government email / security

So I had a question that sort of branches off from the Hillary thread, but deserves a discussion of its own.

In the USA, how do we - as a republic - both secure our government email communications AND preserve them for recordkeeping / FOIA requests / historical archiving?

I'm a tech guy so this crosses my mind a lot.  Don't worry, this isn't a question about the tech, but rather it's about how we handle government oversight.

In the civilian world, on the simple side, most of us email back and forth and things sit on a Microsoft or Yahoo or Google server.  Maybe your ISP's server.  In any case it is all plain text and anyone in the system can read it.  If the FBI subpoenas one of those companies, they just hand it all over.  If someone hacks it, they can read everything.

If you want to secure your email, you can set up a public/private key and encrypt.  In a nutshell, only you and the recipient can read the contents of that email.  Done properly, it is pretty robust and safe.

Now look at the government context.  Let's take someone like the Secretary of State.  Lots of email correspondence with lots of different government officials.  A lot of those have multiple recipients.  My gut feeling is the government servers should have a similar setup.  Everyone gets their own key.  All the public keys go on a government server.  Anyone official correspondence gets encrypted.  Only the intended recipients can read it.  And you can verify that the person who sent it is, indeed, who you think it is.  That's the "easy" part.

So now, someone like the FBI wants to legally read a bunch of email which they were not involved with.  This means we need a master key to decrypt it.

Who owns that key?Who secures it from foreign actors?
How do we avoid abuse?

Separation of powers is a major design feature of the Constitution.  Is a master key too much power concentrated in one place?

I'm not a government official at that level, so I honestly don't know what kind of system they have in place.  This is really the NSA's wheelhouse, but they are focused on *foreign* surveillance.  So I'm asking how you would approach this.