1 files changed, 53 insertions, 0 deletions
diff --git a/src/lib/math/numbertheory/jacobi.cpp b/src/lib/math/numbertheory/jacobi.cpp
new file mode 100644
index 000000000..fcccc80e5
--- /dev/null
+++ b/src/lib/math/numbertheory/jacobi.cpp
@@ -0,0 +1,53 @@
+/*
+* Jacobi Function
+* (C) 1999-2007 Jack Lloyd
+*
+* Distributed under the terms of the Botan license
+*/
+
+#include <botan/numthry.h>
+
+namespace Botan {
+
+/*
+* Calculate the Jacobi symbol
+*/
+s32bit jacobi(const BigInt& a, const BigInt& n)
+   {
+   if(a.is_negative())
+      throw Invalid_Argument("jacobi: first argument must be non-negative");
+   if(n.is_even() || n < 2)
+      throw Invalid_Argument("jacobi: second argument must be odd and > 1");
+
+   BigInt x = a, y = n;
+   s32bit J = 1;
+
+   while(y > 1)
+      {
+      x %= y;
+      if(x > y / 2)
+         {
+         x = y - x;
+         if(y % 4 == 3)
+            J = -J;
+         }
+      if(x.is_zero())
+         return 0;
+
+      size_t shifts = low_zero_bits(x);
+      x >>= shifts;
+      if(shifts % 2)
+         {
+         word y_mod_8 = y % 8;
+         if(y_mod_8 == 3 || y_mod_8 == 5)
+            J = -J;
+         }
+
+      if(x % 4 == 3 && y % 4 == 3)
+         J = -J;
+      std::swap(x, y);
+      }
+   return J;
+   }
+
+}