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255 lines
8.0 KiB
Go
255 lines
8.0 KiB
Go
package p751
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import . "v2ray.com/core/external/github.com/cloudflare/sidh/internal/isogeny"
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// 2*p751
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var ()
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//------------------------------------------------------------------------------
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// Implementtaion of FieldOperations
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//------------------------------------------------------------------------------
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// Implements FieldOps
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type fp751Ops struct{}
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func FieldOperations() FieldOps {
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return &fp751Ops{}
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}
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func (fp751Ops) Add(dest, lhs, rhs *Fp2Element) {
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fp751AddReduced(&dest.A, &lhs.A, &rhs.A)
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fp751AddReduced(&dest.B, &lhs.B, &rhs.B)
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}
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func (fp751Ops) Sub(dest, lhs, rhs *Fp2Element) {
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fp751SubReduced(&dest.A, &lhs.A, &rhs.A)
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fp751SubReduced(&dest.B, &lhs.B, &rhs.B)
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}
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func (fp751Ops) Mul(dest, lhs, rhs *Fp2Element) {
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// Let (a,b,c,d) = (lhs.a,lhs.b,rhs.a,rhs.b).
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a := &lhs.A
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b := &lhs.B
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c := &rhs.A
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d := &rhs.B
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// We want to compute
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//
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// (a + bi)*(c + di) = (a*c - b*d) + (a*d + b*c)i
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//
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// Use Karatsuba's trick: note that
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//
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// (b - a)*(c - d) = (b*c + a*d) - a*c - b*d
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//
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// so (a*d + b*c) = (b-a)*(c-d) + a*c + b*d.
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var ac, bd FpElementX2
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fp751Mul(&ac, a, c) // = a*c*R*R
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fp751Mul(&bd, b, d) // = b*d*R*R
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var b_minus_a, c_minus_d FpElement
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fp751SubReduced(&b_minus_a, b, a) // = (b-a)*R
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fp751SubReduced(&c_minus_d, c, d) // = (c-d)*R
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var ad_plus_bc FpElementX2
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fp751Mul(&ad_plus_bc, &b_minus_a, &c_minus_d) // = (b-a)*(c-d)*R*R
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fp751X2AddLazy(&ad_plus_bc, &ad_plus_bc, &ac) // = ((b-a)*(c-d) + a*c)*R*R
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fp751X2AddLazy(&ad_plus_bc, &ad_plus_bc, &bd) // = ((b-a)*(c-d) + a*c + b*d)*R*R
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fp751MontgomeryReduce(&dest.B, &ad_plus_bc) // = (a*d + b*c)*R mod p
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var ac_minus_bd FpElementX2
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fp751X2SubLazy(&ac_minus_bd, &ac, &bd) // = (a*c - b*d)*R*R
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fp751MontgomeryReduce(&dest.A, &ac_minus_bd) // = (a*c - b*d)*R mod p
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}
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func (fp751Ops) Square(dest, x *Fp2Element) {
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a := &x.A
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b := &x.B
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// We want to compute
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//
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// (a + bi)*(a + bi) = (a^2 - b^2) + 2abi.
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var a2, a_plus_b, a_minus_b FpElement
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fp751AddReduced(&a2, a, a) // = a*R + a*R = 2*a*R
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fp751AddReduced(&a_plus_b, a, b) // = a*R + b*R = (a+b)*R
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fp751SubReduced(&a_minus_b, a, b) // = a*R - b*R = (a-b)*R
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var asq_minus_bsq, ab2 FpElementX2
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fp751Mul(&asq_minus_bsq, &a_plus_b, &a_minus_b) // = (a+b)*(a-b)*R*R = (a^2 - b^2)*R*R
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fp751Mul(&ab2, &a2, b) // = 2*a*b*R*R
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fp751MontgomeryReduce(&dest.A, &asq_minus_bsq) // = (a^2 - b^2)*R mod p
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fp751MontgomeryReduce(&dest.B, &ab2) // = 2*a*b*R mod p
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}
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// Set dest = 1/x
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//
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// Allowed to overlap dest with x.
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//
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// Returns dest to allow chaining operations.
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func (fp751Ops) Inv(dest, x *Fp2Element) {
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a := &x.A
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b := &x.B
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// We want to compute
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//
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// 1 1 (a - bi) (a - bi)
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// -------- = -------- -------- = -----------
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// (a + bi) (a + bi) (a - bi) (a^2 + b^2)
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//
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// Letting c = 1/(a^2 + b^2), this is
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//
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// 1/(a+bi) = a*c - b*ci.
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var asq_plus_bsq primeFieldElement
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var asq, bsq FpElementX2
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fp751Mul(&asq, a, a) // = a*a*R*R
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fp751Mul(&bsq, b, b) // = b*b*R*R
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fp751X2AddLazy(&asq, &asq, &bsq) // = (a^2 + b^2)*R*R
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fp751MontgomeryReduce(&asq_plus_bsq.A, &asq) // = (a^2 + b^2)*R mod p
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// Now asq_plus_bsq = a^2 + b^2
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// Invert asq_plus_bsq
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inv := asq_plus_bsq
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inv.Mul(&asq_plus_bsq, &asq_plus_bsq)
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inv.P34(&inv)
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inv.Mul(&inv, &inv)
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inv.Mul(&inv, &asq_plus_bsq)
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var ac FpElementX2
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fp751Mul(&ac, a, &inv.A)
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fp751MontgomeryReduce(&dest.A, &ac)
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var minus_b FpElement
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fp751SubReduced(&minus_b, &minus_b, b)
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var minus_bc FpElementX2
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fp751Mul(&minus_bc, &minus_b, &inv.A)
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fp751MontgomeryReduce(&dest.B, &minus_bc)
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}
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// In case choice == 1, performs following swap in constant time:
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// xPx <-> xQx
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// xPz <-> xQz
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// Otherwise returns xPx, xPz, xQx, xQz unchanged
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func (fp751Ops) CondSwap(xPx, xPz, xQx, xQz *Fp2Element, choice uint8) {
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fp751ConditionalSwap(&xPx.A, &xQx.A, choice)
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fp751ConditionalSwap(&xPx.B, &xQx.B, choice)
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fp751ConditionalSwap(&xPz.A, &xQz.A, choice)
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fp751ConditionalSwap(&xPz.B, &xQz.B, choice)
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}
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// Converts values in x.A and x.B to Montgomery domain
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// x.A = x.A * R mod p
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// x.B = x.B * R mod p
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func (fp751Ops) ToMontgomery(x *Fp2Element) {
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var aRR FpElementX2
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// convert to montgomery domain
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fp751Mul(&aRR, &x.A, &p751R2) // = a*R*R
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fp751MontgomeryReduce(&x.A, &aRR) // = a*R mod p
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fp751Mul(&aRR, &x.B, &p751R2)
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fp751MontgomeryReduce(&x.B, &aRR)
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}
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// Converts values in x.A and x.B from Montgomery domain
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// a = x.A mod p
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// b = x.B mod p
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//
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// After returning from the call x is not modified.
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func (fp751Ops) FromMontgomery(x *Fp2Element, out *Fp2Element) {
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var aR FpElementX2
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// convert from montgomery domain
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copy(aR[:], x.A[:])
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fp751MontgomeryReduce(&out.A, &aR) // = a mod p in [0, 2p)
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fp751StrongReduce(&out.A) // = a mod p in [0, p)
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for i := range aR {
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aR[i] = 0
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}
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copy(aR[:], x.B[:])
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fp751MontgomeryReduce(&out.B, &aR)
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fp751StrongReduce(&out.B)
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}
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//------------------------------------------------------------------------------
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// Prime Field
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//------------------------------------------------------------------------------
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// Represents an element of the prime field F_p in Montgomery domain
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type primeFieldElement struct {
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// The value `A`is represented by `aR mod p`.
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A FpElement
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}
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// Set dest = lhs * rhs.
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//
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// Allowed to overlap lhs or rhs with dest.
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//
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// Returns dest to allow chaining operations.
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func (dest *primeFieldElement) Mul(lhs, rhs *primeFieldElement) *primeFieldElement {
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a := &lhs.A // = a*R
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b := &rhs.A // = b*R
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var ab FpElementX2
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fp751Mul(&ab, a, b) // = a*b*R*R
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fp751MontgomeryReduce(&dest.A, &ab) // = a*b*R mod p
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return dest
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}
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// Set dest = x^(2^k), for k >= 1, by repeated squarings.
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//
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// Allowed to overlap x with dest.
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//
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// Returns dest to allow chaining operations.
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func (dest *primeFieldElement) Pow2k(x *primeFieldElement, k uint8) *primeFieldElement {
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dest.Mul(x, x)
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for i := uint8(1); i < k; i++ {
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dest.Mul(dest, dest)
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}
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return dest
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}
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// Set dest = x^((p-3)/4). If x is square, this is 1/sqrt(x).
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//
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// Allowed to overlap x with dest.
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//
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// Returns dest to allow chaining operations.
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func (dest *primeFieldElement) P34(x *primeFieldElement) *primeFieldElement {
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// Sliding-window strategy computed with Sage, awk, sed, and tr.
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//
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// This performs sum(powStrategy) = 744 squarings and len(mulStrategy)
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// = 137 multiplications, in addition to 1 squaring and 15
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// multiplications to build a lookup table.
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//
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// In total this is 745 squarings, 152 multiplications. Since squaring
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// is not implemented for the prime field, this is 897 multiplications
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// in total.
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powStrategy := [137]uint8{5, 7, 6, 2, 10, 4, 6, 9, 8, 5, 9, 4, 7, 5, 5, 4, 8, 3, 9, 5, 5, 4, 10, 4, 6, 6, 6, 5, 8, 9, 3, 4, 9, 4, 5, 6, 6, 2, 9, 4, 5, 5, 5, 7, 7, 9, 4, 6, 4, 8, 5, 8, 6, 6, 2, 9, 7, 4, 8, 8, 8, 4, 6, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 2}
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mulStrategy := [137]uint8{31, 23, 21, 1, 31, 7, 7, 7, 9, 9, 19, 15, 23, 23, 11, 7, 25, 5, 21, 17, 11, 5, 17, 7, 11, 9, 23, 9, 1, 19, 5, 3, 25, 15, 11, 29, 31, 1, 29, 11, 13, 9, 11, 27, 13, 19, 15, 31, 3, 29, 23, 31, 25, 11, 1, 21, 19, 15, 15, 21, 29, 13, 23, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 3}
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initialMul := uint8(27)
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// Build a lookup table of odd multiples of x.
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lookup := [16]primeFieldElement{}
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xx := &primeFieldElement{}
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xx.Mul(x, x) // Set xx = x^2
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lookup[0] = *x
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for i := 1; i < 16; i++ {
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lookup[i].Mul(&lookup[i-1], xx)
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}
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// Now lookup = {x, x^3, x^5, ... }
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// so that lookup[i] = x^{2*i + 1}
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// so that lookup[k/2] = x^k, for odd k
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*dest = lookup[initialMul/2]
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for i := uint8(0); i < 137; i++ {
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dest.Pow2k(dest, powStrategy[i])
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dest.Mul(dest, &lookup[mulStrategy[i]/2])
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}
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return dest
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}
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