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LIGHTWEIGHT CRYPTOGRAPHY Cryptographic Engineering for a Pervasive World

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LIGHTWEIGHT CRYPTOGRAPHY Cryptographic Engineering for a Pervasive World DISSERTATION for the degree Doktor-Ingenieur Faculty of Electrical Engineering and Information Technology Ruhr-University Bochum, Germany Axel York Poschmann Bochum, February 2009 To my parents and Katja. Author’s contact information: [email protected] Thesis Advisor: Prof. Dr.-Ing. Christof Paar Secondary Referee: Dr. Matthew J.B. Robshaw Thesis submitted: February 4, 2009 Thesis defense: April 30, 2009 “As light as a feather, and as hard as dragon-scales” Bilbo Baggins in “The Lord of the Rings: The Fellowship of the Ring”. vii Abstract Increasingly, everyday items are enhanced to pervasive devices by embedding computing power and their interconnection leads to Mark Weiser’s famous vision of ubiquitous comput- ing (ubicomp), which is widely believed to be the next paradigm in information technology. The mass deployment of pervasive devices promises on the one hand many benefits (e.g. opti- mized supply-chains), but on the other hand, many foreseen applications are security sensitive (military, financial or automotive applications), not to mention possible privacy issues. Even worse, pervasive devices are deployed in a hostile environment, i.e. an adversary has physical access to or control over the devices, which enables the whole field of physical attacks. Not only the adversary model is different for ubicomp, but also its optimisation goals are signifi- cantly different from that of traditional application scenarios: high throughput is usually not an issue but power, energy and area are sparse resources. Due to the harsh cost constraints for ubicomp applications only the least required amount of computing power will be realized. If computing power is fixed and cost are variable, Moore’s Law leads to the paradox of an increasing demand for lightweight solutions. In this Thesis different approaches are followed to investigate new lightweight cryptographic designs for block ciphers, hash functions and asymmetric identification schemes. A strong fo- cus is put on lightweight hardware implementations that require as few area (measured in Gate Equivalents (GE)) as possible. We start by scrutinizing the Data Encryption Standard (DES)—a standardized and well-investigated algorithm—and subsequently slightly modify it (yielding DESL) to decrease the area requirements. Then we start from scratch and design a complete new algorithm, called PRESENT, where we could build upon the results of the first step. A va- riety of implementation results of PRESENT—both in software and hardware—using different design strategies and different platforms is presented. Our serialized ASIC implementation (1; 000 GE) is the smallest published and enabled PRESENT to be considered as a suitable can- didate for the upcoming ISO/IEC standard on lightweight cryptography (ISO/IEC JTC1 SC27 WG2). Inspired by these implementation results, we propose several lightweight hash func- tions that are based on PRESENT in Davies-Meyer-mode (DM-PRESENT-80, DM-PRESENT-128) and in Hirose-mode (H-PRESENT-128). For their security level of 64 (DM-PRESENT-80, DM- PRESENT-128) and 128 bits (H-PRESENT-128) the implementation results are the smallest pub- lished. Finally, we use PRESENT in output feedback mode (OFB) as a pseudo-random number generator within the asymmetric identification scheme crypto-GPS. Its design trade-offs are discussed and the implementation results of different architectures (starting from 2; 181 GE) are backed with figures from a manufactured prototype ASIC. We conclude that block ciphers drew level with stream-ciphers with regard to low area re- quirements. Consequently, hash functions that are based on block ciphers can be implemented efficiently in hardware as well. Though it is not easy to obtain lightweight hash functions with a digest size of greater or equal to 160 bits. Given the required parameters, it is very unlikely that the NIST SHA-3 hash competition will lead to a lightweight approach. Hence, lightweight hash functions with a digest size of greater or equal to 160 bits remain an open research prob- lem. Keywords. Lightweight Cryptography, Design, Embedded Systems, Hardware, ASIC, S- boxes, Block cipher, Hash Function, Pervasive Security, IT Security. ix Kurzfassung Alltagsgegenstände werden zunehmend durch das Einbetten von Prozessoren zu pervasiven Geräten erweitert und ihre Vernetzung führt zu Mark Weiser’s berühmter Vision des Ubiq- uitous Computing (ubicomp), das gemeinhin als neues IT-Paradigma angenommen wird. Der erwartete Nutzen ist einerseits vielversprechend (z.B. optimierte Supply-Chains), jedoch sind andererseits viele der skizzierten Szenarien (z.B. fürs Militär, für Banken oder für die Auto- mobilbranche) sicherheitskritisch. Schlimmer noch, durch den Einsatz in „feindlicher” Umge- bung, hat ein möglicher Angreifer volle physikalische Kontrolle über die Geräte, wodurch die gesamte Klasse der physikalischen Angriffe überhaupt erst ermöglicht wird. Abschließend sei noch auf die Gefahren für die Privatsphäre und anderer Bürgerrechte durch die Allgegenwär- tigkeit von eingebetteten Systemen hingewiesen. Sicherheit ist also von zentraler Bedeutung. Nicht nur das Angreifermodell von ubicomp, auch seine Optimierungsziele unterscheiden sich deutlich von denen traditioneller IT-Systeme: einerseits geringer Durchsatz, aber ander- erseits starke Beschränkungen hinsichtlich des Strom-, Energie-, und Flächenverbrauchs. Be- dingt durch die scharfen Kostenvorgaben wird stets nur das Minimum der benötigten Rechen- bzw. Speicherkapazität realisiert, wodurch Moore’s Law konträr interpretiert werden muss: da die Rechenkapazität fix und die Kosten variabel sind führt Moore’s Law zu dem Paradoxon einer konstanten oder sogar steigenden Nachfrage nach hocheffizienten Implementierungen. In dieser Dissertation werden verschiedene Ansätze verfolgt um hocheffiziente Implemen- tierungen von kryptographischen Primitiven wie Blockchiffren, Hashfunktionen und asym- metrischen Identifikationssystemen zu untersuchen. Der Fokus liegt dabei auf hocheffizienten Hardwarerealisierungen, die so wenig Fläche wie möglich—gemessen in Gatter Äquivalenten (GE)—verbrauchen. Zuerst wird der Data Encryption Standard (DES)—ein standardisierter und gut-untersuchter Algorithmus—effizient implementiert und um den Flächenverbrauch weiter zu verringern wird er anschließend geringfügig verändert (DESL). Im nächsten Schritt wird ein komplett neuer Algorithmus (PRESENT) entworfen. Hierbei konnte auf Ergebnisse der vorherigen Untersuchungen aufgebaut werden. Verschiedenste Hard- und Softwarere- alisierungen von PRESENT für unterschiedliche Plattformen werden vorgestellt, wobei unser Hardwarerealisiserung (1; 000 GE) die kleinste bekannte Hardwarerealisierung einer kryp- tographischen Primitive mit angemessener Sicherheit darstellt. Diese Ergebnisse führten dazu, dass PRESENT als geeigneter Kandidat für den zukünftigen ISO/IEC Standard für Lightweight Cryptography (ISO/IEC JTC1 SC27 WG2) gehandelt wird. Auf diesen Ergebnissen aufbauend, werden neue hocheffiziente Hashfunktionen, die auf PRESENT im Davies-Meyer-Modus (DM- PRESENT-80, DM-PRESENT-128) und im Hirose-Modus (H-PRESENT-128) basieren, vorgestellt. Für die jeweiligen Sicherheitslevel von 64 (DM-PRESENT-80, DM-PRESENT-128)) bzw. 128 Bit (H-PRESENT-128) sind usere Implementierungen diejenigen mit dem geringsten Flächenver- brauch. Schließlich kommt PRESENT im Output-Feedback-Modus als Pseudozufallszahlen- generator innerhalb des asymmetrischen Identifikationssystems crypto-GPS zum Einsatz. Ver- schiedene Architekturen werden vorgestellt und die Implementierungsergebnisse werden durch die Zahlen eines speziell gefertigten ASIC-Prototypen von crypto-GPS ergänzt. Die Ergebnisse dieser Dissertation lassen den Schluss zu, dass im Hinblick auf effieziente Hardwarerealisierungen Blockchiffren mit Stromchiffren gleichgezogen sind. Dadurch lassen sich Hashfunktionen, die auf Blockchiffren basieren, ebenfalls hocheffizient implementieren. Dieses trifft jedoch nicht auf Hashfunktionen mit Ausgabelängen von 160 oder mehr Bits zu. Berücksichtigt man die Parameter des NIST SHA-3 Hashfunktions-Wettbewerbs, ist es sehr xi unwahrscheinlich, dass hieraus eine hocheffiziente Hashfunktion resultiert und folglich bleibt diese Forschungsfrage weiterhin offen. Schlüsselworte. Hocheffiziente Kryptographie, Entwurf, Eingebette Systeme, Hardware, ASIC, S-Box, Blockchiffre, Hashfunktion, Pervasive Sicherheit, IT-Sicherheit. xii Acknowledgement This Thesis is the outcome of three years of research at the Embedded Security group1 at the Horst Görtz Institute for IT Security at the Ruhr University Bochum. During this time I got the chance to work with friends and therefore could combine many times work and spare time. It offered me a smooth transition from a students life to the working world. The results would not have been possible without collaboration with many researchers and colleagues. Therefore I would like to briefly acknowledge a subset of all the interesting people I met in the past years. First of all I would like to say Danke! to my supervisor Christof Paar for his great work in supervising, guiding and mentoring me in a very friendly and cooperative way. Secondly, I would like to say Danke! to Irmgard Kühn for coping with all the administrative stuff and all the nice coffee chats that we had. Thank you! Matt Robshaw for being my Thesis reader, and for all the exciting research projects we had. Danke! Gregor Leander for all the funny hours that we spent in your office on joint research
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