CMSC 414 Computer (and Network) Security Lecture 2

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CMSC 414Computer (and Network) SecurityLecture
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• Jonathan Katz

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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

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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

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An Overview of Computer Security
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Basic components

• Confidentiality
• Integrity
• Availability

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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

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Security goals

• Prevention
• Detection
• Response/recovery

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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

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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?

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More assumptions

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

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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

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System development, contd

• 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

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System development (summary)

1. Threat analysis
2. Policy
3. Specification
4. Design
5. Implementation
6. (Operation/maintenance/monitoring?)

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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

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Human factors

• E.g., passwords
• Outsider vs. insider attacks
• Software misconfiguration
• Not applying security patches
• Social engineering

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Everything you wanted to know about cryptography
But perhaps were afraid to ask
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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!

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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

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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

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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

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Private-key cryptography

• For confidentiality
• Private-key (symmetric-key) encryption
• For data integrity
• Message authentication codes
• (sometimes called cryptographic checksums)

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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 Alices key
• Techniques for this first developed in the 70s

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Public-key cryptography

• For confidentiality
• Public-key encryption
• For data integrity
• Digital signatures

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To review

• Confidentiality
• Private-key encryption (schemes)
• Public-key encryption (schemes)
• Integrity
• Message authentication (codes)
• Digital signature (schemes)
• We will discuss authentication later

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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

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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)

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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)

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Confidentiality
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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

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Overview, continued

• Public-key cryptography
• A word about security
• Some basic number theory
• RSA and El Gamal
• Some attacksand some fixes

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K
shared info
K
m C EK(m)
m DK(C)
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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 dont discuss (for now) how they do this
• Plaintext - encryption - ciphertext - decryption
• Decryption must recover the message!