I currently informed myself about encryption a lot. And I wonder, whether it would be good to toggle some bits (XOR and bitmasks) at a known position in the encrypted byte array and to toggle them again before decrypting them.
Because even if you know the algorithm and the key it wouldn't be possible to decrypt them propably without knowing where to toggle the bits wouldn't it?
The bit-toggling becomes part of the algorithm, so if "they know the algorithm" comes to include which bits were toggled.
It does become marginally harder to find out that "the algorithm", but this gain is small. If they can get their hands on the key, I think your problem is somewhere else...
There are some disadvantages to this as well. First, you may introduce security flaws in the system. I don't think this will happen, but I don't know I won't, and in security you should assume you might cause security flaws unless you know you won't.
The second problem is that if you make a mistake here somewhere it is possible to corrupt data. Of course, rigid testing will make sure that a mistake won't make it to production, but it just isn't as safe as using the functionality of a security library.
Lastly, there is the problem that your code and data will be harder to work with. If you need to work with it in the future, or someone has to work with it, it'll probably take more effort than it otherwise would have.
Those aren't big things, but I'd say more than the gain. At the end of the day, this is little more than "security through obscurity", so no, I wouldn't say it is a good idea.