[Cryptography] check-summed keys in secret ciphers?

[ianG]

On 29/09/13 16:01 PM, Jerry Leichter wrote:
> ...e.g., according to Wikipedia, BATON is a block cipher with a key length of 320 bits (160 of them checksum bits - I'd guess that this is an overt way for NSA to control who can use stolen equipment, as it will presumably refuse to operate at all with an invalid key). ...


I'm not really understanding the need for checksums on keys.  I can sort 
of see the battlefield requirement that comms equipment that is stolen 
can't then be utilized in either a direct sense (listening in) or 
re-sold to some other theater.

But it still doesn't quite work.  It seems antithetical to NSA's 
obsession with security at Suite A levels, if they are worried about the 
gear being snatched, they shouldn't have secret algorithms in them at all.

Using checksums also doesn't make sense, as once the checksum algorithm 
is recovered, the protection is dead.  I would have thought a HMAC 
approach would be better, but this then brings in the need for a 
centralised key distro approach.  Ok, so that is typically how 
battlefield codes work -- one set for everyone -- but I would have 
thought they'd have moved on from the delivery SPOF by now.



> Cryptographic challenge:  If you have a sealed, tamper-proof box that implements, say, BATON, you can easily have it refuse to work if the key presented doesn't checksum correctly.  In fact, you'd likely have it destroy itself if presented with too many invalid keys.  NSA has always been really big about using such sealed modules for their own algorithms.  (The FIPS specs were clearly drafted by people who think in these terms.  If you're looking at them while trying to get software certified, many of the provisions look very peculiar.  OK, no one expects your software to be potted in epoxy ("opaque in the ultraviolet" - or was it infrared?); but they do expect various kinds of isolation that just affect the blocks on a picture of your software's implementation; they have no meaningful effect on security, which unlike hardware can't enforce any boundaries between the blocks.)
>
> Anyway, this approach obviously depends on the ability of the hardware to resist attacks.  Can one design an algorithm which is inherently secure against such attacks?  For example, can one design an algorithm that's strong when used with valid keys but either outright fails (e.g., produces indexes into something like S-boxes that are out of range) or is easily invertible if used with invalid keys (e.g., has a key schedule that with invalid keys produces all 0's after a certain small number of rounds)?  You'd need something akin to asymmetric cryptography to prevent anyone from reverse-engineering the checksum algorithm from the encryption algorithm, but I know of no fundamental reason why that couldn't be done.


It also seems a little overdone to do that in the algorithm.  Why not 
implement a kill switch with a separate parallel system?  If one is 
designing the hardware, then one has control over these things.

I guess then I really don't understand the threat they are trying to 
address here.

Any comments from the wider audience?

iang