Split Montgomery reduction into two functions, the core algorithm linked

as C (for replacing by asm later), and another that performs a subtract
if needed (inside powm_mnt.cpp). That way an asm version of the Montgomery
algorithm won't have to deal with calling other functions.

2 files changed, 13 insertions, 4 deletions

diff --git a/src/mp_asm.cpp b/src/mp_asm.cpp
index ec2e31011..02f75f281 100644
--- a/src/mp_asm.cpp
+++ b/src/mp_asm.cpp
@@ -199,7 +199,7 @@ void bigint_simple_mul(word z[], const word x[], u32bit x_size,
 /*************************************************
 * Montgomery Reduction Algorithm                 *
 *************************************************/
-void montgomery_reduce(word z[], u32bit z_size,
+void bigint_monty_redc(word z[], u32bit z_size,
                        const word x[], u32bit x_size, word u)
    {
    for(u32bit j = 0; j != x_size; ++j)
@@ -227,9 +227,6 @@ void montgomery_reduce(word z[], u32bit z_size,
          carry = !z_j[k];
          }
       }
-
-   if(bigint_cmp(z + x_size, x_size + 1, x, x_size) >= 0)
-      bigint_sub2(z + x_size, x_size + 1, x, x_size);
    }
 
 }
diff --git a/src/powm_mnt.cpp b/src/powm_mnt.cpp
index ad1ac312c..d28bfacfe 100644
--- a/src/powm_mnt.cpp
+++ b/src/powm_mnt.cpp
@@ -12,6 +12,18 @@ namespace Botan {
 namespace {
 
 /*************************************************
+* Montgomery Reduction                           *
+*************************************************/
+void montgomery_reduce(word z[], u32bit z_size,
+                       const word x[], u32bit x_size, word u)
+   {
+   bigint_monty_redc(z, z_size, x, x_size, u);
+
+   if(bigint_cmp(z + x_size, x_size + 1, x, x_size) >= 0)
+      bigint_sub2(z + x_size, x_size + 1, x, x_size);
+   }
+
+/*************************************************
 * Try to choose a good window size               *
 *************************************************/
 u32bit choose_window_bits(u32bit exp_bits, u32bit,