# Notes - Computer Security MT24, DES

> Source: https://ollybritton.com/notes/uni/part-b/mt24/computer-security/notes/des/ · Updated: 2025-06-04 · Tags: uni, notes

- [Course - Computer Security MT24](https://ollybritton.com/notes/uni/part-b/mt24/computer-security/)
	- [Notes - Computer Security MT24, Symmetric key ciphers](https://ollybritton.com/notes/uni/part-b/mt24/computer-security/notes/symmetric-key-ciphers/)
	- [Notes - Computer Security MT24, Feistel structures](https://ollybritton.com/notes/uni/part-b/mt24/computer-security/notes/feistel-structures/)

### 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 $IP : \lbrace 0, 1 \rbrace^{64} \to \lbrace 0, 1 \rbrace^{64}$
- 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)
	- $E : \lbrace 0, 1\rbrace ^{32} \to \lbrace 0, 1\rbrace ^{48}$ is fixed "expansion permutation", which permutes and duplicates some bits
	- $S : \lbrace 0, 1 \rbrace^{48} \to \lbrace 0, 1 \rbrace^{32}$ is a function which applies "$S$-boxes" $S_1, S_2, \ldots, S_8 : \lbrace 0, 1\rbrace ^6 \to \lbrace 0, 1\rbrace ^4$ and concatenates outputs $S(x) = S_1(x_{[1:6]}) \parallel S_2(x_{[7:12]}) \parallel \cdots \parallel S_n(x_{[43:48]})$ into one 32-bit block. Responsible for *confusion*.
- $P : \lbrace 0, 1\rbrace ^{32} \to \lbrace 0, 1\rbrace ^{32}$ is a fixed permutation of the 32 bits. Responsible for *diffusion*.
- Undergoes an final fixed permutation $FP : \lbrace 0, 1 \rbrace^{64} \to \lbrace 0, 1 \rbrace^{64}$

![Pasted image 20250110150328.png](https://ollybritton.com/assets/attachments/img/Pasted image 20250110150328.png)

How many rounds in the Fiestel structure in DES?::

16

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