Rating:
TL;DR
The MP3 does not hide the flag in decoded PCM samples or ID3 metadata. The encoder stores encrypted flag bytes in the MP3 frame side-info field main_data_begin.
The supplied song.mp3 contains a sequence of 9-bit main_data_begin values. Reversing the encoder's cumulative/reset transform gives a 68-byte AES-CTR ciphertext. The AES-CTR keystream is controlled by a weak seed:
time.Now().UnixMilli() % 1024
So there are only 1024 possible keystreams. Patch the encoder to force each seed, encrypt a known plaintext of the same length, recover the keystream, and XOR it with the target ciphertext.
Initial inspection
