session: change PrivateToken interface methods

This commit replaces PublicKey() and SignData() methods of PrivateToken
with PrivateKey() in order to have the ability to sign data with
session key using service package functions.

session/private.go

@@ -4,8 +4,6 @@ import (
 	"crypto/ecdsa"
 	"crypto/elliptic"
 	"crypto/rand"
-
-	crypto "github.com/nspcc-dev/neofs-crypto"
 )
 
 type pToken struct {
@@ -30,14 +28,9 @@ func NewPrivateToken(validUntil uint64) (PrivateToken, error) {
 	}, nil
 }
 
-// Sign signs data with session private key.
-func (t *pToken) Sign(data []byte) ([]byte, error) {
-	return crypto.Sign(t.sessionKey, data)
-}
-
-// PublicKey returns a binary representation of the session public key.
-func (t *pToken) PublicKey() []byte {
-	return crypto.MarshalPublicKey(&t.sessionKey.PublicKey)
+// PrivateKey returns a binary representation of the session public key.
+func (t *pToken) PrivateKey() *ecdsa.PrivateKey {
+	return t.sessionKey
 }
 
 func (t *pToken) Expired(epoch uint64) bool {

session/private_test.go

@@ -1,35 +1,16 @@
 package session
 
 import (
-	"crypto/rand"
 	"testing"
 
-	crypto "github.com/nspcc-dev/neofs-crypto"
 	"github.com/stretchr/testify/require"
 )
 
-func TestPrivateToken(t *testing.T) {
+func TestPToken_PrivateKey(t *testing.T) {
 	// create new private token
 	pToken, err := NewPrivateToken(0)
 	require.NoError(t, err)
-
-	// generate data to sign
-	data := make([]byte, 10)
-	_, err = rand.Read(data)
-	require.NoError(t, err)
-
-	// sign data via private token
-	sig, err := pToken.Sign(data)
-	require.NoError(t, err)
-
-	// check signature
-	require.NoError(t,
-		crypto.Verify(
-			crypto.UnmarshalPublicKey(pToken.PublicKey()),
-			data,
-			sig,
-		),
-	)
+	require.NotNil(t, pToken.PrivateKey())
 }
 
 func TestPToken_Expired(t *testing.T) {

session/types.go

@@ -10,14 +10,8 @@ import (
 
 // PrivateToken is an interface of session private part.
 type PrivateToken interface {
-	// PublicKey must return a binary representation of session public key.
-	PublicKey() []byte
-
-	// Sign must return the signature of passed data.
-	//
-	// Resulting signature must be verified by crypto.Verify function
-	// with the session public key.
-	Sign([]byte) ([]byte, error)
+	// PrivateKey must return session private key.
+	PrivateKey() *ecdsa.PrivateKey
 
 	// Expired must return true if and only if private token is expired in the given epoch number.
 	Expired(uint64) bool