Der Spiegel has published today an excellent summary of what some of Edward Snowden's revelations show about the difficulty (or, generally, ease) with which the NSA and collaborating intelligence services can track, decrypt, and correlate different means of online communication
. An interesting slice: The NSA and its allies routinely intercept [HTTPS] connections -- by the millions. According to an NSA document, the agency intended to crack 10 million intercepted https connections a day by late 2012. The intelligence services are particularly interested in the moment when a user types his or her password. By the end of 2012, the system was supposed to be able to "detect the presence of at least 100 password based encryption applications" in each instance some 20,000 times a month.
For its part, Britain's GCHQ collects information about encryption using the TLS and SSL protocols -- the protocols https connections are encrypted with -- in a database called "FLYING PIG." The British spies produce weekly "trends reports" to catalog which services use the most SSL connections and save details about those connections. Sites like Facebook, Twitter, Hotmail, Yahoo and Apple's iCloud service top the charts, and the number of catalogued SSL connections for one week is in the many billions -- for the top 40 sites alone. ...
The NSA also has a program with which it claims it can sometimes decrypt the Secure Shell protocol (SSH). This is typically used by systems administrators to log into employees' computers remotely, largely for use in the infrastructure of businesses, core Internet routers and other similarly important systems. The NSA combines the data collected in this manner with other information to leverage access to important systems of interest.