[#53] util: Add SaltXOR function

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

pkg/util/salt.go

@@ -0,0 +1,17 @@
+package util
+
+// SaltXOR xors bits of data with salt
+// repeating salt if necessary.
+func SaltXOR(data, salt []byte) (result []byte) {
+	result = make([]byte, len(data))
+	ls := len(salt)
+	if ls == 0 {
+		copy(result, data)
+		return
+	}
+
+	for i := range result {
+		result[i] = data[i] ^ salt[i%ls]
+	}
+	return
+}