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OS11: Security

Including parts of Chapter 11 and Section 9.6.3 of [Hai19]

(Usage hints for this presentation)

Computer Structures and Operating Systems 2019
Dr. Jens Lechtenbörger (License Information)

DBIS Group
Prof. Dr. Gottfried Vossen
Chair for Computer Science
Dept. of Information Systems
WWU Münster, Germany

1 Introduction

1.1 OS Plan

  • Wrap-up (Wk 28)


1.2 Today’s Core Questions

  • How can I ensure that my downloaded software has not been manipulated?
  • What is e-mail self-defense?

1.3 Learning Objectives

  • Explain confidentiality and integrity as security goals
    • Discuss differences between end-to-end and hop-by-hop goals
  • Explain use of hash values and digital signatures for integrity protection and discuss their differences

1.4 Retrieval Practice

  • Security — So far
    • Hardware building blocks
    • Process as major OS abstraction
      • Virtual address spaces
        • Isolate processes from each other
      • Access rights

1.5 Information Security

  • Safety: Protection against unintended/natural/random events
    • (Not focus here; requires proper management, involves training, redundancy, and insurances)
  • Security: Protection against deliberate attacks/threats
    • Protection of security goals for objects and services against attackers

1.5.1 Security Goals

  • Classical security goals: CIA triad
    • Confidentiality
      • Only intended recipient can access information
      • Typically guaranteed by encryption mechanisms
        • (Or, e.g., with envelopes and protecting laws)
    • Integrity
      • Detection of unauthorized modification
      • Typically guaranteed by cryptographic checksumming mechanisms
        • (Or, e.g., with signatures and/or seals)
    • Availability
      • Information and functionality available when requested
      • Supported by redundancy
    • Further goals
      • Accountability, authenticity, anonymity, (non-) deniability, …

1.5.2 Relativity

  • Security is relative
    • You need to define your goals and risks for specific pieces of information, e.g.:
      • How much confidentiality for course slides vs course exam?
      • Apparently, it’s easy to keep the slides “secure”
        • Harder for the exam
    • Also: Who is the attacker with what resources?
      • Appropriate security mechanism, typically with risk acceptance
  • Security via design process and management
    • BSI (Germany) and ISO standards
    • Topic in its own right

1.5.3 Attacker Models

  • Sample classifications of attackers
    • Strategy
      • Targeted (specialized, looks for “weakest link”)
        • E.g., espionage, blackmailing
      • Opportunistic (standardized, looks for “weakest target”)
        • E.g., phishing, extortion, bot/zombie creation (DDoS, spam, bitcoin mining, proxy)
    • Financial resources
    • Compute capacity
    • Time
    • Knowledge (insider and position?)

1.6 Design Principles for Secure Systems

  • Selected principles based on [SS75]
    • Fail-safe defaults (whitelisting): If no explicit permission, then deny
    • Least privilege (need to know): Subject has only those privileges that are necessary for given task
    • Economy of mechanism: Security mechanisms should be as simple as possible
    • Complete mediation: All accesses need to be checked
    • Open design: Security should not depend on secrecy; instead open reviewing
    • Separation of privilege: Permission not based on single condition
    • Psychological acceptability: Security should not hinder usage
    • And more, see [SS75] or [Hai19]

1.7 End-to-End Security

  • Security goals may have varying scope
    • Hop-by-hop
    • End-to-end
  • Integrity and confidentiality are end-to-end goals
    • Beware: That’s not generally understood!
      • (See next slide…)
    • Consider hop-by-hop confidentiality
      • Alice wants to send confidential message M to Bob via one hop, Eve
        • Alice encrypts M for Eve, sends encrypted M to Eve
        • Eve decrypts M, encrypts M for Bob, sends encrypted M to Bob
      • Security gain or loss? (Compared to what?)
    • Hop-by-hop integrity similarly

1.7.1 (Counter-) Example: De-Mail

  • De-Mail is a German approach defining legally binding, “secure” e-mail

    Hop-to-hop security of e-mail

    • General picture
      • Strong (hop-by-hop) security for each of the three blue links
      • Plaintext at both providers (and broken approach towards integrity, see [Lec11])
        • End-to-end encryption allowed
        • Digital signatures used in special cases

2 Cryptography

2.1 Key Notions

  • Cryptography = Art of “secret writing”
  • Set of mathematical functions
    • Cryptographic hash functions
    • Classes of encryption algorithms
      • Symmetric, secret key: en- and decryption use the same shared secret key
      • Asymmetric, public key: participants own pairs of secret (decryption, signature creation) and public (encryption, signature verification) keys
      • Hybrid: asymmetric initialization, symmetric encryption
    • Basis for various security mechanisms
  • Performance
    • Hashing > Symmetric Enc. > Asymmetric Enc.
      • (One can hash more data per second than one can encrypt)
      • (One can encrypt more data per second symmetrically than asymmetrically)

2.1.1 Basic Assumptions

  • Fundamental Tenet of Cryptography from [KPS02]
    • “If lots of smart people have failed to solve a problem, then it probably won’t be solved (soon).”
    • The problem to solve here: Break specific crypto algorithm
      • If that did not happen for a long time, probably the algorithm is strong
      • (Lots of crypto algorithms come without security proof)
  • Kerckhoffs’ Principle (1883)
    • Security of crypto systems should not depend upon secrecy of en- and decryption functions (but on secrecy of the used keys)
    • “Open Design” principle from [SS75]
      • Not respected in national security/military/intelligence settings in Germany
        • From Enigma through Libelle (approved for “Streng geheim”; developed by BSI, not published)
    • Opposite: Security through obscurity

2.1.2 Names

  • Alice and Bob; Charlie, Carol, Dave, …
    • Communicate frequently
    • Value their privacy
    • Have limited trust in third parties
    • Appeared to be subversive individuals in the past
      • Growing understanding in general public
    • And, of course, politically correct names instead of “A” and “B”
  • Eve, Mallory, Trudy
    • Eavesdropper, malicious attacker, intruder

2.1.3 Notation

  • M, C: Message and ciphertext (encrypted messages)
  • K: Key (random bits, maybe with certain structure)
  • E, D: En- and decryption functions
  • KAB: Secret key shared between Alice and Bob
  • KA-: Alice’s private key
  • KA+: Alice’s public key
  • K(M): Message M encrypted with key K (if function E is clear from context)
  • [M]K: Message M signed with key K

2.2 GnuPG

  • GNU Privacy Guard
    • Free software for (e-mail) encryption and digital signatures
    • Implementation of OpenPGP standard
      • Secure e-mail based on hybrid cryptography
    • In addition, lots of cryptographic algorithms via command line
      • gpg --version … gpg (GnuPG) 2.1.13 … Öff. Schlüssel: RSA, ELG, DSA, ECDH, ECDSA, EDDSA Verschlü.: IDEA, 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH, CAMELLIA128, CAMELLIA192, CAMELLIA256 Hash: SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
    • Start by creating key pair: gpg --gen-key

2.2.1 E-Mail Self-Defense

  • My suggestion: Try out OpenPGP
    • Create key pair, upload public key to server, send/receive encrypted (possibly signed) e-mails
  • More specifically, follow Email Self-Defense
    • GnuPG and Thunderbird with Enigmail plugin
    • Of course, other implementations exist
      • The choice is yours
    • Note: That guide contains instructions concerning the e-mail robot Edward, which can reply to your encrypted (and signed) test e-mails

2.3 (Cryptographic) Hash Functions

  • Hash function (or message digest): One way function H
    • Input: Message M (bit string of arbitrary length)
    • Output: Hash value H(M) (bit string of fixed length)
    • Collision: Different messages mapped to same hash value
  • Cryptographic hash value ≈ digital fingerprint
    • Collision resistant (different hash values for different messages)
    • Weak collision resistance of hash function H
      • Given message M it is computationally infeasible to generate M’ such that H(M) = H(M’)
        • (Computationally infeasible means that attackers should not be able to create collisions due to resource limitations)
    • Strong collision resistance of hash function H
      • Computationally infeasible to generate M and M’ such that H(M) = H(M’)

2.3.1 On Collision Resistance

  • Later: Hash values are essence of digital signatures
    • Consider contract between Alice and Mallory
      • “Mallory buys Alice’s used car for 20,000€”
        • Contract’s text is message M
        • Digital signatures of Alice and Mallory created from H(M)
    • Suppose H not weakly collision resistant
      • Mallory may be able to create M’ with price of 1€ such that H(M) = H(M’)
      • As H(M) = H(M’) there is no proof who signed what contract
  • Birthdays, collisions, and probability
    • Hash people to their birthdays (day and month, without year)
    • (a) Weak collision resistance: Anyone sharing your birthday?
    • (b) Strong collision resistance: Any pair sharing any birthday?

2.3.2 Hash Applications

  • Avoidance of plain text passwords
  • Integrity tests
  • Digital signatures

2.3.3 Hash Standards

2.3.4 Sample Message and Fingerprints

Hi Bob,

let's get started tomorrow!

Best wishes
  • Sample hash values with GnuPG
    • gpg --print-md SHA1 alice.txt
      • alice.txt: 6FC1 F66C 598B D776 BA37 1A5C 2605 06CB 4CF9 0B89
    • gpg --print-md SHA256 alice.txt
      • alice.txt: 84E500CB 388EE799 05F50557 43C5481B 08B0BF17 1A2AE843 F4A197AD 2BA68D2E
  • (Besides, specialized hashing tools exist, e.g., sha256sum)

2.4 JiTT Assignment

Submit a solution for the following task in Learnweb.

2.4.1 Ticket to Exam

Discuss the correctness of the following statements.

  • If messages of unbounded size are hashed cryptographically, an infinite amount of hash collisions is guaranteed.
  • If a hash function is weakly collision resistant, it is computationally infeasible to compute hash collisions.
  • If a hash function is strongly collision resistant, it is computationally infeasible to compute hash collisions.
  • If I download a piece of software along with its hash value produced by a weakly collision resistant function and if the downloaded software has that precise hash value, I can be pretty sure that I obtained the “correct” software (without accidental or malicious changes).

2.5 Symmetric Encryption

  • Sender and recipient share secret key, KAB
  • Encryption of plaintext message M with KAB into ciphertext C
    • C = E(KAB, M) = KAB (M)
      • Bits of M and KAB are mixed and shuffled using reversible functions (e.g., XOR, bit shift)
      • Simplest, yet provably secure case: One-time pad with XOR of random bit string and M
  • Decryption with same key KAB
    • M = D(KAB, E(KAB, M))
    • Notice: Need to exchange secret key ahead of time
  • Typical symmetric algorithms: AES, 3DES

2.6 Intuition of Asymmetric Encryption

  • Participants own key pairs
    • Private key, e.g., KB-: secret
    • Public key, e.g., KB+: public / published
    • En- and decryption based on “hard” mathematical problems
  • Think of key pair as safe/vault with numeric key pad
    • Open safe = public key
      • Everybody can deposit messages and lock the safe
    • Opening combination = private key
      • Only the owner can open the safe and retrieve messages

2.7 Asymmetric Encryption

  • Participants own key pairs
    • Private key, e.g., KB-: secret
    • Public key, e.g., KB+: public / published
  • Encryption of message for Bob with Bob’s public key
    • C = E(KB+, M) = KB+ (M)
    • Notice: No secret key exchange necessary
  • Decryption with Bob’s secret key
    • D(KB-, KB+(M)) = KB-(C) = M
    • Notice: Only Bob can do this
  • Challenge: Reliable distribution of public keys

2.7.1 Sample Asymmetric Algorithms

  • Diffie-Hellman Key Exchange (1976)
    • Used, e.g., in IPsec, SSL/TLS, Tor, OTR
      • RFC 7568, June 2015: SSLv3 MUST NOT be used
  • RSA (Rivest, Shamir, Adleman 1978; Turing award 2002)
    • Most famous, PGP, GnuPG
  • ElGamal (1984)
    • Based on Diffie-Hellman
    • GnuPG and newer PGP variants
  • Elliptic curves
    • Newest class, shorter keys
    • GnuPG

2.7.2 GnuPG: Hybrid Encryption

  • Create asymmetric key pair
    • gpg --gen-key
    • Various options/alternatives
  • Encryption for Bob
    • gpg -e -a -r Bob file
      • Creates file.asc; more precisely:
      • Creates random secret key KAB
      • Symmetric encryption of file with KAB
        • Specific algorithm obtained from Bob’s public key
      • Asymmetric encryption of KAB with KB+
        • Beware! No naïve encryption, but, e.g., PKCS #1
      • Result: KB+(KAB) + KAB(file)
        • (“+” between ciphertexts denotes string concatenation)

3 Message Integrity

3.1 Situation and Goal

  • Alice sends message M to Bob
    • (Parts of) Network controlled by unknown parties (Eve and Mallory)
  • Goals of integrity
    • Bob is sure that M came from Alice
      • Notice: Need authentication!
    • Bob can detect modifications to M
  • Non-goals: Alice cannot be sure
    • that no third party receives M
    • that Bob receives M
    • that Bob receives M in unchanged form

3.2 General Idea

  • Alice sends message along with its fingerprint
    • Recall: A hash value is not good enough
    • Instead: Use some ingredient that is unknown to the attacker
  • Bob receives message and fingerprint and verifies whether both match
    • If message changed by Mallory, he cannot produce a matching fingerprint
  • Typical techniques
    • Message authentication codes
      • E.g., Alice and Bob share secret KAB, concatenate that to message before hashing
    • Digital signatures (next slides)

3.3 Digital Signatures

  • Based on asymmetric cryptography
    • En- and decryption reversed
  • Basic idea
    • Signature created by encryption with private key: KA-(M)
      • Only Alice can create this!
    • Verification via decryption with public key: D(KA+, KA-(M))
      • Everyone can do this as public key is public!
  • Practice: Encrypt hash value of M, e.g., KA-(SHA-3(M))
    • Recall
      • Performance
      • Hash collisions

3.3.1 Some Details of Digital Signatures (1/2)

  • Signing of M by Alice with private key KA-
    • Signature S = KA-(h(M))
      • Only Alice can do this
    • Transmit signed message [M]KA- = M + S = message + signature
      • (“+” is concatenation)

Creation of digital signature

3.3.2 Some Details of Digital Signatures (2/2)

  • [M] received by Bob
  • Verification whether [M] sent by Alice and unchanged along the way

    Verification of digital signature

    • Split [M]: [M] = M’ + S’
    • Hash M’: H = h(M’)
    • Decrypt S’: H’ = KA+(S’)
      • Bob needs public key of Alice to do this
      • Everyone can do this
    • Verify H = H’

3.3.3 GnuPG: Digital Signatures

  • gpg --sign -a -b alice.txt
    • Creates alice.txt.asc
  • gpg --verify alice.txt.asc
    • Expects to be verified content as alice.txt
    • Verifies signature
    • Frequently used to verify integrity of downloads

3.4 Electronic Signatures

  • “Signatures” of varying legal impact in IT environments
    • Different types, e.g., simple (e.g., sender’s name in e-mail), advanced (digital signature as discussed above), qualified
    • Qualified electronic signatures may replace paper based signatures (e.g., dismissal, invoice)
      • Subset of advanced electronic signatures
      • Based on qualified certificates (with qualified electronic signature, issued by accredited organization; law prescribes rules concerning infrastructure and processes)
      • Created on secure signature-creation devices (nPA may store qualified certificate; additional reader necessary)

3.5 JiTT Assignments

Answer the following questions in Learnweb.

3.5.1 Asymmetric Cryptography

  1. Mark the correct statements.
    • If the hash function used to create a digital signature produces a hash collision for messages M and M’, recipients will not know whether M was signed or M’ or both.
    • Alice encrypts messages to Bob with her public key.
    • Alice encrypts messages to Bob with her private key.
    • Alice encrypts messages to Bob with Bob’s public key.
    • Alice encrypts messages to Bob with Bob’s private key.
    • Alice needs her public key to sign messages.
    • Alice needs her private key to sign messages.
    • Alice needs Bob’s public key to sign messages addressed to him.
    • Alice needs Bob’s private key to sign messages addressed to him.
    • Bob needs Alice’s public key to verify her signatures.
    • Bob needs Alice’s private key to verify her signatures.
    • I generated a key pair with GnuPG and sent an encrypted e-mail.

3.5.2 Preparation and Feedback

  1. To prepare the in-class meeting, please bring along some colored pens, e.g., text markers.
  2. What did you find difficult or confusing about the contents of the presentation? Please be as specific as possible. For example, you could describe your current understanding (which might allow us to identify misunderstandings), ask questions that allow us to help you, or suggest improvements (maybe on GitLab). You may submit individual questions as response to this task or ask questions in our Riot room and the Learnweb forum. Most questions turn out to be of general interest; please do not hesitate to ask and answer in forum and Riot room. If you created additional original content that might help others (e.g., a new exercise, an experiment, explanations concerning relationships with different courses, …), please share.

4 In-Class Meeting

4.1 Asymmetric Cryptography with Colors

  • Based on Security Protocol Game by Len Hamey
  • Instructions
    • Form groups of 3
      • Every group member with a different color
    • Mark message with your color to sign
    • Place message into colored envelope to encrypt

4.2 (In-) Security News

4.3 Basic OS Security Services

4.3.1 Service Overview (1/2)

  • Rights management, authorization
  • Logging
    • Who did what when?
    • (Not considered in following)
  • Basic cryptographic services
    • Offering selection of above techniques: a/symmetric techniques, hashing

4.3.2 Service Overview (2/2)

  • Identification/Authentication
    • Identification: Claim of identity
      • I’m Bob …
    • Authentication: Proof of identity
      • My password is “p@ssw0rd”
        • (Bad idea, easily broken!)
  • Integrity protection

4.3.3 Authentication

  • Proof of identity
    • Something the individual knows
      • Password, PIN, answers to security questions
    • Something the individual possesses
      • Smartcard, iTAN
    • Something the individual is
      • Static biometrics, e.g., fingerprint, iris scan
    • Something the individual does
      • Dynamic biometrics, e.g., voice or typing pattern
  • Necessary prerequisite to enforce access rights
    • Who is allowed to perform what operation on what resource?

4.3.4 Two-Factor Authentication

  • Combinations of above categories
    • Physical banking
      • Bank card (possession) plus PIN (knowledge)
    • Online banking
      • Password for login (knowledge) plus mTAN or iTAN (possession)
    • Beware: Must keep factors separate
      • Do not record PIN on card
      • Do not perform online banking on device that receives mTAN

4.4 Passwords

4.4.1 Authentication with Passwords

  • Randomness of passwords
    • Measured via entropy
    • Ca. 4 bits per character in natural language
      • (Contrast this with 8 bits, which are necessary for typical representation)
    • Thus, 32 bits per 8 character password
      • Instead of 64 bits if characters were randomly distributed
      • 232 is about a factor of 109 smaller than 264
  • Consequently, security of memorizable passwords is limited

4.4.2 Password Advice

4.4.3 Password Checking

  • Never store passwords in plain text
  • Typical setup
    • System stores (salted – next slide) hash value of password
      • GNU/Linux: /etc/shadow
      • Windows: Security Accounts Manager (SAM) or Active Directory (AD)
  • Verification
    • System computes hash value for entered password string
    • Compare computed to stored value
  • (Alternative, e.g., Kerberos)
    • Use hash value of password as symmetric encryption key

4.4.4 Sample Password Attacks

4.4.5 Brute Force and Salting

  • Brute force = Try “all” combinations
    • Various heuristics, implemented in tools
      • Try common patterns first
        • Databases of stolen passwords (e.g., “123456”)
        • Variations of dictionary words (e.g., “p@ssw0rd” above)
      • Rainbow tables: precomputed hash values
      • Counter measure: Salting
        • Concatenate password with random number (=salt), then compute hash
        • Store random number and hash

4.5 Key Security Best Practices

  • Consult others
  • Adopt a holistic risk-management perspective
  • Deploy firewalls and make sure they are correctly configured
  • Deploy anti-virus software
  • Keep all your software up to date
  • Deploy an IDS
  • Assume all network communications are vulnerable
  • … (see Sec. 11.8 in [Hai19])

5 Conclusions

5.1 Summary

  • Security is complex, requires design and management
  • Cryptography provides foundation for lots of security mechanisms
    • Don’t implement cryptographic protocols yourselves!
    • Use proven tools, e.g., GnuPG
  • Asymmetric crypto with key pairs
    • Public key for encryption and signature verification
    • Private key for decryption and signature creation
  • Hash functions and digital signatures for integrity


License Information

This document is part of an Open Educational Resource (OER) course on Operating Systems. Source code and source files are available on GitLab under free licenses.

Except where otherwise noted, this work, “OS11: Security”, is © 2017, 2018, 2019 by Jens Lechtenbörger, published under the Creative Commons license CC BY-SA 4.0.

No warranties are given. The license may not give you all of the permissions necessary for your intended use.

In particular, trademark rights are not licensed under this license. Thus, rights concerning third party logos (e.g., on the title slide) and other (trade-) marks (e.g., “Creative Commons” itself) remain with their respective holders.