Some block cipher modes, such as OFB, CTR, or CFB, turn a block cipher into a stream cipher. The idea behind stream ciphers is to produce a pseudorandom keystream which is then XORed with the plaintext. One advantage of stream ciphers is that they can work of plaintext of arbitrary length, with no padding required.

OFB is an obscure cipher mode, with no real benefits these days over using CTR. This challenge introduces an unusual property of OFB.

Play at https://aes.cryptohack.org/symmetry