DHM: blind call to mpi_inv_mod() on secret value
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/dhm.c b/library/dhm.c
index 7c30c84..5396deb 100644
--- a/library/dhm.c
+++ b/library/dhm.c
@@ -386,6 +386,9 @@
int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
{
int ret;
+ mbedtls_mpi R;
+
+ mbedtls_mpi_init( &R );
/*
* Don't use any blinding the first time a particular X is used,
@@ -422,11 +425,21 @@
/* Vi = random( 2, P-1 ) */
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 ) );
+ /* Vf = Vi^-X mod P
+ * First compute Vi^-1 = R * (R Vi)^-1, (avoiding leaks from inv_mod),
+ * then elevate to the Xth power. */
+ MBEDTLS_MPI_CHK( dhm_random_below( &R, &ctx->P, f_rng, p_rng ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &ctx->Vf, &ctx->Vi, &R ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->Vf, &ctx->Vf, &ctx->P ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_inv_mod( &ctx->Vf, &ctx->Vf, &ctx->P ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &ctx->Vf, &ctx->Vf, &R ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->Vf, &ctx->Vf, &ctx->P ) );
+
MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &ctx->Vf, &ctx->Vf, &ctx->X, &ctx->P, &ctx->RP ) );
cleanup:
+ mbedtls_mpi_free( &R );
+
return( ret );
}