Use async in ElGamal encryption and Nyberg-Rueppel verification

1 files changed, 29 insertions, 6 deletions

diff --git a/src/math/numbertheory/powm_fw.cpp b/src/math/numbertheory/powm_fw.cpp
index b764ee7aa..e4272f20d 100644
--- a/src/math/numbertheory/powm_fw.cpp
+++ b/src/math/numbertheory/powm_fw.cpp
@@ -77,17 +77,40 @@ BigInt Fixed_Window_Exponentiator::execute() const
    {
    const u32bit exp_nibbles = (exp.bits() + window_bits - 1) / window_bits;
 
-   BigInt x = 1;
-   for(u32bit j = exp_nibbles; j > 0; --j)
+   if(exp_nibbles == 0)
+      return 1;
+
+   BigInt x1 = 1;
+
+   for(u32bit j = 0; j != exp_nibbles / 2; ++j)
+      {
+      for(u32bit k = 0; k != window_bits; ++k)
+         x1 = reducer.square(x1);
+
+      u32bit nibble = exp.get_substring(window_bits*(exp_nibbles-1-j),
+                                        window_bits);
+
+      if(nibble)
+         x1 = reducer.multiply(x1, g[nibble-1]);
+      }
+
+   for(u32bit k = 0; k != window_bits; ++k)
+      x1 = reducer.square(x1);
+   BigInt x2 = 1;
+
+   for(u32bit j = exp_nibbles / 2; j != exp_nibbles; ++j)
       {
       for(u32bit k = 0; k != window_bits; ++k)
-         x = reducer.square(x);
+         x2 = reducer.square(x2);
+
+      u32bit nibble = exp.get_substring(window_bits*(exp_nibbles-1-j),
+                                        window_bits);
 
-      u32bit nibble = exp.get_substring(window_bits*(j-1), window_bits);
       if(nibble)
-         x = reducer.multiply(x, g[nibble-1]);
+         x2 = reducer.multiply(x2, g[nibble-1]);
       }
-   return x;
+
+   return reducer.multiply(x1, x2);
    }
 
 /*