PUZZLE #9975: DES Feistel Network (diff 5)

des learn difficulty: 5/7 simulate author: system unsolved

32-bit simplified Feistel, 4 rounds. Key: b3. Ciphertext: 4a5f5a54. Reverse the rounds to recover the plaintext.

DATA

N Rounds	`4`
Key	`179`
Ciphertext Hex	`4a5f5a54`
Plaintext Len	`4`
Hint	`Reverse Feistel, 4 rounds. Subkey per round = (0xb3 + r*17) & 0xFFFF. S-box0: [[1,0,3,2],[3,2,1,0],[0,2,1,3],[3,1,3,2]]; S-box1: [[0,1,2,3],[2,0,1,3],[3,0,1,2],[2,1,0,3]]. f(R,subkey): XOR R with subkey, then substitute each 4-bit nibble: even nibbles use S-box0, odd use S-box1. Row=bit3+bit2, Col=bit1+bit0. Feistel round: L'=R, R'=L XOR f(R,subkey). Decrypt by reversing rounds.`
Answer Format	`lowercase word, no spaces or punctuation`

{
  "type": "des",
  "n_rounds": 4,
  "key": 179,
  "ciphertext_hex": "4a5f5a54",
  "plaintext_len": 4,
  "hint": "Reverse Feistel, 4 rounds. Subkey per round = (0xb3 + r*17) & 0xFFFF. S-box0: [[1,0,3,2],[3,2,1,0],[0,2,1,3],[3,1,3,2]]; S-box1: [[0,1,2,3],[2,0,1,3],[3,0,1,2],[2,1,0,3]]. f(R,subkey): XOR R with subkey, then substitute each 4-bit nibble: even nibbles use S-box0, odd use S-box1. Row=bit3+bit2, Col=bit1+bit0. Feistel round: L'=R, R'=L XOR f(R,subkey). Decrypt by reversing rounds.",
  "answer_format": "lowercase word, no spaces or punctuation"
}

author's note: Pool fill: des diff 5