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How do you “shred” the data on nonvolatile memory? Labs has an answer


By Curt Hopkins, Managing Editor, Hewlett Packard Enterprise Labs

Recognition from ASPLOS continues to rain down on Hewlett Packard Labs.

Labs security researchers Pratyusa Manadhata, Stuart Haber and Bill Horne, along with two colleagues from North Carolina State University, Amro Awad and Yan Solihin, have had their paper accepted to ASPLOS 2016, taking place this April in Georgia.

The paper, “Silent Shredder: Zero-Cost Shredding for Secure Non-Volatile Main Memory Controllers,” details an efficient way of dealing with data deletion in future non-volatile memory (NVM) computers – i.e., The Machine. Silent Shredder is their team’s project code name for the new process they have developed.

When a standard DRAM computer “shreds” data (deleting the earlier work so new work can be over-written on that space), it essentially writes zeroes over it, said Stuart Haber. This prevents later reading of the data by other processes.

In the presence of encryption

However, a property of NVM is that it can be overwritten fewer times than standard memory. If this is not taken into account, simple overwriting will age these new memory components before their time. Haber, Pratyusa, and the others have created an extremely clever way around that.

As Manadhata put it, “Conventional encryption increases the number of writes and hence reduces the lifetime of NVM. Silent Shredder works in the presence of encryption, and manipulates the so called initialization vectors (IVs) used in encryption to reduce the number of writes.”

The key benefit of their process, he said, is to extend the life time of non-volatile memory. The experiments the authors conducted indicate that improvement runs about 50% over normal use. Its secondary benefit is improvement in the reading of cache lines and increase in overall system performance.

Elegant, yet understated

Haber, Pratyusa, and company have devised a way to avoid a large proportion of over-writing memory due to shredding.

Instead of attempting to overwrite the whole of a region of encrypted memory, their process rewrites only the last eight bits of the IV. Just eight bits is enough to ensure no one can read the shredded materials – it is as thoroughly unreadable as if you’d burned the whole thing out with a blowtorch.

But a further change was necessary in order to insure that their new shredding mechanism would be compatible with existing systems. Those systems expect a newly allocated page of memory to consist entirely of zeroes. 

“We redesigned the memory controller so that it would recognize – from its IV –  that a particular page has been shredded,” said Haber, “and would return to the caller a page of zeroes instead of a page of unintelligible encrypted data from memory.”

Creating an elegant method of shredding that increases the lifetime of the NVM in an encryption environment was the main thrust in the development of Silent Shredder. But that is not its only benefit. In the paper’s additional use cases, the authors note the utility of Silent Shredder in virtual machines, in initializing large blocks of memory, and in zero initializing managed languages like Java and C#, as well as those which are starting to use zero initialization, like C++.

Most of all, the authors believe Silent Shredder will hasten the adoption of NVM and NVM-based computing devices. The ASPLOS conference organizers clearly agree that the implications of their innovation deserves further discussion. 

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Managing Editor, Hewlett Packard Labs