$[$Protection and Security: $15\%]$

$(1)$ What is the principle of least privilege? How does Sun Microsystems OS implement it$?(4\%)$

$(2)$ Why language$-$based protection may not be secure$?3\%)$

$(3)$ How do polymorphic and tunneling viruses bypass the detection of antivirus software? $(4\%)$

$(4)$ How does the digital$-$signature algorithm work? $(2\%)$

$(5)$ How does DDoS work? $(2\%)$