Feistel Network

A Symmetric Algorithm used to create Block-Cipher. Creates a Round Function

Round Function

• Let $F(\text{datablock},\text{subkey})$ be the Round Function
• Let $K_0,\dots ,K_n$ be sub-keys for rounds $0,\dots ,n$ respectively

Encryption

• Split plaintext block into two equal pieces $L_0,R_0$
• For each round, $i=0,\dots ,n$ compute:
  • $L_{i+1}=R_i$
  • $R_{i+1}=L_i\oplus F(R_i,K_i)$
• Ciphertext is $(R_{n+1},L_{n+1})$

Decryption

• For given ciphertext is $(R_{n+1},L_{n+1})$
• For each round $i=n,n-1,\dots ,0$
  • $R_i=L_{i+1}$
  • $L_i=R_{i+1}\oplus F(L_{i+1},K_i)$’
• $(L_0,R_0)$ is the plaintext again