DHM: make drawing of blinding value a function
In the next commit, we'll need to draw a second random value, in order to
blind modular inversion. Having a function for that will avoid repetition.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/dhm.c b/library/dhm.c
index d14b729..6938e3d 100644
--- a/library/dhm.c
+++ b/library/dhm.c
@@ -336,6 +336,32 @@
}
/*
+ * Pick a random R in the range [2, M) for blinding purposes
+ */
+static int dhm_random_below( mbedtls_mpi *R, const mbedtls_mpi *M,
+ int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
+{
+ int ret, count;
+
+ count = 0;
+ do
+ {
+ MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( R, mbedtls_mpi_size( M ), f_rng, p_rng ) );
+
+ while( mbedtls_mpi_cmp_mpi( R, M ) >= 0 )
+ MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( R, 1 ) );
+
+ if( count++ > 10 )
+ return( MBEDTLS_ERR_MPI_NOT_ACCEPTABLE );
+ }
+ while( mbedtls_mpi_cmp_int( R, 1 ) <= 0 );
+
+cleanup:
+ return( ret );
+}
+
+
+/*
* Use the blinding method and optimisation suggested in section 10 of:
* KOCHER, Paul C. Timing attacks on implementations of Diffie-Hellman, RSA,
* DSS, and other systems. In : Advances in Cryptology-CRYPTO'96. Springer
@@ -344,7 +370,7 @@
static int dhm_update_blinding( mbedtls_dhm_context *ctx,
int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
{
- int ret, count;
+ int ret;
/*
* Don't use any blinding the first time a particular X is used,
@@ -379,18 +405,7 @@
*/
/* Vi = random( 2, P-1 ) */
- count = 0;
- do
- {
- MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &ctx->Vi, mbedtls_mpi_size( &ctx->P ), f_rng, p_rng ) );
-
- while( mbedtls_mpi_cmp_mpi( &ctx->Vi, &ctx->P ) >= 0 )
- MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( &ctx->Vi, 1 ) );
-
- if( count++ > 10 )
- return( MBEDTLS_ERR_MPI_NOT_ACCEPTABLE );
- }
- while( mbedtls_mpi_cmp_int( &ctx->Vi, 1 ) <= 0 );
+ MBEDTLS_MPI_CHK( dhm_random_below( &ctx->Vi, &ctx->P, f_rng, p_rng ) );
/* Vf = Vi^-X mod P */
MBEDTLS_MPI_CHK( mbedtls_mpi_inv_mod( &ctx->Vf, &ctx->Vi, &ctx->P ) );