True, but computer time is a finite resource. Thus gubmint agencies who wish to snoop encrypted messages must pick and choose messages that are likely to be interesting.
Let's say that Mahmoud sends Ali an encrypted message that's 45,258 bytes long, and Bob sends Alice an encrypted message that's also 45,258 bytes long. Agent Smith wants to catch Mahmoud and Ali, but not waste his time on Bob and Alice. Without some prior knowledge of who sent which message to whom, Agent Smith risks seeing Mahmoud and Ali slip through his fingers because he used his computer time to decode Bob's grocery shopping list to Alice.
The lesson for the rest of us, assuming that Agent "O" is more interested in catching Americans than he is catching Mahmoud and Ali, is to encrypt everything. OK Agent "O", which message is the plans for the secret tea party meeting and which is Sarah Palin's Møøsë pie recipe?
“True, but computer time is a finite resource. Thus gubmint agencies who wish to snoop encrypted messages must pick and choose messages that are likely to be interesting.”
Thats why they use key words. By focusing on only a few words it allows them to monitor everything. When the flags show they dig deeper.
The phone network has been modified to allow them to do this. I’m guessing the internet has also since everything goes through core routers at some point.