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Security

Yih-Kuen Tsay

Dept. of Information Management

National Taiwan University

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Introduction

• Security Needs– Secrecy, integrity, etc.– Arise from the desire to share resources

• Security Policies– Specify who are authorized to access what resources– Independent of the technology used

• Security Mechanisms– Enforce security policies

• Security Models– Help understand and analyze the above

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

The Evolution of Security Needs

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Components of a Security Model

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

The Enemy in Network Security

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Familiar Names in the Security Literature

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Classes of Security Threats

• Leakage– Acquisition of information by unauthorized

parties

• Tampering (Modification)– Unauthorized alteration of information

• Vandalism– Interference with the proper operation without

gain to the perpetrator

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Methods of Attack

• Eavesdropping– Release of message contents and traffic

analysis

• Masquerading• Message Tampering (Modification)

– Man-in-the-middle attack

• Replaying• Denial of Service• Mobile Code

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Designing Secure Systems

• Use best standards available

• Informal analysis and checks

• Formal validation

• Security logs and auditing

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Security Requirements

• Secrecy (Confidentiality)

• Data Integrity

• Authentication

• Non-repudiation

• Availability

• …

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Source: W. Stallings, “Cryptography and Network Security”

The Secret-Key Encryption Model

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Source: W. Stallings, “Cryptography and Network Security”

The Public-Key Encryption Model

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Source: W. Stallings, “Cryptography and Network Security”

The Public-Key Authentication Model

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Notational Conventions

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Performance of Cryptographic Algorithms

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

A Scheme of Cipher Block Chaining

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

A Stream Cipher

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Digital Signatures with Secret Keys

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Digital Signatures with Public Keys

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Alice’s Bank Account Certificate

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

A Public Key Certificate of Bob’s Bank

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

The Needham-Schroeder Authentication Protocol

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Kerberos

• Developed at MIT

• For protecting networked services

• Based on the Needham-Schroeder protocol

• Current version: Kerberos Version 5

• Source code available

• Also used in OSF DCE, Windows 2000, ...

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Kerberos Architecture

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Distributed Information Systems Distributed Information Systems 20042004 SecuritySecurity -- -- 2525Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

The Kerberos Protocol

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

auth(C) contains C,t.

ticket(C,S) contains C,S,t1,t2,KCS.

The Kerberos Protocol (cont.)

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The Secure Sockets Layer (SSL)

• Originated by Netscape, now a nonproprietary standard (SSLv3)

• Provides secure end-to-end communications

• Operates between TCP/IP (or any other reliable transport protocol) and the application

• Built into most browsers and servers

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

The SSL Protocol Stack

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How SSL Works

• Sessions between a client and a server are established by the Handshake Protocol

• A session defines a set of security parameters, including peer certificate, cipher spec, and master secret

• Multiple connections can be established within a session, each defining further security parameters such as keys for encryption and authentication

• Security parameters dictate how application data are processed by the SSL Record Protocol into TCP segments

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Security Functions of SSL

• Confidentiality: using one of DES, Triple DES, IDEA, RC2, RC4, …

• Integrity: using MAC with MD5 or SHA-1

• Authentication: using X.509v3 digital certificates

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The SSL Handshake Protocol

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

The SSL Record Protocol

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Micropayments

• The price of some goods may be lower than the standard transaction fees

• Micropayments offer a way for selling small-value products and services

• Technology providers: eCharge (via phone bills), Qpass (monthly bills), Millicent (prepay electronic cash), ...

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The Millicent Scrip Scheme

• Scrip is a form of digital cash valid only for a specific vender.

• Format:

• Scrip is generated and distributed by brokers.

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Source: G. Coulouris et al., Distributed Systems: Concepts and Design, Third Edition.

Millicent Architecture