Computer Laboratory

Technical reports

Microelectronic security measures

Philip Christopher Paul

February 2013, 177 pages

This technical report is based on a dissertation submitted January 2009 by the author for the degree of Doctor of Philosophy to the University of Cambridge, Pembroke College.

Some figures in this document are best viewed in colour. If you received a black-and-white copy, please consult the online version if necessary.

Abstract

In this dissertation I propose the concept of tamper protection grids for microelectronic security devices made from organic electronic materials. As security devices have become ubiquitous in recent years, they are becoming targets for criminal activity. One general attack route to breach the security is to carry out physical attack after depackaging a device. Commercial security devices use a metal wire mesh within the chip to protect against these attacks. However, as a microchip is physically robust, the mesh is not affected by depackaging.

As a better way of protecting security devices against attacks requiring the chip package to be removed, I investigate a protection grid that is vulnerable to damage if the packaging is tampered with. The protection grid is connected directly to standard bond pads on the microchip, to allow direct electronic measurements, saving the need for complex sensor structures. That way, a security device can monitor the package for integrity, and initiate countermeasures if required.

The feasibility of organic tamper protection grids was evaluated. To establish the viability of the concept, a fabrication method for these devices was developed, the sensitivity to depackaging was assessed, and practical implementation issues were evolved. Inkjet printing was chosen as fabrication route, as devices can be produced at low cost while preserving flexibility of layout. A solution to the problem of adverse surface interaction was found to ensure good print quality on the hydrophobic chip surface. Standard contacts between chip and grid are non-linear and degrade between measurements, however it was shown that stable ohmic contacts are possible using a silver buffer layer. The sensitivity of the grid to reported depackaging methods was tested, and improvements to the structure were found to maximise damage to the grid upon tampering with the package. Practical issues such as measurement stability with temperature and age were evaluated, as well as a first prototype to assess the achievable measurement accuracy. The evaluation of these practical issues shows directions for future work that can develop organic protection grids beyond the proof of concept.

Apart from the previously mentioned invasive attacks, there is a second category of attacks, non-invasive attacks, that do not require the removal of the chip packaging. The most prominent non-invasive attack is power analysis in which the power consumption of a device is used as oracle to reveal the secret key of a security device. Logic gates were designed and fabricated with data-independent power consumption in each clock cycle. However, it is shown that this is not sufficient to protect the secret key. Despite balancing the discharged capacitances in each clock cycle, the power consumed still depends on the data input. While the overall charge consumed in each clock cycle matches to a few percent, differences within a clock cycle can easily be measured. It was shown that the dominant cause for this imbalance is early propagation, which can be mitigated by ensuring that evaluation in a gate only takes place after all inputs are present. The second major source of imbalance are mismatched discharge paths in logic gates, which result in data-dependent evaluation times of a gate. This source of imbalance is not as trivial to remove, as it conflicts with balancing the discharged capacitances in each clock cycle.

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BibTeX record

@TechReport{UCAM-CL-TR-829,
  author =	 {Paul, Philip Christopher},
  title = 	 {{Microelectronic security measures}},
  year = 	 2013,
  month = 	 feb,
  url = 	 {http://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-829.pdf},
  institution =  {University of Cambridge, Computer Laboratory},
  number = 	 {UCAM-CL-TR-829}
}