Ok some progress report. I spent a good day testing this with Steveice10. TuxSH also helped with this. (mainly trying patches on the Launcher SRL on TwlBg)
I believe sig checks for DSi SRLs (DSiWare) have been patched out. (Anything using SHA1 hashes I believe have been patched)
Even got a patch that disables the DS Cart White list function entirely so any old DS flashcart (that isn't blocked by save chip detection) to work! I personally tested this by patching the relevent data in the FIRM binary of TWL_FIRM. (after fixing hashes, repacking, and re-encrypting. This works from emunand!)
https://gist.github.com/Steveice10/8a6965c59192ac970832
So those inclined to try it may do so. I have the pre-patched TWL_FIRM CIA. But obviously I'm not going to share it here.
I tested this by modifying the banner of a DSi game. Some random blackjack game.
This breaks the SHA1 HMAC hash for the banner section (which is signed by the way, so it's protected the same way as the rest of the SRL). This causes it to not show up on home menu. But that's good. I just started it from DevMenu instead and it boots! So the sig check worked on allowing a custom banner. (though Home Menu won't let you see it.
). This means one can probably edit other sections of the SRL (and maybe build a custom one or do some cool things like modifying Sudoku to have access to Slot-1.
.
This may or may not make it's way into CFW. Just note not to modify TWL_FIRM on sysnand. Unless you don't plan on booting any DS games outside of CFW. Doing it on Emunand will work provided you are not using rxTools as your CFW. Patches may be slightly different for n3DS version of TWL_FIRM. But I think the only difference with that is title id and encryption of Arm9 section.
Dev TWL apps like TWLNandFiler still don't work though. Those are probably broken for other reasons that aren't sig related. Maybe size or header settings retail consoles don't like with that one.
