[#150] signature: Add scheme

Allow `SignOption` to set 2 parameters:
1. Default signature scheme, which is used in case scheme is
   unspecified.
2. Restrict scheme option which also checks that scheme is either
   unspecified or equal to the restricted scheme. This is only used
   for verification and is necessary because some of the signatures
   are used in smart-contracts.

Also provide signature struct to sign/verify functions in helpers.

The constant names differ a bit from those in API because of linter
complaints.

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

util/signature/options.go

@@ -9,38 +9,65 @@ import (
 	"math/big"
 
 	"github.com/nspcc-dev/neo-go/pkg/crypto/keys"
+	"github.com/nspcc-dev/neofs-sdk-go/signature"
 )
 
 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
+	defaultScheme  signature.Scheme
+	restrictScheme signature.Scheme
 }
 
-func defaultCfg() *cfg {
-	return &cfg{
-		signFunc:   sign,
-		verifyFunc: verify,
+func getConfig(opts ...SignOption) *cfg {
+	cfg := &cfg{
+		defaultScheme:  signature.ECDSAWithSHA512,
+		restrictScheme: signature.Unspecified,
+	}
+
+	for i := range opts {
+		opts[i](cfg)
+	}
+
+	return cfg
+}
+
+func sign(scheme signature.Scheme, key *ecdsa.PrivateKey, msg []byte) ([]byte, error) {
+	switch scheme {
+	case signature.ECDSAWithSHA512:
+		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
+	case signature.RFC6979WithSHA256:
+		p := &keys.PrivateKey{PrivateKey: *key}
+		return p.Sign(msg), nil
+	default:
+		panic("unsupported scheme")
 	}
 }
 
-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
+func verify(scheme signature.Scheme, key *ecdsa.PublicKey, msg []byte, sig []byte) error {
+	switch scheme {
+	case signature.ECDSAWithSHA512:
+		h := sha512.Sum512(msg)
+		r, s := unmarshalXY(sig)
+		if r != nil && s != nil && ecdsa.Verify(key, h[:], r, s) {
+			return nil
+		}
+		return ErrInvalidSignature
+	case signature.RFC6979WithSHA256:
+		p := (*keys.PublicKey)(key)
+		h := sha256.Sum256(msg)
+		if p.Verify(sig, h[:]) {
+			return nil
+		}
+		return ErrInvalidSignature
+	default:
+		return ErrInvalidSignature
 	}
-	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.
@@ -71,21 +98,7 @@ func unmarshalXY(data []byte) (x *big.Int, y *big.Int) {
 
 func SignWithRFC6979() SignOption {
 	return func(c *cfg) {
-		c.signFunc = signRFC6979
-		c.verifyFunc = verifyRFC6979
+		c.defaultScheme = signature.RFC6979WithSHA256
+		c.restrictScheme = signature.RFC6979WithSHA256
 	}
 }
-
-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
-}