Rule 21 allows any senator at his own discression to take the senate into secret session as long as his request has one second.
It should be pretty obviuos that a senator and a pal could take the senate into secret session and then filibuster on an issue that requires secrecy. They could just debate national security.
But as i read rule 22 the only way to stop rule 21 is with a cloture vote on a request to delay consideration indefinitely . And that takes 16 senators to reqest a cloture vote, and 60 senators to pass the cloture.
The senate rules are fairly simple. they can be printed in just 61 pages. They are very permissive and allow members to get away with tons of stuff not allowed in the house.
For most of our history the Senate has been the most exculsive club in the world. And yes all members were club members. The realigment of the parties has changed that to a place where all senators will have to carry a club.