To say it another way: If I encrypt the sentence "My password is GOD" would it be easier to decrypt if I told you the beginning of the encrypted data is "My password is "? So if some hacker person knows what the application data is when it's decrypted does that make it easier to decrypt the application + text?
what you are describing is called a "Known Plaintext Attack" in crypto terms.
the bottom line is that it helps, but for good encryption algorithms
, it doesn't help much, and won't significantly help an attacker.
in generic terms it can help an attacker tell when he found the right key if he/she is just trying trillions of them, but they could probably tell that just by looking at letter frequencies almost as easily. there are weak encryption algorithms for which knowing the plaintext could help you, but AES used by fsekrit is certainly not one of them.
so in short, knowing the start of the plaintext is not going to help anyone.
The application and the text are stored in one file, does that mean the application is also encrypted?
fsekrit is "compressed" with pecompact to make it even smaller, but as i understand it (f0dder will clarify if i'm wrong), the encrypted data is just appended to the exe basically - so it's not really encrypted or part of the encryption process, so in this case your scenario doesn't apply anyway.