// Decode an Edwards curve point into the given x,y coordinates.
// Returns an error if the input does not denote a valid curve point.
// Note that this does NOT check if the point is in the prime-order subgroup:
// an adversary could create an encoding denoting a point
// on the twist of the curve, or in a larger subgroup.
// However, the "safecurves" criteria (http://safecurves.cr.yp.to)
// ensure that none of these other subgroups are small
// other than the tiny ones represented by the cofactor;
// hence Diffie-Hellman exchange can be done without subgroup checking
// without exposing more than the least-significant bits of the scalar.
func (c *curve) decodePoint(bb []byte, x, y *nist.Int) error {

	// Convert from little-endian
	//fmt.Printf("decoding:\n%s\n", hex.Dump(bb))
	b := make([]byte, len(bb))
	util.Reverse(b, bb)

	// Extract the sign of the x-coordinate
	xsign := uint(b[0] >> 7)
	b[0] &^= 0x80

	// Extract the y-coordinate
	y.V.SetBytes(b)
	y.M = &c.P

	// Compute the corresponding x-coordinate
	if !c.solveForX(x, y) {
		return errors.New("invalid elliptic curve point")
	}
	if c.coordSign(x) != xsign {
		x.Neg(x)
	}

	return nil
}

// Encode an Edwards curve point.
// We use little-endian encoding for consistency with Ed25519.
func (c *curve) encodePoint(x, y *nist.Int) []byte {

	// Encode the y-coordinate
	b, _ := y.MarshalBinary()

	// Encode the sign of the x-coordinate.
	if y.M.BitLen()&7 == 0 {
		// No unused bits at the top of y-coordinate encoding,
		// so we must prepend a whole byte.
		b = append(make([]byte, 1), b...)
	}
	if c.coordSign(x) != 0 {
		b[0] |= 0x80
	}

	// Convert to little-endian
	util.Reverse(b, b)
	//fmt.Printf("encoding %s,%s:\n%s\n", x.String(), y.String(),
	//		hex.Dump(b))
	return b
}

// Equality test for two Points on the same curve.
// We can avoid inversions here because:
//
//	(X1/Z1,Y1/Z1) == (X2/Z2,Y2/Z2)
//		iff
//	(X1*Z2,Y1*Z2) == (X2*Z1,Y2*Z1)
//
func (P1 *projPoint) Equal(CP2 abstract.Point) bool {
	P2 := CP2.(*projPoint)
	var t1, t2 nist.Int
	xeq := t1.Mul(&P1.X, &P2.Z).Equal(t2.Mul(&P2.X, &P1.Z))
	yeq := t1.Mul(&P1.Y, &P2.Z).Equal(t2.Mul(&P2.Y, &P1.Z))
	return xeq && yeq
}

// Parameters defining Curve1174, as specified in:
// Bernstein et al, "Elligator: Elliptic-curve points indistinguishable
// from uniform random strings"
// http://elligator.cr.yp.to/elligator-20130828.pdf
//
func Param1174() *Param {
	var p Param
	var mi nist.Int

	p.Name = "1174"
	p.P.SetBit(zero, 251, 1).Sub(&p.P, big.NewInt(9))
	p.Q.SetString("45330879683285730139092453152713398835", 10)
	p.Q.Sub(&p.P, &p.Q).Div(&p.Q, big.NewInt(4))
	p.R = 4
	p.A.SetInt64(1)
	p.D.SetInt64(-1174)

	// Full-group generator is (4/V,3/5)
	mi.InitString("4", "19225777642111670230408712442205514783403012708409058383774613284963344096", 10, &p.P)
	p.FBX.Set(&mi.V)
	mi.InitString("3", "5", 10, &p.P)
	p.FBY.Set(&mi.V)

	// Elligator1 parameter s for Curve1174 (Elligator paper section 4.1)
	p.Elligator1s.SetString("1806494121122717992522804053500797229648438766985538871240722010849934886421", 10)

	return &p
}

// Initialize Elligator 1 parameters given magic point s
func (el *el2param) init(ec *curve, u *big.Int) *el2param {
	el.ec = ec
	el.u.Init(u, &ec.P)

	// Compute the parameters for the Montgomery conversion:
	// A = 2(a+d)/(a-d)
	// B = 4/(a-d)
	// See Bernstein et al, "Twisted Edwards Curves", theorem 3.2
	// http://eprint.iacr.org/2008/013.pdf
	var amd nist.Int
	amd.Sub(&ec.a, &ec.d) // t = a-d
	el.A.Add(&ec.a, &ec.d).Add(&el.A, &el.A).Div(&el.A, &amd)
	el.B.Init64(4, &ec.P).Div(&el.B, &amd)

	// Other precomputed constants
	el.sqrtB.Sqrt(&el.B)
	el.negA.Neg(&el.A)
	el.pp3d8.Add(&ec.P, big.NewInt(3)).Div(&el.pp3d8, big.NewInt(8))
	el.pm1d2.Sub(&ec.P, big.NewInt(1)).Div(&el.pm1d2, big.NewInt(2))
	el.sqrtm1.Init64(-1, &ec.P).Sqrt(&el.sqrtm1)

	return el
}

// Subtract points.
func (P *extPoint) Sub(CP1, CP2 abstract.Point) abstract.Point {
	P1 := CP1.(*extPoint)
	P2 := CP2.(*extPoint)
	X1, Y1, Z1, T1 := &P1.X, &P1.Y, &P1.Z, &P1.T
	X2, Y2, Z2, T2 := &P2.X, &P2.Y, &P2.Z, &P2.T
	X3, Y3, Z3, T3 := &P.X, &P.Y, &P.Z, &P.T
	var A, B, C, D, E, F, G, H nist.Int

	A.Mul(X1, X2)
	B.Mul(Y1, Y2)
	C.Mul(T1, T2).Mul(&C, &P.c.d)
	D.Mul(Z1, Z2)
	E.Add(X1, Y1).Mul(&E, F.Sub(Y2, X2)).Add(&E, &A).Sub(&E, &B)
	F.Add(&D, &C)
	G.Sub(&D, &C)
	H.Mul(&P.c.a, &A).Add(&B, &H)
	X3.Mul(&E, &F)
	Y3.Mul(&G, &H)
	T3.Mul(&E, &H)
	Z3.Mul(&F, &G)
	return P
}

// Convert a point to string representation.
func (c *curve) pointString(x, y *nist.Int) string {
	return fmt.Sprintf("(%s,%s)", x.String(), y.String())
}

// Elligator 1 forward-map from representative to Edwards curve point.
// Currently a straightforward, unoptimized implementation.
// See section 3.2 of the Elligator paper.
func (el *el1param) HideDecode(P point, rep []byte) {
	ec := el.ec
	var t, u, u2, v, Chiv, X, Y, x, y, t1, t2 nist.Int

	l := ec.PointLen()
	if len(rep) != l {
		panic("el1Map: wrong representative length")
	}

	// Take the appropriate number of bits from the representative.
	b := make([]byte, l)
	copy(b, rep)
	b[0] &^= el.padmask() // mask off the padding bits
	t.InitBytes(b, &ec.P)

	// u = (1-t)/(1+t)
	u.Div(t1.Sub(&ec.one, &t), t2.Add(&ec.one, &t))

	// v = u^5 + (r^2-2)u^3 + u
	u2.Mul(&u, &u)                   // u2 = u^2
	v.Mul(&u2, &u2)                  // v = u^4
	v.Add(&v, t1.Mul(&el.r2m2, &u2)) // v = u^4 + (r^2-2)u^2
	v.Add(&v, &ec.one).Mul(&v, &u)   // v = u^5 + (r^2-2)u^3 + u

	// X = Chi(v)u
	chi(&Chiv, &v)
	X.Mul(&Chiv, &u)

	// Y = (Chi(v)v)^((q+1)/4) Chi(v) Chi(u^2+1/c^2)
	t1.Add(&u2, &el.invc2)
	chi(&t1, &t1) // t1 = Chi(u^2+1/c^2)
	Y.Mul(&Chiv, &v)
	Y.Exp(&Y, &el.pp1d4).Mul(&Y, &Chiv).Mul(&Y, &t1)

	// x = (c-1)sX(1+X)/Y
	x.Add(&ec.one, &X).Mul(&X, &x).Mul(&el.cm1s, &x).Div(&x, &Y)

	// y = (rX-(1+X)^2)/(rX+(1+X)^2)
	t1.Mul(&el.r, &X)                 // t1 = rX
	t2.Add(&ec.one, &X).Mul(&t2, &t2) // t2 = (1+X)^2
	y.Div(u.Sub(&t1, &t2), v.Add(&t1, &t2))

	// Sanity-check
	if !ec.onCurve(&x, &y) {
		panic("elligator1 produced invalid point")
	}

	P.initXY(&x.V, &y.V, ec.self)
}

// Elligator 2 reverse-map from point to uniform representative.
// Returns nil if point has no uniform representative.
// See section 5.3 of the Elligator paper.
func (el *el2param) HideEncode(P point, rand cipher.Stream) []byte {
	edx, edy := P.getXY()
	var x, y, r, xpA, t1 nist.Int

	// convert Edwards to Montgomery coordinates
	el.ed2mont(&x, &y, edx, edy)

	// condition 1: x != -A
	if x.Equal(&el.negA) {
		return nil // x = -A, no representative
	}

	// condition 2: if y=0, then x=0
	if y.V.Sign() == 0 && x.V.Sign() != 0 {
		return nil // y=0 but x!=0, no representative
	}

	// condition 3: -ux(x+A) is a square
	xpA.Add(&x, &el.A)
	t1.Mul(&el.u, &x).Mul(&t1, &xpA).Neg(&t1)
	if math.Jacobi(&t1.V, t1.M) < 0 {
		return nil // not a square, no representative
	}

	if y.V.Cmp(&el.pm1d2) <= 0 { // y in image of sqrt function
		r.Mul(&xpA, &el.u).Div(&x, &r)
	} else { // y not in image of sqrt function
		r.Mul(&el.u, &x).Div(&xpA, &r)
	}
	r.Neg(&r)
	el.sqrt(&r, &r)

	// Sanity check on result
	if r.V.Cmp(&el.pm1d2) > 0 {
		panic("el2: r too big")
	}

	// Map representative to a byte-string by padding the upper byte.
	// This assumes that the prime c.P is close enough to a power of 2
	// that the adversary will never notice the "missing" values;
	// this is true for the class of curves Elligator1 was designed for.
	rep, _ := r.MarshalBinary()
	padmask := el.padmask()
	if padmask != 0 {
		var pad [1]byte
		rand.XORKeyStream(pad[:], pad[:])
		rep[0] |= pad[0] & padmask
	}
	return rep
}

// Optimized point doubling for use in scalar multiplication.
func (P *projPoint) double() {
	var B, C, D, E, F, H, J nist.Int

	B.Add(&P.X, &P.Y).Mul(&B, &B)
	C.Mul(&P.X, &P.X)
	D.Mul(&P.Y, &P.Y)
	E.Mul(&P.c.a, &C)
	F.Add(&E, &D)
	H.Mul(&P.Z, &P.Z)
	J.Add(&H, &H).Sub(&F, &J)
	P.X.Sub(&B, &C).Sub(&P.X, &D).Mul(&P.X, &J)
	P.Y.Sub(&E, &D).Mul(&F, &P.Y)
	P.Z.Mul(&F, &J)
}

// Add two points using the basic unified addition laws for Edwards curves:
//
//	x' = ((x1*y2 + x2*y1) / (1 + d*x1*x2*y1*y2))
//	y' = ((y1*y2 - a*x1*x2) / (1 - d*x1*x2*y1*y2))
//
func (P *basicPoint) Add(P1, P2 abstract.Point) abstract.Point {
	E1 := P1.(*basicPoint)
	E2 := P2.(*basicPoint)
	x1, y1 := E1.x, E1.y
	x2, y2 := E2.x, E2.y

	var t1, t2, dm, nx, dx, ny, dy nist.Int

	// Reused part of denominator: dm = d*x1*x2*y1*y2
	dm.Mul(&P.c.d, &x1).Mul(&dm, &x2).Mul(&dm, &y1).Mul(&dm, &y2)

	// x' numerator/denominator
	nx.Add(t1.Mul(&x1, &y2), t2.Mul(&x2, &y1))
	dx.Add(&P.c.one, &dm)

	// y' numerator/denominator
	ny.Sub(t1.Mul(&y1, &y2), t2.Mul(&x1, &x2).Mul(&P.c.a, &t2))
	dy.Sub(&P.c.one, &dm)

	// result point
	P.x.Div(&nx, &dx)
	P.y.Div(&ny, &dy)
	return P
}

// Pick a [pseudo-]random curve point with optional embedded data,
// filling in the point's x,y coordinates
// and returning any remaining data not embedded.
func (c *curve) pickPoint(P point, data []byte, rand cipher.Stream) []byte {

	// How much data to embed?
	dl := c.pickLen()
	if dl > len(data) {
		dl = len(data)
	}

	// Retry until we find a valid point
	var x, y nist.Int
	var Q abstract.Point
	for {
		// Get random bits the size of a compressed Point encoding,
		// in which the topmost bit is reserved for the x-coord sign.
		l := c.PointLen()
		b := make([]byte, l)
		rand.XORKeyStream(b, b) // Interpret as little-endian
		if data != nil {
			b[0] = byte(dl)       // Encode length in low 8 bits
			copy(b[1:1+dl], data) // Copy in data to embed
		}
		util.Reverse(b, b) // Convert to big-endian form

		xsign := b[0] >> 7                    // save x-coordinate sign bit
		b[0] &^= 0xff << uint(c.P.BitLen()&7) // clear high bits

		y.M = &c.P // set y-coordinate
		y.SetBytes(b)

		if !c.solveForX(&x, &y) { // Corresponding x-coordinate?
			continue // none, retry
		}

		// Pick a random sign for the x-coordinate
		if c.coordSign(&x) != uint(xsign) {
			x.Neg(&x)
		}

		// Initialize the point
		P.initXY(&x.V, &y.V, c.self)
		if c.full {
			// If we're using the full group,
			// we just need any point on the curve, so we're done.
			return data[dl:]
		}

		// We're using the prime-order subgroup,
		// so we need to make sure the point is in that subgroup.
		// If we're not trying to embed data,
		// we can convert our point into one in the subgroup
		// simply by multiplying it by the cofactor.
		if data == nil {
			P.Mul(P, &c.cofact) // multiply by cofactor
			if P.Equal(c.null) {
				continue // unlucky; try again
			}
			return data[dl:]
		}

		// Since we need the point's y-coordinate to make sense,
		// we must simply check if the point is in the subgroup
		// and retry point generation until it is.
		if Q == nil {
			Q = c.self.Point()
		}
		Q.Mul(P, &c.order)
		if Q.Equal(c.null) {
			return data[dl:]
		}

		// Keep trying...
	}
}

// Given a y-coordinate, solve for the x-coordinate on the curve,
// using the characteristic equation rewritten as:
//
//	x^2 = (1 - y^2)/(a - d*y^2)
//
// Returns true on success,
// false if there is no x-coordinate corresponding to the chosen y-coordinate.
//
func (c *curve) solveForX(x, y *nist.Int) bool {
	var yy, t1, t2 nist.Int

	yy.Mul(y, y)                     // yy = y^2
	t1.Sub(&c.one, &yy)              // t1 = 1 - y^-2
	t2.Mul(&c.d, &yy).Sub(&c.a, &t2) // t2 = a - d*y^2
	t2.Div(&t1, &t2)                 // t2 = x^2
	return x.Sqrt(&t2)               // may fail if not a square
}

// Check the validity of the T coordinate
func (P *extPoint) checkT() {
	var t1, t2 nist.Int
	if !t1.Mul(&P.X, &P.Y).Equal(t2.Mul(&P.Z, &P.T)) {
		panic("oops")
	}
}

// Compute the square root function,
// specified in section 5.5 of the Elligator paper.
func (el *el2param) sqrt(r, a *nist.Int) {
	var b, b2 nist.Int
	b.Exp(a, &el.pp3d8) // b = a^((p+3)/8); b in {a,-a}

	b2.Mul(&b, &b) // b^2 = a?
	if !b2.Equal(a) {
		b.Mul(&b, &el.sqrtm1) // b*sqrt(-1)
	}

	if b.V.Cmp(&el.pm1d2) > 0 { // |b|
		b.Neg(&b)
	}

	r.Set(&b)
}

// Convert from Montgomery form (u,v) to Edwards (x,y) via:
//
//	x = sqrt(B)u/v
//	y = (u-1)/(u+1)
//
func (el *el2param) mont2ed(x, y, u, v *nist.Int) {
	ec := el.ec
	var t1, t2 nist.Int
	x.Mul(u, &el.sqrtB).Div(x, v)
	y.Div(t1.Sub(u, &ec.one), t2.Add(u, &ec.one))
}

// Convert point from Twisted Edwards form: ax^2+y^2 = 1+dx^2y^2
// to Montgomery form: v^2 = u^3+Au^2+u
// via the equivalence:
//
//	u = (1+y)/(1-y)
//	v = sqrt(B)u/x
//
// where A=2(a+d)/(a-d) and B=4(a-d)
//
// Beware: the Twisted Edwards Curves paper uses B as a factor for v^2,
// whereas the Elligator 2 paper uses B as a factor for the last u term.
//
func (el *el2param) ed2mont(u, v, x, y *nist.Int) {
	ec := el.ec
	var t1, t2 nist.Int
	u.Div(t1.Add(&ec.one, y), t2.Sub(&ec.one, y))
	v.Mul(u, &el.sqrtB).Div(v, x)
}

// Test if a supposed point is on the curve,
// by checking the characteristic equation for Edwards curves:
//
//	a*x^2 + y^2 = 1 + d*x^2*y^2
//
func (c *curve) onCurve(x, y *nist.Int) bool {
	var xx, yy, l, r nist.Int

	xx.Mul(x, x) // xx = x^2
	yy.Mul(y, y) // yy = y^2

	l.Mul(&c.a, &xx).Add(&l, &yy) // l = a*x^2 + y^2
	r.Mul(&c.d, &xx).Mul(&r, &yy).Add(&c.one, &r)
	// r = 1 + d*x^2*y^2
	return l.Equal(&r)
}

// Optimized point doubling for use in scalar multiplication.
// Uses the formulae in section 3.3 of:
// https://www.iacr.org/archive/asiacrypt2008/53500329/53500329.pdf
func (P *extPoint) double() {
	X1, Y1, Z1, T1 := &P.X, &P.Y, &P.Z, &P.T
	var A, B, C, D, E, F, G, H nist.Int

	A.Mul(X1, X1)
	B.Mul(Y1, Y1)
	C.Mul(Z1, Z1).Add(&C, &C)
	D.Mul(&P.c.a, &A)
	E.Add(X1, Y1).Mul(&E, &E).Sub(&E, &A).Sub(&E, &B)
	G.Add(&D, &B)
	F.Sub(&G, &C)
	H.Sub(&D, &B)
	X1.Mul(&E, &F)
	Y1.Mul(&G, &H)
	T1.Mul(&E, &H)
	Z1.Mul(&F, &G)
}

// Initialize a twisted Edwards curve with given parameters.
// Caller passes pointers to null and base point prototypes to be initialized.
func (c *curve) init(self abstract.Group, p *Param, fullGroup bool,
	null, base point) *curve {
	c.self = self
	c.Param = *p
	c.full = fullGroup
	c.null = null

	// Edwards curve parameters as ModInts for convenience
	c.a.Init(&p.A, &p.P)
	c.d.Init(&p.D, &p.P)

	// Cofactor
	c.cofact.Init64(int64(p.R), &c.P)

	// Determine the modulus for scalars on this curve.
	// Note that we do NOT initialize c.order with Init(),
	// as that would normalize to the modulus, resulting in zero.
	// Just to be sure it's never used, we leave c.order.M set to nil.
	// We want it to be in a ModInt so we can pass it to P.Mul(),
	// but the scalar's modulus isn't needed for point multiplication.
	if fullGroup {
		// Scalar modulus is prime-order times the ccofactor
		c.order.V.SetInt64(int64(p.R)).Mul(&c.order.V, &p.Q)
	} else {
		c.order.V.Set(&p.Q) // Prime-order subgroup
	}

	// Useful ModInt constants for this curve
	c.zero.Init64(0, &c.P)
	c.one.Init64(1, &c.P)

	// Identity element is (0,1)
	null.initXY(zero, one, self)

	// Base point B
	var bx, by *big.Int
	if !fullGroup {
		bx, by = &p.PBX, &p.PBY
	} else {
		bx, by = &p.FBX, &p.FBY
		base.initXY(&p.FBX, &p.FBY, self)
	}
	if by.Sign() == 0 {
		// No standard base point was defined, so pick one.
		// Find the lowest-numbered y-coordinate that works.
		//println("Picking base point:")
		var x, y nist.Int
		for y.Init64(2, &c.P); ; y.Add(&y, &c.one) {
			if !c.solveForX(&x, &y) {
				continue // try another y
			}
			if c.coordSign(&x) != 0 {
				x.Neg(&x) // try positive x first
			}
			base.initXY(&x.V, &y.V, self)
			if c.validPoint(base) {
				break // got one
			}
			x.Neg(&x) // try -bx
			if c.validPoint(base) {
				break // got one
			}
		}
		//println("BX: "+x.V.String())
		//println("BY: "+y.V.String())
		bx, by = &x.V, &y.V
	}
	base.initXY(bx, by, self)

	// Uniform representation encoding methods,
	// only useful when using the full group.
	// (Points taken from the subgroup would be trivially recognizable.)
	if fullGroup {
		if p.Elligator1s.Sign() != 0 {
			c.hide = new(el1param).init(c, &p.Elligator1s)
		} else if p.Elligator2u.Sign() != 0 {
			c.hide = new(el2param).init(c, &p.Elligator2u)
		}
		// XXX Elligator Squared
	}

	// Sanity checks
	if !c.validPoint(null) {
		panic("invalid identity point " + null.String())
	}
	if !c.validPoint(base) {
		panic("invalid base point " + base.String())
	}

	return c
}