Security ProtocolsSecurity ProtocolsAnalysis Analysis

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ReadingReading

This Class: – Modelling and Analysis of Security Protocols:

chapters 0.9-0.12– C. Meadows: Formal Methods for Cryptographic

Protocol Analysis: Emerging Issues and Trends, http://citeseer.ist.psu.edu/meadows03formal.html

Next class: – Modelling and Analysis of Security Protocols:

chapter 1

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Cryptographic ProtocolsAttackers’ capabilitiesSecurity?

– Hostile environment

Vulnerabilities– Weakness of cryptography– Incorrect specifications

What is Protocol AnalysisWhat is Protocol Analysis

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Cryptographic ProtocolsCryptographic ProtocolsTwo or more partiesCommunication over insecure networkCryptography used to achieve goal

– Exchange secret keys– Verify identity (authentication)– Secure transaction processing

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Emerging Properties of Emerging Properties of ProtocolsProtocols

Greater interoperation Negotiation of policy Greater complexity Group-oriented protocols Emerging security threats

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Attackers’ CapabilitiesAttackers’ Capabilities

Read trafficModify trafficDelete trafficPerform cryptographic operationsControl over network principals

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AttacksAttacks

Known attacks – Can be picked up by careful inspection

Nonintuitive attacks– Not easily apparent– May not depend on flaws or weaknesses of

cryptographic algs. – Use variety of methods, e.g., statistical analysis,

subtle properties of crypto algs., etc.

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Formal MethodsFormal Methods

Combination of a mathematical or logical model of a system and its requirements and

Effective procedures for determining whether a proof that a system satisfies its requirements is correct.

Can be automated!Can be automated!

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Example: Needham-Example: Needham-SchroederSchroeder

Famous simple example (page 30-31)– Protocol published and known for 10 years– Gavin Lowe discovered unintended property while

preparing formal analysis using FDR system

Subsequently rediscovered by every analysis method

From: J. Mitchell

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Needham-Schroeder CryptoNeedham-Schroeder Crypto

Nonces – Fresh, Random numbers

Public-key cryptography – Every agent A has

Public encryption key Ka Private decryption key Ka-1

– Main properties Everyone can encrypt message to A Only A can decrypt these messages

From: J. Mitchell

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Needham-Schroeder Key ExchangeNeedham-Schroeder Key Exchange

{ A, NonceA }

{ NonceA, NonceB }

{ NonceB}

Ka

Kb

On execution of the protocol, A and B are guaranteed mutual authentication and secrecy.

A B

Kb

From: J. Mitchell

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Needham Schroeder Needham Schroeder properties properties

Responder correctly authenticated– When initiator A completes the protocol apparently with

Honest responder B, it must be that B thinks he ran the protocol with A

Initiator correctly authenticated– When responder B completes the protocol apparently with

Honest initiator A, it must be that A thinks she ran the protocol with B

Initiator Nonce secrecy– When honest initiator completes the protocol with honest peer,

intruder does not know initiators nonce.

From: J. Mitchell

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Anomaly in Needham-SchroederAnomaly in Needham-Schroeder

A E

B

{ A, NA }

{ A, NA }{ NA, NB }

{ NA, NB }

{ NB }

Ke

KbKa

Ka

Ke

Evil agent E trickshonest A into revealingprivate key NB from B

Evil E can then fool B

[Lowe]

From: J. Mitchell

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Requirements and PropertiesRequirements and Properties

Authentication – Authentication, Secrecy

Trading– Fairness

Special applications (e.g., voting) – Anonymity and Accountability

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Security AnalysisSecurity AnalysisUnderstand system requirements Model

– System – Attacker

Evaluate security properties– Under normal operation (no attacker)– In the presence of attacker

Security results: under given assumptions about system and about the capabilities of the attackers.

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Explicit intruder modelExplicit intruder model

Intruder Model

AnalysisTool

Formal Protocol

Informal Protocol

Description

Find error

From: J. Mitchell

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Protocol Analysis SpectrumProtocol Analysis Spectrum

Low High

Hig

hL

owSo

ph

isti

cati

on

of

atta

ck

s

Protocol complexity

Mur

FDR

NRLAthena

Hand proofs

Paulson

Bolignano

BAN logic

Spi-calculus

Poly-time calculus

Model checking

Symbolic methods (MSR)

Protocol logic

From: J. Mitchell

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Analysis of Discrete SystemsAnalysis of Discrete Systems

Properties of discrete systems– Requirements– Attackers

Attack: sequence of finite set of operationsEvaluate different paths an attacker may

takeState the environmental assumptions

precisely

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First Analysis MethodFirst Analysis Method Dolev-Yao Set of polynomial-time algorithms for deciding

security of a restricted class of protocols First to develop formal model of environment in

which– Multiple executions of the protocol can be running

concurrently– Cryptographic algorithms considered as “black boxes”– Includes intrudes model

Tools based on Dolev-Yao– NRL protocol analyzer– Longley-Rigby tool

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Model checkingModel checking

Two components– Finite state system– Specification of properties

Exhaustive search the state space to determine security

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Theorem ProverTheorem Prover

Theorems: properties of protocolsProve or check proofs automaticallyCould find flaws not detected by manual

analysisDo not give counterexamples like the model

checkers

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LogicLogic

Burrows, Abadi, and Needham (BAN) logic Logic of belief Set of modal operators: describing the relationship

of principal to data Set of possible beliefs Inference rules Seems to be promising but weaker than state

exploration tools and theorem proving (higher level abstraction)

Next weekNext weekCSPCSP