Feistel Network: The Structure Behind DES

Block & Rounds

Rounds to run 8

Flip input bit # 0

Playback speed 1.0×

Live State

Round

0 / 8

Round key

Track A (original)

L=-- R=--

Track B (1 bit flipped)

L=-- R=--

Avalanche0 / 16 bits differ (0%)

How it works

A Feistel network (the design behind DES and many other block ciphers) splits a block into two halves L and R and runs identical rounds that never need to be invertible themselves — only the overall swap-and-XOR structure does:

L(i+1) = R(i)
R(i+1) = L(i) XOR F(R(i), K(i))

Here the 16-bit block is L₈‖R₈ (8 bits each). The round function F rotates R left by 3 bits, XORs with the round key K(i), substitutes each 4-bit nibble through a fixed S-box, then reverses the bit order:

F(R,K) = P( SBOX4( ROL3(R) XOR K ) )
K(i)   = ROL8(masterKey, 3·i mod 8) XOR (i · 0x1B mod 256)

Rounds slider — how many identical rounds the block passes through before the simulation stops (real DES uses 16).
Flip input bit — choose which single bit of Track B's plaintext differs from Track A's, then compare the two ciphertexts after the same number of rounds.
Avalanche meter — the Hamming distance between Track A and Track B's current state as a percentage of all 16 bits. A strong cipher should push this toward 50% after only a handful of rounds — flipping one input bit should look like flipping a coin for every output bit.
Play / Step — advance round by round; each cube in the grid is one bit, lit when it is 1, and Track B's cubes glow red where they disagree with Track A at that position.

Real-world relevance: DES (1977) used exactly this L/R-swap structure with a 32-bit half-block and 16 rounds; 3DES, Blowfish and Twofish all extend the same idea. AES, by contrast, is a substitution-permutation network, not a Feistel network — which is why an AES round updates the whole block at once instead of swapping halves.

Bit = 1 Bit = 0 Track B differs from A

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