mpi: harden swap_cond functions against EM leakage

* mpi/mpih-const-time.c (vzero, vone): New.
(_gcry_mpih_swap_cond): Use two masks for selecting output.
* mpi/mpiutil.c (vzero, vone): New.
(_gcry_mpi_swap_cond): Use to masks for selecting output.
--

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

1 files changed, 23 insertions, 11 deletions

diff --git a/mpi/mpiutil.c b/mpi/mpiutil.c
index 86b8361e..a1ac1c43 100644
--- a/mpi/mpiutil.c
+++ b/mpi/mpiutil.c
@@ -46,6 +46,11 @@
 /* Constants allocated right away at startup.  */
 static gcry_mpi_t constants[MPI_NUMBER_OF_CONSTANTS];
 
+/* These variables are used to generate masks from conditional operation
+ * flag parameters.  Use of volatile prevents compiler optimizations from
+ * converting AND-masking to conditional branches.  */
+static volatile mpi_limb_t vzero = 0;
+static volatile mpi_limb_t vone = 1;
 
 
 const char *
@@ -608,8 +613,12 @@ _gcry_mpi_swap_cond (gcry_mpi_t a, gcry_mpi_t b, unsigned long swap)
 {
   mpi_size_t i;
   mpi_size_t nlimbs;
-  mpi_limb_t mask = ((mpi_limb_t)0) - swap;
-  mpi_limb_t x;
+  mpi_limb_t mask1 = vzero - swap;
+  mpi_limb_t mask2 = swap - vone;
+  mpi_limb_t *ua = a->d;
+  mpi_limb_t *ub = b->d;
+  mpi_limb_t xa;
+  mpi_limb_t xb;
 
   if (a->alloced > b->alloced)
     nlimbs = b->alloced;
@@ -620,18 +629,21 @@ _gcry_mpi_swap_cond (gcry_mpi_t a, gcry_mpi_t b, unsigned long swap)
 
   for (i = 0; i < nlimbs; i++)
     {
-      x = mask & (a->d[i] ^ b->d[i]);
-      a->d[i] = a->d[i] ^ x;
-      b->d[i] = b->d[i] ^ x;
+      xa = ua[i];
+      xb = ub[i];
+      ua[i] = (xa & mask2) | (xb & mask1);
+      ub[i] = (xa & mask1) | (xb & mask2);
     }
 
-  x = mask & (a->nlimbs ^ b->nlimbs);
-  a->nlimbs = a->nlimbs ^ x;
-  b->nlimbs = b->nlimbs ^ x;
+  xa = a->nlimbs;
+  xb = b->nlimbs;
+  a->nlimbs = (xa & mask2) | (xb & mask1);
+  b->nlimbs = (xa & mask1) | (xb & mask2);
 
-  x = mask & (a->sign ^ b->sign);
-  a->sign = a->sign ^ x;
-  b->sign = b->sign ^ x;
+  xa = a->sign;
+  xb = b->sign;
+  a->sign = (xa & mask2) | (xb & mask1);
+  b->sign = (xa & mask1) | (xb & mask2);
 }