[Xbox-linux-hacking] .xbe signing process

[mike_cj <mi...@te...>]

I'm a software developer that's interested in this whole "project B" 
things as a challenge, and also as I'd like to be able to develop my 
own software for my X-Box (i.e. I'd like to be able to run stuff I'd 
developed using OpenXDK or something similar) without mod'ing my 'box. 
I think I may have some ideas about how to create a .xbe that the Xbox 
kernel will be fooled into loading, but I don't know enough about the 
Xbox kernel's verification/signing process yet to know if my ideas are 
stupid or not :-).  As I hinted above, I'm fairly unkeen on mod'ing my 
XBox, but really all I need is a few more details to expand on the 
excellent "XBox Security Concept" doc' by Franz Lehner.  I think I've 
got a good grasp on the most of it, but I could really use a little 
clarificaton of more details as to what is meant by the following  -

"When the Xbox loads an XBE file, it first checks whether the header 
is "valid".
This is done by comparing the calculated SHA-1 hash against the 
decrypted RSA signature.
The RSA signature is "padded" in the format 01FFFFFFFFFFFFFF.......FF00
(HASH 20bytes)"

1. I assume the SHA1 hash referred to is calculated on the whole xbe 
header, including the RSA Signature, TLS and library version structures 
etc. ?

2. The "decrypted RSA signature" - is this the 256 bytes from the Image 
header modified by the public key (i.e use the public key to "encrypt 
it"), or is the hash value calculated from the header padded to 256 
bytes (as described), encrypted using the public key, and then compared 
to the 256 bytes in the image header ? I guess my question could be 
answered by 'is the 256 byte RSA signature therefore "cipher text" ?' 
or 'when you say "decrypt Signature" don't you mean "encrypt 
signature" ?' as the RSA algorithm decrypts with the private key, which 
isn't on the XBox anywhere (as I understand it).

As it stands, I struggle to see how, even with the private key, the RSA 
signature can be created at all, as the value of the signature seems to 
depend on the SHA1 hash on data which includes it's own value, creating 
the proverbial chicken and egg scenario - as it is described anyway 
(i.e it seems to suggest you need the hash to calculate the signature, 
but you can't calculate the hash until you have the signature).

If I had some way of reading a signed .xbe on my PC, I could probably 
answer these questions myself through experimentation, but, as I am 
very reluctant to open the case on my XBox, I don't have any way of 
examining any such files in a binary manner (i.e other than by playing 
them on my XBox :-) )  Is there any known (legal) source of .xbe's 
distributed on media other than the proprietry DVD format ? Is there 
any way to read XBox media on a PC ?

cheers,

mj.


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