EC_AffinePoint API improvements

doc/api_ref/ecc.rst

@@ -256,10 +256,38 @@ Only curves over prime fields are supported.
 
       Fixed base scalar multiplication. Constant time with blinding.
 
+   .. cpp:function::  static std::optional<EC_AffinePoint> mul_px_qy(const EC_AffinePoint& p, \
+                          const EC_Scalar& x, \
+                          const EC_AffinePoint& q, \
+                          const EC_Scalar& y, \
+                          RandomNumberGenerator& rng)
+
+      Constant time 2-ary multiscalar multiplication. Returns p*x + q*y, or
+      nullopt if the resulting point was the identity element.
+
    .. cpp:function:: EC_AffinePoint mul(const EC_Scalar& scalar, RandomNumberGenerator& rng, std::vector<BigInt>& ws) const
 
       Variable base scalar multiplication. Constant time with blinding.
 
+   .. cpp:function::  static EC_AffinePoint add(const EC_AffinePoint& p, const EC_AffinePoint& q)
+
+      Elliptic curve point addition.
+
+      .. note::
+
+         This point addition operation is relatively quite expensive since it
+         must convert the point directly from projective to affine coordinates,
+         which requires an expensive field inversion. This is, however,
+         sufficient for protocols which just require a small number of point
+         additions. In the future a type for projective coordinate points may
+         also be added, to better handle protocols which require many point
+         additions. If you are implementing such a protocol using this interface
+         please open an issue on Github.
+
+   .. cpp:function:: EC_AffinePoint negate() const
+
+      Return the negation of this point.
+
    .. cpp:function:: static EC_AffinePoint hash_to_curve_ro(const EC_Group& group, \
                                              std::string_view hash_fn, \
                                              std::span<const uint8_t> input, \

src/cli/perf_ec.cpp

@@ -34,6 +34,7 @@ class PerfTest_EllipticCurve final : public PerfTest {
 
             auto bp_timer = config.make_timer(group_name + " base point mul");
             auto vp_timer = config.make_timer(group_name + " variable point mul");
+            auto add_timer = config.make_timer(group_name + " point addition");
             auto der_uc_timer = config.make_timer(group_name + " point deserialize (uncompressed)");
             auto der_c_timer = config.make_timer(group_name + " point deserialize (compressed)");
             auto mul2_setup_timer = config.make_timer(group_name + " mul2 setup");
@@ -64,6 +65,8 @@ class PerfTest_EllipticCurve final : public PerfTest {
                const auto r2_bytes = r2.serialize_uncompressed();
                BOTAN_ASSERT_EQUAL(r1_bytes, r2_bytes, "Same result for multiplication");
 
+               add_timer->run([&]() { r1.add(r2); });
+
                der_uc_timer->run([&]() { Botan::EC_AffinePoint::deserialize(group, r1_bytes); });
 
                const auto r1_cbytes = r1.serialize_compressed();
@@ -81,6 +84,7 @@ class PerfTest_EllipticCurve final : public PerfTest {
                }
             }
 
+            config.record_result(*add_timer);
             config.record_result(*bp_timer);
             config.record_result(*vp_timer);
             config.record_result(*mul2_setup_timer);

src/lib/math/pcurves/pcurves.h

@@ -191,6 +191,11 @@ class BOTAN_TEST_API PrimeOrderCurve {
                return bytes;
             }
 
+            /**
+            * Point negation
+            */
+            AffinePoint negate() const { return m_curve->point_negate(*this); }
+
             /**
             * Return true if this is the curve identity element (aka the point at infinity)
             */
@@ -243,8 +248,6 @@ class BOTAN_TEST_API PrimeOrderCurve {
 
             ProjectivePoint dbl() const { return m_curve->point_double(*this); }
 
-            ProjectivePoint negate() const { return m_curve->point_negate(*this); }
-
             friend ProjectivePoint operator+(const ProjectivePoint& x, const ProjectivePoint& y) {
                return x.m_curve->point_add(x, y);
             }
@@ -376,7 +379,7 @@ class BOTAN_TEST_API PrimeOrderCurve {
 
       virtual ProjectivePoint point_double(const ProjectivePoint& pt) const = 0;
 
-      virtual ProjectivePoint point_negate(const ProjectivePoint& pt) const = 0;
+      virtual AffinePoint point_negate(const AffinePoint& pt) const = 0;
 
       virtual ProjectivePoint point_add(const ProjectivePoint& a, const ProjectivePoint& b) const = 0;
 

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

@@ -190,7 +190,7 @@ class PrimeOrderCurveImpl final : public PrimeOrderCurve {
          return stash(from_stash(a) + from_stash(b));
       }
 
-      ProjectivePoint point_negate(const ProjectivePoint& pt) const override { return stash(from_stash(pt).negate()); }
+      AffinePoint point_negate(const AffinePoint& pt) const override { return stash(from_stash(pt).negate()); }
 
       bool affine_point_is_identity(const AffinePoint& pt) const override {
          return from_stash(pt).is_identity().as_bool();

src/lib/pubkey/ec_group/ec_apoint.cpp

@@ -111,12 +111,26 @@ EC_AffinePoint EC_AffinePoint::mul(const EC_Scalar& scalar, RandomNumberGenerato
    return EC_AffinePoint(inner().mul(scalar._inner(), rng, ws));
 }
 
-EC_AffinePoint EC_AffinePoint::mul_px_qy(const EC_AffinePoint& p,
-                                         const EC_Scalar& x,
-                                         const EC_AffinePoint& q,
-                                         const EC_Scalar& y,
-                                         RandomNumberGenerator& rng) {
+std::optional<EC_AffinePoint> EC_AffinePoint::mul_px_qy(const EC_AffinePoint& p,
+                                                        const EC_Scalar& x,
+                                                        const EC_AffinePoint& q,
+                                                        const EC_Scalar& y,
+                                                        RandomNumberGenerator& rng) {
    auto pt = p._inner().group()->mul_px_qy(p._inner(), x._inner(), q._inner(), y._inner(), rng);
+   if(pt) {
+      return EC_AffinePoint(std::move(pt));
+   } else {
+      return {};
+   }
+}
+
+EC_AffinePoint EC_AffinePoint::add(const EC_AffinePoint& q) const {
+   auto pt = _inner().group()->affine_add(_inner(), q._inner());
+   return EC_AffinePoint(std::move(pt));
+}
+
+EC_AffinePoint EC_AffinePoint::negate() const {
+   auto pt = this->_inner().group()->affine_neg(this->_inner());
    return EC_AffinePoint(std::move(pt));
 }
 

src/lib/pubkey/ec_group/ec_apoint.h

@@ -26,6 +26,8 @@ class EC_Point;
 class EC_Group_Data;
 class EC_AffinePoint_Data;
 
+/// Elliptic Curve in Affine Representation
+///
 class BOTAN_UNSTABLE_API EC_AffinePoint final {
    public:
       /// Point deserialization. Throws if wrong length or not a valid point
@@ -79,11 +81,27 @@ class BOTAN_UNSTABLE_API EC_AffinePoint final {
       /// Compute 2-ary multiscalar multiplication - p*x + q*y
       ///
       /// This operation runs in constant time with respect to p, x, q, and y
-      static EC_AffinePoint mul_px_qy(const EC_AffinePoint& p,
-                                      const EC_Scalar& x,
-                                      const EC_AffinePoint& q,
-                                      const EC_Scalar& y,
-                                      RandomNumberGenerator& rng);
+      ///
+      /// @returns p*x+q*y, or nullopt if the result was the point at infinity
+      static std::optional<EC_AffinePoint> mul_px_qy(const EC_AffinePoint& p,
+                                                     const EC_Scalar& x,
+                                                     const EC_AffinePoint& q,
+                                                     const EC_Scalar& y,
+                                                     RandomNumberGenerator& rng);
+
+      /// Point addition
+      ///
+      /// Note that this is quite slow since it converts the resulting
+      /// projective point immediately to affine coordinates, which requires a
+      /// field inversion. This can be sufficient when implementing protocols
+      /// that just need to perform a few additions.
+      ///
+      /// In the future a cooresponding EC_ProjectivePoint type may be added
+      /// which would avoid the expensive affine conversions
+      EC_AffinePoint add(const EC_AffinePoint& q) const;
+
+      /// Point negation
+      EC_AffinePoint negate() const;
 
       /// Return the number of bytes of a field element
       ///

src/lib/pubkey/ec_group/ec_inner_data.cpp

@@ -317,16 +317,41 @@ std::unique_ptr<EC_AffinePoint_Data> EC_Group_Data::mul_px_qy(const EC_AffinePoi
       auto qy = q_mul.mul(EC_Scalar_Data_BN::checked_ref(y).value(), rng, order, ws);
 
       auto px_qy = px + qy;
-      px_qy.force_affine();
 
       if(!px_qy.is_zero()) {
+         px_qy.force_affine();
          return std::make_unique<EC_AffinePoint_Data_BN>(shared_from_this(), std::move(px_qy));
       } else {
          return nullptr;
       }
    }
 }
 
+std::unique_ptr<EC_AffinePoint_Data> EC_Group_Data::affine_add(const EC_AffinePoint_Data& p,
+                                                               const EC_AffinePoint_Data& q) const {
+   if(m_pcurve) {
+      auto pt = m_pcurve->point_add_mixed(
+         PCurve::PrimeOrderCurve::ProjectivePoint::from_affine(EC_AffinePoint_Data_PC::checked_ref(p).value()),
+         EC_AffinePoint_Data_PC::checked_ref(q).value());
+
+      return std::make_unique<EC_AffinePoint_Data_PC>(shared_from_this(), pt.to_affine());
+   } else {
+      auto pt = p.to_legacy_point() + q.to_legacy_point();
+      return std::make_unique<EC_AffinePoint_Data_BN>(shared_from_this(), std::move(pt));
+   }
+}
+
+std::unique_ptr<EC_AffinePoint_Data> EC_Group_Data::affine_neg(const EC_AffinePoint_Data& p) const {
+   if(m_pcurve) {
+      auto pt = m_pcurve->point_negate(EC_AffinePoint_Data_PC::checked_ref(p).value());
+      return std::make_unique<EC_AffinePoint_Data_PC>(shared_from_this(), pt);
+   } else {
+      auto pt = p.to_legacy_point();
+      pt.negate();  // negates in place
+      return std::make_unique<EC_AffinePoint_Data_BN>(shared_from_this(), std::move(pt));
+   }
+}
+
 std::unique_ptr<EC_Mul2Table_Data> EC_Group_Data::make_mul2_table(const EC_AffinePoint_Data& h) const {
    if(m_pcurve) {
       EC_AffinePoint_Data_PC g(shared_from_this(), m_pcurve->generator());

src/lib/pubkey/ec_group/ec_inner_data.h

@@ -261,6 +261,10 @@ class EC_Group_Data final : public std::enable_shared_from_this<EC_Group_Data> {
                                                      const EC_Scalar_Data& y,
                                                      RandomNumberGenerator& rng) const;
 
+      std::unique_ptr<EC_AffinePoint_Data> affine_add(const EC_AffinePoint_Data& p, const EC_AffinePoint_Data& q) const;
+
+      std::unique_ptr<EC_AffinePoint_Data> affine_neg(const EC_AffinePoint_Data& p) const;
+
       std::unique_ptr<EC_Mul2Table_Data> make_mul2_table(const EC_AffinePoint_Data& pt) const;
 
       const PCurve::PrimeOrderCurve& pcurve() const {