Avoid needless computation in base point multiply

We computed twice as many values as were used.

3 files changed, 6 insertions, 33 deletions

diff --git a/src/lib/pubkey/ec_group/curve_gfp.cpp b/src/lib/pubkey/ec_group/curve_gfp.cpp
index fba9a419c..a8fdb2906 100644
--- a/src/lib/pubkey/ec_group/curve_gfp.cpp
+++ b/src/lib/pubkey/ec_group/curve_gfp.cpp
@@ -106,12 +106,6 @@ void CurveGFp_Montgomery::from_curve_rep(BigInt& z, secure_vector<word>& ws) con
 void CurveGFp_Montgomery::curve_mul(BigInt& z, const BigInt& x, const BigInt& y,
                                     secure_vector<word>& ws) const
    {
-   if(x.is_zero() || y.is_zero())
-      {
-      z.clear();
-      return;
-      }
-
    if(ws.size() < get_ws_size())
       ws.resize(get_ws_size());
 
@@ -138,12 +132,6 @@ void CurveGFp_Montgomery::curve_mul(BigInt& z, const BigInt& x, const BigInt& y,
 void CurveGFp_Montgomery::curve_sqr(BigInt& z, const BigInt& x,
                                     secure_vector<word>& ws) const
    {
-   if(x.is_zero())
-      {
-      z.clear();
-      return;
-      }
-
    if(ws.size() < get_ws_size())
       ws.resize(get_ws_size());
 
@@ -215,12 +203,6 @@ BigInt CurveGFp_NIST::invert_element(const BigInt& x, secure_vector<word>& ws) c
 void CurveGFp_NIST::curve_mul(BigInt& z, const BigInt& x, const BigInt& y,
                               secure_vector<word>& ws) const
    {
-   if(x.is_zero() || y.is_zero())
-      {
-      z.clear();
-      return;
-      }
-
    if(ws.size() < get_ws_size())
       ws.resize(get_ws_size());
 
@@ -242,12 +224,6 @@ void CurveGFp_NIST::curve_mul(BigInt& z, const BigInt& x, const BigInt& y,
 void CurveGFp_NIST::curve_sqr(BigInt& z, const BigInt& x,
                               secure_vector<word>& ws) const
    {
-   if(x.is_zero())
-      {
-      z.clear();
-      return;
-      }
-
    if(ws.size() < get_ws_size())
       ws.resize(get_ws_size());
 
diff --git a/src/lib/pubkey/ec_group/point_mul.cpp b/src/lib/pubkey/ec_group/point_mul.cpp
index 487fb9884..7acb60b6a 100644
--- a/src/lib/pubkey/ec_group/point_mul.cpp
+++ b/src/lib/pubkey/ec_group/point_mul.cpp
@@ -43,9 +43,9 @@ PointGFp_Base_Point_Precompute::PointGFp_Base_Point_Precompute(const PointGFp& b
    * the size of the prime modulus. In all cases they are at most 1 bit
    * longer. The +1 compensates for this.
    */
-   m_T_bits = round_up(p_bits + PointGFp_SCALAR_BLINDING_BITS + 1, 2);
+   const size_t T_bits = round_up(p_bits + PointGFp_SCALAR_BLINDING_BITS + 1, 2) / 2;
 
-   m_T.resize(3*m_T_bits);
+   m_T.resize(3*T_bits);
 
    m_T[0] = base;
    m_T[1] = m_T[0];
@@ -53,7 +53,7 @@ PointGFp_Base_Point_Precompute::PointGFp_Base_Point_Precompute(const PointGFp& b
    m_T[2] = m_T[1];
    m_T[2].add(m_T[0], ws);
 
-   for(size_t i = 1; i != m_T_bits; ++i)
+   for(size_t i = 1; i != T_bits; ++i)
       {
       m_T[3*i+0] = m_T[3*i - 2];
       m_T[3*i+0].mult2(ws);
@@ -78,17 +78,15 @@ PointGFp PointGFp_Base_Point_Precompute::mul(const BigInt& k,
    const BigInt mask(rng, PointGFp_SCALAR_BLINDING_BITS, false);
    const BigInt scalar = k + group_order * mask;
 
-   const size_t scalar_bits = scalar.bits();
+   size_t windows = round_up(scalar.bits(), 2) / 2;
 
-   BOTAN_ASSERT(scalar_bits <= m_T_bits,
+   BOTAN_ASSERT(windows <= m_T.size() / 3,
                 "Precomputed sufficient values for scalar mult");
 
-   PointGFp R = m_T[0].zero();
-
    if(ws.size() < PointGFp::WORKSPACE_SIZE)
       ws.resize(PointGFp::WORKSPACE_SIZE);
 
-   size_t windows = round_up(scalar_bits, 2) / 2;
+   PointGFp R = m_T[0].zero();
 
    for(size_t i = 0; i != windows; ++i)
       {
diff --git a/src/lib/pubkey/ec_group/point_mul.h b/src/lib/pubkey/ec_group/point_mul.h
index b6e9da3b9..97fd326a2 100644
--- a/src/lib/pubkey/ec_group/point_mul.h
+++ b/src/lib/pubkey/ec_group/point_mul.h
@@ -23,7 +23,6 @@ class PointGFp_Base_Point_Precompute
                    const BigInt& group_order,
                    std::vector<BigInt>& ws) const;
    private:
-      size_t m_T_bits;
       std::vector<PointGFp> m_T;
    };