Applying and Rebuilding Romhacks for /hbg/

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How to apply 3DS romhacks (and rebuild them) - a /hbg/ guide
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Part 1: Dumping the romfs
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In order to apply our patches we're going to need a base to paste those patches on. When you play a game, most of the information and gamecode is in the romfs of the cia. Other parts of the cia are the exefs, which includes the code.bin (responsible for certain things like marts, etc in Pokemon, as well as containing the icon and banner for the game. Different romhacks will supply different files; some edit the code.bin and will give you exefs files, most will give you romfs files. Either way, dumping a game is necessary. There are a few different ways to do this depending on whether you're stuck in userland homebrew (SysNAND 9.3+) or whether you have kernel access (9.2 SysNAND).
Hands down the easiest way is just finding an already decrypted CIA online. You can find these on 3dsiso or 3dschaos, just look for one that's not a "legit" cia or that has been patched to be region free or something, this means they have been decrypted. You can also grab a regular encrypted cia and decrypt it yourself following this guide (https://gbatemp.net/threads/tutorial-how-to-decrypt-extract-and-rebuild-a-cia.388677/)
Other methods include using the homebrew application braindump (https://gbatemp.net/threads/braindump-prerelease-dump-game-contents-on-any-system-version.400920/) which works on 9.3+ SysNAND, or using Uncart which essentially does the same thing but runs via kernel access. Braindump has been known to provide faulty romfs's that fuck up when you recompile though, so use Uncart or just get a decrypted cia online if you can.

In any case, once you have your CIA of your base game, follow these steps. When I tell you to type in a command, omit the quotation marks.
1. Download Asia's Hack Pack (http://bit.ly/1SmThNI) and extract it to a folder somewhere, then place your CIA/CXI in that folder.
2. Run HackingToolkit.exe (any virus alerts are a false positive)
3. Type in "cia" (or "CXI" if you used braindump)
4. Type in the name of your CIA/CXI WITHOUT the file extension
5. Let it extract, when it asks you if you want to decompress the code.bin, choose No ("n")
6. You will be left with a bunch of files, but 3 in particular are important; the romfs folder, the exe folder, and the DecryptedExHeader.bin. Hold onto these, they will provide the basis for your romhack.

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Part 2: Applying your hack
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This part is relatively short and quick. Your hack will usually provide its own instructions on how to go about applying it but the general gist is to take whatever files it gives you and overwrite the correct files in the romfs/exe folders we just generated with the files the hack gives you. Literally just copy/paste them over the top and choose yes to overwrite files. Follow the instructions provided with your hack for this part.

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Part 3.5: Getting the hack working with Hans
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This only really applies if you don't have CFW and have to use Hans for romhacks. If you have CFW and are wanting to make a prepatched .cia, SKIP THIS PART.
Again, usually hacks come with their own instructions for this part so just follow them, this is more of a general guide on what to do.

1. After applying the hack in the previous step, open up HackingToolkit.exe again
2. Type in "RFSB" and hit enter
3. In the window that pops up, click Open then navigate to your romfs folder and select it
4. Hit "Go"
5. Take the .romfs file this process generated and rename it to whatever the instructions of your hack say
6. Put that romfs file in a /hans/ folder at the root of your 3DS SD card
7. If the hack provides a .code file, put that in the folder too
8. Copy-paste the other hans files the hack provides into their respective folders
9. Now you should be able to run this patch from Hans.

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Part 4: Rebuilding a CIA/3DS file
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This is if you have CFW and can install unsigned CIAs, this will install the romhack as a full game to your Home Menu in EmuNAND.

1. Open up HackingToolkit.exe again
2. Type in "3DSB"
3. Click "Open Romfs" and navigate to the romfs folder from before and choose it
4. Do the same for Exefs (using the exe folder this time). You will likely get a dialogue box after selecting your exe folder that asks if you want to compress the code.bin. If you get this popup, choose yes, if you don't then your code.bin is already compressed so don't worry about it.
5. Click "Open Exheader" and navigate to the DecryptedExHeader.bin that we generated earlier and choose it. It will ask you if you want to load the default settings for your game, choose Yes.
6. Click "Choose Save Location" and enter in the name of your file and the .3ds extension. For example, I might name my hack StarSapphire.3ds
7. Click Go and wait for it to finish generating the .3ds file. If you have Gateway or Sky3DS, you're done here, you can use that 3DS file to play. Otherwise read on for how to make a CIA.
8. Download Simple CIA Converter (http://bit.ly/1QcUjFG) and extract it somewhere.
9. Create a folder called roms (if it doesn't exist already) and place your .3ds file in it. If you're compiling multiple romhacks, you can put multiple .3ds files in this folder and the converter will convert them all in one go.
10. Run 3ds_simple_cia.exe and check the "FW Spoof" box.
11. Click the "3DS ROM to CIA" button and navigate to your roms folder from step 9 and select it.
12. At this point the program will appear to freeze up and stop responding, but it's still running. Don't mess with it, just let it do its thing and eventually it will pop up with a box saying "Finished".
13. It will have created a folder called cia and placed all your CIA files in there
14. You can now install these CIAs using FBI/BBM/DevMenu and your romhack should appear on your home menu!