Exactly what @victormr21 said. The ckey.bin, which you obviously acquired from some website, contains the hexadecimal code needed for decrypting Wii U files gathered from NUS. Notepad reads the hex and attempts to translate them into letters, numbers, or symbols based on the format (UTF-8, ASCII) it thinks the file is in.
After using the famous HxD v1.7.7, I could see the content now. It matches that D-beginning string of texts that could be found more easily. However, IDM UltraEdit still shows the garbled text. thx