I am wonder how can psp firmware get updated and not downgraded with the new firmware that comes out.
Isn't it possible if it is based on version number first update it to a hex 1.5 which says like 2.7 than downgrade it back to a real 1.5 version?
I am not too sure how the firmware upgrade works. I dont have a psp yet.
I am about to get one from ebay.
Firmware updates?
No, the problem is that higher firmware versions don't allow homebrew to be run in the first place (because the exploits found to allow homebrew to run get fixed). Thus no downgrader can be written. Then still if homebrew can be run on a firmware (like now on up to 2.6) as long as there's no kernel access for the homebrew, there is also no access to the flash memory (where the OS sits) and so no downgrader can still be written. Kernel access is only possible by exploits found in software that runs in kernel mode, so this won't work for game exploits like the GTA savegame exploit.
This all wouldn't be a problem if we had the 'magic key' to sign homebrew applications so the psp can naturally run them. That's the way sony can create an upgrader and other companys can create official software/games for the psp. However, since Sony doesn't support homebrew they also don't provide an official downgrader, so that's the reason why we have the problem with downgrading from above.
Oh and btw: If you think it would be possible to write your own firmware that pretends to be > 2.6 so you could upgrade to that from any version and then downgrade back from there. That's impossible, because the firmware itself is encrypted with an unknown key and the PSP won't run anything unencrypted from the flash. So still if you'd write anything homebrewn to the flash, you could just burn your PSP straight away - it's just a brick.
This all wouldn't be a problem if we had the 'magic key' to sign homebrew applications so the psp can naturally run them. That's the way sony can create an upgrader and other companys can create official software/games for the psp. However, since Sony doesn't support homebrew they also don't provide an official downgrader, so that's the reason why we have the problem with downgrading from above.
Oh and btw: If you think it would be possible to write your own firmware that pretends to be > 2.6 so you could upgrade to that from any version and then downgrade back from there. That's impossible, because the firmware itself is encrypted with an unknown key and the PSP won't run anything unencrypted from the flash. So still if you'd write anything homebrewn to the flash, you could just burn your PSP straight away - it's just a brick.
As far as i know, (just bought a 2.60 console last week so i haven't had too much time to experiment with it yet) the update process have 3 protections:
1. The firmware checks if UPDATER_VER is greater than the current one.
2. Then perform an unknown (to me) test to see if DATA.PSP is valid, if it isn't it triggers an 80020148 error (note that the firmware 2.60 is a valid prx if the same firmware (2.60) is already running).
3. Then DATA.PSP checks if EBOOT.PBP is valid (maybe with some crc32 or just by checking UPDATER_VER), if it isn't it trigger a UKN90000001 error.
Just wild guesses from some experiments i made, actually.
If you knew that already just consider it as a reminder :D
1. The firmware checks if UPDATER_VER is greater than the current one.
2. Then perform an unknown (to me) test to see if DATA.PSP is valid, if it isn't it triggers an 80020148 error (note that the firmware 2.60 is a valid prx if the same firmware (2.60) is already running).
3. Then DATA.PSP checks if EBOOT.PBP is valid (maybe with some crc32 or just by checking UPDATER_VER), if it isn't it trigger a UKN90000001 error.
Just wild guesses from some experiments i made, actually.
If you knew that already just consider it as a reminder :D