[PATCH] Add mpi_set_cond.

[NIIBE Yutaka]

Hello,

For Curve25519, I added the internal function mpi_swap_cond last year.
I think that we should also have mpi_set_cond.

Adding mpi_set_cond, I revised the implementation of
_gcry_mpi_swap_cond.

OK to commit?  I think that it's worth to have those functions in 1.6.


    mpi: Add mpi_set_cond.

    * mpi/mpiutil.c (_gcry_mpi_set_cond): New.
    (_gcry_mpi_swap_cond): Fix types.
    * src/mpi.h (mpi_set_cond): New.

diff --git a/mpi/mpiutil.c b/mpi/mpiutil.c
index f74dd91..e2e4db9 100644
--- a/mpi/mpiutil.c
+++ b/mpi/mpiutil.c
@@ -482,6 +482,31 @@ _gcry_mpi_set (gcry_mpi_t w, gcry_mpi_t u)
   return w;
 }

+gcry_mpi_t
+_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 = 0UL - !!set;
+  mpi_limb_t x;
+
+  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;
+    }
+
+  x = mask & (w->nlimbs ^ u->nlimbs);
+  w->nlimbs = w->nlimbs ^ x;
+
+  x = mask & (w->sign ^ u->sign);
+  w->sign = w->sign ^ x;
+  return w;
+}
+

 gcry_mpi_t
 _gcry_mpi_set_ui (gcry_mpi_t w, unsigned long u)
@@ -545,10 +570,10 @@ _gcry_mpi_swap (gcry_mpi_t a, gcry_mpi_t b)
 void
 _gcry_mpi_swap_cond (gcry_mpi_t a, gcry_mpi_t b, unsigned long swap)
 {
-  size_t i;
-  size_t nlimbs = a->alloced;
-  unsigned long mask = 0UL - !!swap;
-  unsigned long x;
+  mpi_size_t i;
+  mpi_size_t nlimbs = a->alloced;
+  mpi_limb_t mask = 0UL - !!swap;
+  mpi_limb_t x;

   if (a->alloced != b->alloced)
     log_bug ("mpi_swap_cond: different sizes\n");
diff --git a/src/mpi.h b/src/mpi.h
index 13b5117..0d19f46 100644
--- a/src/mpi.h
+++ b/src/mpi.h
@@ -120,8 +120,11 @@ void _gcry_mpi_immutable_failed (void);
 #define mpi_m_check(a)        _gcry_mpi_m_check ((a))
 #define mpi_const(n)          _gcry_mpi_const ((n))
 #define mpi_swap_cond(a,b,sw)  _gcry_mpi_swap_cond ((a),(b),(sw))
+#define mpi_set_cond(w,u,set)  _gcry_mpi_set_cond ((w),(u),(set))

 void _gcry_mpi_clear( gcry_mpi_t a );
+gcry_mpi_t _gcry_mpi_set_cond (gcry_mpi_t w, const gcry_mpi_t u,
+                               unsigned long swap);
 gcry_mpi_t  _gcry_mpi_alloc_like( gcry_mpi_t a );
 gcry_mpi_t  _gcry_mpi_alloc_set_ui( unsigned long u);
 void _gcry_mpi_m_check( gcry_mpi_t a );
--