You can not select more than 25 topics Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.

152 lines
3.3 KiB

// code originally taken from https://github.com/arnaucube/go-snark (https://github.com/arnaucube/go-snark/blob/master/fields/fq.go), pasted here to ensure compatibility among future changes
package field
import (
"bytes"
"crypto/rand"
"math/big"
)
// Fq is the Z field over modulus Q
type Fq struct {
Q *big.Int // Q
}
// NewFq generates a new Fq
func NewFq(q *big.Int) Fq {
return Fq{
q,
}
}
// Zero returns a Zero value on the Fq
func (fq Fq) Zero() *big.Int {
return big.NewInt(int64(0))
}
// One returns a One value on the Fq
func (fq Fq) One() *big.Int {
return big.NewInt(int64(1))
}
// Add performs an addition on the Fq
func (fq Fq) Add(a, b *big.Int) *big.Int {
r := new(big.Int).Add(a, b)
return new(big.Int).Mod(r, fq.Q)
}
// Double performs a doubling on the Fq
func (fq Fq) Double(a *big.Int) *big.Int {
r := new(big.Int).Add(a, a)
return new(big.Int).Mod(r, fq.Q)
}
// Sub performs a subtraction on the Fq
func (fq Fq) Sub(a, b *big.Int) *big.Int {
r := new(big.Int).Sub(a, b)
return new(big.Int).Mod(r, fq.Q)
}
// Neg performs a negation on the Fq
func (fq Fq) Neg(a *big.Int) *big.Int {
m := new(big.Int).Neg(a)
return new(big.Int).Mod(m, fq.Q)
}
// Mul performs a multiplication on the Fq
func (fq Fq) Mul(a, b *big.Int) *big.Int {
m := new(big.Int).Mul(a, b)
return new(big.Int).Mod(m, fq.Q)
}
func (fq Fq) MulScalar(base, e *big.Int) *big.Int {
return fq.Mul(base, e)
}
// Inverse returns the inverse on the Fq
func (fq Fq) Inverse(a *big.Int) *big.Int {
return new(big.Int).ModInverse(a, fq.Q)
}
// Div performs the division over the finite field
func (fq Fq) Div(a, b *big.Int) *big.Int {
d := fq.Mul(a, fq.Inverse(b))
return new(big.Int).Mod(d, fq.Q)
}
// Square performs a square operation on the Fq
func (fq Fq) Square(a *big.Int) *big.Int {
m := new(big.Int).Mul(a, a)
return new(big.Int).Mod(m, fq.Q)
}
// Exp performs the exponential over Fq
func (fq Fq) Exp(base *big.Int, e *big.Int) *big.Int {
res := fq.One()
rem := fq.Copy(e)
exp := base
for !bytes.Equal(rem.Bytes(), big.NewInt(int64(0)).Bytes()) {
if BigIsOdd(rem) {
res = fq.Mul(res, exp)
}
exp = fq.Square(exp)
rem = new(big.Int).Rsh(rem, 1)
}
return res
}
func (fq Fq) Rand() (*big.Int, error) {
maxbits := fq.Q.BitLen()
b := make([]byte, (maxbits/8)-1)
_, err := rand.Read(b)
if err != nil {
return nil, err
}
r := new(big.Int).SetBytes(b)
rq := new(big.Int).Mod(r, fq.Q)
// r over q, nil
return rq, nil
}
func (fq Fq) IsZero(a *big.Int) bool {
return bytes.Equal(a.Bytes(), fq.Zero().Bytes())
}
func (fq Fq) Copy(a *big.Int) *big.Int {
return new(big.Int).SetBytes(a.Bytes())
}
func (fq Fq) Affine(a *big.Int) *big.Int {
nq := fq.Neg(fq.Q)
aux := a
if aux.Cmp(big.NewInt(int64(0))) == -1 { // negative value
if aux.Cmp(nq) != 1 { // aux less or equal nq
aux = new(big.Int).Mod(aux, fq.Q)
}
if aux.Cmp(big.NewInt(int64(0))) == -1 { // negative value
aux = new(big.Int).Add(aux, fq.Q)
}
} else {
if aux.Cmp(fq.Q) != -1 { // aux greater or equal nq
aux = new(big.Int).Mod(aux, fq.Q)
}
}
return aux
}
func (fq Fq) Equal(a, b *big.Int) bool {
aAff := fq.Affine(a)
bAff := fq.Affine(b)
return bytes.Equal(aAff.Bytes(), bAff.Bytes())
}
func BigIsOdd(n *big.Int) bool {
one := big.NewInt(int64(1))
and := new(big.Int).And(n, one)
return bytes.Equal(and.Bytes(), big.NewInt(int64(1)).Bytes())
}