I was poking around the AGI Wiki, and came across an entry talking about different tools used to decrypt the AGI file in Sierra games. And I'm surprised to see that there a pervasive, mistaken belief that in order to decrypt the file you need a copy of the key from the notorious hidden track of original Sierra disks (either from an original disk, or made available from some other source).
That is NOT TRUE.
The key is already in the encrypted file. The MSDOS executable for AGI includes a large chunk of bytes near the beginning that are all zeros, specifically including from bytes 257 to 384. Since the encryption is a simple XOR function, the corresponding bytes in the encrypted file are exactly the entire key for the third iteration of decryption. To determine the original key, just rotate the the value of those bytes to the left twice. You don't need anything other than the encrypted file.
I created a small app that did this years ago. I think I shared it on MegaTokyo, but of course that's long since gone. The app is still available though, and is also bundled with the latest version of WinAGI. It will quickly and easily decrypt any AGI file without needing any original disk or key string.