forked from Shiloh/githaven
125 lines
3.3 KiB
Go
125 lines
3.3 KiB
Go
package curve25519
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import (
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"crypto/elliptic"
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"math/big"
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"sync"
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)
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var cv25519 cv25519Curve
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type cv25519Curve struct {
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*elliptic.CurveParams
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}
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func copyReverse(dst []byte, src []byte) {
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// Curve 25519 multiplication functions expect scalars in reverse
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// order than PGP. To keep the curve25519Curve type consistent
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// with other curves, we reverse it here.
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for i, j := 0, len(src)-1; j >= 0 && i < len(dst); i, j = i+1, j-1 {
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dst[i] = src[j]
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}
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}
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func copyTruncate(dst []byte, src []byte) {
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lenDst, lenSrc := len(dst), len(src)
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if lenDst == lenSrc {
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copy(dst, src)
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} else if lenDst > lenSrc {
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copy(dst[lenDst-lenSrc:lenDst], src)
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} else if lenDst < lenSrc {
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copy(dst, src[:lenDst])
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}
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}
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func (cv25519Curve) ScalarMult(x1, y1 *big.Int, scalar []byte) (x, y *big.Int) {
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// Assume y1 is 0 with cv25519.
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var dst [32]byte
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var x1Bytes [32]byte
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var scalarBytes [32]byte
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copyTruncate(x1Bytes[:], x1.Bytes())
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copyReverse(scalarBytes[:], scalar)
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scalarMult(&dst, &scalarBytes, &x1Bytes)
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x = new(big.Int).SetBytes(dst[:])
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y = new(big.Int)
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return x, y
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}
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func (cv25519Curve) ScalarBaseMult(scalar []byte) (x, y *big.Int) {
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var dst [32]byte
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var scalarBytes [32]byte
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copyReverse(scalarBytes[:], scalar[:32])
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scalarMult(&dst, &scalarBytes, &basePoint)
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x = new(big.Int).SetBytes(dst[:])
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y = new(big.Int)
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return x, y
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}
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func (cv25519Curve) IsOnCurve(bigX, bigY *big.Int) bool {
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return bigY.Sign() == 0 // bigY == 0 ?
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}
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// More information about 0x40 point format:
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// https://tools.ietf.org/html/draft-koch-eddsa-for-openpgp-00#section-3
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// In addition to uncompressed point format described here:
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// https://tools.ietf.org/html/rfc6637#section-6
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func (cv25519Curve) MarshalType40(x, y *big.Int) []byte {
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byteLen := 32
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ret := make([]byte, 1+byteLen)
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ret[0] = 0x40
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xBytes := x.Bytes()
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copyTruncate(ret[1:], xBytes)
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return ret
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}
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func (cv25519Curve) UnmarshalType40(data []byte) (x, y *big.Int) {
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if len(data) != 1+32 {
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return nil, nil
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}
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if data[0] != 0x40 {
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return nil, nil
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}
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x = new(big.Int).SetBytes(data[1:])
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// Any x is a valid curve point.
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return x, new(big.Int)
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}
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// ToCurve25519 casts given elliptic.Curve type to Curve25519 type, or
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// returns nil, false if cast was unsuccessful.
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func ToCurve25519(cv elliptic.Curve) (cv25519Curve, bool) {
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cv2, ok := cv.(cv25519Curve)
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return cv2, ok
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}
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func initCv25519() {
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cv25519.CurveParams = &elliptic.CurveParams{Name: "Curve 25519"}
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// Some code relies on these parameters being available for
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// checking Curve coordinate length. They should not be used
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// directly for any calculations.
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cv25519.P, _ = new(big.Int).SetString("7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffed", 16)
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cv25519.N, _ = new(big.Int).SetString("1000000000000000000000000000000014def9dea2f79cd65812631a5cf5d3ed", 16)
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cv25519.Gx, _ = new(big.Int).SetString("9", 16)
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cv25519.Gy, _ = new(big.Int).SetString("20ae19a1b8a086b4e01edd2c7748d14c923d4d7e6d7c61b229e9c5a27eced3d9", 16)
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cv25519.BitSize = 256
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}
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var initonce sync.Once
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// Cv25519 returns a Curve which (partially) implements Cv25519. Only
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// ScalarMult and ScalarBaseMult are valid for this curve. Add and
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// Double should not be used.
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func Cv25519() elliptic.Curve {
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initonce.Do(initCv25519)
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return cv25519
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}
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func (curve cv25519Curve) Params() *elliptic.CurveParams {
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return curve.CurveParams
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}
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