peridot/vendor/github.com/cloudflare/circl/math/fp448/fp.go
2024-10-16 12:56:53 +02:00

165 lines
3.9 KiB
Go

// Package fp448 provides prime field arithmetic over GF(2^448-2^224-1).
package fp448
import (
"errors"
"github.com/cloudflare/circl/internal/conv"
)
// Size in bytes of an element.
const Size = 56
// Elt is a prime field element.
type Elt [Size]byte
func (e Elt) String() string { return conv.BytesLe2Hex(e[:]) }
// p is the prime modulus 2^448-2^224-1.
var p = Elt{
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, 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,
}
// P returns the prime modulus 2^448-2^224-1.
func P() Elt { return p }
// ToBytes stores in b the little-endian byte representation of x.
func ToBytes(b []byte, x *Elt) error {
if len(b) != Size {
return errors.New("wrong size")
}
Modp(x)
copy(b, x[:])
return nil
}
// IsZero returns true if x is equal to 0.
func IsZero(x *Elt) bool { Modp(x); return *x == Elt{} }
// IsOne returns true if x is equal to 1.
func IsOne(x *Elt) bool { Modp(x); return *x == Elt{1} }
// SetOne assigns x=1.
func SetOne(x *Elt) { *x = Elt{1} }
// One returns the 1 element.
func One() (x Elt) { x = Elt{1}; return }
// Neg calculates z = -x.
func Neg(z, x *Elt) { Sub(z, &p, x) }
// Modp ensures that z is between [0,p-1].
func Modp(z *Elt) { Sub(z, z, &p) }
// InvSqrt calculates z = sqrt(x/y) iff x/y is a quadratic-residue. If so,
// isQR = true; otherwise, isQR = false, since x/y is a quadratic non-residue,
// and z = sqrt(-x/y).
func InvSqrt(z, x, y *Elt) (isQR bool) {
// First note that x^(2(k+1)) = x^(p-1)/2 * x = legendre(x) * x
// so that's x if x is a quadratic residue and -x otherwise.
// Next, y^(6k+3) = y^(4k+2) * y^(2k+1) = y^(p-1) * y^((p-1)/2) = legendre(y).
// So the z we compute satisfies z^2 y = x^(2(k+1)) y^(6k+3) = legendre(x)*legendre(y).
// Thus if x and y are quadratic residues, then z is indeed sqrt(x/y).
t0, t1 := &Elt{}, &Elt{}
Mul(t0, x, y) // x*y
Sqr(t1, y) // y^2
Mul(t1, t0, t1) // x*y^3
powPminus3div4(z, t1) // (x*y^3)^k
Mul(z, z, t0) // z = x*y*(x*y^3)^k = x^(k+1) * y^(3k+1)
// Check if x/y is a quadratic residue
Sqr(t0, z) // z^2
Mul(t0, t0, y) // y*z^2
Sub(t0, t0, x) // y*z^2-x
return IsZero(t0)
}
// Inv calculates z = 1/x mod p.
func Inv(z, x *Elt) {
// Calculates z = x^(4k+1) = x^(p-3+1) = x^(p-2) = x^-1, where k = (p-3)/4.
t := &Elt{}
powPminus3div4(t, x) // t = x^k
Sqr(t, t) // t = x^2k
Sqr(t, t) // t = x^4k
Mul(z, t, x) // z = x^(4k+1)
}
// powPminus3div4 calculates z = x^k mod p, where k = (p-3)/4.
func powPminus3div4(z, x *Elt) {
x0, x1 := &Elt{}, &Elt{}
Sqr(z, x)
Mul(z, z, x)
Sqr(x0, z)
Mul(x0, x0, x)
Sqr(z, x0)
Sqr(z, z)
Sqr(z, z)
Mul(z, z, x0)
Sqr(x1, z)
for i := 0; i < 5; i++ {
Sqr(x1, x1)
}
Mul(x1, x1, z)
Sqr(z, x1)
for i := 0; i < 11; i++ {
Sqr(z, z)
}
Mul(z, z, x1)
Sqr(z, z)
Sqr(z, z)
Sqr(z, z)
Mul(z, z, x0)
Sqr(x1, z)
for i := 0; i < 26; i++ {
Sqr(x1, x1)
}
Mul(x1, x1, z)
Sqr(z, x1)
for i := 0; i < 53; i++ {
Sqr(z, z)
}
Mul(z, z, x1)
Sqr(z, z)
Sqr(z, z)
Sqr(z, z)
Mul(z, z, x0)
Sqr(x1, z)
for i := 0; i < 110; i++ {
Sqr(x1, x1)
}
Mul(x1, x1, z)
Sqr(z, x1)
Mul(z, z, x)
for i := 0; i < 223; i++ {
Sqr(z, z)
}
Mul(z, z, x1)
}
// Cmov assigns y to x if n is 1.
func Cmov(x, y *Elt, n uint) { cmov(x, y, n) }
// Cswap interchanges x and y if n is 1.
func Cswap(x, y *Elt, n uint) { cswap(x, y, n) }
// Add calculates z = x+y mod p.
func Add(z, x, y *Elt) { add(z, x, y) }
// Sub calculates z = x-y mod p.
func Sub(z, x, y *Elt) { sub(z, x, y) }
// AddSub calculates (x,y) = (x+y mod p, x-y mod p).
func AddSub(x, y *Elt) { addsub(x, y) }
// Mul calculates z = x*y mod p.
func Mul(z, x, y *Elt) { mul(z, x, y) }
// Sqr calculates z = x^2 mod p.
func Sqr(z, x *Elt) { sqr(z, x) }