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Secure encryption systems against side-channel attacks

Posted: 02 Apr 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Cryptography? encryption? decryption? AES-256? Hardware Security Modules?

For example, the University of Bochum in Ruhr has for a long list of attacks on various FPGAs targeting their bitstream decryption engine. Figure 2 shows a probe placed on a bypass capacitor on the motherboard of an Intel server chip as the best location to for attacking the AES-NI hardware contained within this processor.

The trustworthiness of a large number of systems, and indeed of the entire computer and networking infrastructure today, is dependent on the secrecy of a small number of critical keys. These "root" keys are used to directly or indirectly sign and validate the software and updates to the commonly used operating systems and the billions of systems and devices worldwide. These root keys are instrumental in preventing someone from surreptitiously taking over people's machines. Some "root" keys are used to securely identify servers and public keys belonging to different companies, so that users are not fooled into interacting with the websites and services set up by imposters.

Figure 2: Probe location on the motherboard for capturing the EM signal from AES-NI hardware engine on an Intel server chip.

Such critical keys are typically kept within Hardware Security Modules (HSMs), which are systems designed specifically to provide the maximum protection to cryptographic keys from physical attacks. HSMs deploy an active shield and a variety of sensors designed to detect any attempts to tamper with the device to extract its internal, secret keys. Any tamper event device causes the device to erase its keys.

But while many critical HSMs today are kept in physically secure and monitored locations, there is increasing pressure to deploy HSMs in less controlled spaces. Many such deployments have already occurred. Yet, the current generation of HSMs has poor to non-existent protections against side-channel attacks. In fact, some modern HSMs emit EM signals that can be observed from outside the server in which they are installed, and are vulnerable to RSA key recovery from emissions from a single signature operation. Figure 3 shows how an M-field probe placed outside a server containing a PCI-based HSM can capture a signal that can be used to extract the secret RSA private key otherwise kept securely within the HSM.

Figure 3: Photograph showing how a 3cm M-field probe placed outside a server chassis containing a modern PCI express based HSM could be used to capture the EM side-channel signal related to the RSA operation occurring with the HSM.

Emerging standards and requirements
Fortunately, solutions are available for these large-system, SOC, and HSM threats. The common criterion protection profiles for smart-cards that require side-channel protections are already well established, as are EMVCo's standards for payment cards and national and international standards for electronic passports and national ID cards. They can be applied to these other systems.

The recent onslaught of side-channel research and attacks on large systems has already prompted many other industry associations and standards bodies to add side-channel countermeasure and testing requirements into emerging standards. For example, the Payment Card Industry (PCI) standard for Point of Sale terminals (PCI PTS POI Standard), Version 4.0, now requires all point of sale terminals to be physically tested for side-channel vulnerabilities. Over the past few years, all defence systems subject to anti-tamper requirements have needed to demonstrate protections against side-channel attacks. MovieLabs, a highly influential research and development venture started by the six major motion picture studios, has issued a specification for enhanced content protection that requires DRM systems to be side-channel resistant. As a result, many upcoming content protection standards for high-definition video players (e.g., 4K content) will be requiring vendors to defend and test against side-channel attacks.

Other such standards revisions are taking place. The popular FIPS 140-2 standard for cryptographic modules is in the process of being updated and the proposed FIPS 140-3 or the ISO/IEC 19790 standard that would replace it, both specify protection against and testing for side-channel vulnerabilities. Many upcoming security standards, such as those for secure grid and the like, point towards the upcoming FIPS/ISO standards for achieving physical security. In addition, there are several companies and conditional access vendors that are placing side-channel requirements on their vendors.

The discovery of side-channel attacks in the mid-1990s with the initial focus on smart-cards led to a myth that these attacks are applicable only to smart-cards and other limited devices. However, over the past few years, this myth is being debunked as side-channel attacks have been demonstrated on a wide variety of large devices. Emerging standards from the DRM, payment, content protection, mobile, defence, and automotive industries are requiring devices to demonstrate resistance to such attacks. The time is now, therefore, for vendors to begin the process of designing and testing to protect their systems against side-channel attacks.

About the author
Pankaj Rohatgi is the Technical Director for Cryptography Research Division at Rambus.

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