[#290] util: Implement salting writer

Implement wrapper over io.Writer that applies binary salt to written data.

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

pkg/util/salt_test.go

@@ -0,0 +1,34 @@
+package util_test
+
+import (
+	"bytes"
+	"crypto/rand"
+	"testing"
+
+	"github.com/nspcc-dev/neofs-node/pkg/util"
+	"github.com/stretchr/testify/require"
+)
+
+func randData(sz int) []byte {
+	data := make([]byte, sz)
+
+	_, _ = rand.Read(data)
+
+	return data
+}
+
+func TestSaltWriter_Write(t *testing.T) {
+	salt := randData(4)
+	data := randData(15)
+	buf := bytes.NewBuffer(nil)
+
+	w := util.NewSaltingWriter(buf, salt)
+
+	_, err := w.Write(data)
+	require.NoError(t, err)
+
+	require.Equal(t,
+		buf.Bytes(),
+		util.SaltXOR(data, salt),
+	)
+}