Low power AES implementations for RFID Dina Kamel, Francesco Regazzoni, Cédric Hocquet, David Bol, Denis Flandre and François-Xavier Standaert

Outline Overview Design of S-box Subthreshold AES core RFIDs Why AES ? RFID Power budget Design of S-box Technology selection Supply voltage Logic style Subthreshold AES core BCRYPT 2010

RFID General Constraints: 1- Power – few µW 2- Area – few K Gates 3- Latency – ms Speak about RFID / Power management BCRYPT 2010

Technology road map for memories IP vendors provide NVM down till 45 nm targeting several foundries Foundries used to provide NVM down till 0.18 µm White background behind text BCRYPT 2010

Why AES Nowadays RFID are at 180 nm and 130 nm mainly for memory issues The technology trend is pushing for smaller technologies (also for memories) Smaller technologies allow to implement complex algorithms / enhanced functionality 3-D stacking enables mixed technologies e.g. 65 nm logic + 130 nm NVM AES is the standard Not Gate count - Area BCRYPT 2010

Move to 65nm to overcome area problems… 65nm will allow compact AES implementation Widespread use of Low-Power technology flavor Low fabrication costs for high volume production BCRYPT 2010

…low power is still an issue Passive RFIDs are battery less devices Power constraints are still present at 65 nm (leakage) In advanced technologies, such as 65 nm and below, two flavors are developed: General purpose (GP) Low power (LP) BCRYPT 2010

Power budget [A.S.W. Man, RFID Eurasia’07] Power: 4.7 µW Tech.: TSMC 0.18 µm VDD: 1.8 V Sim. results using Power compiler Man: The Hong Kong University of Science and Technology Power Budget for: HF (13.56 MHz): 22.5 µW UHF (900 MHz): 4 µW BCRYPT 2010

How the power is distributed in an 8bit Architecture AES numbers [T. Good, TVLSI’09] BCRYPT 2010

S-box Design The optimized S-box given by [N. Mentens,05] It uses the composite field GF(((22)2)2) Power and delay aspects in light of different parameters: Technology selection Supply voltage Logic style [D. Kamel, ISCAS’09] BCRYPT 2010

S-box design: Technology selection 0.13 µm main properties of Standard VT (SVT) and High VT (HVT) NMOS transistors. Tech. flavor Device type VDD V Tox nm Vt mV Ion µA/ µm Ioff nA/ µm Ig pA/ µm GP SVT 1.2 2 247 670 46 9 HVT 336 537 12 23 x lower + 90 mV [D. Kamel, ISCAS’09] BCRYPT 2010

S-box design: Technology selection 65 nm Main properties of Low VT (LVT), Standard VT (SVT) and High VT (HVT) NMOS transistors in GP and LP technology flavors. Tech. flavor Device type VDD V Tox nm Lpoly Vt mV Ion µA/ µm Ioff nA/ µm Ig GP SVT 1 1.3 45 475 896 62 8.97 HVT 555 740 4.7 6.18 LP LVT 1.2 1.85 57 507 855 4.2 0.0114 SVT 645 702 0.52 0.008 HVT 721 501 0.036 0.0054 [D. Kamel, ISCAS’09] 3 orders of magnitude BCRYPT 2010

Simulation results Power consumption at 100 kHz 8.7 μW/MHz [P. Hamalainen, DSD’06] 3.71 μW Power * 90.6 nW 870 nW 1.8 times less than 166 nW reported by [T. Good,TVLSI’09] using 0.13 μm, 0.75 V 10 times less than 870 nW 7 times less than 630 nW reported by [Feldhofer,05] using 0.35 μm, 1.5 V Igate Ioff Introduce graphs [D. Kamel, ISCAS’09] BCRYPT 2010

Simulation results Delay SVT SVT 2.2 ns 65 nm GP 130 nm HVT Power↓40 LVT HVT SVT 2.35 ns HVT 65 nm LP [D. Kamel, ISCAS’09] BCRYPT 2010

S-box design: Supply voltage Simulations are done using 65 nm LP SVT devices at 100 kHz and at nominal conditions. * 166 nW [T. Good, TVLSI09] at 0.75 V 5 times less than 166 nW reported by [T. Good,TVLSI09] using 0.13 μm, 0.75 V Fine for 100 KHz (large margin) Promising, but robustness ? [D. Kamel, ISCAS’09] BCRYPT 2010

S-box Design: compare different logic families Standard Logic: Static CMOS (S-CMOS) Dynamic Differential Logic: Dynamic Differential Swing Limited Logic (DDSLL) – Protected Logic Why ? Security – more resilient against power analysis attacks BCRYPT 2010

Static CMOS versus Dynamic Differential Swing Limited logic DDSLL XOR A B OUT 1 PPart NMOS Tree SC XOR FeedBack Developed at UCL Completion Signal Current Source [I. Hassoune, the VLSI Journal’07] BCRYPT 2010

[I. Hassoune, the VLSI Journal’07] DDSLL – How does it work ? Pre-charge Evaluation [I. Hassoune, the VLSI Journal’07] BCRYPT 2010

DDSLL is too complex ! # trans for 1 XOR SC DDSLL P transistors 4 9 N transistors 12 Total transistors 8 21 BCRYPT 2010

DDSLL – Sharing principle [I. Hassoune, the VLSI Journal’07] BCRYPT 2010

DDSLL – Sharing principle 1 3 Trans GF(((22)2)2) -> GF(28) + Affine Trans Trans GF(28) -> GF(((22)2)2) The whole DDSLL AES S-box consists of 13 stages The total number of DDSLL S-box transistors is 1.2 times less that of S-CMOS S-box Explain fig # trans for S-box S-CMOS DDSLL Total transistors 1530 1275 BCRYPT 2010

Measurement results of S-CMOS and DDSLL S-boxes S-CMOS S-box DDSLL S-box Area 24 µm 25 µm = 1.53 PS-CMOS PDDSLL 46 µm 46 µm Thanks to lower voltage swing 127 nW 83 nW Meas - 65 Power Delay 3 – 3.2 ns 7.5 – 8.1 ns BCRYPT 2010

Full AES core Base architecture [Feldhofer,05]: Design Target: 128 AES 8 bit data path S-box GF(((22)2)2) Design Target: sub-threshold 65nm 100 kHz Low power BCRYPT 2010

Sub-threshold Design Flow HDL Sub-threshold library Library Constraints Synopsys Designcompiler Synth Group UCL Cadence Encounter P&R [C. Hocquet, FTFC’09] BCRYPT 2010

Design of 65 nm subthreshold library Start point: 65nm library with nominal voltage 1.2V Keep in the library only gates with maximum stack of 2 MOSFETs Re-characterize the library at 0.4V (lowest VDD for 100kHz) Final library: 73 cells [C. Hocquet, FTFC’09] BCRYPT 2010

Results 1.2 V standard library 1.2 V restricted library 0.4 V BCRYPT 2010

Comparison with state of the art Implementation Technology Area [GEs] Max. freq. [MhZ] Power @ 100 kHz [µW] Proposed 1.2 V 65 nm 3500 0.1 @ 0.4 V 0.12 @ 0.4 V [Feldhofer,05] 1.5 V 0.35 µm 3400 80 4.5 [Hamalainen, DSD’06] 1.2 V 0.13 µm 130 3 [Good,TVLSI’09] 5500 12.8 0.692 @ 0.75 V BCRYPT 2010

Conclusions The S-box consumes the largest percentage of power. By choosing the appropriate technology the S-box power can be reduced by more than 1 order of magnitude – from 3.71 µW (0.13 µm) to 90 nW (65 nm - LP) while maintaining same delay Reducing the VDD of S-box from 1.2 V to 0.8 V decreases the power by 60 %, but increases the delay x3 The DDSLL logic reduces the power x1.5 than S-CMOS Subthreshold AES is a good candidate for Ultra Low power RFID applications – 120 nW @ 0.4 V BCRYPT 2010

Thank you BCRYPT 2010