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.