Click the different texts to decode/encode Base64. If you want to put this in your own project, go ahead! You'll need to copy over the "Base64 Engine" sprite. To use this, set the variable "Base64 I/O" to your input, and broadcast the corresponding message (base64[Encode/Decode]IO), waiting for the output. The same variable, "Base64 I/O", will be your output. In case a number list needs to be encoded, I've also exposed a way to turn individual numbers into ASCII characters and back. Use the broadcast "base64[CharacterToByte/ByteToCharacter]" to do this.
This uses costume names to work around the fact that Scratch can't distinguish between uppercase and lowercase. (This was updated 3/15/2022. Before, it used UUEncode as a workaround, but I felt I could do better.)