[#190] Refactor cryptographic functionality

Remove `signature` and `util/signature` packages. Re-implement their
functionality in new `crypto` package. Generalize the approach of
digital signature computation and verification by adding `Signer` and
`PublicKey` primitives similar to standard `crypto` package. Support
already exising in protocol signature schemes.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

crypto/ecdsa/signer.go

@@ -0,0 +1,86 @@
+package neofsecdsa
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/sha512"
+	"fmt"
+
+	"github.com/nspcc-dev/neo-go/pkg/crypto/keys"
+	neofscrypto "github.com/nspcc-dev/neofs-sdk-go/crypto"
+)
+
+// Signer implements neofscrypto.Signer based on ECDSA.
+//
+// Supported schemes:
+//  - neofscrypto.ECDSA_SHA512 (default)
+//  - neofscrypto.ECDSA_DETERMINISTIC_SHA256
+//
+// Instances MUST be initialized with ecdsa.PrivateKey using SetKey.
+type Signer struct {
+	deterministic bool
+
+	key ecdsa.PrivateKey
+}
+
+// SetKey specifies ecdsa.PrivateKey to be used for ECDSA signature calculation.
+func (x *Signer) SetKey(key ecdsa.PrivateKey) {
+	x.key = key
+}
+
+// MakeDeterministic makes Signer to use Deterministic ECDSA scheme
+// (see neofscrypto.ECDSA_DETERMINISTIC_SHA256). By default,
+// neofscrypto.ECDSA_SHA512 is used.
+func (x *Signer) MakeDeterministic() {
+	x.deterministic = true
+}
+
+// Scheme returns signature scheme depending on Signer state:
+//  - neofscrypto.ECDSA_DETERMINISTIC_SHA256 if MakeDeterministic called
+//  - neofscrypto.ECDSA_SHA512 otherwise.
+func (x Signer) Scheme() neofscrypto.Scheme {
+	if x.deterministic {
+		return neofscrypto.ECDSA_DETERMINISTIC_SHA256
+	}
+
+	return neofscrypto.ECDSA_SHA512
+}
+
+// Sign signs data with algorithm depending on Signer state:
+//  - Deterministic ECDSA with SHA-256 hashing if MakeDeterministic called
+//  - ECDSA with SHA-512 hashing, otherwise
+func (x Signer) Sign(data []byte) ([]byte, error) {
+	if x.deterministic {
+		p := keys.PrivateKey{PrivateKey: x.key}
+		return p.Sign(data), nil
+	}
+
+	h := sha512.Sum512(data)
+	r, s, err := ecdsa.Sign(rand.Reader, &x.key, h[:])
+	if err != nil {
+		return nil, err
+	}
+
+	return elliptic.Marshal(elliptic.P256(), r, s), nil
+}
+
+// MaxPublicKeyEncodedSize returns size of the compressed ECDSA public key
+// of the Signer.
+func (x Signer) MaxPublicKeyEncodedSize() int {
+	return 33
+}
+
+// EncodePublicKey encodes ECDSA public key of the Signer in compressed form
+// into buf. Uses exactly MaxPublicKeyEncodedSize bytes of the buf.
+//
+// EncodePublicKey panics if buf length is less than MaxPublicKeyEncodedSize.
+//
+// See also PublicKey.Decode.
+func (x Signer) EncodePublicKey(buf []byte) int {
+	if len(buf) < 33 {
+		panic(fmt.Sprintf("too short buffer %d", len(buf)))
+	}
+
+	return copy(buf, (*keys.PublicKey)(&x.key.PublicKey).Bytes())
+}