Key principles of software design $($first noted in $1974$ paper by Jerome Saltzer and Michael Schroeder $($The Protection of Information in Computer Systems$)$ and in the SafeCode paper$)$ include $($select all that apply$)$:

$1.$ Least privilege

$2.$ Fail safe defaults

$3.$ Complete mediation

$4.$ Threat modeling

$5.$ Economy of mechanism