RC5 Encryption Block for wireless Tire Pressure

Monitor (wTPM)

Victor Wen

EE241 Project Presentation

05/09/2005

Background Federal mandate

Required for 2008 model year vehicles < 10,000lbs Indirect method inaccurate

Use existing ABS sensors Only distinguish relative pressure

Direct method Individual P/T sensor in each tire data broadcasted wirelessly to onboard computer

“I am flat!”“Hmm..I will alert the driver”

RF

Motivation/Limitations

Extreme low power requirement ~4000J for 10 years (typical lithium coin button

battery) RF, sensor and uController consumes power! 10uW design goal sub/near threshold voltage?

Why RC5? Symmetric cipher; low hardware overhead

Authentication needed; only care about my own tires Encryption of data not needed

Parametrizable (choose w8/r8/k16) Well-tested

Grand Scheme

Sensor

uController

RC5

Radio

Raw data

auth. dataData packet

RC5Setup

KeytableROM

RC5Round

input Intermediatevalue authenticated output8

16 16

16

addr After 8 cycles, the encryptioncompletes and the outputbecomes valid

Design/Sim FlowDataflowverilog

Hspice

Libraries0.13um

Structureverilog

DesignCompiler

Verilog2spi SpiceFile

TestVectors

Spice results

Area: 4453 um2

Standard cells: 310

RC5 C code

C results

Eq?

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.3 0.5 0.7 0.9 1.1

Encryption Cost (I)

0

5

10

15

20

25

30

0.3 0.5 0.7 0.9 1.1

Active Encrypting Energy (8 round + 1 round setup)

Vdd (V)

Ene

rgy

(pJ)

Power is freq. dependent; energy per operation more meaningful

Given 4000J total energy,only 0.2322 yr@1.08V and73.764 yr@0.3V cont. operation

Red: 125CBlue: 85C

Encrypting Energy per ns

320X

Encryption Cost (II)

0

20

40

60

80

100

120

140

160

180

200

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

Delay (1 round)

Vdd (V)

Del

ay (

ns)

Vth ~ 0.3V; delay grows drastically near threshold

@0.3V, requires 1.62us for encryption (~600kHz)

0

10

20

30

40

50

60

70

80

90

0.3 0.5 0.7 0.9 1.1

Encryption Cost (III)

Vdd (V)

Red: 125CBlue: 85C

Active Encrypting ED Product (for 1 round)

ED

(1e

-21

J*s)

Sweet spot near0.6 – 0.8V

Given low frequency,choose lowest possiblevoltage

Encryption Cost (IV)

Supply voltage (V) Static energy (J/year) Years of operation*

1.08 276(401) 14.49(9.99)

0.6 100(136) 39.9(29.34)

0.3 19.6(44) 204.3(90.9)

Measured when CLK off Non trivial static energy need to disconnect Vdd

when not in use

*Assume 4000J of total battery energy(…) = circuits at 125C

Conclusion Strong encryption block with low energy

requirement 320x energy difference between 1.08V and

0.3V > 70 yrs cont. operation @0.3V

Future Varying RC5 parameters (w/r/k) Custom design RC5 blocks