[xmly] feat #5: implement xmly decipher

um_crypto/utils/src/base64.rs

@@ -22,3 +22,14 @@ where
 {
     ENGINE.decode(data)
 }
+
+pub fn decode_overwrite<T>(data: &mut T) -> Result<&[u8], DecodeError>
+where
+    T: AsMut<[u8]> + ?Sized,
+{
+    let data = data.as_mut();
+    let decoded = decode(&mut data[..])?;
+    let len = decoded.len();
+    data[..len].copy_from_slice(&decoded);
+    Ok(&data[..len])
+}