[A83] Re: DS Thread Extended onto this list


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[A83] Re: DS Thread Extended onto this list



At 03:28 PM 12/11/2002 -0800, you wrote:
>(Lurk mode off :)
>
>>(See, to  emulate the restriction, you'd have to know how to check for 
>>it, thus how to break it . . . )
>
>Not necesarrily.  Actually checing to see if the os image is valid is 
>probably pretty easy.  I assume the TI-83 does it in software, which means 
>both that VTI already checks the images (by vitrue of emulating the 
>calculator), and also anyone clever as disassembly could figure out how it 
>works.  It's actually signing an OS image that's a bitch.
>
>I assume (though I'm not sure) that the signing system is a variation of 
>very well known and understood public-key encryption.  TI signes their OS 
>with a secret, private-key.  The OS can then be verified using a 
>public-key that can be stored in the TI-83 ROM.  Because of some very 
>complicated math having to do with number theory, the public key can be 
>calculated using the private key, but not the other way around.  This is 
>the way most digital content is securely signed these days.
>
>Thus, even though you can check to see if data is correctly signed, you 
>have no way of actually creating signed data yourself (without years of 
>CPU time spend brute-forcing the crypto).

VTI doesn't emulate the os check.  Just see by changing a string in a ROM 
image.  I wonder, though.  What if you load a hacked os in VTI, then use 
the Send Product Code feature of said OS to transmit to a real calc via the 
virtual link? 





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