Agencies like the NSA are currently able to eavesdrop on anyone with few restrictions only - though other messages are spread by the NSA.
Theoretically cryptography can make that difficult. Hence those agencies speak up for actions like introducing trapdoors to make it possible to get access to everybody's data.
See the U.S. discussion about the Clipper Chip some years ago:
While encryption offers us privacy for the transmission of data, we do not only wish to have it but also need it if we want to transport data which shall not be seen by anyone else but the recipient of our message. Given this, the governments and governmental institutions/organizations fear to lose control. Strict laws are the consequence. The often repeated rumor that the Internet was a sphere of illegality has been proven wrong. Some parts are controlled by law very clearly. One of them is cryptography. Prohibition of cryptography or at least its restriction are considered an appropriate tool against criminality. Or one should say: had been considered that. In the meantime also governmental institutions have to admit that those restrictions most of all work against the population instead against illegal actors. Therefore laws have been changed in many states during the last five years. Even the USA, the Master of cryptography-restriction, liberated its laws in December 1999 to be more open-minded now.
for an insight into the discussion having gone on for years see:
the final text of the new U.S. Encryption Regulations you will find under:
an explanation of the regulations can be found under: