path: root/crypto/secp256k1/curve.go

// Copyright 2010 The Go Authors. All rights reserved.
// Copyright 2011 ThePiachu. All rights reserved.
//
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package secp256k1

import (
    "crypto/elliptic"
    "math/big"
    "unsafe"

    "github.com/ethereum/go-ethereum/common/math"
)

/*
#include "libsecp256k1/include/secp256k1.h"
extern int secp256k1_ext_scalar_mul(const secp256k1_context* ctx, const unsigned char *point, const unsigned char *scalar);
*/
import "C"

// This code is from https://github.com/ThePiachu/GoBit and implements
// several Koblitz elliptic curves over prime fields.
//
// The curve methods, internally, on Jacobian coordinates. For a given
// (x, y) position on the curve, the Jacobian coordinates are (x1, y1,
// z1) where x = x1/z1² and y = y1/z1³. The greatest speedups come
// when the whole calculation can be performed within the transform
// (as in ScalarMult and ScalarBaseMult). But even for Add and Double,
// it's faster to apply and reverse the transform than to operate in
// affine coordinates.

// A BitCurve represents a Koblitz Curve with a=0.
// See http://www.hyperelliptic.org/EFD/g1p/auto-shortw.html
type BitCurve struct {
    P       *big.Int // the order of the underlying field
    N       *big.Int // the order of the base point
    B       *big.Int // the constant of the BitCurve equation
    Gx, Gy  *big.Int // (x,y) of the base point
    BitSize int      // the size of the underlying field
}

func (BitCurve *BitCurve) Params() *elliptic.CurveParams {
    return &elliptic.CurveParams{
        P:       BitCurve.P,
        N:       BitCurve.N,
        B:       BitCurve.B,
        Gx:      BitCurve.Gx,
        Gy:      BitCurve.Gy,
        BitSize: BitCurve.BitSize,
    }
}

// IsOnBitCurve returns true if the given (x,y) lies on the BitCurve.
func (BitCurve *BitCurve) IsOnCurve(x, y *big.Int) bool {
    // y² = x³ + b
    y2 := new(big.Int).Mul(y, y) //y²
    y2.Mod(y2, BitCurve.P)       //y²%P

    x3 := new(big.Int).Mul(x, x) //x²
    x3.Mul(x3, x)                //x³

    x3.Add(x3, BitCurve.B) //x³+B
    x3.Mod(x3, BitCurve.P) //(x³+B)%P

    return x3.Cmp(y2) == 0
}

//TODO: double check if the function is okay
// affineFromJacobian reverses the Jacobian transform. See the comment at the
// top of the file.
func (BitCurve *BitCurve) affineFromJacobian(x, y, z *big.Int) (xOut, yOut *big.Int) {
    zinv := new(big.Int).ModInverse(z, BitCurve.P)
    zinvsq := new(big.Int).Mul(zinv, zinv)

    xOut = new(big.Int).Mul(x, zinvsq)
    xOut.Mod(xOut, BitCurve.P)
    zinvsq.Mul(zinvsq, zinv)
    yOut = new(big.Int).Mul(y, zinvsq)
    yOut.Mod(yOut, BitCurve.P)
    return
}

// Add returns the sum of (x1,y1) and (x2,y2)
func (BitCurve *BitCurve) Add(x1, y1, x2, y2 *big.Int) (*big.Int, *big.Int) {
    z := new(big.Int).SetInt64(1)
    return BitCurve.affineFromJacobian(BitCurve.addJacobian(x1, y1, z, x2, y2, z))
}

// addJacobian takes two points in Jacobian coordinates, (x1, y1, z1) and
// (x2, y2, z2) and returns their sum, also in Jacobian form.
func (BitCurve *BitCurve) addJacobian(x1, y1, z1, x2, y2, z2 *big.Int) (*big.Int, *big.Int, *big.Int) {
    // See http://hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-0.html#addition-add-2007-bl
    z1z1 := new(big.Int).Mul(z1, z1)
    z1z1.Mod(z1z1, BitCurve.P)
    z2z2 := new(big.Int).Mul(z2, z2)
    z2z2.Mod(z2z2, BitCurve.P)

    u1 := new(big.Int).Mul(x1, z2z2)
    u1.Mod(u1, BitCurve.P)
    u2 := new(big.Int).Mul(x2, z1z1)
    u2.Mod(u2, BitCurve.P)
    h := new(big.Int).Sub(u2, u1)
    if h.Sign() == -1 {
        h.Add(h, BitCurve.P)
    }
    i := new(big.Int).Lsh(h, 1)
    i.Mul(i, i)
    j := new(big.Int).Mul(h, i)

    s1 := new(big.Int).Mul(y1, z2)
    s1.Mul(s1, z2z2)
    s1.Mod(s1, BitCurve.P)
    s2 := new(big.Int).Mul(y2, z1)
    s2.Mul(s2, z1z1)
    s2.Mod(s2, BitCurve.P)
    r := new(big.Int).Sub(s2, s1)
    if r.Sign() == -1 {
        r.Add(r, BitCurve.P)
    }
    r.Lsh(r, 1)
    v := new(big.Int).Mul(u1, i)

    x3 := new(big.Int).Set(r)
    x3.Mul(x3, x3)
    x3.Sub(x3, j)
    x3.Sub(x3, v)
    x3.Sub(x3, v)
    x3.Mod(x3, BitCurve.P)

    y3 := new(big.Int).Set(r)
    v.Sub(v, x3)
    y3.Mul(y3, v)
    s1.Mul(s1, j)
    s1.Lsh(s1, 1)
    y3.Sub(y3, s1)
    y3.Mod(y3, BitCurve.P)

    z3 := new(big.Int).Add(z1, z2)
    z3.Mul(z3, z3)
    z3.Sub(z3, z1z1)
    if z3.Sign() == -1 {
        z3.Add(z3, BitCurve.P)
    }
    z3.Sub(z3, z2z2)
    if z3.Sign() == -1 {
        z3.Add(z3, BitCurve.P)
    }
    z3.Mul(z3, h)
    z3.Mod(z3, BitCurve.P)

    return x3, y3, z3
}

// Double returns 2*(x,y)
func (BitCurve *BitCurve) Double(x1, y1 *big.Int) (*big.Int, *big.Int) {
    z1 := new(big.Int).SetInt64(1)
    return BitCurve.affineFromJacobian(BitCurve.doubleJacobian(x1, y1, z1))
}

// doubleJacobian takes a point in Jacobian coordinates, (x, y, z), and
// returns its double, also in Jacobian form.
func (BitCurve *BitCurve) doubleJacobian(x, y, z *big.Int) (*big.Int, *big.Int, *big.Int) {
    // See http://hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-0.html#doubling-dbl-2009-l

    a := new(big.Int).Mul(x, x) //X1²
    b := new(big.Int).Mul(y, y) //Y1²
    c := new(big.Int).Mul(b, b) //B²

    d := new(big.Int).Add(x, b) //X1+B
    d.Mul(d, d)                 //(X1+B)²
    d.Sub(d, a)                 //(X1+B)²-A
    d.Sub(d, c)                 //(X1+B)²-A-C
    d.Mul(d, big.NewInt(2))     //2*((X1+B)²-A-C)

    e := new(big.Int).Mul(big.NewInt(3), a) //3*A
    f := new(big.Int).Mul(e, e)             //E²

    x3 := new(big.Int).Mul(big.NewInt(2), d) //2*D
    x3.Sub(f, x3)                            //F-2*D
    x3.Mod(x3, BitCurve.P)

    y3 := new(big.Int).Sub(d, x3)                  //D-X3
    y3.Mul(e, y3)                                  //E*(D-X3)
    y3.Sub(y3, new(big.Int).Mul(big.NewInt(8), c)) //E*(D-X3)-8*C
    y3.Mod(y3, BitCurve.P)

    z3 := new(big.Int).Mul(y, z) //Y1*Z1
    z3.Mul(big.NewInt(2), z3)    //3*Y1*Z1
    z3.Mod(z3, BitCurve.P)

    return x3, y3, z3
}

func (BitCurve *BitCurve) ScalarMult(Bx, By *big.Int, scalar []byte) (*big.Int, *big.Int) {
    // Ensure scalar is exactly 32 bytes. We pad always, even if
    // scalar is 32 bytes long, to avoid a timing side channel.
    if len(scalar) > 32 {
        panic("can't handle scalars > 256 bits")
    }
    // NOTE: potential timing issue
    padded := make([]byte, 32)
    copy(padded[32-len(scalar):], scalar)
    scalar = padded

    // Do the multiplication in C, updating point.
    point := make([]byte, 64)
    math.ReadBits(Bx, point[:32])
    math.ReadBits(By, point[32:])
    pointPtr := (*C.uchar)(unsafe.Pointer(&point[0]))
    scalarPtr := (*C.uchar)(unsafe.Pointer(&scalar[0]))
    res := C.secp256k1_ext_scalar_mul(context, pointPtr, scalarPtr)

    // Unpack the result and clear temporaries.
    x := new(big.Int).SetBytes(point[:32])
    y := new(big.Int).SetBytes(point[32:])
    for i := range point {
        point[i] = 0
    }
    for i := range padded {
        scalar[i] = 0
    }
    if res != 1 {
        return nil, nil
    }
    return x, y
}

// ScalarBaseMult returns k*G, where G is the base point of the group and k is
// an integer in big-endian form.
func (BitCurve *BitCurve) ScalarBaseMult(k []byte) (*big.Int, *big.Int) {
    return BitCurve.ScalarMult(BitCurve.Gx, BitCurve.Gy, k)
}

// Marshal converts a point into the form specified in section 4.3.6 of ANSI
// X9.62.
func (BitCurve *BitCurve) Marshal(x, y *big.Int) []byte {
    byteLen := (BitCurve.BitSize + 7) >> 3
    ret := make([]byte, 1+2*byteLen)
    ret[0] = 4 // uncompressed point flag
    math.ReadBits(x, ret[1:1+byteLen])
    math.ReadBits(y, ret[1+byteLen:])
    return ret
}

// Unmarshal converts a point, serialised by Marshal, into an x, y pair. On
// error, x = nil.
func (BitCurve *BitCurve) Unmarshal(data []byte) (x, y *big.Int) {
    byteLen := (BitCurve.BitSize + 7) >> 3
    if len(data) != 1+2*byteLen {
        return
    }
    if data[0] != 4 { // uncompressed form
        return
    }
    x = new(big.Int).SetBytes(data[1 : 1+byteLen])
    y = new(big.Int).SetBytes(data[1+byteLen:])
    return
}

var theCurve = new(BitCurve)

func init() {
    // See SEC 2 section 2.7.1
    // curve parameters taken from:
    // http://www.secg.org/collateral/sec2_final.pdf
    theCurve.P, _ = new(big.Int).SetString("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F", 16)
    theCurve.N, _ = new(big.Int).SetString("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141", 16)
    theCurve.B, _ = new(big.Int).SetString("0000000000000000000000000000000000000000000000000000000000000007", 16)
    theCurve.Gx, _ = new(big.Int).SetString("79BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798", 16)
    theCurve.Gy, _ = new(big.Int).SetString("483ADA7726A3C4655DA4FBFC0E1108A8FD17B448A68554199C47D08FFB10D4B8", 16)
    theCurve.BitSize = 256
}

// S256 returns a BitCurve which implements secp256k1.
func S256() *BitCurve {
    return theCurve
}