Eli5: what is a secure phone line?


In spy movies they ask, if one is on a secure line? What is that? Can Theoretically any communication channel could be eavesdropped?

In: 20

Secure line typically means that it is encrypted in some way. Even if someone is listening directly into the call via a tap, all they would hear is gibberish.

Back when phones were analog, you could put an encrypted scrambler on the phone and again it would make what ever you say sounding like a buzzing.

Now a days, you can do similar stuff with software.

Now with the Patriot act. The U.S. Collects a ton of voice and text message data, but it is almost impossible to weed through it all.

Another definition of a secure line is picking up a burner phone or using a phone that the chances of it being tapped are zero. Like borrowing a phone at a gas station in the middle of nowhere.

Hence why it is always better to bug the room someone is calling from then the device itself.

Although you don’t get both sides of the conversation, you at least get enough information to make a professional judgement.

u/craigworknova covers the ELI5 pretty well, but I have made calls on the STU-III and used them in secure mode. Ours was the version with a key you had to turn to initiate the secure mode.



In the context of U.S. government/military, I don’t know about the details other than it’s run through a completely different network, just like how classified data runs through the SIPRNet, a network not connected to the rest of the internet.

The thing is that nobody would ask if the line were secure, because they would already know that, because they’d have to use a special phone, on a special network, in a special room.

What’s annoying is if you get a call on a secure phone, then everyone else in the room has to first hang-up any non-secure phone calls before you can answer the secure one, so an unsecure microphone doesn’t potentially overhear any secure conversations.

It means the channel is encrypted.

The people on each end of the line have a unique ‘key.’ This key is secret and you can’t share it with anyone. Before the message is sent across the line, it is scrambled into gibberish and the only way to unscramble it is by using the key either you or the other person have.

So hypothetically, if someone intercepted the message, all it would be is a big pile of gibberish and they have no way of knowing what it actually says without the key to ‘unlock’ it.