Xbox 360 Timing Attack
The timing attack is used allow machines with kernel 4552 or higher to downgrade to a vulnerable kernel. The purpose is to find the required hash values for the altered CB section in a base kernel (1888). The lockdown counter in the CB section is changed to a higher value to allow the base kernel (1888) to bypass the restrictive lockdown counter check. The check is what makes it impossible to just flash the NAND and downgrade to a vulnerable kernel.
Fault
The memcmp function used to check the CB-auth value is 16 bytes long and done byte-by-byte. Changing one byte will to something false will take a few microseconds longer than a true value. Doing this for each byte will reveal the correct hash. Possibilities: 16 bytes * 256 different possibility for each byte, total 4096 tries.
Possible MS Updates
arnezami:
MS cannot fix this problem by simply changing the memcmp function in a future kernel version. Thats not gonna help them. The weakness is that the byte-wise memcmp function is in the 1888 kernel/bootloader (and they cannot change that one anymore of course). [1]
tmbinc:
sure, microsoft can change the 2BL, and burn a fuse (of the fuseline 2) so that an old 2BL doesn't work anymore... [2]
arnezami:
Ah. Right. If they can indeed burn these fuses at row 2 than you wouldn't be able to run any of the lower kernel versions anymore. [3]
Been thinking about this. I'm now pretty sure when row 2 of the fuses is burned your xbox won't be able to downgrade or run homebrew anymore (it appears the fuse count number is indeed RSA signed). [4]
surrido:
you could if it makes you happy wire a switch to the R6T3 and keep it on while being in live and turn it of when you receive an update. [5]
tmbinc:
No, a switch at R6T3 doesn't help. It's not the resistor which presence can be detected, but the result to the fuse (burned or not). So his question is completely valid: If you remove the resistor, you could end up with an unbootable box after the next update. But at least you could restore a previous flash. (If you want, you could *then* re-attach the resistor, and update again, of course loosing the possibility to downgrade). [6]
