Towards more secure networks

using formal approaches

Dr. Jayaraj PoroorDependSoft Consultingjayaraj@dependsoft.comhttp://dependsoft.com

Presented at Dr. C. R. Rao Institute of Advanced Studies

Jan 22, 2013

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Roadmap for the talk

• Formal Methods

• Cryptographic Protocols

• Formal Modeling

• Modeling Language

• A Cryptographic handshake Protocol

• The ProVerif Tool

✓Motivate

✓ Key Ideas

✗ All details

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Formal Methods

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Formal methods

• The art and science of describing and reasoning about computing systems using rigorous mathematical techniques having well-developed theoretical basis.

Alan Turing Von Neumann John McCarthy

Turing MachineBombe machine

Automata TheoryStored Program Computer

Artificial IntelligenceLISP

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The bridge

THEORY PRACTICEFORMAL

METHODS

ComputabilityComplexity

ExpressivityAutomata

LogicsCalculi

Semantics

SpecificationDesign &DevelopmentSoftware &HardwareSystemsTools

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Why do we need it?

• Gain precise understanding of computing systems▫ Ideas are developed with mathematical rigor

• Rigor pays off▫ Strong guarantees▫ Mechanization of synthesis and analysis

• Theory informs▫ Establishes the limits▫ Provides efficient algorithms

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Reasoning about programs

• What is a correctness property?▫ Safety + Liveness

• Safety▫ Something bad will never happen.

• Liveness▫ Something good will eventually happen.

• Can be expressed mathematically.

Recognizing Safety and Liveness, Alpern and Schneider, Distributed Computing, 1986

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Coffee/tea machine as a finite-state

automata

5

3sec

Now we can reason about safety and liveness!

1Reasoning … is nothing but reckoning, Thomas Hobbes, English Philosopher

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Cryptographic

Protocols

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What are they?

• Concurrent programs that interact via public channels to achieve security objectives.

• “3 lines programs that people still manage to get wrong,” Roger Needham.

• A critical component for ensuring security in today’s distributed systems.▫ e.g., SSL/TLS, SSH, IPSec, SRTP.

Gavin Lowe. 1995. An attack on the Needham-Schroeder public-key authentication protocol. Inf. Process. Lett. 56, 3 (November 1995), 131-133.

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Narrating a simple cryptographic protocol

Many concurrent instances of A’s and B’s are possible

A Brequest, A’s public key

A B

B

Response encrypted with A’s public key

prepareresponse

ADecrypt responsewith A’s private key

No guarantee that the response came from B!

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Security objectives

• Secrecy

• Strong secrecy

• Origin Authentication

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Security objectives

• Integrity

• Authenticated key exchange

• Non-repudiation

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Security objectives

• Fairness

• Anonymity

• Availability1

1Jayaraj Poroor and Bharat Jayaraman, Denial of service attacks on Real-time media through Indirect Contention-in-Hosts, IEEE Internet Computing, 2009.

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The fruit flies of formal methods

research!

Is ToIs LikeIs To

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Formal reasoning

Finite state representation of a

cryptographic protocol

A

S

?

Attacker gets the secret!

Start state

Will attacker get the secret?

Is state A reachable from state S?

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Formal Modeling

of

Cryptographic Protocols

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Modeling assumptions

• What are the capabilities of the attacker?

• What are the assumptions behind modeling cryptographic functions

▫ e.g., AES is cryptanalytically stronger than DES

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Be specific!

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The Dolev Yao model

• All-powerful Intruder▫ Channel is the Intruder▫ Intruder can read, modify, fabricate, replay,

analyze messages

• Perfect cryptography▫ Computational complexity abstracted out

Dolev, D.; Yao, A. C. (1983), "On the security of public key protocols", IEEE trans. on Information Theory IT-29: 198–208

IntruderAlice Bob

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Modeling perfect cryptography

• Functions as symbolic terms

▫ senc(m,k) may be applied to any message m and key k

• Properties of functions as equations

▫ sdec(senc(m,k),k) = m

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Equational reasoning

• Equations:

▫ mix(salt, water) may be applied to any salt

▫ mix(salt, empty) = salt

▫ heat(water) = empty

▫ heat(salt) = salt

▫ heat(mix(a,b)) = mix(heat(a),heat(b))

• What happens if we: heat(mix(salt,water))?

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Equational reasoning

heat(mix(salt,water)) mix(heat(salt), heat(water))

mix(heat(salt), heat(water)) mix(heat(salt),empty)

mix(heat(salt),empty) mix(salt,empty)

mix(salt,empty) salt

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Equational theory

• The set of equalities derivable under

▫ A set of equational axioms +

▫ Rules of equational reasoning (e.g., reflexivity, commutativity)

▫ Proof technique: Matching & rewriting

Klop, Term rewriting systems from Church-Rosser to Knuth-Bendix and beyond, LNCS, 90.

Don Knuth

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Try out

• What do we get?

▫ sdec(sdec(senc(senc(m,k2),k1),k1) ,k2)

• Remember:

▫ sdec(senc(m,k),k) = m

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Soundness & Completeness

• Tells us how a model and the reality (model’s intended meaning) are related.

MODEL REALITY

Soundness

Completeness

The Map is not the Territory!

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Unsoundness

Route Exits

A route exists from B to A is a theoremprovable in our model

however

But route from B to A does notExist in Reality

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Incompleteness

Road ExistsNo route exists in our map

however

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Limits of Dolev-Yao models

• Could be computationally unsound

▫ A provably secure protocol could be computationally breakable (e.g., use of DES)

• Incomplete

▫ A secure protocol need not be provable in the model

▫ undecidable in general for unbounded sessions

3Soundness Limits of Dolev-Yao Models, Backus, Pfitzmann, Waidner, FCC, 2006

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Modeling Language

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Modeling requirements

• Modeling the domain

▫ Unforgeable fresh secrets, nonces

▫ Symbolic encoding of cryptographic functions

⚫enc(M,k), dec(M, pk(k))

⚫dec(enc(m,k),k) = m

⚫xor(xor(m,k),k) = m

• Concurrency and communication

▫ Client parallel with Server

▫ out(channel, encrypted msg), in(channel, msg)

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Computational calculus

• Succinct language for studying a particular class of computation

• Results in well-developed theory▫ Can prove properties of programs

• e.g., λ-calculus: sequential functional-style computation▫ Influenced functional language development over

last 5 decades

Alonzo Church

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π-calculus

• Communication & concurrency are primitive operations

• Milner, Parrow, Walker, 89

• Numerous extensions, applications

▫ Distributed systems

▫ Programming languages

▫ Biomolecular systems

▫ Cryptographic protocols

Robin Milner

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A multithreaded echo server in Java

http://www.javareference.com/jrexamples/viewexample.jsp?id=47

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Multithreaded echo server in Java (2)

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Multithreaded echo server in π-

calculus

let Echo = in(echo, (client,msg));out(client,msg);0

Two threads: Echo | Echo

Three threads: Echo | Echo | Echo

Unbounded threads: !Echo

!in(echo, (client,msg));out(client,msg);0

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Sending a fresh name

• Java:

▫ Read from random sources

▫ Write to socket etc.,

• π-calculus:

▫ new n; out(channel,n)

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The complete π-calculus language

• A process P, Q is

▫ 0

▫ in(c, x); P

▫ out(c, v); P

▫ P | Q

▫ new n; P

▫ if u = v then P else Q

▫ !P

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Execution: Informal vs Formal

G. Plotkin, A Structural Approach to Operational Semantics (1981)

Gordon Plotkin

Java Program

Compiler

Byte code

Executions

JVM

π-calculusProgram

OperationalSemantics

Rules

TransitionGraph

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Sample execution steps

in(echo, (c,x));out(c,x);0 | out(echo, (ret,3));in(ret,y);out(prn,y);0

out(ret,3);0 | in(ret,y).out(prn,y);0

0 | out(prn,3);0

reduction

reduction

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A Cryptographic

Handshake protocol

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A naïve handshake protocol

Bruno Blanchett and Ben Smyth, ProVerif 1.86pl3: Automatic Cryptographic Protocol Verifier, User Manual and Tutorial, Dec 2011

A Bpk(skA)

A B

B

(pk(skB),k)sign( , skB)aenc( , pk(skA))

generate key k

A Bsenc(s,k)

pk(skB) known

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Handshake protocol properties

• Secrecy

▫ Secret s is known only to A and B

• Authentication of A to B

▫ If B believes he has shared key k with A

▫ then B was indeed communicating with A

▫ and A has the shared key k.

• Authentication of B to A

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Murphy’s law!

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Murphy’s law in cyber security!

CENTER FOR

CYBER

SECURITY

If an attack can happen, it will

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Man-in-the-middle attack

A B

I

A B

pk(skI)I B

aenc(sign((pk(skB),k),skB),pk(skI))I B

pk(skA)

A

aenc(sign((pk(skB),k),skB),pk(skA))

senc(s,k)

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The fix

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A Bpk(skA)

A B(pk(skA),pk(skB),k)sign( , skB)aenc( , pk(skA))

A Bsenc(s,k)

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The ProVerif tool

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Short bio

• Follows Dolev-Yao model▫ CryptoVerif: Computational Model

• Input language: Applied π-calculus

• Reconstructs attacks▫ Sound when trace is given

• Successfully verified complex protocols/systems▫ TPM, TLS, JFK, Plutus, electronic voting

Bruno Blanchet. An Efficient Cryptographic Protocol Verifier Based on Prolog Rules. CSFW-14, June 2001.

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ProVerif: attack types

• Reachability properties

▫ Secrecy

• Correspondence assertions

▫ authentication

• Observational equivalence

▫ non-interference

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Symmetric encryption

Constructor name

argument types

Result type

Destructor name

Typed variables

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Horn clause representation

• P(x) if Q1(x) and Q2(x) … and Qn(x)

• grandparent(x,z) if parent(x,y) and parent(y,z)

• attacker(m) if attacker(senc(m,k)) and attacker(k)

• Horn clause logic

▫ Basis for logic programming

▫ Resolution Proof: Unification & Search

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Model components

• Constructors & Destructors for:

▫ Symmetric encryption

▫ Asymmetric encryption

▫ Digital signature

• Process definitions for:

▫ clientA

▫ serverB

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The system

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Conclusion• Formal Methods

▫ Rigorous Methods = Theory + Practice

• Cryptographic Protocols▫ Fruit-flies of formal methods research

• Formal Modeling▫ The Dolev-Yao model

• Modeling Language▫ Applied Pi-calculus

• A Cryptographic handshake Protocol▫ Man-in-the-middle vulnerability

• The ProVerif Tool▫ Uses Horn-clause logic, sound if trace is given

Thank You. Questions?

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jayaraj@am.amrita.edu

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Asymmetric encryption

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Digital Signature

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The client

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The server

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Other Modeling languages/tools

• Murphi

• FDR/CSP

• BAN Logic

• AVISPA

• CryptoVerif