In 11.5, Blizzard has introduced APIs to perform Base64 encoding/decoding ¹. Rarity has its own Lua version, which is somewhat slow. Note: The speed of encoding/decoding item strings isn't a real concern because most strings are short and it doesn't happen while playing.

Nevertheless, if the implementations are compatible it might be worth considering to let Blizzard handle this task. I don't know if their C API can really handle large volumes of data (and there may be some limits in place), but if nothing else it would be interesting to explore the options.

(The encoding scheme could certainly be made more efficient, but due to backwards compatibility breaking changes aren't feasible, nor needed)