The 4th amendment

[intaglio]

48. You are conflating public information with private

A letter in an envelope is private, the envelope and what is written on it is public.

The text of an e-mail is private, the metadata is public.

A conversation on the phone is private, the number you dial and the number you are calling from are public. What is worse is that on a cellphone your approximate location is public information. If you use a smartphone then it is likely that your precise location is public (assuming you have agreed to such tracking data).

If you write or phone or e-mail a drug dealer then there will be probable cause for investigators to read your letter or e-mail or to listen to the phone call. If you have encrypted or scrambled that particular letter, e-mail or voice then the investigators have the right to hold a record of that communication until such time as it can be decrypted.

If you display the numberplate on your car many automatic numberplate recognition systems can track your vehicle, and will alert the police or the DMV if there are violations - the number on the plate is public information.