We understand the need for this feature in certain situations, and we have in the past considered including this in our commercial (enterprise) offering (see http://www.tersus.com/#Id=279). There are various ways to implement this, but it should be understood that none of them are 100% proof, i.e. given enough time/experience/motivation someone could break through any such implementation.
If you are interested, please drop us a line via the contact us page @ http://www.tersus.com/#Id=283.
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