mpi: harden set_cond functions against EM leakage

* mpi/mpih-const-time.c (_gcry_mpih_set_cond): Use two masks for
selecting output.
* mpi/mpiutil.c (_gcry_mpi_set_cond): Use two masks for selecting
output.
--

GnuPG-bug-id: T5330
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>

2 files changed, 18 insertions, 12 deletions

diff --git a/mpi/mpih-const-time.c b/mpi/mpih-const-time.c
index 8b78aabc..3a69e6ba 100644
--- a/mpi/mpih-const-time.c
+++ b/mpi/mpih-const-time.c
@@ -40,13 +40,12 @@ _gcry_mpih_set_cond (mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize,
                      unsigned long op_enable)
 {
   mpi_size_t i;
-  mpi_limb_t mask = ((mpi_limb_t)0) - op_enable;
-  mpi_limb_t x;
+  mpi_limb_t mask1 = vzero - op_enable;
+  mpi_limb_t mask2 = op_enable - vone;
 
   for (i = 0; i < usize; i++)
     {
-      x = mask & (wp[i] ^ up[i]);
-      wp[i] = wp[i] ^ x;
+      wp[i] = (wp[i] & mask2) | (up[i] & mask1);
     }
 }
 
diff --git a/mpi/mpiutil.c b/mpi/mpiutil.c
index a1ac1c43..5320f4d8 100644
--- a/mpi/mpiutil.c
+++ b/mpi/mpiutil.c
@@ -519,23 +519,30 @@ _gcry_mpi_set_cond (gcry_mpi_t w, const gcry_mpi_t u, unsigned long set)
 {
   mpi_size_t i;
   mpi_size_t nlimbs = u->alloced;
-  mpi_limb_t mask = ((mpi_limb_t)0) - set;
-  mpi_limb_t x;
+  mpi_limb_t mask1 = vzero - set;
+  mpi_limb_t mask2 = set - vone;
+  mpi_limb_t xu;
+  mpi_limb_t xw;
+  mpi_limb_t *uu = u->d;
+  mpi_limb_t *uw = w->d;
 
   if (w->alloced != u->alloced)
     log_bug ("mpi_set_cond: different sizes\n");
 
   for (i = 0; i < nlimbs; i++)
     {
-      x = mask & (w->d[i] ^ u->d[i]);
-      w->d[i] = w->d[i] ^ x;
+      xu = uu[i];
+      xw = uw[i];
+      uw[i] = (xw & mask2) | (xu & mask1);
     }
 
-  x = mask & (w->nlimbs ^ u->nlimbs);
-  w->nlimbs = w->nlimbs ^ x;
+  xu = u->nlimbs;
+  xw = w->nlimbs;
+  w->nlimbs = (xw & mask2) | (xu & mask1);
 
-  x = mask & (w->sign ^ u->sign);
-  w->sign = w->sign ^ x;
+  xu = u->sign;
+  xw = w->sign;
+  w->sign = (xw & mask2) | (xu & mask1);
   return w;
 }