Use reduce_below in PointGFp

Improves ECDSA times by 2-3%

2 files changed, 6 insertions, 8 deletions

diff --git a/src/lib/math/bigint/bigint.cpp b/src/lib/math/bigint/bigint.cpp
index 50e93c38d..c822a94e1 100644
--- a/src/lib/math/bigint/bigint.cpp
+++ b/src/lib/math/bigint/bigint.cpp
@@ -260,6 +260,8 @@ void BigInt::reduce_below(const BigInt& p, secure_vector<word>& ws)
    if(ws.size() < p_words + 1)
       ws.resize(p_words + 1);
 
+   clear_mem(ws.data(), ws.size());
+
    for(;;)
       {
       word borrow = bigint_sub3(ws.data(), data(), p_words + 1, p.data(), p_words);
diff --git a/src/lib/math/ec_gfp/point_gfp.cpp b/src/lib/math/ec_gfp/point_gfp.cpp
index 407da9dbe..f054c51ff 100644
--- a/src/lib/math/ec_gfp/point_gfp.cpp
+++ b/src/lib/math/ec_gfp/point_gfp.cpp
@@ -203,8 +203,7 @@ void PointGFp::mult2(std::vector<BigInt>& ws_bn)
 
    m_curve.mul(S, m_coord_x, y_2, monty_ws);
    S <<= 2; // * 4
-   while(S >= p)
-      S -= p;
+   S.reduce_below(p, monty_ws);
 
    m_curve.sqr(a_z4, m_coord_z, monty_ws); // z^2
    m_curve.sqr(tmp, a_z4, monty_ws); // z^4
@@ -213,8 +212,7 @@ void PointGFp::mult2(std::vector<BigInt>& ws_bn)
    m_curve.sqr(M, m_coord_x, monty_ws);
    M *= 3;
    M += a_z4;
-   while(M >= p)
-      M -= p;
+   M.reduce_below(p, monty_ws);
 
    m_curve.sqr(x, M, monty_ws);
    x -= (S << 1);
@@ -223,8 +221,7 @@ void PointGFp::mult2(std::vector<BigInt>& ws_bn)
 
    m_curve.sqr(U, y_2, monty_ws);
    U <<= 3;
-   while(U >= p)
-      U -= p;
+   U.reduce_below(p, monty_ws);
 
    S -= x;
    while(S.is_negative())
@@ -237,8 +234,7 @@ void PointGFp::mult2(std::vector<BigInt>& ws_bn)
 
    m_curve.mul(z, m_coord_y, m_coord_z, monty_ws);
    z <<= 1;
-   if(z >= p)
-      z -= p;
+   z.reduce_below(p, monty_ws);
 
    m_coord_x = x;
    m_coord_y = y;