C SC 481.20 Lecture 15: Security Intro and Cryptographic Foundations
major resource: Computer Networking (4th Edition), Kurose and Ross, Addison Wesley, 2008

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Network Security Intro

• Consider various dimensions of network security
  • Secure transmission of credit card, bank account, or other private information
  • Denial of service attacks
  • Virus and worms
  • Trojan horses
  • Authentication: knowing who you're dealing with
  • Pharming (request to a website is redirected to a different one)
  • impersonation
  • and so forth...
• We focus on sender, receiver, channel and intruder
  • sender, receiver, intruder may be people (Alice, Bob, Trudy - a.k.a. Eve)
  • sender, receiver, intruder may be electronic (browser, server, router, virus)
  • sender, receiver, intruder may be combination of people and electronic
  • We can illustrate using people but same principles/practices apply to electronics
• Major concerns thus are:
  • confidentiality (only Alice and Bob can see the correct message contents)
  • authentication (Bob can be assured message really came from Alice not Trudy)
  • integrity (Bob can be assured message contents were not altered by Trudy)
  • availability (message sending service is available and operational for Alice and Bob)
• The foundation for security is cryptography : principles and practice of hiding information using secret codes

Cryptography

Some terminology

• plaintext: information written in a format understandable to sender and receiver
• encryption: process of transforming plaintext into secret form
• cipher: algorithm for encryption
• ciphertext: information written in encrypted form
• decryption: process of transforming ciphertext back into plaintext
• key: parameter that determines functional output of a cipher
• brute-force attack: attempt to decrypt by trying every possible key value

Traditional, Symmetric, cryptography methods

• Symmetric: both sender and receiver must know the key and cipher, but no one else can
• Date to the age of Julius Caesar ("Caesar ciphers")
• Substitutions (disguise symbols)
• Transposition (re-order symbols)
• Combination of substitution and transposition
• Example: Caesar cipher and key
  • cipher: For a given letter, substitute the letter X positions further along in the alphabet (circular)
  • key: selected value of X
  • How hard would this be to break?
• How can the key be securely exchanged?
• Example: DES
  • Data Encryption Standard
  • Message is encrypted in 64-bit blocks
  • The key is 56 bit number (64 bits when 8 parity bits are added, one per 7 bit group)
  • The 64 bit block is permuted (shuffled) l6 times (16 "rounds"), each permutation based on different 48-bit combination from key, followed by a 17th permutation that is the inverse of the first one
  • If you are interested, I have references that describe specific details of the permutations and selection of the 48 bits for each round.
• DES has been cracked, and enhanced to Triple DES, a.k.a. 3DES, which applies DES three times, e.g. E₃(E₂(E₁(M))), or E₃(D₂(E₁(M))), using two or three different keys
• A technique called Cipher Block Chaining (CBC) is also employed
  • CBC addresses this problem: if same block of plaintext appears multiple times in a message, its DES ciphertext will be the same for each occurrence. This represents a pattern, which gives Trudy an advantage over brute-force.
  • Under CBC, each occurrence of a plaintext block will have different ciphertext. How?
  • Before submitting block I to DES, its plaintext is XORed with the ciphertext of block I-1.
  • The bootstrapping "block 0" is a randomly-generated initialization vector (IV) that needs be known by the receiver or sent with the message
• DES supplanted by AES (Advanced) that uses 128-bit blocks, longer keys (128, 192 or 256 bit)

Public Key Cryptography

• developed by Hellman and Diffie at Stanford in 1976, independently in secret British project
• Essence:
  • Encryption key E is public, everyone can know it
  • Decryption key D is private, only one person can know it
  • Plaintext message M.
  • Characteristics of D and E:
    1. D (private) is "very difficult" to deduce from E
    2. E (public) cannot be broken by plaintext attack
    3. D(E(M)) = M
  • Procedure:
    1. Sender encrypts M using receiver's algorithm and receiver's public key E_R to get E_R(M)
    2. Sender transmits E_R(M)
    3. Receiver decrypts using its private key D_R as follows: D_R(E_R(M)) = M
• Rivest, Shamir, Adleman (RSA), a public-key encryption method
• Start with two very large prime numbers P and Q
• Calculate N = P * Q
• Calculate Z = (P-1) * (Q-1)
• Select E such that E < N and E is relatively prime to Z
• Select D such that E * D mod Z = 1 (e.g., remainder of (E * D) / Z is 1)
• Public key consists of N and E.
• Private key consists of N and D.
• The algorithm:
  • Given plaintext M, ciphertext C = M^E mod N
  • Given ciphertext C, plaintext M = C^D mod N
• RSA is widely used but very computationally expensive (hundreds of times more than DES)
• Public key encryption is often used in conjunction with symmetric encryption: Use public key encryption to communicate the symmetric key between Alice and Bob, who proceed to use symmetric encryption like DES to communicate

Legal Issues

• The U.S. government is quite interested in the export of encryption systems. Why?
• The fear obviously is that enemies will use encryption to communicate plans for hostile action against the U.S.
• One counter-argument is freedom of speech denial (where the algorithms represent speech)
• Encryption exports are currently (2008) regulated by the U.S. Bureau of Industry and Security.
• See http://www.bis.doc.gov/encryption/ (warning: bureaucratic language!)
• Until the mid-90s, encryption systems with key length > 40 bits were considered munition and their export prohibited!
• A 40-bit key is trivial to break via brute force (1 trillion possible keys).
• With rise of the Web and e-commerce, 40-bit keys became an incredible barrier for U.S. companies.
• SSL, for instance, the backbone protocol for e-commerce (https://), uses 128-bit keys
• PGP (Pretty Good Privacy) in the early 90s provided a legal challenge, as it was open source (how to control who downloads it?) and used 128-bit public keys
• Government charged PGP inventor Phil Zimmerman with violating munitions export laws, but gave up after three year legal battle
• Regulations were then relaxed, but even the current (2008) website describes procedure for requesting permission to export symmetric encryption with key length > 64 bits