Java Applet random access storage

Question

I have a java project that uses java.io.RandomAccessFile to manage data loading. It seeks through the file creating a map of key points which can then be loaded as needed later. This works great.

I want to make it run as an applet but it requires security permissions to create a temp file that a downloaded file could be stored in, and that's a huge barrier for it's intended usage.

I think I can spare the memory (a few MB) to store the contents in a memory buffer of some sort and then random access it in the same way I treat local files...

Is there a way to create a temp file without requiring security permissions (I assume not)?

What is the best buffering option? How would I get the contents of a URL based input stream into the buffer, read bytes from it, and be able to record and change the current seek position?