Since 1968, federal law has prohibited the use of bugging devices — secret microphones — to record private conversations. Here’s the relevant text:
[A]ny person who . . . intentionally intercepts, endeavors to intercept, or procures any other person to intercept or endeavor to intercept, any . . . oral . . . communication [is guilty of a crime and commits a civil violation] [18 U.S.C. 2511(1)(a)]
Here are the definitions of two key terms, “oral communication” and “intercept”:
“oral communication” means any oral communication uttered by a person exhibiting an expectation that such communication is not subject to interception under circumstances justifying such expectation [18 U.S.C. 2510(2)]
“intercept” means the aural or other acquisition of the contents of any . . . oral communication through the use of any electronic, mechanical, or other device [18 U.S.C. 2510(4)]
The basic idea is to criminalize listening in on someone’s private conversation using a recording device. The law applies both to the government and to private parties, and it provides for both criminal and civil remedies. On the whole, it’s a sensible criminal and civil law.