Cryptography Algorithms

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Cryptography Algorithms

• Basics
• Block cipher, stream cipher
• Block size, key size
• Symmetric Algorithm
• DES (block size, key size)
• Feistel network
• Asymmetric Algorithm
• Full-fledged algorithm (Encryption/Decryption,
  Key exchange, digital signature)
• RSA key size, relation with block size
• ECC (Elliptic curve cryptography) (refer to
  WS 10.3-10.4)
• ElGamal (used on GnuPG)
• Key exchange only algorithm
• Diffie-Hellman (refer to WS 10.1)
• Digital Signature only algorithm
• DSA (digital signature algorithm)
• DSA works with SHA hash algorithms as a DSS
  (digital signature standard)
• Hash Algorithms
• MD5 SHA family (digest lengths)

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Block Cipher Principle

• Feistel network
• Relationship between the number of keys and the
  number of transformation from plaintext to
  ciphertext in block cipher

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Modes of Operation

• Structure of each mode
• Properties and Applications of each mode
• Confidentiality (e.g., Ciphertext via ECB mode
  reveals the pattern in the plaintext)
• Error propagation
• Block vs. Stream-based operation
• applications
• No integrity protection from any of the five
  modes of operation
• CBC mode could be used for MAC generation
• MAC needs to append to the plaintext
• For both MAC generation and encryption, two keys
  need to be used

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Message Authentication Code and Digital Signature

• Intuition/Basic Requirement of MAC design
• Security Requirement of Hash Algorithm design
• Generation of MAC ? Data integrity/Message
  authentication
• CBC mode DES
• Hash symmetric key encryption
• Hash secret key (no encryption required)
• This structure leads to HMAC design
• HMAC
• Generation of Digital Signature ? Data
  integrity/Message authentication
  Non-repudiation
• Hash asymmetric key encryption (using private
  key)
• DSA?DSS
• Usage of MAC and Hash

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Usage of MAC
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Usage of Hash Function
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Comparison

• Computation efficiency
• Hash gt symmetric encryption gt asymmetric
  encryption
• Message Authentication Code
• CBC-based
• Hash-based
• Encrypt the hash code
• Hash the message key
• HMAC
• Digital Signature
• Encrypt the hash code
• Digital signature standard

faster
faster
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Authentication and Key Management

• Distribution of Secret Key
• Using a KDC
• Using a previously shared master secret key
• Using asymmetric key
• Distribution of Public Key
• Using a CA with public key certificate
• Authentication Protocol and System

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In A Nutshell
Public Keys
Session Keys
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Mid-Term Exam

• Basic knowledge
• E.g., key size, block size, etc
• Understand the key algorithm/structure/protocol
  design properties
• E.g., Feistel network, MAC generation, modes of
  operation, key distribution, authentication
• Understand the usage of crypto primitives
• Identify flaws in the protocol design and usage
  of crypto primitives
• Identify security requirement in network
  applications and design solutions