- Keys are derrived based on a key expansion algorithm that takes as input an unsigned short (16 bit) seed. I'm not
- totally sure what expansion algorithm this is, or if it's something totally custom, but for now here's a snippet of
- python code that implements the expansion:
+ Keys are derrived based on a key expansion algorithm that takes as input an unsigned short (16 bit) seed. This
+ algorithm is fundamentally an LSFR-based PRNG, used to generate a stream of bytes, which becomes the key. The
+ & 0xffffffff is not strictly necessary but is convenient in python in order to avoid
+ seed blowing up in size.
@@ -51,26 +52,17 @@
def amAuthDiskInitKey(seed):
key = bytearray(0x8000)
- for i in range(0x8000):
- uVar1 = (seed * 2 >> 4 ^ seed * 2) >> 10 & 2 | seed << 2
+ seed <<= 1
+ for i in range(len(key)):
+ x = seed
- uVar2 = uVar1 * 2
- uVar3 = ((seed << 2) >> 4 ^ uVar1) >> 10 & 2 | uVar2
- uVar1 = uVar3 * 2
- uVar3 = (uVar2 >> 4 ^ uVar3) >> 10 & 2 | uVar1
+ for _ in range(8):
+ x |= ((((seed >> 4) ^ x) >> 11) & 1)
+ seed <<= 1
+ x <<= 1
+ key[i] = (x >> 1) & 0xff
- uVar2 = uVar3 * 2
- uVar3 = (uVar1 >> 4 ^ uVar3) >> 10 & 2 | uVar2
- uVar1 = uVar3 * 2
- uVar3 = (uVar2 >> 4 ^ uVar3) >> 10 & 2 | uVar1
-
- uVar2 = uVar3 * 2
- uVar3 = (uVar1 >> 4 ^ uVar3) >> 10 & 2 | uVar2
- uVar1 = uVar3 * 2
- uVar2 = (uVar2 >> 4 ^ uVar3) >> 10 & 2 | uVar1
-
- seed = uVar2 | (uVar1 >> 4 ^ uVar2) >> 11 & 1
- key[i] = seed & 0xff
+ seed = x & 0xffffffff
return key
{% endhighlight %}