Feistel Cipher: Rounds and the Avalanche Effect

Block & Key

Plaintext block (16-bit hex)

Master key (16-bit hex)

Rounds 6

Show avalanche twin (flip one input bit)

Bit to flip bit 0 (MSB)

Live State

Tower A (original) Tower B (1 bit flipped)

Round

0 / 6

Hamming distance

0 / 16

A: 0100 1011 | 0010 1110

B: 0100 1010 | 0010 1110

Ciphertext (current round, hex)

A=4B2E B=4A2E

How it works

A Feistel network splits a block into two halves L, R and repeats a simple update instead of designing one complicated reversible function. This is the structure behind DES and many other block ciphers:

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

Because L only ever moves sideways and R is combined by XOR, the whole round is invertible for any round function F — you don't need F itself to have an inverse, which is what makes the construction so useful.

This demo's F (educational, not cryptographically secure) XORs the 8-bit half with the round key, splits the result into two 4-bit nibbles, substitutes each nibble through a fixed non-linear S-box, then rotates the combined byte left by one bit:

F(R, K) = rotl1( Sbox(hi(R⊕K)) << 4 | Sbox(lo(R⊕K)) )

Rounds — more rounds mix the S-box's non-linearity through more of the block; real ciphers use this same idea with 16+ rounds.
Avalanche twin — flips one bit of the plaintext before encryption and runs it through the identical key schedule. A strong cipher should scramble roughly half the output bits (Hamming distance ≈ 8 of 16) after only a few rounds even though the inputs differ by a single bit.
Tower — each 4×4 layer is one round's 16-bit state (top half = L, bottom half = R); step upward to watch the two towers diverge.

Real-world relevance: DES, Blowfish, and Twofish are all Feistel networks; AES uses a related but non-Feistel substitution-permutation structure covered on this site's AES simulation.

bit = 1 (tower A) bit = 0 (tower A)

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