2 files changed, 25 insertions, 0 deletions
diff --git a/src/lib/math/numbertheory/mp_numth.cpp b/src/lib/math/numbertheory/mp_numth.cpp
index 6eb938286..3373b9ee7 100644
--- a/src/lib/math/numbertheory/mp_numth.cpp
+++ b/src/lib/math/numbertheory/mp_numth.cpp
@@ -71,4 +71,18 @@ BigInt sub_mul(const BigInt& a, const BigInt& b, const BigInt& c)
    return r;
    }
 
+/*
+* Multiply-Subtract Operation
+*/
+BigInt mul_sub(const BigInt& a, const BigInt& b, const BigInt& c)
+   {
+   if(c.is_negative() || c.is_zero())
+      throw Invalid_Argument("mul_sub: Third argument must be > 0");
+
+   BigInt r = a;
+   r *= b;
+   r -= c;
+   return r;
+   }
+
 }
diff --git a/src/lib/math/numbertheory/numthry.h b/src/lib/math/numbertheory/numthry.h
index e1e6c65f6..591b61f6a 100644
--- a/src/lib/math/numbertheory/numthry.h
+++ b/src/lib/math/numbertheory/numthry.h
@@ -37,6 +37,17 @@ BigInt BOTAN_DLL sub_mul(const BigInt& a,
                          const BigInt& c);
 
 /**
+* Fused multiply-subtract
+* @param a an integer
+* @param b an integer
+* @param c an integer
+* @return (a*b)-c
+*/
+BigInt BOTAN_DLL mul_sub(const BigInt& a,
+                         const BigInt& b,
+                         const BigInt& c);
+
+/**
 * Return the absolute value
 * @param n an integer
 * @return absolute value of n