Wiretapping and Cryptography Today:
[a government eavesdropper can] exploit the fact that the cleartext (or the keys to decrypt it) for almost all encrypted traffic today is typically available, somewhere, on a general-purpose computer that is exposed to government access, either explicitly or through surreptitious means. And as systems become more sophisticated and incorporate more features, the exposure of cleartext and keys to third party access tends to increase correspondingly.
Take, for example, that most ubiquitous instrument of criminal (and legitimate) communication, the cellular phone. In the 1990's, most cellular calls were transmitted over the air as unencrypted analog signals, easily intercepted, by police and curious neighbor alike, with an inexpensive radio receiver. Today cellular signals are almost always encrypted, making over-the-air interception a losing proposition. But the 2010 wiretap reports tells us that the majority of law enforcement wiretaps were for cellular calls, and that encryption was not a barrier. This is because, by 2010, investigators had moved on from over-the-air interception. They found ways instead to tap cellphones at the endpoint where plaintext is available (in the cellular phone company where the call connects to the wireline network).