Hacking How to find decryption key from decrypted and encrypted .carc file?

[NitroShell]

I have 2 versions of a file, 1 works in Ermii Kart DS and the other one works in Mario Kart DS, they both contain the same data but are a bit different in how they are stored, I am wondering can I find a sort of key from the decrypted file to decrypt all the other encrypted files in Ermii Kart DS to work in the normal version of Mario Kart DS? I'm not sure if the correct term I should use is "encrypted" and "decrypted" but anyways you know what I mean of course the data size is different so I think it may be another thing than encrypted, more like another format.