Base64

Decoding a multi-layer encoding chain: Base64 followed by Base91.

Extracting a flag hidden inside a PDF stream using FlateDecode and base64.

A 4.9 MB rules PDF hides 360 fake 1×1 image XObjects holding base64 chunks that reassemble into an encrypted ZIP; the password is hidden in the rules text, inside is a Singularity .sif whose squashfs holds the flag.