Reverse-engineering? What a wonderful opportunity to procrastinate! 
So, you still have not explained very clearly what an FSYS file is. A bit of googling led me to believe it is a video/audio file used primarily in Gamecube Pokemon games? Secondly, am I right in saying the FSYS file type is not well documented?
Some searching made me come to this (http://forum.xentax.com/viewtopic.php?f=10&t=8890), some guy has started something similar. You should definitely check this out if you haven't already.
In his pseudo-code, you can see the following structure:
You may already have got this far, but hopefully this will get you started in the right way.
And those numbers at offset 06 & 07? Well, they do not seem that important if the other person's doc. is to be believed, as they fit into a temp (ie. dummy) variable. Perhaps some metadata, maybe information about the compression, or file version information.
As for LZSS, that is a classic LZ compression scheme, which basically is an improved LZ77 with less redundant data sent.
I see you are using Windows from your screenshots, but let me say this: there is a reason Linux is called a hacker's OS. There are many little utilities available to encrypt/decrypt strings on the command line, among other things. You should be fine with a good hex editor, and I recommend something to prototype quickly (not C++; a scripting language like python, maybe) for experiments.
Sorry if I have oversimplified or repeated stuff you know already.
Good luck.
EDIT: As per the C++ debate; C++ is a very complex language. It is one of the only languages not to have a context-free grammar representation, because of its many ambiguities. It is also one of the slowest to compile. BUT it is definitely one the most powerful you can use: native, object-oriented, template meta-programming, ... There is a reason to the fact it is the #1 language in video games, high-performance programs, and many other areas.
My 2 cents: perhaps C# would have been a wiser choice for a GUI program. Or at least, the GUI side of it, it is much simpler. C++ is a bit more work, but you seem to have something good already, so carry on, good sir.
So, you still have not explained very clearly what an FSYS file is. A bit of googling led me to believe it is a video/audio file used primarily in Gamecube Pokemon games? Secondly, am I right in saying the FSYS file type is not well documented?
Some searching made me come to this (http://forum.xentax.com/viewtopic.php?f=10&t=8890), some guy has started something similar. You should definitely check this out if you haven't already.
In his pseudo-code, you can see the following structure:
Code:
struct fsys_file_header {
char magic[4]; //FSYS
uint32_t t1;
uint32_t t2;
uint32_t file_size; //DATA_COUNT?
uint32_t t3;
uint32_t t4;
uint32_t info_offset; //start address of info header
uint32_t data_offset; //DATA_BASE, start address of data
uint32_t file_size;
};
struct fsys_info_header {
uint32_t array_offset;
uint32_t name_table:
uint32_t base_offset;
};
struct fsys_fdat_entry {
uint32_t data_hash; // CRC-32? You probably know more than me.
uint32_t offset;
uint32_t size;
uint32_t t1;
uint32_t t2;
uint32_t z_size;
uint32_t t3;
uint32_t t4;
uint32_t t5;
uint32_t name_offset
};
/* ETC.... */And those numbers at offset 06 & 07? Well, they do not seem that important if the other person's doc. is to be believed, as they fit into a temp (ie. dummy) variable. Perhaps some metadata, maybe information about the compression, or file version information.
As for LZSS, that is a classic LZ compression scheme, which basically is an improved LZ77 with less redundant data sent.
I see you are using Windows from your screenshots, but let me say this: there is a reason Linux is called a hacker's OS. There are many little utilities available to encrypt/decrypt strings on the command line, among other things. You should be fine with a good hex editor, and I recommend something to prototype quickly (not C++; a scripting language like python, maybe) for experiments.
Sorry if I have oversimplified or repeated stuff you know already.
Good luck.
EDIT: As per the C++ debate; C++ is a very complex language. It is one of the only languages not to have a context-free grammar representation, because of its many ambiguities. It is also one of the slowest to compile. BUT it is definitely one the most powerful you can use: native, object-oriented, template meta-programming, ... There is a reason to the fact it is the #1 language in video games, high-performance programs, and many other areas.
My 2 cents: perhaps C# would have been a wiser choice for a GUI program. Or at least, the GUI side of it, it is much simpler. C++ is a bit more work, but you seem to have something good already, so carry on, good sir.