Why are MSI files designed like this? Why allow for just a specific part of the file to be signed? (I have no technical experience dealing with them.)
- Headers describing a section, including the length of a section.
- Prefixing each section with a size.
- Using a marker pattern (such as a null byte) to mark the end of a section of data.
The consequence of using the first two techniques is that it's entirely possible to add data to the end of the format, since a parser would just read the last section and stop (assuming the format supports knowing what the last section is).
I developed something like this to put an md5 hash in a comment in the first line of a code module at a place I worked. It simply read the file, removed that line, and hashed the rest of the file data to generate the hash for comparison.
This was just to stop programmers changing the code without submitting it to a secondary system for validation/tracking (easy to work around, but that shows intent to bypass policies when we see it in the repo).