Notes - Computer Security MT24, DES cryptosystem


Flashcards

@Define the components of DES:

  • Symmetric or asymmetric?
  • Plaintext length?
  • Key length?
  • How is the plaintext scrambled, and where does confusion/diffusion come in?

  • DES is symmetric key cipher
  • 64-bit plaintext
  • 56-bit key
  • Undergoes an initial fixed permutation
  • Consists of 16 round Feistel network:
    • $L _ {n+1} = R _ n$
    • $R _ {n+1} = F(K _ {n+1}, R _ n) \oplus L _ n$
  • Each subkey $K _ n$ is a different selection of 48 bits from the key
  • $F(K, R) = P(S(K \oplus E(R)))$ (funky kangaroos rap, publicly squawking karaoke every rave)
  • $E : \{0, 1\}^{32} \to \{0, 1\}^{48}$ is fixed “expansion permutation”, which permutes and duplicates some bits
  • $S$ is a function which applies “$S$-boxes” $S _ 1, S _ 2, \ldots, S _ 8 : \{0, 1\}^6 \to \{0, 1\}^4$ and concatenates outputs into a 32-bit block. Responsible for confusion.
  • $P : \{0, 1\}^{32} \to \{0, 1\}^{32}$ is a fixed permutation of the 32 bits. Responsible for diffusion.




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