_select_comb( const ecp_group *grp, ecp_point *R,
- const ecp_point T[], unsigned char i )
+ ecp_point T[], unsigned char t_len,
+ unsigned char i )
{
int ret;
+ unsigned char ii, j;
- /* Ignore the "sign" bit */
- MPI_CHK( ecp_copy( R, &T[ ( i & 0x7Fu ) >> 1 ] ) );
+ /* Ignore the "sign" bit and scale down */
+ ii = ( i & 0x7Fu ) >> 1;
- /* Restore the Z coordinate */
+ /* Read the whole table to thwart cache-based timing attacks */
+ for( j = 0; j < t_len; j++ )
+ {
+ MPI_CHK( mpi_safe_cond_assign( &R->X, &T[j].X, j == ii ) );
+ MPI_CHK( mpi_safe_cond_assign( &R->Y, &T[j].Y, j == ii ) );
+ }
+
+ /* The Z coordinate is always 1 */
MPI_CHK( mpi_lset( &R->Z, 1 ) );
/* Safely invert result if i is "negative" */
@@ -1409,7 +1418,7 @@
* Cost: d A + d D + 1 R
*/
static int ecp_mul_comb_core( const ecp_group *grp, ecp_point *R,
- const ecp_point T[],
+ ecp_point T[], unsigned char t_len,
const unsigned char x[], size_t d,
int (*f_rng)(void *, unsigned char *, size_t),
void *p_rng )
@@ -1422,14 +1431,14 @@
/* Start with a non-zero point and randomize its coordinates */
i = d;
- MPI_CHK( ecp_select_comb( grp, R, T, x[i] ) );
+ MPI_CHK( ecp_select_comb( grp, R, T, t_len, x[i] ) );
if( f_rng != 0 )
MPI_CHK( ecp_randomize_coordinates( grp, R, f_rng, p_rng ) );
while( i-- != 0 )
{
MPI_CHK( ecp_double_jac( grp, R, R ) );
- MPI_CHK( ecp_select_comb( grp, &Txi, T, x[i] ) );
+ MPI_CHK( ecp_select_comb( grp, &Txi, T, t_len, x[i] ) );
MPI_CHK( ecp_add_mixed( grp, R, R, &Txi ) );
}
@@ -1447,8 +1456,8 @@
int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
{
int ret;
- unsigned char w, m_is_odd, p_eq_g;
- size_t pre_len, d, i;
+ unsigned char w, m_is_odd, p_eq_g, pre_len, i;
+ size_t d;
unsigned char k[COMB_MAX_D + 1];
ecp_point *T;
mpi M, mm;
@@ -1542,7 +1551,7 @@
* Go for comb multiplication, R = M * P
*/
ecp_comb_fixed( k, d, w, &M );
- ecp_mul_comb_core( grp, R, T, k, d, f_rng, p_rng );
+ MPI_CHK( ecp_mul_comb_core( grp, R, T, pre_len, k, d, f_rng, p_rng ) );
/*
* Now get m * P from M * P and normalize it