Cryptography Principles

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Definition of a Secure Cipher

A cryptosystem is secure if the best‐known attack requires as much work as an exhaustive key search.

• In other words, no shortcut attack is known
• a cipher can never offer more security than an exhaustive key search,
  • so the key size could be considered its “advertised” level of security
• If a shortcut attack is known,
  • the algorithm fails to provide its advertised level of security
• in practice, we must select a cipher that is:
  • secure
  • and has a large enough key space so that an exhaustive key search is impractical

Security Principles

• Information Security
• CIA Triad

Cryptographic Principles

• Cryptographic Primitive
• Message Authentication Code (MAC)
• Hashed Message Authentication Codes (HMAC)

Fundamental Properties of Cryptographic Algorithms

• Kerckhoff’s Principles
• Models of Cryptography Security Proof
• Perfect Secrecy
• Security Strength of Cryptographic Algorithms