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Section: New Results

Reconfigurable Architecture Design

Design Flow and Run-Time Management for Compressed FPGA Configurations

Participants : Olivier Sentieys, Christophe Huriaux.

Almost since the creation of the first SRAM-based FPGAs there has been a desire to explore the benefits of partially reconfiguring a portion of an FPGA at run-time while the remainder of design functionality continues to operate uninterrupted. Currently, the use of partial reconfiguration imposes significant limitations on the FPGA design: reconfiguration regions must be constrained to certain shapes and sizes and, in many cases, bitstreams must be precompiled before application execution depending on the precise region of the placement in the fabric. We developed an FPGA architecture that allows for seamless translation of partially-reconfigurable regions, even if the relative placement of fixed-function blocks within the region is changed.

In [42] we proposed a design flow for generating compressed configuration bit-streams abstracted from their final position on the logic fabric. Those configurations can then be decoded and finalized in real-time and at run-time by a dedicated reconfiguration controller to be placed at a given physical location. The VTR framework has been expanded to include bit-stream generation features. A bit-stream format is proposed to take part of our approach and the associated decoding architecture was designed. We analyzed the compression induced by our coding method and proved that compression ratios of at least 2.5× can be achieved on the 20 largest MCNC benchmarks. The introduction of clustering which aggregates multiple routing resources together showed compression ratio up to a factor of 10×, at the cost of a more complex decoding step at runtime. The VBS approach can provide increased online relocation capabilities using a decoding algorithm capable of decoding the VBS on-the-fly during the task migration.

Run-Time Approximation under Performance Constraints in OFDM Wireless Receivers

Participants : Olivier Sentieys, Fernado Cladera.

Mobile wireless channels are characterized by time-varying multipath propagation, noise, and interference effects. To cope with these rapid variations of channel parameters, wireless receivers are designed with a significant performance margin to be able to reach a given link quality (BER - Bit Error Rate), even for the worst-case channel conditions. Indeed, one of the steps during the design phase is the choice of the architecture bit-width, and the smallest wordlength that ensures the correct behaviour of the receiver is usually chosen. In [39] , an adaptive precision OFDM receiver is proposed. Significant energy savings come from varying at run time processing bit-width, based on estimation of channel conditions, without compromising BER constraints. To validate the energy savings, the energy consumption of basic operators has been obtained from real measurements for different bit-widths on a FPGA and a processor using soft SIMD. Results show that up to 62% of the dynamic energy consumption can be saved using this adaptive technique. The algorithms proposed for the low complexity selector used to choose the processing word-length at run time, without modifying the standard OFDM frame, are detailed in [38] .

Optical Interconnections for 3D Multiprocessor Architectures

Participants : Jiating Luo, Pham Van Dung, Cédric Killian, Daniel Chillet, Olivier Sentieys.

To address the issue of interconnection bottleneck in multiprocessor on a single chip, we study how an Optical Network-on-Chip (ONoC) can leverage 3D technology by stacking a specific photonics die. The objectives of this study target: i) the definition of a generic architecture including both electrical and optical components, ii) the interface between electrical and optical domains, iii) the definition of strategies (communication protocol) to manage this communication medium, and iv) new techniques to manage and reduce the power consumption of optical communications. The first point is required to ensure that electrical and optical components can be used together to define a global architecture. Indeed, optical components are generally larger than electrical components, so a trade-off must be found between the size of optical and electrical parts. For example, if the need in terms of communications is high, several waveguides and wavelengths must be necessary, and can lead to an optical area larger than the footprint of a single processor. In this case, a solution is to connect (through the optical NoC) clusters of processors rather than each single processor. For the second point, we study how the interface can be designed to take applications needs into account. From the different possible interface designs, we extract a high-level performance model of optical communications from losses induced by all optical components to efficiently manage Laser parameters. Then, the third point concerns the definition of high-level mechanisms which can handle the allocation of the communication medium for each data transfer between tasks. This part consists in defining the protocol of wavelength allocation. Indeed, the optical wavelengths are a shared resource between all the electrical computing clusters and are allocated at run time according to application needs and quality of service. The last point concerns the definition of techniques allowing to reduce the power consumption of on-chip optical communications. The power of each Laser can be dynamically tuned in the optical/electrical interface at run time for a given targeted bit-error-rate. Due to the relatively high power consumption of such integrated Laser, we study how to define adequate policies able to adapt the laser power to the signal losses.

In [44] , we proposed a wavelength reservation protocol handled by an Optical Network Interface (ONI) Manager for reconfigurable ONoC based on shared waveguide. It allows to efficiently allocate, at runtime, the optical communication channels for a manycore architecture. We described the ONI manager architecture and reservation protocol. Synthesis results in a 28nm FDSOI technology demonstrated that our interface can support a clock frequency up to 550 MHz with 6 wavelengths managed. From these results, we can be optimistic about the scaling of the ONoC and its capacity to manage a large number of processors and more wavelengths.

In [55] , we explored the trade-off among channel bandwidth alternatives, performance, area and power. We showed that the channel size has a strong impact on the system performance and cost. We employed synthetic and real application traffic executed on the GEM5 simulator. As a result, we show that different channel bandwidths can improve the execution time of an application up to 75%, while including low area and power penalties.

Arithmetic Operators for Cryptography and Fault-Tolerance

Participants : Arnaud Tisserand, Emmanuel Casseau, Nicolas Veyrat-Charvillon, Karim Bigou, Franck Bucheron, Jérémie Métairie, Gabriel Gallin.

Arithmetic Operators for Fast and Secure Cryptography.

Our paper [36] , presented at CHES, describes a new RNS modular multiplication algorithm for efficient implementations of ECC over GF(p). Thanks to the proposition of RNS-friendly Mersenne-like primes, the proposed RNS algorithm requires 2 times less moduli than the state-of-art ones, leading to 4 times less precomputations and about 2 times less operations. FPGA implementations of our algorithm are presented, with area reduced up to 46 %, for a time overhead less than 10 %. Other RNS algorithms and implementations have been presented at RAIM [66] .

Scalar recoding is popular to speed up ECC (elliptic curve cryptography) scalar multiplication: non-adjacent form, double-base number system, multi-base number system (MBNS). Ensuring uniform computation profiles is an efficient protection against some side channel attacks (SCA) in embedded systems. Typical ECC scalar multiplication methods use two point operations (addition and doubling) scheduled according to secret scalar digits. Euclidean addition chains (EAC) offer a natural SCA protection since only one point operation is used. Computing short EACs is considered as a very costly operation and no hardware implementation has been reported yet. We designed an hardware recoding unit for short EACs which works concurrently to scalar multiplication. It has been integrated in an in-house ECC processor on various FPGAs. The implementation results show similar computation times compared to non-protected solutions, and faster ones compared to typical protected solutions (e. g. 18 % speed-up over 192 b Montgomery ladder). A paper [62] has been presented at Compas conference.

In a collaboration with University College Cork (Ireland), we worked on the design of secure multipliers for asymmetric cryptography using asynchronous circuits. A common paper has been published at ASYNC Conference [37] . In this paper, a specially adjusted Latch-less Asynchronous Charge Sharing Logic (LACSL) is developed to inherently defend such architecture against DPA attacks. The proposed logic provides input data independent low-power/energy consumption which is attributed to interleaved charge sharing stages with non-static elements involved in the data path. A 32-bit LACSL Montgomery Multiplier (case study) is extensively tested through HSPICE simulations and great consistency in power/energy consumption is achieved. The normalized energy deviation and normalized standard deviation are only 0.048 and 0.011, respectively. Compared with the original ACSL implementation, besides the impressive energy coherence, 42% energy saving is demonstrated plus that the leakage power is 3.5 times smaller. Furthermore, the scalability of the proposed multiplier is explored where 64- bit, 128-bit and 256-bit designs are implemented. Again, great energy consistency is found with the highest deviation being 0.5%.

In collaboration with D. Pamula, we worked on fast and secure finite field multipliers for GF(2m) arithmetic, a paper has been presented at DSD conference [53] . It presents details on fast and secure GF(2m) multipliers dedicated to elliptic curve cryptography applications. Presented design approach aims at high efficiency and security against side channel attacks of a hardware multi- plier. The security concern in the design process of a GF(2m) multiplier is quite a novel concept. Basing on the results obtained in course of conducted research it is argued that, as well as efficiency of the multiplier impacts the efficiency of the cryptoprocessor, the security level of the multiplier impacts the security level of the whole cryptoprocessor. Thus the goal is to find a tradeoff, to compromise efficiency, in terms of speed and area, and security of the multiplier. We intend to secure the multiplier by masking the operation, either by uniformization or by randomization of the power consumption of the device during its work. The design methodology is half automated. The analyzed field sizes are the standard ones, which ensure that a cryptographic system is mathematically safe. The described architecture is based on principles of Mastrovito multiplication method. It is very flexible and enables to improve the resistance against side channel attacks without degrading the multiplier efficiency.

In a collaboration with G. Abozaid (EJUST University Egypt), we worked on the FPGA implementation of arithmetic operators for very large numbers (millions of bits) in fully homomorphic encryption (FHE) applications. A journal paper has been published in IEEE Embedded Systems Letters [18] .

ECC Crypto-Processor with Protections Against SCA.

A dedicated processor for elliptic curve cryptography (ECC) is under development. Functional units for arithmetic operations in GF(2m) and GF(p) finite fields and 160-600-bit operands have been developed for FPGA implementation. Several protection methods against side channel attacks (SCA) have been studied. The use of some number systems, especially very redundant ones, allows one to change the way some computations are performed and then their effects on side channel traces. This work is done in the PAVOIS project. An ASIC version of the processor is under development and should be sent for fabrication in the beginning of 2016.

A. Tisserand has been invited speaker at the conference on elliptic curve cryptography (ECC): "Hardware Accelerators for ECC and HECC" [29] .

Arithmetic Operators and Crypto-Processor for HECC.

In the HAH project, we study and prototype efficient arithmetic algorithms for hyperelliptic curve cryptography for hardware implementations (on FPGA circuits). We study new advanced arithmetic algorithms and representations of numbers for efficient and secure implementations of HECC in hardware. First results have been published in Compas conference [60] and RAIM workshop [68] .

Arithmetic Operators for Fault Tolerance.

In the ARDyT and Reliasic projects, we work on computation algorithms, representations of numbers and hardware implementations of arithmetic operators with integrated fault detection (and/or fault tolerance) capabilities. The target arithmetic operators are: adders, subtracters, multipliers (and variants of multiplications by constants, square, FMA, MAC), division, square-root, approximations of the elementary functions. We study two approaches: residue codes and specific bit-level coding in some redundant number systems for fault detection/tolerance integration at the arithmetic operator/unit level. FPGA prototypes are under development.