Add Solinas reduction for P-192

Only faster on 32-bit, it seems. On x86-32 provides a ~20% speedup

src/lib/math/pcurves/pcurves_secp192r1/pcurves_secp192r1.cpp

@@ -6,6 +6,7 @@
 
 #include <botan/internal/pcurves_instance.h>
 
+#include <botan/internal/pcurves_solinas.h>
 #include <botan/internal/pcurves_wrap.h>
 
 namespace Botan::PCurve {
@@ -14,6 +15,87 @@ namespace {
 
 namespace secp192r1 {
 
+template <typename Params>
+class Secp192r1Rep final {
+   public:
+      static constexpr auto P = Params::P;
+      static constexpr size_t N = Params::N;
+      typedef typename Params::W W;
+
+      constexpr static std::array<W, N> redc(const std::array<W, 2 * N>& z) {
+         const int64_t X00 = get_uint32(z.data(), 0);
+         const int64_t X01 = get_uint32(z.data(), 1);
+         const int64_t X02 = get_uint32(z.data(), 2);
+         const int64_t X03 = get_uint32(z.data(), 3);
+         const int64_t X04 = get_uint32(z.data(), 4);
+         const int64_t X05 = get_uint32(z.data(), 5);
+         const int64_t X06 = get_uint32(z.data(), 6);
+         const int64_t X07 = get_uint32(z.data(), 7);
+         const int64_t X08 = get_uint32(z.data(), 8);
+         const int64_t X09 = get_uint32(z.data(), 9);
+         const int64_t X10 = get_uint32(z.data(), 10);
+         const int64_t X11 = get_uint32(z.data(), 11);
+
+         const int64_t S0 = X00 + X06 + X10;
+         const int64_t S1 = X01 + X07 + X11;
+         const int64_t S2 = X02 + X06 + X08 + X10;
+         const int64_t S3 = X03 + X07 + X09 + X11;
+         const int64_t S4 = X04 + X08 + X10;
+         const int64_t S5 = X05 + X09 + X11;
+
+         std::array<W, N> r = {};
+
+         SolinasAccum sum(r);
+
+         sum.accum(S0);
+         sum.accum(S1);
+         sum.accum(S2);
+         sum.accum(S3);
+         sum.accum(S4);
+         sum.accum(S5);
+         const auto S = sum.final_carry(0);
+
+         BOTAN_DEBUG_ASSERT(S <= 3);
+
+         const auto correction = p192_mul_mod_192(S);
+         W borrow = bigint_sub2(r.data(), N, correction.data(), N);
+
+         bigint_cnd_add(borrow, r.data(), N, P.data(), N);
+
+         return r;
+      }
+
+      constexpr static std::array<W, N> one() { return std::array<W, N>{1}; }
+
+      constexpr static std::array<W, N> to_rep(const std::array<W, N>& x) { return x; }
+
+      constexpr static std::array<W, N> wide_to_rep(const std::array<W, 2 * N>& x) { return redc(x); }
+
+      constexpr static std::array<W, N> from_rep(const std::array<W, N>& z) { return z; }
+
+   private:
+      // Return (i*P-192) % 2**192
+      //
+      // Assumes i is small
+      constexpr static std::array<W, N> p192_mul_mod_192(W i) {
+         static_assert(WordInfo<W>::bits == 32 || WordInfo<W>::bits == 64);
+
+         // For small i, multiples of P-192 have a simple structure so it's faster to
+         // compute the value directly vs a (constant time) table lookup
+
+         auto r = P;
+
+         if constexpr(WordInfo<W>::bits == 32) {
+            r[2] -= i;
+            r[0] -= i;
+         } else {
+            r[1] -= i;
+            r[0] -= i;
+         }
+         return r;
+      }
+};
+
 // clang-format off
 class Params final : public EllipticCurveParameters<
    "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFFFFFFFFFFFF",
@@ -26,7 +108,13 @@ class Params final : public EllipticCurveParameters<
 
 // clang-format on
 
+#if BOTAN_MP_WORD_BITS == 32
+// Secp192r1Rep works for 64 bit also, but is at best marginally faster at least
+// on compilers/CPUs tested so far
+class Curve final : public EllipticCurve<Params, Secp192r1Rep> {};
+#else
 class Curve final : public EllipticCurve<Params> {};
+#endif
 
 }  // namespace secp192r1