Hallelujah! PSP dev Yoshihiro has released his much anticipated 5.55 and 6.00 Game Decrypter. Which means that users running custom firmware 5.50GEN-B2 and 5.00 M33-6 will be able to play most of the games that are currently on the market, without upgrading to an official firmware.
Some of the bigger titles that work include Gran Turismo, Persona, Disgaea 2, and Soul Calibur Broken Destiny.
Forum members at GBAtemp are maintaining a list of compatible and incompatible games.
Here are the tools you are going to need:
Download: Yoshihiro’s Game Decrypter
Download: UMDGen v4.00
Update: GBAtemp member Absynthe was kind enough to allow us to post his detailed instructions for getting Yoshihiro’s Game Decrypter to play nicely with your PSP. Here they are, with a few revisions from AnalogMan (steps 14-17):
1. Download Yoshihiro’s Decrypter (http://www.pspgen.com/game-decripter-by-yoshihiro-pack-rtu-telechargement-190007.html, you need to be registered to download).
2. Unrar it.
3. Copy the files in the PSP/GAME/ folder.
4. Pick a ISO file.
5. Download UMDGEN (http://www.pspgen.com/divers/telecharger-145702.html).
6. Open your ISO with UMDGEN and browse to PSP_GAME/SYSDIR.
7. Right click EBOOT.BIN.
8. Extract it somewhere.
9. Copy it to the root of your memory stick.
10. Unplug the PSP and load Yoshihiro’s Decrypter.
11. Press X and wait a few seconds.
12. When the PSP has restarted, re-plug the USB cable and connect it to your PC.
13. In your memory stick root, there should be a new folder called “DECRYPTOR”, and inside of it there should be a file called EBOOT.BIN. Take it and copy it somewhere safe. Do not mix it with the EBOOT.BIN you previously extracted from the ISO.
14. Open UMDGEN again, load again your ISO, go to File->Filelist->Export, type “Filelist”, and save it somewhere.
15. Go to PSP_GAME/SYSDIR, right-click EBOOT.BIN and select delete.
16. Right-click somewhere in the SYSDIR folder and choose Add -> Existing File(s)…, and browse to the EBOOT.BIN you found in the DECRYPTOR folder.
17. Now go again to File->Filelist, and this time select Import. Choose the Filelist you previously exported, and click “Yes”.
18. Click the “Save” button in the toolbar, select “Uncompressed (*.iso)”, name it and wait a few minutes.
19. Copy the newly-created .iso file into the ISO folder of your PSP.
20. Unplug and start it. It should work. If it doesn’t, try updating your custom firmware and/or changing drivers (this can be done by pressing select and by scrolling to the “UMD ISO MODE” voice. If the menu doesn’t show up, power off your PSP, keep the R trigger pressed and power it on again. A recovery menu should appear. Go to Configuration, and look for the voice “Use VshMenu”. Change its parameter to “VSHMENU”, then go back and reboot the PSP. Now the menu should appear. Try changing the drivers, it may work. If it doesn’t, just pray that someone smarter than us comes up with a solution).
Thanks guys!
From the README:
hi guyz this tool is here for let you use your original game backup
on CFW 5.50 gen B-2 .
how i can use this tool ?
1 :lets go first you need to install the GameZDecryptZ folder in your X:PSP/GAME folder
2 : put your EBOOT.BIN extracted from UMDGEN PSP_GAME/SYSDIR and put it on root of your memory stick
3 : now you can boot game decrypter now you can press X 5 seconds later it's done 
4 : connect your usb cable check on root of your memory stick you have a new folder called decryptor
5 : copy the eboot.bin from the decryptor folder inside umdgen and rebuild your iso
Voilà
Enjoy it Yoshihiro
PS : not all games working because some games use some fact header on prx files
Update: Fixed link to compatible games list, found a better one.
