I recognize the similarity between phone calls and Tor; however, I too think there is a difference in the expectation of privacy.<p>I like your analogy with aerial surveillance, but partially because I think you are wrong. It is my understanding that aerial views available to the public, such as those available to a commercial helicopter, do not require a warrant. However, if the police go beyond public capabilities with high zoom cameras it may require a warrant. Similarly, if the state goes beyond the capabilities of the public in obtaining my IP address, they should require a warrant. The question then becomes, &quot;Does the CMU project constitute knowledge generally available to the public?&quot; I would argue the negative, but I could see it answered either way.