Posted on January 21, 2019 by Abhirath Mahipal
In the context of a call things like who did you call, location, duration of the call, the number of times you called up the person etc. Anything that describes additional parameters about the content (recording of the phone call in this case) can be termed as metadata.
State of the art privacy mechanisms even go to the extent of hiding your screen size and resolution as it can be used to identify a whistleblower, journalist etc amongst a pool of their suspects. Ganesh mentioned about an interesting flaw in browsers called Canvas Fingerprinting.
Some systems offer strong privacy guarantees but do not scale. Some systems broadcast every message to every user. Other systems use Private Information Retrieval (PIR) to hide metadata. Simplest PIR technique is to send the entire database to every user who requests for information. This way observing parties will not get to know which particular information (or message from a recipient) they were trying to access.
Systems that scale do not hide metadata and a persistant adversary can easily correlate activities with a particular individual.