sigsh is a non-interactive, signature requiring and verifying command
interpreter. More accurately, it is a signature verification wrapper
around a given shell. It reads input in PKCS#7 format from standard in,
verifies the signature and, if the signature matches, pipes the decoded
input into the command interpreter.

Related:
	NetBSD's Veriexec
	MS Powershell ExecutionPolicy
	OpenBSD's "Stephanie" / TPE
		http://packetfactory.openwall.net/projects/stephanie/index.html
	Linux Trusted Path Execution