The Lightweight Block Ciphers Zoo Nathan Keller Bar Ilan University Lightweight Crypto Day, Haifa University,

Talk outline Block ciphers in general – definition and some history DES (and Feistel constructions in general) AES (and SP networks in general) Attacks on AES (related-key, side channel) Lightweight crypto in general The target applications (RFID tags, WSNs) Why block ciphers and not stream ciphers? Design criteria Hardware vs. Software efficiency

Talk outline (cont.) The lightweight block ciphers (LBC) zoo The ciphers (divided to groups) Specific examples (Present, Piccolo, Ktantan) Special features in LBC design Key schedule (simplicity, on-the-fly computation) Decryption (is possible or not, involution structures) Simple operations

Talk outline (cont.) Theoretical foundations Generalized Even-Mansour constructions Different security models ?! Cryptanalysis Are the attacks on lightweight block ciphers different?

History See auxiliary presentation.

Lightweight Crypto in General What are the target platforms? Radio-frequency identification (RFID) is the wireless non-contact use of radio-frequency electromagnetic fields to transfer data, for the purposes of automatically identifying and tracking tags attached to objects.radio-frequencyelectromagnetic fields A wireless sensor network (WSN) of spatially distributed autonomous sensors to monitor physical or environmental conditions, such as temperature, sound, pressure, etc. and to cooperatively pass their data through the network to a main location.autonomoussensorstemperature soundpressure Various other platforms.

Why block ciphers? Security – it is believed that our understanding in block cipher design is better than in stream cipher design. Various modes of operations possible (e.g., can act as a stream cipher in the CBC mode). Other reasons.

Design criteria Speed (a.k.a throughput, or cycle count) A single block or many blocks? Memory consumption (SW) ROM use (usually between 300 to 2000 bytes). RAM use (a few dozens of bytes). Area consumption (HW) Measured in Gate Equivalents, between Energy consumption Correlated with cycle count.

Hardware/Software efficiency Some of the ciphers are especially optimized for hardware/software, while others try to achieve good performance on both platforms. Comparative studies show that they are quite inherent tradeoffs between software and hardware efficiency. So, should we try to optimize both in the same primitive, or different primitives for hardware and software are better?

The lightweight block ciphers Zoo SP networks: AES (various lightweight implementations). mCrypton [Lim05] (lightweight version of Crypton). Present [Bogdanov+07] (the most well-known one, accepted as an ISO standard). PrintCipher [Knudsen+10] (Uses the ability to “print” the circuit on the RFID tag, 3*3 S-boxes). Klein [Gong+11] (similar to Present, more effective). LED [Guo+11] (Hardware oriented, no key schedule). Prince [Borghoff+12] (involution structure, no claims against related-key attacks, similar to Present). Zorro [Gerard+13] (designed to resist side-channel attacks, similar to LED but less S-boxes).

The lightweight block ciphers Zoo Feistel (or generalized Feistel) constructions: TEA, XTEA [Needham+94] (Very simple code, no S- boxes, the same key used each round. TEA was vulnerable to related-key attacks, fixed in XTEA). DESL,DESXL [Poschmann06] (A variant of DES, using the same S-box in all places. In DESXL, independent keys are XORed before and after encryption). SEA [Standaert+05] (variable block/key size). Hight [Hong+06] (8-branch Feistel-II). MIBS [Izadi+09] (Similar to Camellia). Piccolo [Shibutani+11] (4-branch Feistel-II, with a byte transformation after each round).

The lightweight block ciphers Zoo Feistel (or generalized Feistel) constructions: LBlock [Wu+11] (4-bit S-boxes and wiring). Twine [Suzaki+12] (16-branch Feistel-II, similar to LBlock). Stream-cipher like constructions: KATAN, KTANTAN [deCanniere+09] (Based on two NLFSRs that influence each other. The full state after many steps is the ciphertext. In KTANTAN, the key is burnt into the device).

Specific examples SP network Present Feistel construction Piccolo (see auxiliary presentation) Stream-cipher like construction KTANTAN

Special features in LBC design Simple operations and small S-boxes In many of the target devices, only simple operations are supported (e.g., not multiplication). As a result, most of the designs are based on modular additions, rotations and xors (except for S-boxes). The S-boxes are small (4*4 or even 3*3). Very simple key schedule Allows on-the-fly computation, including the decryption direction (as opposed to IDEA). The key schedule is mostly linear, and in some of the ciphers (LED, Zorro, CGEN, PrintCipher, Prince) is absent at all!

Special features in LBC design Decryption Some of the ciphers (e.g., Present) prefer to not allow decryption, in order to save space on the device. If decryption is allowed, there is a try to make it as similar to encryption as possible: Feistel constructions Involutional elements (ranging from S-boxes to the entire cipher, like in Iceberg, SEA, Prince). Re-using of components In order to save space, there is a try to re-use components: The same S-box is used for all purposes. Key generation uses the same procedure as encryption.

Theoretical foundations Due to the simplicity of the key schedule, some of the lightweight block ciphers can be viewed as a special case of the Generalized Even-Mansour construction. This allows to achieve security bounds (under some assumptions, of course ). Very extensive research in the last three years.

Theoretical foundations Attack model issues Several of the lightweight designs base their security claims on the anticipated threats in lightweight environment: The amount of available data is limited. Related-key attacks are not considered a threat. Is this a good practice?

Cryptanalysis Are the attacks on LBC similar to the attacks on classical block ciphers? In general, yes. In particular, almost no proposal was broken. But there are a few differences: In LBC, the complexities are lower (e.g., 80-bit keys), so we are closer to being practical. The simple key schedules lead to new kinds of attacks, which weren’t relevant in classical ciphers.

Thanks for your attention This was just the appetizer, all the real stuff will be in the next talks (and was in Roberto’s talk, of course )