[Intro. to Computer Security Course Note] Ch 2

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Ch2. Cryptographic Tools

Confidentiality with Symmetric Encryption

Symmetric Encryption

  • Universal technique for providing confidentiality for transmitted of stored data
  • Single-key encryption

2 Requirements

  • Strong encryption algo.
  • Sender and receiver must have obtained copies of the key in a secure fashion and must keep the key secret

Attacking

  • Cryptanalytic attacks
    • Rely on
      1. nature of algo.
      2. general characteristics of plaintext
      3. sample plaintext-ciphertext pairs
    • Exploit the characteristics of the algo. to deduce a specific plaintext or key
  • Brute-force attacks
    • On average half of all possible keys must be tried to achieve success
    • Knowledge about the expected plaintext!

Symmetric Block Excryption Algo.

  • DES Encryption Standard
    • Key length too short
      • 56 bits -> 2^56^ = 7.2 x 10^16^ possible keys (很快就破了)
    • e.g. data encryption algo.
  • Triple DES (3DES)
    • Repeats basic DES algo. 3 times using 2 or 3 unique keys
      (key size: 112 or 168 bits)
    • Key length 比較長
    • Underlying algo. is tje same as DES
    • Drawbacks
      • Not effetient (3 次 DES，瘋子)
      • 64-bit block size -> not effecient / secure
  • Advanced Encryption Standard (AES)
    • Widely available in commercial products
    • 128-bit data and 128 / 192 / 256 bit keys

Practical Security Issues

  • Symmetric encryption: applied to a unit of data larger than a single 64-bit or 128-bit block
    e.g., E-mail messages, network packets, database records
  • Simplest approach: electronic codebook (ECB)
    • Multiple-block encryption
    • Each block of plaintext is encrypted using the same key
  • Issue: cryptanalysts may exploit regularities in the plaintext
  • How to overcome the weakness of ECB
    • Cipher Block Chaining (CBC)

Block / Stream Ciphers

  • Block Cipher
    • Can reuse key
    • More common
    • Usage: file transfer, e-mail, database
  • Stream Cipher
    • Faster (XOR), less code
    • Usage: communication

Meseage Authentication and Hash Function

2 Major Aspects

  • Message content: not altered
  • Message source: authentic

Another Aspect

  • Messafe timeliness / sequence: not artificially delayed or replayed

Can we use symmetric encryption ?

  • Not necessary. content may not need to be encrpyted

Message Authentication w/o Encrpytion -> Auth. tag

  • Message authentication code (MAC)
    • Use a secret key to generate small block of data
    • Assumption: two communicating parties share the secret key
    • Does message authentication really need encrpytion of the message ?
      • NO!!
      • 不需要 reversible
      • Only need a way to generate a tag which can verify message
  • One-way hash function
    • The hash value ensures only unaltered contents. How about authentic source ?

Hash Function w/ Symmetric Encryption

Hash Function w/ Public-key Encryption

Hash Function w/o Encryption: Keyed Hash MAC

Secure Hash Functions

  • Easy to compute
    • Making both hardware and software implementations practical
  • One-way (pre-image resistant)
    • For any given code h, it is computationally infeasible to find x such that H(x) = h
  • Second pre-image (weak collision) resistant
    • For any given block x, it is computationally infeasible to find y != x with H(y) = H(x)
  • Collision (strong collision) resistant
    • It is computationally infeasible to find any pair (x, y) such that H(y) = H(x)

Security of Hash Functions

  • 2 attack approaches
    • Cryptanalysis
      exploits logical weaknesses in algo.
    • Brute-force
      depends solely on the length of the hash code
  • Security strength against brute-force attacks
    • Preimage resistant: 2^n^
    • Second preimage resistant: 2^n^
    • Collision resistant: 2^n/2^
  • Most widely used hash algo.: Secure Hash Algo. (SHA)
  • Applications of hash functions
    • Message authentication
    • Digital signature
    • Passwords
    • Intrusion detection

Public-Key Encryption

Asymmetric algo.
Based on mathematical functions

3 Common Misconceptions about Public-key Encryption

  • More secure than symmetric ones
  • A general purpose technique that has made symmetric ones obsolete
  • Key distribution is trivial

Requirements for Public-key Cryptosystems

  • Computationally easy
    • To create key pairs
    • For sender knowing public key to encrypt messages
    • For receiver knowing private key to decrypt ciphertext
  • Computationally infeasible for opponebt knowing public key
    • To determine private key
    • To recover original message which is encrypted by public key

Encryption Algo. and Applications

  • RSA: widely acepted and implemented
  • Diffie-Hellman
  • Digital Signature Standard: 就只能處理 digital signature
  • Elliptic Curve: offer equal security for a far smaller bit size

Digital Signatures and Key Management

Digital Signature

Public-key Certificates: secure distribution of public keys

  • Public key distribution
    • Any person can release his / her public key
    • But, anyone can forge such a public announcement
  • Solution: public-key certificate
    • Certificate: a public key + a user ID of the key owner
    • The whole block signed by a trusted third party, CA (Certificate Authority)
    • Certificate also includes CA info. and validity period
  • X.509 standard

Symmetric Key Exchange using Public-key Encryption: temporary key creation for message encryption

  • Diffie-Hellman key exchange
    • No authentication of two communicating partners

Digital Envelopes: distribution of secret keys


Can also be used to deliver a symmetric key only

Random and Pseudorandom Numbers

2 Distinct Requirements of Random Numbers

  • Randomness
    • Uniform distribution (easy to verify)
    • Independence (difficult to verify)
  • Unpredictability