Current Flattening in Software and Hardware for Security Applications

Radu Muresan

CODES+ISSS'04, September 8-10, 2004, Stockholm, Sweden 1

Current Flattening in Software and Hardware for Security Applications

Authors: R. Muresan, C. GebotysPresentation By: Radu Muresan

Radu Muresan


Outline Introduction Power analysis attacks (PAAs)

Definitions, examples, countermeasures Current flattening technique

Definition, methodology, implementations

Current flattening as a countermeasure against PAAs

Results and conclusions

Radu Muresan


Introduction Embedded

systems are increasingly used in security applications

The software and the hardware components must be secure against all threats

Current flattening is a potential countermeasure against PAAs Secret-key Cryptosystem: ke = kd

Public – key Cryptosystem: ke ≠ kd

Encryption

Decryption

Ciphertext

Message Channel

Plaintext

Plaintext

KeyGeneration

kd

ke

Key Channel

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What is a Power Analysis Attack ? Side-channel attacks

exploit correlation between secret parameters and variations in timing, power consumption, and other emanations from cryptographic devices to reveal secret keys

CryptographicDevice

R

Currentor

PowerMeasurement

Power Supply

Attacker’s Point

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Example, DPA Attack on a Scalar Multiplication Algorithm for EC Protocols

DPA, uses correlation between power consumption and specific key-dependent bits

kP, scalar multiplication Double-and-add approach, binary k(2) = (kn-1,...,k0)

kP1,kP2,...,kPn => Ci(t) = power kn-1 = 1; After the first iteration => Q[0] = 2Pi Second iteration

If kn-2=1 => Q[1] = 4P If kn-2=0 => Q[1] = 5P

g(t)=<Ci(t)>i=1,...,k|si=1 - <Ci(t)>i=1,...,k|si=0

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Example, DPA Attack on a Scalar Multiplication Algorithm for EC Protocols

A peak is observed when 4Pi are computed by the card

No peak is observed when 4Pi are never computed by the card

Simulated correlation function between the points 4Pi and power consumptionCi(t) when kn-2 = 0.

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Countermeasures Against PAAs Against timing attacks

Equalizing; Randomizing; Blinding Against simple power analysis attacks

Avoiding; Creating; Symmetric Against differential power analysis attacks

Randomization; Blinding Hardware: non-deterministic techniques

Against all PAAs Proposed: current flattening technique

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What is Current Flattening? Current flattening

targets a flat (emission free) current consumption measured at an attacker’s point of a cryptographic device

CryptographicDevice

Attacker’s Point

CurrentFlattening(internal)

ExternalCurrentFilteringDevices

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Behind Current Flattening Current

consumption in a processor is a function of: The hardware

architecture The instruction type The instruction

sequencing Data manipulated

Examples of current dynamics

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Software Method Program execution is

composed of two types of cycles charging; discharging

Code transformations are generated for classes of instructions

Current measurements used for determining code transformations

ClassFLATTEN0 [ALU Units]

FLATTENi [ALU Units]

ALU 1 1

NOP 0 0

CONTROL 4 3

Example

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Hardware Method Pipeline

current flattening module

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Hardware Method Feedbac

k current module

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Does Current Flattening Protect Against PAAs?

Software method does not support DPA due to the fact that the program to data dependencies are not covered

Hardware method has potential to cover all PAAs Supports real-time current adjustment at

the clock frequency Covers both current to data and to

instruction dependencies

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Results of Software Flattening The experiments

used the polymulNIST.asm implementation of an EC scalar multiplication (kP), where: P a fixed point

on a known elliptic curve

k a secret key Target processor:

Motorola SC140 DSP Real current measurements

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Results of Software Flattening Pk-Pk current

variation reduced by 70 to 78%

Energy consumption increased by 71 to 74%

Execution time increased by up to 135%

Ver-sion

StartTime [μs]

End Time[μs]

EnergyProg.[μJ]

Mean[mA]

S[mA]

Pk-Pk[mA]

M1 25 681 209.9 0.168 18.85 67.7

M2 25 1569 363.2 0.124 1.60 20.3

M4 25 1545 359.9 0.125 1.91 16.8

M7 25 1569 365.5 0.125 1.98 14.3

Data analysis for software flattening

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Results of Hardware Flattening Instantaneous

current simulation for polymul.asm polymul.asm is a

subroutine of polymulNIST.asm

polymul.asm is a target of PAAs

Target system Motorola SC140

DSP plus the Feedback Current Module

Current simulation and real current measurement

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Results of Hardware Flattening Pk-Pk current

variation reduced by 94 to 97%

Energy consumption increased by up to 16%

Execution time increased by up to 29%

Wave-form

RunTime [μs]

Energy[μJ]

Max[mA]

S[mA]

Pk-Pk

[mA]

blue 21.3 6.04 167.7 2.47 22.4

red 21.3 6.14 168.5 1.87 19.2

mag. 23.7 6.47 144.0 0.38 1.2

black 27.5 7.01 134.0 0.13 0.6

Data analysis for hardware flattening

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Conclusions The paper presented the mechanisms

of the internal current flattening technique (ICF)

ICF controls power consumption and current variation Countermeasure against PAAs

Limitations Increased execution time and energy

consumption

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Future Work

Investigate an ASIC implementation of the PAAR architecture

Methods to improve the performance and energy consumption of implementations using ICF

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References Used for the Presentation Slides 3 and 4

W. Mao, “Modern Cryptography”, Prentice Hall, 2004 O. Kommerling, M. G. Kuhn, “Design principles for

tamper-resistant smartcard processors”, In Workshop on Smartcard Technology 1999

Slides 5, 6 and 7 J-S. Coron, “Resistance against dpa for elliptic curve

cryptosystems”, CHES’99 P. Kocher, et al., “Differential power analysis”, In

CRYPTO’99 Slide 9

R. Muresan, C. Gebotys, “Instantaneous current modeling in a complex vliw processor core”, In ACM TECS, 2004

Radu Muresan


THANK YOU!

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