this post was submitted on 10 Aug 2023
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The problem is that when you compress before you encrypt, the file size becomes a source of data about the contents. If an attacker has control of part of the data - say - a query string, they can use that to repeatedly add things to your data and see how the size changes as a result.
So it sounds like compression before encryption should only be done in specific circumstances because it can be a security issue depending on use case, but encryption before compression should never be done because it will almost always increase the size of the file