[#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.go

@@ -1,8 +1,18 @@
 package util
 
+import (
+	"io"
+)
+
 // SaltXOR xors bits of data with salt
 // repeating salt if necessary.
-func SaltXOR(data, salt []byte) (result []byte) {
+func SaltXOR(data, salt []byte) []byte {
+	return SaltXOROffset(data, salt, 0)
+}
+
+// SaltXOROffset xors bits of data with salt starting from off byte
+// repeating salt if necessary.
+func SaltXOROffset(data, salt []byte, off int) (result []byte) {
 	result = make([]byte, len(data))
 	ls := len(salt)
 	if ls == 0 {
@@ -11,7 +21,37 @@ func SaltXOR(data, salt []byte) (result []byte) {
 	}
 
 	for i := range result {
-		result[i] = data[i] ^ salt[i%ls]
+		result[i] = data[i] ^ salt[(i+off)%ls]
 	}
 	return
 }
+
+type saltWriter struct {
+	w io.Writer
+
+	off int
+
+	salt []byte
+}
+
+// NewSaltingWriter returns io.Writer instance that applies
+// salt to written data and write the result to w.
+func NewSaltingWriter(w io.Writer, salt []byte) io.Writer {
+	if len(salt) == 0 {
+		return w
+	}
+
+	return &saltWriter{
+		w:    w,
+		salt: salt,
+	}
+}
+
+func (w *saltWriter) Write(data []byte) (int, error) {
+	if dataLen := len(data); dataLen > 0 {
+		data = SaltXOROffset(data, w.salt, w.off)
+		w.off += dataLen
+	}
+
+	return w.w.Write(data)
+}

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),
+	)
+}