Is there a way to encrypt design under Quartus?

"what is the extension of the bitstream file?"

It's .sof, .pof, .jam, etc... depending on what tool you're using and how you want to configure the device. It's what Altera calls Programming File (generated by the Assembler by default).

"...I am wondering does there exist QXP file..."

This is how Altera does design partitioning. Refer to Altera's resources on design partitioning and floorplanning. For a start, you can use your whole entire design as a single partition and export it as a QXP partition. Later, you may want to try exporting only part of your project as a QXP partition. Check out "Project > Export Design Partition..." from the menu bar. The default is a QXP extension.

Have fun...