Adds polynomials and Lagrange polynomials.

Author: armfazh

group/group.go

@@ -101,6 +101,8 @@ type Scalar interface {
 	Set(x Scalar) Scalar
 	// Copy returns a new scalar equal to the receiver.
 	Copy() Scalar
+	// IsZero returns true if the receiver is equal to zero.
+	IsZero() bool
 	// IsEqual returns true if the receiver is equal to x.
 	IsEqual(x Scalar) bool
 	// SetUint64 set the receiver to x, and returns the receiver.

group/group_test.go

@@ -314,7 +314,8 @@ func testScalar(t *testing.T, testTimes int, g group.Group) {
 
 	c.Inv(a)
 	c.Mul(c, a)
-	if !one.IsEqual(c) {
+	c.Sub(c, one)
+	if !c.IsZero() {
 		test.ReportError(t, c, one, a)
 	}
 }

group/ristretto255.go

@@ -8,6 +8,7 @@ import (
 
 	r255 "github.com/bwesterb/go-ristretto"
 	"github.com/cloudflare/circl/expander"
+	"github.com/cloudflare/circl/internal/conv"
 )
 
 // Ristretto255 is a quotient group generated from the edwards25519 curve.
@@ -203,9 +204,9 @@ func (e *ristrettoElement) UnmarshalBinary(data []byte) error {
 }
 
 func (s *ristrettoScalar) Group() Group              { return Ristretto255 }
-func (s *ristrettoScalar) String() string            { return fmt.Sprintf("0x%x", s.s.Bytes()) }
+func (s *ristrettoScalar) String() string            { return conv.BytesLe2Hex(s.s.Bytes()) }
 func (s *ristrettoScalar) SetUint64(n uint64) Scalar { s.s.SetUint64(n); return s }
-
+func (s *ristrettoScalar) IsZero() bool              { return s.s.IsNonZeroI() == 0 }
 func (s *ristrettoScalar) IsEqual(x Scalar) bool {
 	return s.s.Equals(&x.(*ristrettoScalar).s)
 }

group/short.go

@@ -274,6 +274,10 @@ type wScl struct {
 func (s *wScl) Group() Group              { return s.wG }
 func (s *wScl) String() string            { return fmt.Sprintf("0x%x", s.k) }
 func (s *wScl) SetUint64(n uint64) Scalar { s.fromBig(new(big.Int).SetUint64(n)); return s }
+func (s *wScl) IsZero() bool {
+	return subtle.ConstantTimeCompare(s.k, make([]byte, (s.wG.c.Params().BitSize+7)/8)) == 1
+}
+
 func (s *wScl) IsEqual(a Scalar) bool {
 	aa := s.cvtScl(a)
 	return subtle.ConstantTimeCompare(s.k, aa.k) == 1

math/polynomial/polynomial.go

@@ -0,0 +1,147 @@
+// Package polynomial provides representations of polynomials over the scalars
+// of a group.
+package polynomial
+
+import "github.com/cloudflare/circl/group"
+
+// Polynomial stores a polynomial over the set of scalars of a group.
+type Polynomial struct {
+	// Internal representation is in polynomial basis:
+	// Thus,
+	//     p(x) = \sum_i^k c[i] x^i,
+	// where k = len(c)-1 is the degree of the polynomial.
+	c []group.Scalar
+}
+
+// New creates a new polynomial given its coefficients in ascending order.
+// Thus,
+//     p(x) = \sum_i^k c[i] x^i,
+// where k = len(c)-1 is the degree of the polynomial.
+//
+// The zero polynomial has degree equal to -1 and can be instantiated passing
+// nil to New.
+func New(coeffs []group.Scalar) (p Polynomial) {
+	if l := len(coeffs); l != 0 {
+		p.c = make([]group.Scalar, l)
+		for i := range coeffs {
+			p.c[i] = coeffs[i].Copy()
+		}
+	}
+
+	return
+}
+
+func (p Polynomial) Degree() int {
+	i := len(p.c) - 1
+	for i > 0 && p.c[i].IsZero() {
+		i--
+	}
+	return i
+}
+
+func (p Polynomial) Evaluate(x group.Scalar) group.Scalar {
+	px := x.Group().NewScalar()
+	if l := len(p.c); l != 0 {
+		px.Set(p.c[l-1])
+		for i := l - 2; i >= 0; i-- {
+			px.Mul(px, x)
+			px.Add(px, p.c[i])
+		}
+	}
+	return px
+}
+
+// LagrangePolynomial stores a Lagrange polynomial over the set of scalars of a group.
+type LagrangePolynomial struct {
+	// Internal representation is in Lagrange basis:
+	// Thus,
+	//     p(x) = \sum_i^k y[i] L_j(x), where k is the degree of the polynomial,
+	//     L_j(x) = \prod_i^k (x-x[i])/(x[j]-x[i]),
+	//     y[i] = p(x[i]), and
+	//     all x[i] are different.
+	x, y []group.Scalar
+}
+
+// NewLagrangePolynomial creates a polynomial in Lagrange basis given a list
+// of nodes (x) and values (y), such that:
+//     p(x) = \sum_i^k y[i] L_j(x), where k is the degree of the polynomial,
+//     L_j(x) = \prod_i^k (x-x[i])/(x[j]-x[i]),
+//     y[i] = p(x[i]), and
+//     all x[i] are different.
+// It panics if one of these conditions does not hold.
+//
+// The zero polynomial has degree equal to -1 and can be instantiated passing
+// (nil,nil) to NewLagrangePolynomial.
+func NewLagrangePolynomial(x, y []group.Scalar) (l LagrangePolynomial) {
+	if len(x) != len(y) {
+		panic("lagrange: invalid length")
+	}
+
+	if !areAllDifferent(x) {
+		panic("lagrange: x[i] must be different")
+	}
+
+	if n := len(x); n != 0 {
+		l.x, l.y = make([]group.Scalar, n), make([]group.Scalar, n)
+		for i := range x {
+			l.x[i], l.y[i] = x[i].Copy(), y[i].Copy()
+		}
+	}
+
+	return
+}
+
+func (l LagrangePolynomial) Degree() int { return len(l.x) - 1 }
+
+func (l LagrangePolynomial) Evaluate(x group.Scalar) group.Scalar {
+	px := x.Group().NewScalar()
+	tmp := x.Group().NewScalar()
+	for i := range l.x {
+		LjX := baseRatio(uint(i), l.x, x)
+		tmp.Mul(l.y[i], LjX)
+		px.Add(px, tmp)
+	}
+
+	return px
+}
+
+// LagrangeBase returns the j-th Lagrange polynomial base evaluated at x.
+// Thus, L_j(x) = \prod (x - x[i]) / (x[j] - x[i]) for 0 <= i < k, and i != j.
+func LagrangeBase(jth uint, xi []group.Scalar, x group.Scalar) group.Scalar {
+	if jth >= uint(len(xi)) {
+		panic("lagrange: invalid index")
+	}
+	return baseRatio(jth, xi, x)
+}
+
+func baseRatio(jth uint, xi []group.Scalar, x group.Scalar) group.Scalar {
+	num := x.Copy()
+	num.SetUint64(1)
+	den := x.Copy()
+	den.SetUint64(1)
+
+	tmp := x.Copy()
+	for i := range xi {
+		if uint(i) != jth {
+			num.Mul(num, tmp.Sub(x, xi[i]))
+			den.Mul(den, tmp.Sub(xi[jth], xi[i]))
+		}
+	}
+
+	return num.Mul(num, den.Inv(den))
+}
+
+func areAllDifferent(x []group.Scalar) bool {
+	m := make(map[string]struct{})
+	for i := range x {
+		k, err := x[i].MarshalBinary()
+		if err != nil {
+			panic(err)
+		}
+		if _, exists := m[string(k)]; exists {
+			return false
+		}
+		m[string(k)] = struct{}{}
+	}
+	return true
+}

math/polynomial/polynomial_test.go

@@ -0,0 +1,131 @@
+package polynomial_test
+
+import (
+	"testing"
+
+	"github.com/cloudflare/circl/group"
+	"github.com/cloudflare/circl/internal/test"
+	"github.com/cloudflare/circl/math/polynomial"
+)
+
+func TestPolyDegree(t *testing.T) {
+	g := group.P256
+
+	t.Run("zeroPoly", func(t *testing.T) {
+		p := polynomial.New(nil)
+		test.CheckOk(p.Degree() == -1, "it should be -1", t)
+		p = polynomial.New([]group.Scalar{})
+		test.CheckOk(p.Degree() == -1, "it should be -1", t)
+	})
+
+	t.Run("constantPoly", func(t *testing.T) {
+		c := []group.Scalar{
+			g.NewScalar().SetUint64(0),
+			g.NewScalar().SetUint64(0),
+		}
+		p := polynomial.New(c)
+		test.CheckOk(p.Degree() == 0, "it should be 0", t)
+	})
+
+	t.Run("linearPoly", func(t *testing.T) {
+		c := []group.Scalar{
+			g.NewScalar().SetUint64(0),
+			g.NewScalar().SetUint64(1),
+			g.NewScalar().SetUint64(0),
+		}
+		p := polynomial.New(c)
+		test.CheckOk(p.Degree() == 1, "it should be 1", t)
+	})
+}
+
+func TestPolyEval(t *testing.T) {
+	g := group.P256
+	c := []group.Scalar{
+		g.NewScalar(),
+		g.NewScalar(),
+		g.NewScalar(),
+	}
+	c[0].SetUint64(5)
+	c[1].SetUint64(5)
+	c[2].SetUint64(2)
+	p := polynomial.New(c)
+
+	x := g.NewScalar()
+	x.SetUint64(10)
+
+	got := p.Evaluate(x)
+	want := g.NewScalar()
+	want.SetUint64(255)
+	if !got.IsEqual(want) {
+		test.ReportError(t, got, want)
+	}
+}
+
+func TestLagrange(t *testing.T) {
+	g := group.P256
+	c := []group.Scalar{
+		g.NewScalar(),
+		g.NewScalar(),
+		g.NewScalar(),
+	}
+	c[0].SetUint64(1234)
+	c[1].SetUint64(166)
+	c[2].SetUint64(94)
+	p := polynomial.New(c)
+
+	x := []group.Scalar{g.NewScalar(), g.NewScalar(), g.NewScalar()}
+	x[0].SetUint64(2)
+	x[1].SetUint64(4)
+	x[2].SetUint64(5)
+
+	y := []group.Scalar{}
+	for i := range x {
+		y = append(y, p.Evaluate(x[i]))
+	}
+
+	zero := g.NewScalar()
+	l := polynomial.NewLagrangePolynomial(x, y)
+	test.CheckOk(l.Degree() == p.Degree(), "bad degree", t)
+
+	got := l.Evaluate(zero)
+	want := p.Evaluate(zero)
+
+	if !got.IsEqual(want) {
+		test.ReportError(t, got, want)
+	}
+
+	// Test Kronecker's delta of LagrangeBase.
+	// Thus:
+	//    L_j(x[i]) = { 1, if i == j;
+	//                { 0, otherwise.
+	one := g.NewScalar()
+	one.SetUint64(1)
+	for j := range x {
+		for i := range x {
+			got := polynomial.LagrangeBase(uint(j), x, x[i])
+
+			if i == j {
+				want = one
+			} else {
+				want = zero
+			}
+
+			if !got.IsEqual(want) {
+				test.ReportError(t, got, want)
+			}
+		}
+	}
+
+	// Test that inputs are different length
+	err := test.CheckPanic(func() { polynomial.NewLagrangePolynomial(x, y[:1]) })
+	test.CheckNoErr(t, err, "should panic")
+
+	// Test that nodes must be different.
+	x[0].Set(x[1])
+	err = test.CheckPanic(func() { polynomial.NewLagrangePolynomial(x, y) })
+	test.CheckNoErr(t, err, "should panic")
+
+	// Test LagrangeBase wrong index
+	err = test.CheckPanic(func() { polynomial.LagrangeBase(10, x, zero) })
+	test.CheckNoErr(t, err, "should panic")
+}