ethwallet: swap out btcec.S256 with crypto.S256

[attente]

ethkit/go-ethereum/crypto/signature_nocgo.go

Lines 82 to 84 in cdebd57

	if prv.Curve != S256() {
	return nil, errors.New("private key curve is not secp256k1")
	}

ethkit/go-ethereum/crypto/signature_nocgo.go

Lines 161 to 196 in cdebd57

	// S256 returns an instance of the secp256k1 curve.
	func S256() EllipticCurve {
	return btCurve{btcec.S256()}
	}
	type btCurve struct {
	*btcec.KoblitzCurve
	}
	// Marshal converts a point given as (x, y) into a byte slice.
	func (curve btCurve) Marshal(x, y *big.Int) []byte {
	byteLen := (curve.Params().BitSize + 7) / 8
	ret := make([]byte, 1+2*byteLen)
	ret[0] = 4 // uncompressed point
	x.FillBytes(ret[1 : 1+byteLen])
	y.FillBytes(ret[1+byteLen : 1+2*byteLen])
	return ret
	}
	// Unmarshal converts a point, serialised by Marshal, into an x, y pair. On
	// error, x = nil.
	func (curve btCurve) Unmarshal(data []byte) (x, y *big.Int) {
	byteLen := (curve.Params().BitSize + 7) / 8
	if len(data) != 1+2*byteLen {
	return nil, nil
	}
	if data[0] != 4 { // uncompressed form
	return nil, nil
	}
	x = new(big.Int).SetBytes(data[1 : 1+byteLen])
	y = new(big.Int).SetBytes(data[1+byteLen:])
	return
	}

Upstream go-ethereum started embedding the original curve in a new type with custom marshaling, causing the original curve check to fail when running with CGO_ENABLED=0.