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Completing the Group interface and hash to curve.
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@armfazh
armfazh committed Jun 3, 2022
1 parent 45008f1 commit 3bf0eae
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310 changes: 222 additions & 88 deletions ecc/goldilocks/goldilocks.go
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@@ -1,4 +1,4 @@
// Package goldilocks provides arithmetic operations on the Goldilocks curve.
// Package goldilocks provides arithmetic operations on the Goldilocks (edwards448) curve.
//
// Goldilocks Curve
//
Expand All @@ -19,111 +19,142 @@ package goldilocks

import (
"crypto/subtle"
"encoding/binary"
"errors"
"math/bits"

"github.com/cloudflare/circl/internal/ted448"
ted "github.com/cloudflare/circl/ecc/goldilocks/internal/ted448"
fp "github.com/cloudflare/circl/math/fp448"
)

type Scalar = ted448.Scalar
// Point defines a point on the Goldilocks curve using extended projective
// coordinates. For any affine point (x,y) it holds x = X/Z, y = Y/Z, and
// T = Ta*Tb = X*Y/Z.
type Point ted.Point

type Point ted448.Point
// Identity returns the identity point.
func Identity() Point { return Point(ted.Identity()) }

// EncodingSize is the size (in bytes) of an encoded point on the Goldilocks curve.
const EncodingSize = fp.Size + 1
// Generator returns the generator point.
func Generator() Point { return Point{X: genX, Y: genY, Z: fp.One(), Ta: genX, Tb: genY} }

// ErrInvalidDecoding alerts of an error during decoding a point.
var ErrInvalidDecoding = errors.New("invalid decoding")
// Order returns the number of points in the prime subgroup in little-endian order.
func Order() []byte { r := ted.Order(); return r[:] }

// Decode if succeeds constructs a point by decoding the first
// EncodingSize bytes of data.
func (P *Point) Decode(data *[EncodingSize]byte) error {
x, y := &fp.Elt{}, &fp.Elt{}
isByteZero := subtle.ConstantTimeByteEq(data[EncodingSize-1]&0x7F, 0x00)
signX := int(data[EncodingSize-1] >> 7)
copy(y[:], data[:fp.Size])
p := fp.P()
isLessThanP := isLessThan(y[:], p[:])
// ParamD is the D parameter of the Goldilocks curve, D=-39081 in Fp.
func ParamD() fp.Elt { return paramD }

u, v := &fp.Elt{}, &fp.Elt{}
one := fp.One()
fp.Sqr(u, y) // u = y^2
fp.Mul(v, u, &paramD) // v = dy^2
fp.Sub(u, u, &one) // u = y^2-1
fp.Sub(v, v, &one) // v = dy^2-a
isQR := fp.InvSqrt(x, u, v) // x = sqrt(u/v)
isValidXSign := 1 - (fp.IsZero(x) & signX)
fp.Neg(u, x) // u = -x
fp.Cmov(x, u, uint(signX^fp.Parity(x))) // if signX != x mod 2
func (P Point) String() string { return ted.Point(P).String() }
func (P *Point) ToAffine() { (*ted.Point)(P).ToAffine() }
func (P *Point) Neg() { (*ted.Point)(P).Neg() }
func (P *Point) IsEqual(Q *Point) int { return (*ted.Point)(P).IsEqual((*ted.Point)(Q)) }
func (P *Point) Double() { P.Add(P) }
func (P *Point) Add(Q *Point) {
// Formula as in Eq.(5) of "Twisted Edwards Curves Revisited" by
// Hisil H., Wong K.KH., Carter G., Dawson E. (2008)
// https://doi.org/10.1007/978-3-540-89255-7_20
// Formula for curves with a=1.
Px, Py, Pz, Pta, Ptb := &P.X, &P.Y, &P.Z, &P.Ta, &P.Tb
Qx, Qy, Qz, Qta, Qtb := &Q.X, &Q.Y, &Q.Z, &Q.Ta, &Q.Tb

b0 := isByteZero
b1 := isLessThanP
b2 := isQR
b3 := isValidXSign
b := uint(subtle.ConstantTimeEq(int32(8*b3+4*b2+2*b1+b0), 0xF))
fp.Cmov(&P.X, x, b)
fp.Cmov(&P.Y, y, b)
fp.Cmov(&P.Ta, x, b)
fp.Cmov(&P.Tb, y, b)
fp.Cmov(&P.Z, &one, b)
if b == 0 {
return ErrInvalidDecoding
}
return nil
a, b, c, d := &fp.Elt{}, &fp.Elt{}, &fp.Elt{}, &fp.Elt{}
e, f, g, h := &fp.Elt{}, &fp.Elt{}, &fp.Elt{}, &fp.Elt{}
ee, ff := &fp.Elt{}, &fp.Elt{}

fp.Mul(a, Px, Qx) // A = x1*x2
fp.Mul(b, Py, Qy) // B = y1*y2
fp.Mul(c, Pta, Ptb) // C = d*t1*t2
fp.Mul(c, c, Qta) //
fp.Mul(c, c, Qtb) //
fp.Mul(c, c, &paramD) //
fp.Mul(d, Pz, Qz) // D = z1*z2
fp.Add(ee, Px, Py) // x1+y1
fp.Add(ff, Qx, Qy) // x2+y2
fp.Mul(e, ee, ff) // E = (x1+y1)*(x2+y2)-A-B
fp.Sub(e, e, a) //
fp.Sub(e, e, b) //
fp.Sub(f, d, c) // F = D-C
fp.Add(g, d, c) // g = D+C
fp.Sub(h, b, a) // H = B-A
fp.Mul(Px, e, f) // X = E * F
fp.Mul(Py, g, h) // Y = G * H
fp.Mul(Pz, f, g) // Z = F * G
P.Ta, P.Tb = *e, *h // T = E * H
}

// Encode sets data with the unique encoding of the point P.
func (P *Point) Encode(data *[EncodingSize]byte) error {
x, y, invZ := &fp.Elt{}, &fp.Elt{}, &fp.Elt{}
fp.Inv(invZ, &P.Z) // 1/z
fp.Mul(x, &P.X, invZ) // x/z
fp.Mul(y, &P.Y, invZ) // y/z
fp.Modp(x)
fp.Modp(y)
data[EncodingSize-1] = (x[0] & 1) << 7
return fp.ToBytes(data[:fp.Size], y)
func (P *Point) CMov(Q *Point, b uint) {
fp.Cmov(&P.X, &Q.X, b)
fp.Cmov(&P.Y, &Q.Y, b)
fp.Cmov(&P.Z, &Q.Z, b)
fp.Cmov(&P.Ta, &Q.Ta, b)
fp.Cmov(&P.Tb, &Q.Tb, b)
}

// ScalarBaseMult calculates P = kG, where G is the generator of the Goldilocks
// curve. This function runs in constant time.
func (P *Point) ScalarBaseMult(k *Scalar) {
k4 := &Scalar{}
divBy4(k4, k)
var Q ted448.Point
ted448.ScalarBaseMult(&Q, k4)
// TODO: recheck if this works for any scalar, likely yes.
k4 := &ted.Scalar{}
divBy4ModOrder(k4, &k.k)
var Q ted.Point
ted.ScalarBaseMult(&Q, k4)
push(P, &Q)
}

// CombinedMult calculates P = mG+nQ, where G is the generator of the Goldilocks
// curve. This function does NOT run in constant time as is only used for
// signature verification.
func (P *Point) CombinedMult(m, n *Scalar, Q *Point) {
m4, n4 := &Scalar{}, &Scalar{}
divBy4(m4, m)
divBy4(n4, n)
var R, phiQ ted448.Point
var m4, n4 ted.Scalar
divBy4ModOrder(&m4, &m.k)
divBy4ModOrder(&n4, &n.k)
var R, phiQ ted.Point
pull(&phiQ, Q)
ted448.CombinedMult(&R, m4, n4, &phiQ)
ted.CombinedMult(&R, &m4, &n4, &phiQ)
push(P, &R)
}

func (P *Point) Neg() { fp.Neg(&P.X, &P.X); fp.Neg(&P.Ta, &P.Ta) }
func (P *Point) ScalarMult(k *Scalar, Q *Point) {
var T [4]Point
T[0] = Identity()
T[1] = *Q
T[2] = *Q
T[2].Double()
T[3] = T[2]
T[3].Add(Q)

// Order returns a scalar with the order of the group.
func Order() Scalar { return ted448.Order() }
var R Point
kMod4 := int32(k.k[0] & 0b11)
for i := range T {
R.CMov(&T[i], uint(subtle.ConstantTimeEq(int32(i), kMod4)))
}

// divBy4 calculates z = x/4 mod order.
func divBy4(z, x *Scalar) { z.Mul(x, &invFour) }
var kDiv4 ted.Scalar
for i := 0; i < ScalarSize-1; i++ {
kDiv4[i] = (k.k[i+1] << 6) | (k.k[i] >> 2)
}
kDiv4[ScalarSize-1] = k.k[ScalarSize-1] >> 2

var phikQ, phiQ ted.Point
pull(&phiQ, Q)
ted.ScalarMult(&phikQ, &kDiv4, &phiQ)
push(P, &phikQ)
P.Add(&R)
}

// divBy4ModOrder calculates z = x/4 mod order.
func divBy4ModOrder(z, x *ted.Scalar) {
z.Mul(x, &invFour)
}

// pull calculates Q = Iso4(P), where P is a Goldilocks point and Q is a ted448 point.
func pull(Q *ted448.Point, P *Point) { isogeny4(Q, (*ted448.Point)(P), true) }
func pull(Q *ted.Point, P *Point) { isogeny4(Q, (*ted.Point)(P), true) }

// push calculates Q = Iso4^-1(P), where P is a ted448 point and Q is a Goldilocks point.
func push(Q *Point, P *ted448.Point) { isogeny4((*ted448.Point)(Q), P, false) }
func push(Q *Point, P *ted.Point) { isogeny4((*ted.Point)(Q), P, false) }

// isogeny4 is a birational map between ted448 and Goldilocks curves.
func isogeny4(Q, P *ted448.Point, isPull bool) {
func isogeny4(Q, P *ted.Point, isPull bool) {
Px, Py, Pz := &P.X, &P.Y, &P.Z
a, b, c, d, e, f, g, h := &Q.X, &Q.Y, &Q.Z, &fp.Elt{}, &Q.Ta, &Q.X, &Q.Y, &Q.Tb
fp.Add(e, Px, Py) // x+y
Expand All @@ -147,6 +178,78 @@ func isogeny4(Q, P *ted448.Point, isPull bool) {
fp.Mul(&Q.Y, g, h) // Y = G * H, // T = E * H
}

type Scalar struct{ k ted.Scalar }

func (z Scalar) String() string { return z.k.String() }
func (z *Scalar) Add(x, y *Scalar) { z.k.Add(&x.k, &y.k) }
func (z *Scalar) Sub(x, y *Scalar) { z.k.Sub(&x.k, &y.k) }
func (z *Scalar) Mul(x, y *Scalar) { z.k.Mul(&x.k, &y.k) }
func (z *Scalar) Neg(x *Scalar) { z.k.Neg(&x.k) }
func (z *Scalar) Inv(x *Scalar) { z.k.Inv(&x.k) }
func (z *Scalar) IsEqual(x *Scalar) int { return subtle.ConstantTimeCompare(z.k[:], x.k[:]) }
func (z *Scalar) SetUint64(n uint64) { z.k = ted.Scalar{}; binary.LittleEndian.PutUint64(z.k[:], n) }

// UnmarshalBinary recovers the scalar from its byte representation in big-endian order.
func (z *Scalar) UnmarshalBinary(b []byte) error { return z.k.UnmarshalBinary(b) }

// MarshalBinary returns the scalar byte representation in big-endian order.
func (z *Scalar) MarshalBinary() ([]byte, error) { return z.k.MarshalBinary() }

// ToBytesLE returns the scalar byte representation in little-endian order.
func (z *Scalar) ToBytesLE() []byte { return z.k.ToBytesLE() }

// ToBytesBE returns the scalar byte representation in big-endian order.
func (z *Scalar) ToBytesBE() []byte { return z.k.ToBytesBE() }

// FromBytesLE stores z = x mod order, where x is a number stored in little-endian order.
func (z *Scalar) FromBytesLE(x []byte) { z.k.FromBytesLE(x) }

// FromBytesBE stores z = x mod order, where x is a number stored in big-endian order.
func (z *Scalar) FromBytesBE(x []byte) { z.k.FromBytesBE(x) }

var (
// genX is the x-coordinate of the generator of Goldilocks curve.
genX = fp.Elt{ // little-endian
0x5e, 0xc0, 0x0c, 0xc7, 0x2b, 0xa8, 0x26, 0x26,
0x8e, 0x93, 0x00, 0x8b, 0xe1, 0x80, 0x3b, 0x43,
0x11, 0x65, 0xb6, 0x2a, 0xf7, 0x1a, 0xae, 0x12,
0x64, 0xa4, 0xd3, 0xa3, 0x24, 0xe3, 0x6d, 0xea,
0x67, 0x17, 0x0f, 0x47, 0x70, 0x65, 0x14, 0x9e,
0xda, 0x36, 0xbf, 0x22, 0xa6, 0x15, 0x1d, 0x22,
0xed, 0x0d, 0xed, 0x6b, 0xc6, 0x70, 0x19, 0x4f,
}
// genY is the y-coordinate of the generator of Goldilocks curve.
genY = fp.Elt{ // little-endian
0x14, 0xfa, 0x30, 0xf2, 0x5b, 0x79, 0x08, 0x98,
0xad, 0xc8, 0xd7, 0x4e, 0x2c, 0x13, 0xbd, 0xfd,
0xc4, 0x39, 0x7c, 0xe6, 0x1c, 0xff, 0xd3, 0x3a,
0xd7, 0xc2, 0xa0, 0x05, 0x1e, 0x9c, 0x78, 0x87,
0x40, 0x98, 0xa3, 0x6c, 0x73, 0x73, 0xea, 0x4b,
0x62, 0xc7, 0xc9, 0x56, 0x37, 0x20, 0x76, 0x88,
0x24, 0xbc, 0xb6, 0x6e, 0x71, 0x46, 0x3f, 0x69,
}
// paramD is the D parameter of the Goldilocks curve, D=-39081 in Fp.
paramD = fp.Elt{ // little-endian
0x56, 0x67, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xfe, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
}
// invFour is 1/4 mod order, where order = ted.Order().
invFour = ted.Scalar{ // little-endian
0x3d, 0x11, 0xd6, 0xaa, 0xa4, 0x30, 0xde, 0x48,
0xd5, 0x63, 0x71, 0xa3, 0x9c, 0x30, 0x5b, 0x08,
0xa4, 0x8d, 0xb5, 0x6b, 0xd2, 0xb6, 0x13, 0x71,
0xfa, 0x88, 0x32, 0xdf, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x0f,
}
)

// isLessThan returns 1 if 0 <= x < y, and assumes that slices are of the
// same length and are interpreted in little-endian order.
func isLessThan(x, y []byte) int {
Expand All @@ -159,25 +262,56 @@ func isLessThan(x, y []byte) int {
return ((xi - yi) >> (bits.UintSize - 1)) & 1
}

var (
// invFour is 1/4 mod order, where order = ted448.Order().
invFour = Scalar{
0x3d, 0x11, 0xd6, 0xaa, 0xa4, 0x30, 0xde, 0x48,
0xd5, 0x63, 0x71, 0xa3, 0x9c, 0x30, 0x5b, 0x08,
0xa4, 0x8d, 0xb5, 0x6b, 0xd2, 0xb6, 0x13, 0x71,
0xfa, 0x88, 0x32, 0xdf, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x0f,
}
// paramD is the D parameter of the Goldilocks curve, D=-39081 in Fp.
paramD = fp.Elt{
0x56, 0x67, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xfe, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
// Decode if succeeds constructs a point by decoding the first
// EncodingSize bytes of data.
func (P *Point) Decode(data *[EncodingSize]byte) error {
x, y := &fp.Elt{}, &fp.Elt{}
isByteZero := subtle.ConstantTimeByteEq(data[EncodingSize-1]&0x7F, 0x00)
signX := int(data[EncodingSize-1] >> 7)
copy(y[:], data[:fp.Size])
p := fp.P()
isLessThanP := isLessThan(y[:], p[:])

u, v := &fp.Elt{}, &fp.Elt{}
one := fp.One()
fp.Sqr(u, y) // u = y^2
fp.Mul(v, u, &paramD) // v = dy^2
fp.Sub(u, u, &one) // u = y^2-1
fp.Sub(v, v, &one) // v = dy^2-a
isQR := fp.InvSqrt(x, u, v) // x = sqrt(u/v)
isValidXSign := 1 - (fp.IsZero(x) & signX)
fp.Neg(u, x) // u = -x
fp.Cmov(x, u, uint(signX^fp.Parity(x))) // if signX != x mod 2

b0 := isByteZero
b1 := isLessThanP
b2 := isQR
b3 := isValidXSign
b := uint(subtle.ConstantTimeEq(int32(8*b3+4*b2+2*b1+b0), 0xF))
fp.Cmov(&P.X, x, b)
fp.Cmov(&P.Y, y, b)
fp.Cmov(&P.Ta, x, b)
fp.Cmov(&P.Tb, y, b)
fp.Cmov(&P.Z, &one, b)
if b == 0 {
return ErrInvalidDecoding
}
return nil
}

// Encode sets data with the unique encoding of the point P.
func (P *Point) Encode(data *[EncodingSize]byte) error {
P.ToAffine()
data[EncodingSize-1] = (P.X[0] & 1) << 7
return fp.ToBytes(data[:fp.Size], &P.Y)
}

const (
// EncodingSize is the size (in bytes) of an encoded point on the Goldilocks curve.
EncodingSize = fp.Size + 1
// ScalarSize is the size (in bytes) of scalars.
ScalarSize = ted.ScalarSize
)

// ErrInvalidDecoding alerts of an error during decoding a point.
var ErrInvalidDecoding = errors.New("goldilocks: invalid point decoding")

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