pedersen-libsnark/ecgadget.hpp

#include "libsnark/gadgetlib1/gadgets/basic_gadgets.hpp"

using namespace libsnark;

// Double the EC point (inx,iny) to yield (outx,outy).  The input point
// must not be the point at infinity.
template<typename FieldT>
class ec_double_gadget : public gadget<FieldT> {
private:
  pb_variable<FieldT> lambda, inxsq;
public:
  const pb_variable<FieldT> outx, outy, inx, iny;

  ec_double_gadget(protoboard<FieldT> &pb,
              const pb_variable<FieldT> &outx,
              const pb_variable<FieldT> &outy,
              const pb_variable<FieldT> &inx,
              const pb_variable<FieldT> &iny) : 
    gadget<FieldT>(pb, "ec_double_gadget"), outx(outx), outy(outy),
        inx(inx), iny(iny)
  {
    // Allocate variables to protoboard
    // The strings (like "x") are only for debugging purposes
	  
    lambda.allocate(this->pb, "lambda");
    inxsq.allocate(this->pb, "inxsq");
  }

  void generate_r1cs_constraints()
  {
    // inxsq = inx * inx
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>(inx, inx, inxsq));

    // 2 * iny * lambda = 3 * inxsq - 3  (a = -3 on our curve)
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>(2 * iny, lambda, 3 * inxsq - 3));

    // outx = lambda^2 - 2 * inx
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>(lambda, lambda, outx + 2 * inx));

    // outy = lambda * (inx - outx) - iny
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>(lambda, inx - outx, outy + iny));

  }

  void generate_r1cs_witness()
  {
    this->pb.val(inxsq) = this->pb.val(inx) * this->pb.val(inx);
    this->pb.val(lambda) = (this->pb.val(inxsq) * 3 - 3) * (this->pb.val(iny) * 2).inverse();
    this->pb.val(outx) = this->pb.val(lambda).squared() - this->pb.val(inx) * 2;
    this->pb.val(outy) = this->pb.val(lambda) * (this->pb.val(inx) - this->pb.val(outx)) - this->pb.val(iny);
  }
};

// Add nothing, G, H, or G+H to the EC point (inx,iny) to yield
// (outx,outy).  The input point must not be the point at infinity.
// The two input bits addG and addH control what is added.
template<typename FieldT>
class ec_add_GH_gadget : public gadget<FieldT> {
private:
  pb_variable<FieldT> lambda, sumx, sumy, move;
  const FieldT Gx, Gy, Hx, Hy, GHx, GHy;
public:
  const pb_variable<FieldT> outx, outy, inx, iny, addG, addH;

  ec_add_GH_gadget(protoboard<FieldT> &pb,
              const pb_variable<FieldT> &outx,
              const pb_variable<FieldT> &outy,
              const pb_variable<FieldT> &inx,
              const pb_variable<FieldT> &iny,
              const pb_variable<FieldT> &addG,
              const pb_variable<FieldT> &addH) : 
    gadget<FieldT>(pb, "ec_add_GH_gadget"),
    Gx(0),
    Gy("11977228949870389393715360594190192321220966033310912010610740966317727761886"),
    Hx(1),
    Hy("21803877843449984883423225223478944275188924769286999517937427649571474907279"),
    GHx("2864090850787705444524344020850508438903451433901276387624248428140647539638"),
    GHy("3350168998338968221269367365107720885864670493693161027931048546881356285970"),
    outx(outx), outy(outy), inx(inx), iny(iny), addG(addG), addH(addH)
  {
    // Allocate variables to protoboard
    // The strings (like "x") are only for debugging purposes
	  
    lambda.allocate(this->pb, "lambda");
    sumx.allocate(this->pb, "sumx");
    sumy.allocate(this->pb, "sumy");
    move.allocate(this->pb, "move");
  }

  void generate_r1cs_constraints()
  {
    // Strategy: if addG = addH = 0, we compute some nonsense but throw
    // it away later.  Otherwise, the coordinates of addG * G + addH * H
    // are a _linear_ function of addG and addH (since G and H are global
    // constants)

    // In particular:
    // G = (0, 11977228949870389393715360594190192321220966033310912010610740966317727761886)
    // H = (1, 21803877843449984883423225223478944275188924769286999517937427649571474907279)
    // G+H = (2864090850787705444524344020850508438903451433901276387624248428140647539638, 3350168998338968221269367365107720885864670493693161027931048546881356285970)
    // so the point to add is ( (GHx - Hx) * addG + (GHx - Gx) * addH + (Gx + Hx - GHx), (GHy - Hy) * addG + (GHy - Gy) * addH + (Gy + Hy - GHy))
    // = (2864090850787705444524344020850508438903451433901276387624248428140647539637 * addG + 2864090850787705444524344020850508438903451433901276387624248428140647539638 * addH - 2864090850787705444524344020850508438903451433901276387624248428140647539637, -18453708845111016662153857858371223389324254275593838490006379102690118621309 * addG - 8627059951531421172445993229082471435356295539617750982679692419436371475916 * addH + 30430937794981406055869218452561415710545220308904750500617120069007846383195)

    // (addx - inx) * lambda = addy - iny
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>((GHx - Hx) * addG + (GHx - Gx) * addH + (Gx + Hx - GHx) - inx, lambda, (GHy - Hy) * addG + (GHy - Gy) * addH + (Gy + Hy - GHy) - iny));

    // sumx = lambda^2 - (addx + inx)
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>(lambda, lambda, sumx + (GHx - Hx) * addG + (GHx - Gx) * addH + (Gx + Hx - GHx) + inx));

    // sumy = lambda * (inx - sumx) - iny
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>(lambda, inx - sumx, sumy + iny));

    // Now we want to conditionally move the sum.  We want that
    // outx = (addG || addH) ? sumx : inx
    // outy = (addG || addH) ? sumy : iny

    // so we compute move = addG || addH, and then
    // outx = inx + (sumx - inx) * move
    // outy = iny + (sumy - iny) * move

    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>(1 - addG, 1 - addH, 1 - move));
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>(sumx - inx, move, outx - inx));
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>(sumy - iny, move, outy - iny));

  }

  void generate_r1cs_witness()
  {
    FieldT addxval = (GHx - Hx) * this->pb.val(addG) + (GHx - Gx) * this->pb.val(addH) + (Gx + Hx - GHx);
    FieldT addyval = (GHy - Hy) * this->pb.val(addG) + (GHy - Gy) * this->pb.val(addH) + (Gy + Hy - GHy);
    this->pb.val(lambda) = (addyval - this->pb.val(iny)) * (addxval - this->pb.val(inx)).inverse();
    this->pb.val(sumx) = this->pb.val(lambda).squared() - (addxval + this->pb.val(inx));
    this->pb.val(sumy) = this->pb.val(lambda) * (this->pb.val(inx) - this->pb.val(sumx)) - this->pb.val(iny);

    bool aG = this->pb.val(addG) != FieldT(0);
    bool aH = this->pb.val(addH) != FieldT(0);
    this->pb.val(move) = aG || aH;
    this->pb.val(outx) = this->pb.val(aG || aH ? sumx : inx);
    this->pb.val(outy) = this->pb.val(aG || aH ? sumy : iny);
  }
};

// Compute a*G + b*H as (outx, outy), given a and b as bit vectors.
// a and b must be of the same size.  The _caller_ is responsible for
// proving that the elements of avec and bvec are bits.
template<typename FieldT>
class ec_pedersen_vec_gadget : public gadget<FieldT> {
private:
  pb_variable_array<FieldT> accumx, accumy, daccumx, daccumy;
  pb_variable<FieldT> lambda;
  std::vector<ec_double_gadget<FieldT> > doublers;
  std::vector<ec_add_GH_gadget<FieldT> > adders;
  const FieldT Cx, Cy, CGx, CGy, CHx, CHy, CGHx, CGHy;
  FieldT m2nCx, m2nCy;

  // Compute m2nC = -2^n * C.  We can precomute the answers for values
  // of n we expect to get.
  void compute_m2nC(FieldT &m2nCx, FieldT &m2nCy, size_t n)
  {
    if (n == 253) {
        m2nCx = FieldT("2630025903576807331238993847875694711243784786568881628418508626984487096258");
        m2nCy = FieldT("17628834417659968531880949658739649785248429713924280788649629869316127047701");
    } else {
        // Invariant: m2iC = -2^i * C
        FieldT m2iCx = Cx;
        FieldT m2iCy = -Cy;
        size_t i = 0;
        while (i < n) {
            FieldT xsq = m2iCx.squared();
            FieldT lambda = (xsq * 3 - 3) * (m2iCy * 2).inverse();
            FieldT m2iCxo = lambda.squared() - m2iCx * 2;
            FieldT m2iCyo = lambda * (m2iCx - m2iCxo) - m2iCy;
            m2iCx = m2iCxo;
            m2iCy = m2iCyo;
            ++i;
        }
        m2nCx = m2iCx;
        m2nCy = m2iCy;
    }
  }

public:
  const pb_variable<FieldT> outx, outy;
  const pb_variable_array<FieldT> avec, bvec;

  ec_pedersen_vec_gadget(protoboard<FieldT> &pb,
              const pb_variable<FieldT> &outx,
              const pb_variable<FieldT> &outy,
              const pb_variable_array<FieldT> &avec,
              const pb_variable_array<FieldT> &bvec) :
    gadget<FieldT>(pb, "ec_pedersen_vec_gadget"),
    Cx(2),
    Cy("4950745124018817972378217179409499695353526031437053848725554590521829916331"),
    CGx("4998993376791159436553350546778310121346937620672073819457843493128326049156"),
    CGy("11119675827304465476900978353730540420130346377889406728458325551400357147144"),
    CHx("19614539896004018833724771305328960655474424364705508053472946746883341111010"),
    CHy("9853241351900213537247225242092949438866383394579783148395572971112906592855"),
    CGHx("10755582242294898568680134375159803731902153202607833320871336755950640390928"),
    CGHy("3110667473759844579409644567672992116704859238881299917617768683686288881761"),
    outx(outx), outy(outy), avec(avec), bvec(bvec)
  {
    // Allocate variables to protoboard
    // The strings (like "x") are only for debugging purposes
	  
    size_t numbits = avec.size();
    accumx.allocate(this->pb, numbits, "accumx");
    accumy.allocate(this->pb, numbits, "accumy");
    daccumx.allocate(this->pb, numbits-1, "daccumx");
    daccumy.allocate(this->pb, numbits-1, "daccumy");
    lambda.allocate(this->pb, "lambda");

    for (size_t i = 0; i < numbits-1; ++i) {
        doublers.emplace_back(this->pb, daccumx[i], daccumy[i],
            accumx[i], accumy[i]);
        adders.emplace_back(this->pb, accumx[i+1], accumy[i+1],
            daccumx[i], daccumy[i], avec[numbits-2-i], bvec[numbits-2-i]);
    }

    compute_m2nC(m2nCx, m2nCy, numbits-1);
  }

  void generate_r1cs_constraints()
  {
    // Strategy: We do a basic double-and-add, using the variant of a
    // single double and an add of one of (O, G, H, G+H) at each step.
    // *However*, there's a twist.  Our doubling and adding routines
    // don't handle the point at infinity O as the point to add to or to
    // double.  (Adding O as one of the four options above is fine.)
    // Normally, the double-and-add algorithm starts with an accumulator
    // of O, and that won't work for us.  So instead, we start the
    // accumulator at a different base point C, whose discrete log
    // with respect to the (G,H) basis is unknown.  Then we'll end up
    // with an extra 2^n * C in the accumulator (where n is the number
    // of doublings we do), so at the end, we'll add the (constant!)
    // point -2^n * C to get the final result.  That the discrete log of
    // C is unknown means we won't encounter O along the way, either (if
    // we did, we could compute the DL of C in the (G,H) basis).

    // For the first bit, we just precompute C, C+G, C+H, C+G+H and use
    // the top bit of a and b to choose which one to start with.

    // accumx[0] = Cx + (CGx - Cx) * avec[numbits-1] + (CHx - Cx) *
    // bvec[numbits-1] + (CGHx - CGx - CHx + Cx) *
    // avec[numbits-1]*bvec[numbits-1] (and similarly for y)

    // We could possibly optimize this later by computing the a*b
    // product once, but then we'd have to pass a large linear
    // combination to a gadget, which it probably doesn't like?  It
    // would save just one constraint, so probably not so important?
    size_t numbits = avec.size();
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>((CGHx - CGx - CHx + Cx) * avec[numbits-1], bvec[numbits-1], accumx[0] - (Cx + (CGx - Cx) * avec[numbits-1] + (CHx - Cx) * bvec[numbits-1])));
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>((CGHy - CGy - CHy + Cy) * avec[numbits-1], bvec[numbits-1], accumy[0] - (Cy + (CGy - Cy) * avec[numbits-1] + (CHy - Cy) * bvec[numbits-1])));

    // After that, for each remaining bit of a and b, we use the
    // ec_double_gadget and the ec_add_GH_gadget to accumulate
    // the answer.
    for (size_t i = 0; i < numbits-1; ++i) {
        doublers[i].generate_r1cs_constraints();
        adders[i].generate_r1cs_constraints();
    }

    // Finally, we add the constant point -2^n * C to the result, where
    // n = numbits-1
    // (m2nCx - accumx[numbits-1]) * lambda = m2nCy - accumy[numbits-1]
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>(m2nCx - accumx[numbits-1], lambda, m2nCy - accumy[numbits-1]));
    // outx = lambda^2 - (m2nCx + accumx[numbits-1])
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>(lambda, lambda, outx + m2nCx + accumx[numbits-1]));
    // outy = lambda * (accumx[numbits-1] - outx) - accumy[numbits-1]
    this->pb.add_r1cs_constraint(r1cs_constraint<FieldT>(lambda, accumx[numbits-1] - outx, outy + accumy[numbits-1]));

  }

  void generate_r1cs_witness()
  {
    size_t numbits = avec.size();
    this->pb.val(accumx[0]) = Cx + (CGx - Cx) * this->pb.val(avec[numbits-1]) + (CHx - Cx) * this->pb.val(bvec[numbits-1]) + (CGHx - CGx - CHx + Cx) * this->pb.val(avec[numbits-1])*this->pb.val(bvec[numbits-1]);
    this->pb.val(accumy[0]) = Cy + (CGy - Cy) * this->pb.val(avec[numbits-1]) + (CHy - Cy) * this->pb.val(bvec[numbits-1]) + (CGHy - CGy - CHy + Cy) * this->pb.val(avec[numbits-1])*this->pb.val(bvec[numbits-1]);

    for (size_t i = 0; i < numbits-1; ++i) {
        doublers[i].generate_r1cs_witness();
        adders[i].generate_r1cs_witness();
    }

    this->pb.val(lambda) = (m2nCy - this->pb.val(accumy[numbits-1])) *
                           (m2nCx - this->pb.val(accumx[numbits-1])).inverse();
    this->pb.val(outx) = this->pb.val(lambda).squared() -
                         (m2nCx + this->pb.val(accumx[numbits-1]));
    this->pb.val(outy) = this->pb.val(lambda) *
                         (this->pb.val(accumx[numbits-1]) - this->pb.val(outx))
                         - this->pb.val(accumy[numbits-1]);
    
  }
};

// Compute a*G + b*H as (outx, outy), given a and b as field elements.
template<typename FieldT>
class ec_pedersen_gadget : public gadget<FieldT> {
private:
  pb_variable_array<FieldT> avec, bvec;
  std::vector<packing_gadget<FieldT> > packers;
  std::vector<ec_pedersen_vec_gadget<FieldT> > vecgadget;

public:
  const pb_variable<FieldT> outx, outy, a, b;

  ec_pedersen_gadget(protoboard<FieldT> &pb,
              const pb_variable<FieldT> &outx,
              const pb_variable<FieldT> &outy,
              const pb_variable<FieldT> &a,
              const pb_variable<FieldT> &b) :
    gadget<FieldT>(pb, "ec_pedersen_gadget"),
    outx(outx), outy(outy), a(a), b(b)
  {
    // Allocate variables to protoboard
    // The strings (like "x") are only for debugging purposes
	  
    FieldT minus1(-1);
    size_t numbits = FieldT::num_bits;
    avec.allocate(this->pb, numbits, "a");
    bvec.allocate(this->pb, numbits, "b");
    packers.emplace_back(this->pb, avec, a);
    packers.emplace_back(this->pb, bvec, b);
    vecgadget.emplace_back(this->pb, outx, outy, avec, bvec);
  }

  void generate_r1cs_constraints()
  {
    packers[0].generate_r1cs_constraints(true);
    packers[1].generate_r1cs_constraints(true);
    vecgadget[0].generate_r1cs_constraints();
  }

  void generate_r1cs_witness()
  {
    packers[0].generate_r1cs_witness_from_packed();
    packers[1].generate_r1cs_witness_from_packed();
    vecgadget[0].generate_r1cs_witness();
  }
};