CMSC 414Computer (and Network) Security

Lecture 2

Jonathan Katz

Two papers linked from webpage

“Reflections on trusting trust”

“Managed security monitoring”

Both leave a fairly negative impression of security…

…at the very least, they show that security is not easy, and cannot just be applied as a “magic bullet”

“Managed security monitoring”

(Summarize article)– Is the state of network security really this bad?

(Arguably, yes)– Although network monitoring and risk

management are important, security is too– Security is not an ends unto itself

• If you really want to be secure, disconnect yourself from the Internet

An Overview of ComputerSecurity

Basic components

Confidentiality

Integrity

Availability

Policy vs. mechanism

Security policy– Statement of what is and is not allowed

Security mechanism– Method for enforcing a security policy

One is meaningless without the other…

Problems when combining security policies of multiple organizations

Security goals

Prevention

Detection

Response/recovery

Assumptions and trust

Example: assume that all employees are trustworthy, and do not represent a threat

Assumptions underlie any security mechanism – Important to recognize and evaluate these

assumptions

Example

Assumption: locks cannot be picked– What if a locksmith is around?– What if this locksmith is trustworthy?– Why do we assume that she is trustworthy?

More assumptions

Two assumptions are (almost) always made:– Policy defines the intended level of security– Mechanism correctly implements policy

System development

Determine threats; develop policy

Give specification of the system– Desired functionality of the system

– If specification is ambiguous, vulnerabilities can result

– An imprecise specification is useless…

Design the system– Design system satisfying the specification

– Difficult (but not impossible) to verify

System development, cont’d…

Implementation– Create a system satisfying the design– Impossible to fully verify correctness

• Software complexity

• Unknown inputs

• Unverified tools

– “Testing” after the fact• Subject to limitations of the tests

System development (summary)

1. Threat analysis

2. Policy

3. Specification

4. Design

5. Implementation

6. (Operation/maintenance/monitoring?)

Cost-benefit analysis

Important to evaluate what level of security is necessary/appropriate– Cost of mounting a particular attack vs. value

of attack to an adversary– Cost of damages from an attack vs. cost of

defending against the attack– Likelihood of a particular attack

Human factors

E.g., passwords…

Outsider vs. insider attacks

Software misconfiguration

Not applying security patches

Social engineering

Everything you wanted to know about cryptography*

*But perhaps were afraid to ask…

Caveat

Everything I present will be (relatively) informal– But I will try not to say anything that is an

outright lie…

Cryptography is about precise definitions, formal models, and rigorous proofs of security (which we will not cover here)– If you want more details, take CMSC 456!

Attacks

Crypto deals primarily with three goals:– Confidentiality– Integrity (of data)– Authentication (of resources, people, systems)

Other goals also considered– E.g., non-repudiation– E-cash (e.g., double spending)– General secure multi-party computation

Private- vs. public-key

For many security goals, there are two types of cryptographic algorithms– Private-key / shared-key / symmetric-key /

secret-key– Public-key

Private-key cryptography

The parties communicating share a completely random and secret key– Main point: key is not known to an attacker– This key must be shared (somehow) before they

communicate

All “classical” cryptosystems are private-key based

Can also be used for secure storage

Private-key cryptography

For confidentiality:– Private-key (symmetric-key) encryption

For data integrity:– Message authentication codes– (sometimes called cryptographic checksums)

Public-key cryptography

One party (Alice) generates both a public key and a private key (or secret key)

The public key is published; the private key is kept secret– An attacker knows the public key!

The other communicating party (Bob) need not have any key of his own; knows Alice’s key

Techniques for this first developed in the 70’s

Public-key cryptography

For confidentiality:– Public-key encryption

For data integrity:– Digital signatures

To review…

Confidentiality:– Private-key encryption (schemes)– Public-key encryption (schemes)

Integrity:– Message authentication (codes)– Digital signature (schemes)

We will discuss authentication later

Private- vs. public-key I

Disadvantages of private-key– Need to securely share a key

• If you can share a key securely, why not just share the message itself?

• What if not possible?

• Need to know who you want to communicate with in advance!

– O(n2) blowup in storage

Private- vs. public-key II

Why study private-key at all?– Private-key is much more efficient (3 orders of

magnitude)– Public-key crypto is “harder” to get right

• Needs stronger assumptions, more math

– Can combine private-key with public-key to get the best of both worlds (for encryption)

Private- vs. public-key III

More disadvantages of public-key crypto– Public-key crypto still requires secure

distribution and binding of public keys (PKI)• May (sometimes) be just as hard as sharing a key

– Not clear who you are communicating with (for public-key encryption)

Confidentiality

Overview

Private-key encryption– Attack model– “Trivial” systems

• Show why the problem is hard

• Show methods of attack

• Convince you not to use “home-brewed” techniques

– What do we mean by security?– Block ciphers and modern-day techniques

Overview, continued…

Public-key cryptography– A word about security– Some basic number theory– RSA and El Gamal– Some attacks…and some fixes

Alice Bob

shared infoK K

Alice

K

Bob

K

mC = EK(m)

C1

m = DK(C)

In more detail…

Alice and Bob share a key K– Must be shared securely– Must be completely random– Must be kept completely secret from attacker– We don’t discuss (for now) how they do this

Plaintext - encryption - ciphertext - decryption

Decryption must recover the message!