File formats are great fun to RE. I worked at a place that used a proprietary business management app. One day I was bored and started looking at its data files with a hex editor and saw some patterns like the titles of records in the app were spaced at exact multiples of X bytes apart on the data file. Oh! Fixed width records! Huh, look at that: right after the title, the next two bytes look like the hex value of the record number show in the app. Guess that's how it stores those! If I click this checkbox, this one byte changes from 0x00 to 0x01. Hey there! After enough experimentation I had the whole thing mapped to structs in code and was able to build reports that the original app couldn't support.

There wasn't anything illegal about that. I'm sure the vendor would've preferred we pay them to make reports for us but nothing legally prevented it.