Ciphertexts XOR - A Reused Nonce Leaks the XOR

Reused keystream makes the ciphertext XOR match the plaintext XOR.

highlighted = computed this step

XOR the two ciphertexts

Ciphertext A XOR ciphertext B gives 0x000000000800.

\text{cipher XOR}=0x000000000800

Plaintext A XOR plaintext B gives the same bytes 0x000000000800.

\text{plain XOR}=0x000000000800

Because the toy keystream was reused, XORing the ciphertexts cancels that same keystream.

\text{reused keystream}=1

The ciphertext pair leaks the XOR difference between the plaintext pair.

\text{changed bytes}=1