Microwavik: Start with the simplest ciphers that fit in a few kilobytes, like a stream cipher that can be implemented in a handful of loops. Keep the key schedule small, reuse the same buffer for input and output, and avoid dynamic allocation. Stick to bit‑wise ops, avoid floating point, and compile with optimisations for size. If you need to try a block cipher, pick something like TinyAES or an even smaller variant, and run it in a single pass over the data, feeding it from a memory‑mapped file so you never hold the whole plaintext in RAM. With only 512 KB, you’ll have to process in chunks and keep state minimal – no recursion, no large tables. That’s the only way to squeeze any cracking work into a single‑core, 64‑bit, 512‑KB box.