[#53] util: move signature package from neofs-api-go

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

util/signature/options.go

@@ -0,0 +1,91 @@
+package signature
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/sha256"
+	"crypto/sha512"
+	"math/big"
+
+	"github.com/nspcc-dev/neo-go/pkg/crypto/keys"
+)
+
+var curve = elliptic.P256()
+
+type cfg struct {
+	signFunc   func(key *ecdsa.PrivateKey, msg []byte) ([]byte, error)
+	verifyFunc func(key *ecdsa.PublicKey, msg []byte, sig []byte) error
+}
+
+func defaultCfg() *cfg {
+	return &cfg{
+		signFunc:   sign,
+		verifyFunc: verify,
+	}
+}
+
+func sign(key *ecdsa.PrivateKey, msg []byte) ([]byte, error) {
+	h := sha512.Sum512(msg)
+	x, y, err := ecdsa.Sign(rand.Reader, key, h[:])
+	if err != nil {
+		return nil, err
+	}
+	return elliptic.Marshal(elliptic.P256(), x, y), nil
+}
+
+func verify(key *ecdsa.PublicKey, msg []byte, sig []byte) error {
+	h := sha512.Sum512(msg)
+	r, s := unmarshalXY(sig)
+	if r != nil && s != nil && ecdsa.Verify(key, h[:], r, s) {
+		return nil
+	}
+	return ErrInvalidSignature
+}
+
+// unmarshalXY converts a point, serialized by Marshal, into an x, y pair.
+// It is an error if the point is not in uncompressed form.
+// On error, x,y = nil.
+// Unlike the original version of the code, we ignore that x or y not on the curve
+// --------------
+// It's copy-paste elliptic.Unmarshal(curve, data) stdlib function, without last line
+// of code.
+// Link - https://golang.org/pkg/crypto/elliptic/#Unmarshal
+func unmarshalXY(data []byte) (x *big.Int, y *big.Int) {
+	if len(data) != PublicKeyUncompressedSize {
+		return
+	} else if data[0] != 4 { // uncompressed form
+		return
+	}
+
+	p := curve.Params().P
+	x = new(big.Int).SetBytes(data[1:PublicKeyCompressedSize])
+	y = new(big.Int).SetBytes(data[PublicKeyCompressedSize:])
+
+	if x.Cmp(p) >= 0 || y.Cmp(p) >= 0 {
+		x, y = nil, nil
+	}
+
+	return
+}
+
+func SignWithRFC6979() SignOption {
+	return func(c *cfg) {
+		c.signFunc = signRFC6979
+		c.verifyFunc = verifyRFC6979
+	}
+}
+
+func signRFC6979(key *ecdsa.PrivateKey, msg []byte) ([]byte, error) {
+	p := &keys.PrivateKey{PrivateKey: *key}
+	return p.Sign(msg), nil
+}
+
+func verifyRFC6979(key *ecdsa.PublicKey, msg []byte, sig []byte) error {
+	p := (*keys.PublicKey)(key)
+	h := sha256.Sum256(msg)
+	if p.Verify(sig, h[:]) {
+		return nil
+	}
+	return ErrInvalidSignature
+}