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*****UPDATE****

Quoted:
With the new 15*** update, M$ has added a new key to their hash calculation for the rc4 key. It's basically just the first 16 bytes of the header, which

 

include the version number, entrypoint, and size. These are all per-CB, per-version, so we cannot take a keystream from a 15*** CBB and use it to make a 14*** CB because the CBA on 14*** is unable to calculate the rc4 key no matter what we change.

What this means:
In order to RGH2 an xbox with 15***, you need either:

1) The cpu_key

2) A previous exploitable dump from the SAME XBOX. Must fit one of the following:
- Phats: 14717, 14719
- Slims: 13146, 13599, 14699, 14717, 14719

Older dumps will NOT WORK with RGH2/RGH3 !

DO NOT UPDATE TO 15*** WITHOUT BACKING UP FIRST!

What do we do now:

We are looking into ways of exploiting the rc4.

To make it clear, the new way of generating the CBB decryption rc4 key is as follows:

Secret = CBA[0x10:0x20]
Ingest = CBB[0x10:0x20] + CPU_Key + CBA[0:6] + 0x0000 + CBA[6:0x10]

Code:

def decrypt_CB_Cpu(CB):
assert cpukey
secret = CB_A[0x10:0x20]
h = hmac.new(secret,None, sha);
h.update(CB[0x10:0x20]);
h.update(cpukey);

v = struct.unpack(">h", CB_A[0x6:0x8])[0]
print " * checking flag: %X" % v
if( v & 0x1000):
print " ** Using new encryption scheme"
h.update(CB_A[0:0x6] + "\x00\x00" + CB_A[8:0x10]);

key = h.digest()[0:0x10]
CB = CB[0:0x10] +key+ RC4.new(key).decrypt(CB[0x20:])
return CB

Source: http://team-xecuter.com/forums/showthread.php?t=87967

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