L0. Introduction

Rocky K. C. Chang, January 2013

The Internet is inherently insecure.

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Internet backbone infrastructure: DoS, worm Routing protocols (BGP): route hijacking DNS: poisoning, DoS Core Internet protocols (e.g., IP, TCP/UDP, HTTP):

eavesdropping, modification, authentication LAN security: eavesdropping, modification,

authentication Host security (e.g., Web servers, database): DoS,

authentication, phishing, malicious software implant, identity and data theft, data exfiltration, etc.

Internet security is inherently complex.

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A packet goes through many hops and links. Involve from the physical layer and up.

Physical layer security Network security System security Application security

Complexity in software and protocols Software ages Some protocol fields are never tested.

The weakest link The human factor The success of Internet makes things worse. Security verses privacy (anonymity) How to measure security?

Internet security is more than cryptography.

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Cryptography is not the solution to many security problems, e.g., software exploit, DoS.

The vulnerability could come from the implementations of the cryptographic algorithms.

Cryptography affects performance. Ease of use

Security involves

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Threats: potential violation of security Policies

Security policies: trust and access control Confidentiality policies: The Bell-LaPadula model Integrity policies: Clark-Wilson integrity model Hybrid policies: Chinese Wall models

Design and implementation Identity representation, access control lists, information

flow, etc Encryption and key management Authentication (human, user account, machine, service)

Security involves

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How to ascertain how well a system meets its security goals? Assurance, system evaluation (TCSEC)

Miscellaneous, e.g., Viruses, worms, software security Auditing Intrusion detection System security Network security User security

This course is not about

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Cryptography, the art of secret writing, Writing computer viruses and worms, Special techniques of attacking and defending, The lower layer security measures, System security, Biometrics, Application-specific security …

This course is about

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Understand the 3 fundamental cryptographic functions used in network security.

Understand the issues involved when applying the cryptographic functions to the network protocols.

Understand the main elements in securing today’s Internet infrastructure.

Exposed to some current Internet security problems.

Purposes of network security

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Confidentiality (or secrecy): Prevent others from reading information shared between two participants.

Authentication: Verify someone’s or something’s identity. Message integrity: Assure that the message received has

not be altered since it was generated by a legitimate source.

Nonrepudiation: A sender should not be able to falsely deny later that he sent a message.

Legitimate (and authorized) usage: Ensure that the network and system resources are properly utilized.

Possible threats

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Obtaining information for … Secrecy, authentication

Modifying information for … Authentication, message integrity

Stealing information for … Secrecy, authentication, legitimate usage

Lying electronically for … Nonrepudiation

Backmail for … Secrecy, legitimate usage, message integrity

Revenge for … Legitimate usage, message integrity

Testing for … Legitimate usage, message integrity

Contracted for … Secrecy, authentication, legitimate usage, message integrity

Fun for … Secrecy, authentication, legitimate usage, message integrity

The goals of security

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Prevention: Confidentiality, source authentication, nonrepudiation, and

legitimate usage Active countermeasures

Detection: Message authentication, nonrepudiation, and legitimate usage Active and passive countermeasures

Recovery: Legitimate usage Rely on the detection.

Traceback: Locate the actual attack source(s).

Scope of considerations

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Two cases The secrecy, message integrity, authentication, and

nonrepudiation services are provided by some cryptographic functions.

Denial-of-service, worms, viruses, etc Scope:

Concern mainly communication between two parties (group communication security is another important topic).

Concern attacks against protocols, not those against cryptographic algorithms or cryptographic techniques used to implement the algorithms.

Cryptography

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Plaintext (encryption) ciphertext Ciphertext (decryption) plaintext What is the secret?

The cryptographic algorithm (restricted algorithm) The cryptographic algorithm is not a secret, but the key is.

Level of security the length of the key the time of discovering the key using brute force

The security problem is reduced to the securing of the key.

Types of attacks

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Passive attacks (eavesdropping), e.g., ciphertext-only attacks (recognizable plaintext attacks)

Fred has seen some ciphertext. known-plaintext attacks

Fred has obtained some <plaintext, ciphertext> pairs. chosen-plaintext attacks

Fred can choose any plaintext he wants. Active attacks, e.g.,

pretend to be someone else introduce new messages in the protocol delete existing messages substituting one message for another replay old messages

Three cryptographic functions Hash functions: require 0 key Secret key functions: require 1 key Public key functions: require 2 keys

Secret keyfunctions

Public keyfunctions

Hashfunctions

Secrecyservice

Authenticationservice

Messageintegrity service

Nonrepudiationservice

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Secret key (symmetric) cryptography

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Given: Alice and Bob agree on a secret key cryptosystem. Alice and Bob agree on a key (secret) K.

Encryption and decryption using the key. Alice encrypts M with K: K{M} Bob decrypts K{M} with K M

Problems: Keys must be distributed in secret. Compromising keys means compromising all aspects of security. The number of keys is not scalable to the user population size.

Usages of the secret key cryptography Transmitting over an insecure channel Secure storage on insecure media Authentication:

Challenge-response authentication with shared secret

Message integrity check

rA

rB encrypted with K

rA encrypted with K

rB

Alice Bob

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Public key (asymmetric) cryptography

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Given: Alice and Bob agree on a public key cryptosystem. Alice owns a pair of public key and private key, and Bob

knows Alice’s public key, which is not a secret. Encryption using the public key and decryption using

the private key. Alice encrypts M with Bob’s public key: {M}Bob

Bob decrypts {M}Bob with its private key M Generate a digital signature on a message:

Alice signs M with its private key: [M]Alice.

Bob verifies Alice’s signature on [M]Alice with Alice’s public key.

Usages of the public key cryptography Problems:

Public-key algorithms are slow. Secret key algorithms are at least 1,000 times faster.

Obtain the public key reliably. Usages:

Transmitting over an insecure channel Secure storage on insecure media (difference as compared with the secret key

cryptography?) Authentication:

Nonrepudiation with the digital signatures.

r encrypted with Bob’s public key

decrypted with Bob’s private key

Alice Bob

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Hash functions

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A hash (message digest or one-way function) produces a short, fixed-sized output h(m) for a message m.

Properties: One-way functions are relatively easy to compute, i.e.,

given x and compute h(x). However, given h(x), it is significantly harder to compute

x. It is computationally infeasible to find two inputs that hash

to the same value.

Usages of hash functions

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Password hashing Message integrity

Keyed hash: compute h(message | key) and send the result with the message.

Message fingerprinting Downline load security Digital signature efficiency

Securing the Internet

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IP Security (IPSec) TCP and UDP insecurity SSL/TLS DNS security Firewalls DoS attacks and the countermeasures Buffer overflow attacks and the

countermeasures Wireless LAN security

Acknowledgments

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This set of notes is based on C. Kaufman, R. Perlman, and M. Speciner, Network

Security: Private Communication in Public World, Second Edition, Prentice Hall PTR, 2002.

L. Peterson and B. Davie, Computer Networks: A Systems Approach, Morgan Kaufmann, 2000.

B. Schneier. Applied Cryptography, Second Edition, Wiley, 1996.

M. Bishop, Introduction to Computer Security, Addison Wesley, 2005.