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COMP2700 Cheat Sheet

Table of Contents

Introduction

Foundations of Computer Security

  • Slide 41
  • Prevention -> Take measures that prevent your assets from being damaged
  • Detection -> Take measures so that you can detect when, how and by whom an asset has been damaged by
  • Reaction -> Take measures so that you can recover from damage to your assets

Security Objectives

  • Slide 44
  • Confidentiality - Slide 45
    • Privacy - Slide 46
  • Integrity - Slide 49
  • Availability - Slide 52
  • Authenticity
  • Accountability - Slide 53
    • Non-Repudiation - Slide 54
    • Reliability and Safety - Slide 55

Design Decisions

  • Slide 59
  1. Where to focus the protection mechanism? - Slide 60
  2. Where to place the protection mechanism? - Slide 61
    • Human-machine Scale - Slide 62, 64
      • Concentric protection rings. Centre -> more generic, outside -> more abstract (individual)
    • Onion Model - Slide 63
    • Data vs Information - Slide 65
  3. Complexity vs Assurance - Slide 69
  4. Centralized vs decentralized control - Slide 70
  5. Protection of the 'layer' below - Slide 71

Security Principles

  • No hard and fast rules
  1. Simplicity - Keep it simple - Slide 79
  2. Open Design - Security should not depend on secrecy (Security by obscurity is frowned upon) - Slide 80
  3. Compartmentalisation - Organize resources into isolated groups of simialr needs - Slide 81
  4. Minimum exposure - Minimize attack surface presented to an adversary - Slide 82
  5. Least Privilege - Any component (and user) should operate with the least set of privileges to complete its job - Slide 83
  6. Min Trust, Max Trustworthiness - Slide 84
  7. Secure, Fail-safe Defaults - System should start and return t oa secure state in event of a failure - Slide 85
  8. Complete mediation - Access to any object must be monitored and controlled - Slide 86
  9. No Single Point of Failure - Build redundancy wherever and whenever possible - Slide 87
  10. Tracability - Log security system events - Slide 88
  11. Generating Secrets - Maximize entropy of secrets - Slide 89
  12. Usability - Design usable security mechanisms - Slide 90

Security Management

  • Slide 94

  • Security is a people problem

    • Cannot be solved by technology alone

  • Security measures need to be clear and well defined

  • Security-awareness training needs to be part of the management process

  • Security Policy: A statement that defines the security objectives of an organization - Slide 95

    • States what needs to be protected and how

  • Measuring Security

    1. Obtain values for security relevant factors (measurement)
    2. Consolidate measurements into single value (security metrics) - Slide 96 & 97

  • Risk Analysis - Slide 100

    • Comprehensive
    • Specific

    Risk  Assets  ∗  Threads  ∗  Vulnerabilities Risk \; Assets \; * \; Threads \; * \; Vulnerabilities

    • Calculation

      • Slide 115

    • Risk factors include: - Slide 103

      • Vulterability
      • Environment
      • Asset

    • Attacks

      • Likelihood depends on motivations

    • Quantitative or Qualitative? - Slide 104

      • Quantitative Risk Analysis -> Values taken from a mathematical domain
      • Qualitative Risk Analysis -> Values taken from domains without underlying mathematical structure

    • Assets

      • Should be identified and valued
        • Valuation (Monetary replacement costs) - Slide 106
      • Slide 105 for examples

    • Vulnerabilities - Slide 107 & 108

      • Weaknesses of a system, either exploitable by accident or intentionally

    • Threats - Slide 110

    • Mitigation - Slide 117

Identification and Authentication -- Slide 122

Authentication

  • A process where one party (verifier) is assured of the identity of a second party (claimant) in protocol
    • Basis
      • What you know
      • What you have
      • Who you are

Protocols

  • Slide 124
  • Weak Authentication -> Unilateral
  • Strong Authentication -> Mutual Authentication required

Passwords

  • Hashes - Slide 125
  • Storage - Slide 128
  • Verification image - Slide 130
  • Entropy - Slide 135
    • If given the choice to add more symbols or make it longer, make it longer

Salting

  • Added to password before applying hash function
  • A random string
  • Stored in plaintext along with password
    • Linux Crypt Hash
  • Attacker would need to pre-compute 2n2^n hashes of the same password for a nn-bit salt

Policies

  • Slide 152

Biometrics

  • Biometric systems must include an enrolment process
  • Should not use biometrics for the sole basis for authentication
  • Verification -> 1:1 - Comparison checks whether there is a match for a given user - Slide 163
  • Identification -> 1:n - Comparison tries to identify the user from a database of n users - Slide 164
Failure Rates

  • A value between 0 (no matching features) or 1 (all features match) should be assigned

  • Slide 165 & 167

  • False positive -> Accept wrong user -> Security Probelem

  • False negative -> Reject legitimate user -> Usability Problem

Scenario Analysis

  • Images in formula sheet

  • Failure-to-capture-rate (FTC) -> Frequency of failing to capture a sample

  • Failure-to-extract-rate (FTX) -> Frequency of failing to extract a feature from a sample

  • Failure-to-acquire rate (FTA) -> Frequency of failing to acquire a biometric feature

  • False accept rate (FAR) -> Rate for entrie biometric scheme

  • False reject rate (FRR)

Identification

  • False positive Identification rate for database with n persons -> Error rate increases as database size increases

Access Control -- Slide 176

  • Security Policy -> Statement of what is and what isn't allowed - Slide 179

    • Requester -> Principal (slide 183)
    • Object -> Resource requested
    • Reference monitor -> Abstract machine enforcing access control

  • Security Mechanism -> Method, tool, or procedure for enforcing a security policy

  • Authentication -> Reference monitor verifies identity of the principal

  • Authorization -> Reference monitor decides if access is granted or not

Access Operations

  • Slide 184

Ownership

  • Discretionary access control (DAC) -> Define an owner for each resource and let the owner set the policy

  • Mandatory Access Control (MAC) -> Impose system wide policies on who are allowed access to what

Reference Monitors and Hardware Security -- Slide 202

  • Reference Monitor - Slide 205

    • Complete mediation -> Must always be invoked

    • Tamper-proof -> Reference validation mechanism must be tamper-proof

    • Verifiable -> Small enough to be analysed and tested

    • Stuff on OS security

Unix Security -- Slide 233

  • Discretionary access control -> Owner, group, other

  • Principals -> UIDs and GIDs - Slide 239

  • Permissions - http://permissions-calculator.org/

    • Default 0666 for new file
    • Umask and calculations - Slide 267
      • Common umask options - Slide 268

Software Security -- Slide 302

  • Common Vulnerabilities - Slide 326

  • Type Confusion - Slide 373

  • Defenses - Slide 397

    • Safer functions
    • Canaries
    • Least Privilege

SOLUTION SET

https://tom.busby.ninja/understanding-cryptography-full-solution-set/

Intro Cryptography -- Slide 415

Symmetric Cryptography

  • Encryption and decryption are inverse operations if the same key K is used on both sides

    • Key must be transmitted via secure channel

  • Kerckhoff's Principle

A cryptosystem should be secure even if the attacker (Oscar) knows all details about the system with the exception of the secret key

  • An adversary only needs to succeed with one attack

Modular Arithmetic

  • Usually the smallest non-negative integer r is chosen as the remainder
Calculating Modular Remainder
  • Modular division - Slide 442

The Ring

The ring ZmZ_m

Rules on slide 444

  • Closure -> Adding or multiplying any two numbers always results in a number in the ring
  • Adding and multiplying are asssociative
    • Addition is also cumulative
  • Distributive law holds
  • Neutral element 0 with respect to addition
  • Additive inverse element
  • Neutral element 1 with respect to multiplication
  • Multiplicative inverse exists for some but not all elements in ZmZ_m
    • A ring is a structure which we can always add, subtract and multiply. We can only divide by certain elements (namely those for which a multiplicative inverse exists)
    • Multiplicative inverse exists only if:

Stream Ciphers & Block Ciphers -- Slide 455

Stream vs Block

  • Stream ciphers encrypt bits individually

  • Lightweight

  • Block ciphers always encrypt a full block

Stream Ciphers

  • Encryption and decryption are the same function - Slide 458

  • Encryption: -> yi=esi(xi)=xi+si  mod  2y_i = e_{s_i}(x_i) = xi + si \; mod \; 2

  • Decryption: -> xi=esi(yi)=yi+si  mod  2x_i = e_{s_i}(y_i) = yi + si \; mod \; 2

Synchronous vs Asychronous

  • Security depends entirely on key stream sis_i

  • Synchronous -> Key stream depends entirely on the key (+ maybe an IV)

  • Asychronous -> Key stream depends also on the ciphertext

Random Number Generators

  • Slide 463

Block Ciphers

  • Confusion -> Encryption operation where relationship between ciphertext and plaintext is obscured

    • Substitution commonly used to achieve this

  • Diffusion -> The influence of one plaintext symbol is spread over many ciphertext symbols to hide statistical properties of the plaintext

    • Bit permutation commonly used

  • Idea is to use both, as by themselves they cannot provide security

Galois Field

Group -- Slide 485

  • Every element has an inverse −a-a such that a+(−a)=0  mod  ma + (-a) = 0 \; mod \; m

Field -- Slide 487

  • Finite Field -> Finite number of elements. Number of elements is the order or cardinality of the field
Finite Field Theorem

  • Prime Field

    • Given a prime pp, the integer ring ZpZ_p is a prime field, denoted by GF(p)GF(p)
      • Elements are 0,...,p−10,...,p-1
      • Addition and multiplication always performed modulo pp
      • Multiplicative inverse always exists for non-zero elements of ZpZ_p
    • Example on slide 490

  • Extensition Field

    • Slide 492
    • AES finite field contains 256 elements, GF(28)GF(2^8) -> Extensition field of GF(2)GF(2)
    • Addition -> Slide 493
    • Multiplication -> Slide 494
    • Inversion -> Slide 497

AES -- Slide 501

AES Encryption Modes -- Slide 521

Hash and MAC -- Slide 535

  • Digital Signatures - Slide 541

    • Signing -> Requires private key
    • Signature Verification -> Requires public key

  • Hash Functions - Slide 547

    • Three security properties

      a. Preimage resistance -> Computationally infeasible to find input xx such that h(x)=zh(x)=z, i.e. h(x)h(x) is one-way

      b. Second preimage resistance -> Computationally infeasible to find any x2x_2 such that x1≠x_1 \ne x_2$

      c. Collision resistance -> Computationally infeasible to find pairs x1≠x2x_1 \ne x_2 such that h(x1)=h(x2)h(x_1) = h(x_2)

Euclidean Algorithm

  • Slide 597

RSA

  • Slide 606

Public-Key and Discrete Logarithm -- Slide 618

  • Order of an element - Slide 624

Digital Signatures -- Slide 644

Key Establishment -- Slide 669

  • N2N^2 Distribution Problem -- Slide 677

  • Certificates -- Slide 689

Kerberos -- Slide 697