tl;dr:1. Notice repeating pattern in place of palette, which suggests zeros2. Notice key is repeated many times3. Notice that you can unxor the key from the flag using the above4. Notice you can brute-force decryption, by encrypting `00000A`, `000000B`... and comparing with ciphertext5. Brute-force the number of flags looking for `tRNS` and `IDAT` in decrypted data6. Brute-force entire flag7. Recover PNG
Full writeup: https://github.com/p4-team/ctf/tree/master/2019-11-16-asis-finals/true_zero