yaralyzer VS CyberChef

for the ultra low level the Didier Stevens tools mentioned in the OP are rock solid, but for anything sort of in the middle zone - allowing you to work with the logical structure, having a consistent API, etc. etc. - yeah there's not much out there, which is why I ended up making The Pdfalyzer (and The Yaralyzer, which was basically just a side effect).

Just released 0.4.0 which has some new features and small improvements over the 0.1.0 I posted originally... CHANGELOG.md

Then we take the encrypted text and use CyberChef to decrypt it.

Let's go to CyberChef and insert our pieces of evidence.

A parameter was changed from '2AMBCgIQBg' to 'CgIIAdgDAQ%3D%3D' which is just the correct base64 encoding they should have been using the entire time.

I don't think this was a hostile action by Google, I think someone just added better input validation for security reasons and it accidently broke the bad requests they were sending.

https://gchq.github.io/CyberChef/#recipe=URL_Decode()From_Ba...

❗This is indeed the flag, but the text is encrypted with Base64. Usually, the presence of padding character "=" indicates that's Base64 type of encoding (but that's only one of the hints). To decrypt it, we can use CyberChef. Copy-paste the text and we either:

It uses a combination of magic bytes (like the `file` command), entropy analysis and character frequency detection to determine whether an output is likely to be of interest to the user.

The file type mechanism is written here[0]. There's a list of all signatures we detect here[1].

[0] https://github.com/gchq/CyberChef/blob/master/src/core/lib/F...

I think this can be deciphered using CyberChef...