Add functions to reduce integers mod the order to EC_Group

This allows calculating the Barett reduction params just once,
when the group is initialized, then sharing them across all
operations which use that group.

2 files changed, 65 insertions, 9 deletions

diff --git a/src/lib/pubkey/ec_group/ec_group.cpp b/src/lib/pubkey/ec_group/ec_group.cpp
index 0c2cd566c..beff90eec 100644
--- a/src/lib/pubkey/ec_group/ec_group.cpp
+++ b/src/lib/pubkey/ec_group/ec_group.cpp
@@ -35,9 +35,10 @@ class EC_Group_Data final
          m_base_point(m_curve, g_x, g_y),
          m_order(order),
          m_cofactor(cofactor),
+         m_mod_order(order),
          m_oid(oid),
          m_p_bits(p.bits()),
-         m_p_bytes(p.bytes())
+         m_order_bits(order.bits())
          {
          }
 
@@ -60,18 +61,30 @@ class EC_Group_Data final
       BigInt g_y() const { return m_base_point.get_affine_y(); }
 
       size_t p_bits() const { return m_p_bits; }
-      size_t p_bytes() const { return m_p_bytes; }
+      size_t p_bytes() const { return (m_p_bits + 7) / 8; }
+
+      size_t order_bits() const { return m_order_bits; }
+      size_t order_bytes() const { return (m_order_bits + 7) / 8; }
 
       const CurveGFp& curve() const { return m_curve; }
       const PointGFp& base_point() const { return m_base_point; }
 
+      BigInt mod_order(const BigInt& x) const { return m_mod_order.reduce(x); }
+
+      BigInt multiply_mod_order(const BigInt& x, const BigInt& y) const
+         {
+         return m_mod_order.multiply(x, y);
+         }
+
    private:
       CurveGFp m_curve;
       PointGFp m_base_point;
       BigInt m_order;
       BigInt m_cofactor;
+      Modular_Reducer m_mod_order;
       OID m_oid;
-      size_t m_p_bits, m_p_bytes;
+      size_t m_p_bits;
+      size_t m_order_bits;
    };
 
 class EC_Group_Data_Map final
@@ -246,6 +259,9 @@ std::shared_ptr<EC_Group_Data> EC_Group::BER_decode_EC_group(const uint8_t bits[
          .end_cons()
          .verify_end();
 
+      if(p.bits() < 64 || p.is_negative() || a.is_negative() || b.is_negative() || order <= 0 || cofactor <= 0)
+         throw Decoding_Error("Invalid ECC parameters");
+
       std::pair<BigInt, BigInt> base_xy = Botan::OS2ECP(base_pt.data(), base_pt.size(), p, a, b);
 
       return ec_group_data().lookup_or_create(p, a, b, base_xy.first, base_xy.second, order, cofactor, OID());
@@ -348,6 +364,16 @@ size_t EC_Group::get_p_bytes() const
    return data().p_bytes();
    }
 
+size_t EC_Group::get_order_bits() const
+   {
+   return data().order_bits();
+   }
+
+size_t EC_Group::get_order_bytes() const
+   {
+   return data().order_bytes();
+   }
+
 const BigInt& EC_Group::get_p() const
    {
    return data().p();
@@ -378,6 +404,16 @@ const BigInt& EC_Group::get_cofactor() const
    return data().cofactor();
    }
 
+BigInt EC_Group::mod_order(const BigInt& k) const
+   {
+   return data().mod_order(k);
+   }
+
+BigInt EC_Group::multiply_mod_order(const BigInt& x, const BigInt& y) const
+   {
+   return data().multiply_mod_order(x, y);
+   }
+
 const OID& EC_Group::get_curve_oid() const
    {
    return data().oid();
diff --git a/src/lib/pubkey/ec_group/ec_group.h b/src/lib/pubkey/ec_group/ec_group.h
index 1dc839540..b4b0ec9b3 100644
--- a/src/lib/pubkey/ec_group/ec_group.h
+++ b/src/lib/pubkey/ec_group/ec_group.h
@@ -133,6 +133,16 @@ class BOTAN_PUBLIC_API(2,0) EC_Group final
       size_t get_p_bytes() const;
 
       /**
+      * Return the size of group order in bits (same as get_order().bits())
+      */
+      size_t get_order_bits() const;
+
+      /**
+      * Return the size of p in bytes (same as get_order().bytes())
+      */
+      size_t get_order_bytes() const;
+
+      /**
       * Return the prime modulus of the field
       */
       const BigInt& get_p() const;
@@ -159,6 +169,22 @@ class BOTAN_PUBLIC_API(2,0) EC_Group final
       */
       const BigInt& get_order() const;
 
+      /*
+      * Reduce x modulo the order
+      */
+      BigInt mod_order(const BigInt& x) const;
+
+      /*
+      * Reduce (x*y) modulo the order
+      */
+      BigInt multiply_mod_order(const BigInt& x, const BigInt& y) const;
+
+      /**
+      * Return the cofactor
+      * @result the cofactor
+      */
+      const BigInt& get_cofactor() const;
+
       /**
       * Return the OID of these domain parameters
       * @result the OID as a string
@@ -172,12 +198,6 @@ class BOTAN_PUBLIC_API(2,0) EC_Group final
       const OID& get_curve_oid() const;
 
       /**
-      * Return the cofactor
-      * @result the cofactor
-      */
-      const BigInt& get_cofactor() const;
-
-      /**
       * Return a point on this curve with the affine values x, y
       */
       PointGFp point(const BigInt& x, const BigInt& y) const;