[#57] services/object: Implement private key storage

Implement storage that provides access to local node key and session keys
through session token.

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

pkg/services/object/util/key.go

@@ -0,0 +1,41 @@
+package util
+
+import (
+	"crypto/ecdsa"
+
+	"github.com/nspcc-dev/neofs-api-go/pkg/token"
+	"github.com/nspcc-dev/neofs-node/pkg/services/session/storage"
+	"github.com/pkg/errors"
+)
+
+// KeyStorage represents private key storage of the local node.
+type KeyStorage struct {
+	key *ecdsa.PrivateKey
+
+	tokenStore *storage.TokenStore
+}
+
+// NewKeyStorage creates, initializes and returns new KeyStorage instance.
+func NewKeyStorage(localKey *ecdsa.PrivateKey, tokenStore *storage.TokenStore) *KeyStorage {
+	return &KeyStorage{
+		key:        localKey,
+		tokenStore: tokenStore,
+	}
+}
+
+// GetKey returns private key of the node.
+//
+// If token is not nil, session private key is returned.
+// Otherwise, node private key is returned.
+func (s *KeyStorage) GetKey(token *token.SessionToken) (*ecdsa.PrivateKey, error) {
+	if token != nil {
+		pToken := s.tokenStore.Get(token.OwnerID(), token.ID())
+		if pToken == nil {
+			return nil, errors.Wrapf(storage.ErrNotFound, "(%T) could not get session key", s)
+		}
+
+		return pToken.SessionKey(), nil
+	}
+
+	return s.key, nil
+}