Interview with LulzSec's Topiary
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I've often wondered how often this happens when law enforcement are trying to catch child pornographers and such.
Or maybe they just don't waste - or admit to - their resources for such low profile targets?
Bingo, it's all about cost-effectiveness. The NSA likely has a large variety of SSL certificate keys and other keys for decrypting things, and the continuing usage of them depends on their perceived security.
They can use all of their decryption keys to read everything and provide it as evidence to the courts, but it'll tip the world off that some keys aren't secure and the NSA's targets would defend themselves accordingly.
It's a catch-22 -- they need to use decryption keys for gathering evidence for convictions, but there is such a thing as using too many. They're a resource that takes longer to build up than CPU/storage/memory/network IO.