security protocols analysis
DESCRIPTION
Security Protocols Analysis. Reading. 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: - PowerPoint PPT PresentationTRANSCRIPT
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