edwards25519

package
v0.0.0-...-b0909d3 Latest Latest
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 Go to latest Published: Aug 31, 2015 License: BSD-3-Clause Imports: 0 Imported by: 7

Documentation

Overview

Package edwards25519 implements operations in GF(2**255-19) and on an Edwards curve that is isomorphic to curve25519. See http://ed25519.cr.yp.to/.

Index

Constants

This section is empty.

Variables

View Source 
var A = FieldElement{
	486662, 0, 0, 0, 0, 0, 0, 0, 0, 0,
}
View Source 
var SqrtM1 = FieldElement{
	-32595792, -7943725, 9377950, 3500415, 12389472, -272473, -25146209, -2005654, 326686, 11406482,
}

Functions

func FeAdd 

func FeAdd(dst, a, b *FieldElement)

func FeCMove 

func FeCMove(f, g *FieldElement, b int32)

Replace (f,g) with (g,g) if b == 1; replace (f,g) with (f,g) if b == 0.

Preconditions: b in {0,1}.

func FeCombine 

func FeCombine(h *FieldElement, h0, h1, h2, h3, h4, h5, h6, h7, h8, h9 int64)

func FeCopy 

func FeCopy(dst, src *FieldElement)

func FeFromBytes 

func FeFromBytes(dst *FieldElement, src *[32]byte)

func FeInvert 

func FeInvert(out, z *FieldElement)

func FeIsNegative 

func FeIsNegative(f *FieldElement) byte

func FeIsNonZero 

func FeIsNonZero(f *FieldElement) int32

func FeMul 

func FeMul(h, f, g *FieldElement)

FeMul calculates h = f * g Can overlap h with f or g.

Preconditions: 

|f| bounded by 1.1*2^26,1.1*2^25,1.1*2^26,1.1*2^25,etc.
|g| bounded by 1.1*2^26,1.1*2^25,1.1*2^26,1.1*2^25,etc.

Postconditions: 

|h| bounded by 1.1*2^25,1.1*2^24,1.1*2^25,1.1*2^24,etc.

Notes on implementation strategy:

Using schoolbook multiplication. Karatsuba would save a little in some cost models.

Most multiplications by 2 and 19 are 32-bit precomputations; cheaper than 64-bit postcomputations.

There is one remaining multiplication by 19 in the carry chain; one *19 precomputation can be merged into this, but the resulting data flow is considerably less clean.

There are 12 carries below. 10 of them are 2-way parallelizable and vectorizable. Can get away with 11 carries, but then data flow is much deeper.

With tighter constraints on inputs can squeeze carries into int32.

func FeNeg 

func FeNeg(h, f *FieldElement)

FeNeg sets h = -f

Preconditions: 

|f| bounded by 1.1*2^25,1.1*2^24,1.1*2^25,1.1*2^24,etc.

Postconditions: 

|h| bounded by 1.1*2^25,1.1*2^24,1.1*2^25,1.1*2^24,etc.

func FeOne 

func FeOne(fe *FieldElement)

func FeSquare 

func FeSquare(h, f *FieldElement)

FeSquare calculates h = f*f. Can overlap h with f.

Preconditions: 

|f| bounded by 1.1*2^26,1.1*2^25,1.1*2^26,1.1*2^25,etc.

Postconditions: 

|h| bounded by 1.1*2^25,1.1*2^24,1.1*2^25,1.1*2^24,etc.

func FeSquare2 

func FeSquare2(h, f *FieldElement)

FeSquare2 sets h = 2 * f * f

Can overlap h with f.

Preconditions: 

|f| bounded by 1.65*2^26,1.65*2^25,1.65*2^26,1.65*2^25,etc.

Postconditions: 

|h| bounded by 1.01*2^25,1.01*2^24,1.01*2^25,1.01*2^24,etc.

See fe_mul.c for discussion of implementation strategy.

func FeSub 

func FeSub(dst, a, b *FieldElement)

func FeToBytes 

func FeToBytes(s *[32]byte, h *FieldElement)

FeToBytes marshals h to s. Preconditions: 

|h| bounded by 1.1*2^25,1.1*2^24,1.1*2^25,1.1*2^24,etc.

Write p=2^255-19; q=floor(h/p). Basic claim: q = floor(2^(-255)(h + 19 2^(-25)h9 + 2^(-1))).

Proof: 

Have |h|<=p so |q|<=1 so |19^2 2^(-255) q|<1/4.
Also have |h-2^230 h9|<2^230 so |19 2^(-255)(h-2^230 h9)|<1/4.

Write y=2^(-1)-19^2 2^(-255)q-19 2^(-255)(h-2^230 h9).
Then 0<y<1.

Write r=h-pq.
Have 0<=r<=p-1=2^255-20.
Thus 0<=r+19(2^-255)r<r+19(2^-255)2^255<=2^255-1.

Write x=r+19(2^-255)r+y.
Then 0<x<2^255 so floor(2^(-255)x) = 0 so floor(q+2^(-255)x) = q.

Have q+2^(-255)x = 2^(-255)(h + 19 2^(-25) h9 + 2^(-1))
so floor(2^(-255)(h + 19 2^(-25) h9 + 2^(-1))) = q.

func FeZero 

func FeZero(fe *FieldElement)

func GeDoubleScalarMultVartime 

func GeDoubleScalarMultVartime(r *ProjectiveGroupElement, a *[32]byte, A *ExtendedGroupElement, b *[32]byte)

GeDoubleScalarMultVartime sets r = a*A + b*B where a = a[0]+256*a[1]+...+256^31 a[31]. and b = b[0]+256*b[1]+...+256^31 b[31]. B is the Ed25519 base point (x,4/5) with x positive.

func GeScalarMultBase 

func GeScalarMultBase(h *ExtendedGroupElement, a *[32]byte)

GeScalarMultBase computes h = a*B, where 

a = a[0]+256*a[1]+...+256^31 a[31]
B is the Ed25519 base point (x,4/5) with x positive.

Preconditions: 

a[31] <= 127

func PreComputedGroupElementCMove 

func PreComputedGroupElementCMove(t, u *PreComputedGroupElement, b int32)

func ScMulAdd 

func ScMulAdd(s, a, b, c *[32]byte)

Input: 

a[0]+256*a[1]+...+256^31*a[31] = a
b[0]+256*b[1]+...+256^31*b[31] = b
c[0]+256*c[1]+...+256^31*c[31] = c

Output: 

s[0]+256*s[1]+...+256^31*s[31] = (ab+c) mod l
where l = 2^252 + 27742317777372353535851937790883648493.

func ScReduce 

func ScReduce(out *[32]byte, s *[64]byte)

Input: 

s[0]+256*s[1]+...+256^63*s[63] = s

Output: 

s[0]+256*s[1]+...+256^31*s[31] = s mod l
where l = 2^252 + 27742317777372353535851937790883648493.

Types

type CachedGroupElement 

type CachedGroupElement struct {
	Z, T2d FieldElement
	// contains filtered or unexported fields
}

type CompletedGroupElement 

type CompletedGroupElement struct {
	X, Y, Z, T FieldElement
}

func (*CompletedGroupElement) ToExtended 

func (p *CompletedGroupElement) ToExtended(r *ExtendedGroupElement)

func (*CompletedGroupElement) ToProjective 

func (p *CompletedGroupElement) ToProjective(r *ProjectiveGroupElement)

type ExtendedGroupElement 

type ExtendedGroupElement struct {
	X, Y, Z, T FieldElement
}

func (*ExtendedGroupElement) Double 

func (p *ExtendedGroupElement) Double(r *CompletedGroupElement)

func (*ExtendedGroupElement) FromBytes 

func (p *ExtendedGroupElement) FromBytes(s *[32]byte) bool

func (*ExtendedGroupElement) ToBytes 

func (p *ExtendedGroupElement) ToBytes(s *[32]byte)

func (*ExtendedGroupElement) ToCached 

func (p *ExtendedGroupElement) ToCached(r *CachedGroupElement)

func (*ExtendedGroupElement) ToProjective 

func (p *ExtendedGroupElement) ToProjective(r *ProjectiveGroupElement)

func (*ExtendedGroupElement) Zero 

func (p *ExtendedGroupElement) Zero()

type FieldElement 

type FieldElement [10]int32

FieldElement represents an element of the field GF(2^255 - 19). An element t, entries t[0]...t[9], represents the integer t[0]+2^26 t[1]+2^51 t[2]+2^77 t[3]+2^102 t[4]+...+2^230 t[9]. Bounds on each t[i] vary depending on context.

type PreComputedGroupElement 

type PreComputedGroupElement struct {
	// contains filtered or unexported fields
}

func (*PreComputedGroupElement) Zero 

func (p *PreComputedGroupElement) Zero()

type ProjectiveGroupElement 

type ProjectiveGroupElement struct {
	X, Y, Z FieldElement
}

func (*ProjectiveGroupElement) Double 

func (p *ProjectiveGroupElement) Double(r *CompletedGroupElement)

func (*ProjectiveGroupElement) ToBytes 

func (p *ProjectiveGroupElement) ToBytes(s *[32]byte)

func (*ProjectiveGroupElement) Zero 

func (p *ProjectiveGroupElement) Zero()

Source Files

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