Botan::VartimeMul2Table< C, W > Class Template Reference

#include <pcurves_impl.h>

Public Types
using	AffinePoint = typename C::AffinePoint
using	ProjectivePoint = typename C::ProjectivePoint
using	Scalar = typename C::Scalar

Public Member Functions
ProjectivePoint	mul2_vartime (const Scalar &s1, const Scalar &s2) const
	VartimeMul2Table (const AffinePoint &p, const AffinePoint &q)

Static Public Attributes
static constexpr size_t	WindowBits = W

Detailed Description

template<typename C, size_t W>
class Botan::VartimeMul2Table< C, W >

Definition at line 1565 of file pcurves_impl.h.

Member Typedef Documentation

AffinePoint

template<typename C, size_t W>

using Botan::VartimeMul2Table< C, W >::AffinePoint = typename C::AffinePoint

Definition at line 1573 of file pcurves_impl.h.

ProjectivePoint

template<typename C, size_t W>

using Botan::VartimeMul2Table< C, W >::ProjectivePoint = typename C::ProjectivePoint

Definition at line 1574 of file pcurves_impl.h.

Scalar

template<typename C, size_t W>

using Botan::VartimeMul2Table< C, W >::Scalar = typename C::Scalar

Definition at line 1572 of file pcurves_impl.h.

Constructor & Destructor Documentation

VartimeMul2Table()

template<typename C, size_t W>

Botan::VartimeMul2Table< C, W >::VartimeMul2Table ( const AffinePoint & p, const AffinePoint & q )

inline

Definition at line 1576 of file pcurves_impl.h.

                                                                   :
            m_table(to_affine_batch<C, true>(mul2_setup<C, W>(p, q))) {}

References Botan::mul2_setup(), and Botan::to_affine_batch().

Member Function Documentation

mul2_vartime()

template<typename C, size_t W>

ProjectivePoint Botan::VartimeMul2Table< C, W >::mul2_vartime ( const Scalar & s1, const Scalar & s2 ) const

inline

Variable time 2-ary multiplication

A common use of 2-ary multiplication is when verifying the commitments of an elliptic curve signature. Since in this case the inputs are all public, there is no problem with variable time computation.

TODO in the future we could use joint sparse form here.

Definition at line 1588 of file pcurves_impl.h.

                                                                             {
         constexpr size_t Windows = (Scalar::BITS + WindowBits - 1) / WindowBits;
 
         const UnblindedScalarBits<C, W> bits1(s1);
         const UnblindedScalarBits<C, W> bits2(s2);
 
         const bool s1_is_zero = s1.is_zero().as_bool();
         const bool s2_is_zero = s2.is_zero().as_bool();
 
         // Conditional ok: this function is variable time
         if(s1_is_zero && s2_is_zero) {
            return ProjectivePoint::identity();
         }
 
         auto [w_0, first_nonempty_window] = [&]() {
            for(size_t i = 0; i != Windows; ++i) {
               const size_t w_1 = bits1.get_window((Windows - i - 1) * WindowBits);
               const size_t w_2 = bits2.get_window((Windows - i - 1) * WindowBits);
               const size_t window = w_1 + (w_2 << WindowBits);
               // Conditional ok: this function is variable time
               if(window > 0) {
                  return std::make_pair(window, i);
               }
            }
            // We checked for s1 == s2 == 0 above, so we must see a window eventually
            BOTAN_ASSERT_UNREACHABLE();
         }();
 
         BOTAN_ASSERT_NOMSG(w_0 > 0);
         auto accum = ProjectivePoint::from_affine(m_table[w_0 - 1]);
 
         for(size_t i = first_nonempty_window + 1; i < Windows; ++i) {
            accum = accum.dbl_n(WindowBits);
 
            const size_t w_1 = bits1.get_window((Windows - i - 1) * WindowBits);
            const size_t w_2 = bits2.get_window((Windows - i - 1) * WindowBits);
 
            const size_t window = w_1 + (w_2 << WindowBits);
 
            // Conditional ok: this function is variable time
            if(window > 0) {
               accum += m_table[window - 1];
            }
         }
 
         return accum;
      }

References BOTAN_ASSERT_NOMSG, BOTAN_ASSERT_UNREACHABLE, Botan::UnblindedScalarBits< C, WindowBits >::get_window(), and WindowBits.

Member Data Documentation

WindowBits

template<typename C, size_t W>

size_t Botan::VartimeMul2Table< C, W >::WindowBits = W

staticconstexpr

Definition at line 1570 of file pcurves_impl.h.

Referenced by mul2_vartime().

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The documentation for this class was generated from the following file:

• src/lib/math/pcurves/pcurves_impl/pcurves_impl.h