Use of TPM chip for RNG?

Anne & Lynn Wheeler lynn at garlic.com
Mon Jul 3 12:41:05 EDT 2006


Peter Gutmann wrote:
> You have to be pretty careful here.  Most of the TPM chips are just rebadged
> smart cards, and the RNGs on those are often rather dubious.  A standard
> technique is to repeatedly encrypt some stored seed with an onboard block
> cipher (e.g. DES) as your "RNG".  Beyond the obvious attacks (DES as a PRNG
> isn't particularly strong) there are the usual paranoia concerns (how do we
> know the manufacturer doesn't keep a log of the seed and key?) and stupidity
> concerns (all devices use the same hardwired key, which some manufacturers
> have done in the past).  There are also active attacks possible, e.g. request
> values from the device until the EEPROM locks up, after which you get constant
> "random" values.  Finally, some devices have badly-designed challenge-response
> protocols that give you an infinite amount of RNG output to analyse, as well
> as helping cycle the RNG to lockup.

One of the issues for a long time for that class of chips is whether 
on-chip key-gen and/or supported DSA (and/or ECDSA) were in use ... 
processes where reasonable good RNG are integral to the operation.

at one point there was tests for a collection of chips in that class 
that perform 65k power-cycle/RNG operations and found that something 
like 30 percent of the numbers were repeated.

however, at least some of the TPM chips have RNGs that have some level 
of certification (although you might have to do some investigation to 
find out what specific chip is being used for TPM).

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