Your adversary is the person or organization attempting to undermine your security goals. Adversaries can be different, depending on the situation. For instance, you may worry about criminals spying on the network at a cafe, or your classmates logging into your accounts on a shared computer at a school. Often the adversary is hypothetical.


Cookies are a web technology that let websites recognize your browser. Cookies were originally designed to allow sites to offer online shopping carts, save preferences, or keep you logged on to a site. They also enable tracking and profiling so sites can recognize you and learn more about where you go, which devices you use, and what you are interested in – even if you don't have an account with that site, or aren't logged in.


Any kind of information, typically stored in a digital form. Data can include documents, pictures, keys, programs, messages, and other digital information or files.

Internet filtering

Also known as blocking or censoring Internet traffic. Virtual Private Networks or services like Tor are sometimes used to access Internet communications that would otherwise be filtered.

IP address

A device on the Internet needs its own address to receive data, just like a home or business needs a street address to receive physical mail. This address is its IP (Internet Protocol) address. When you connect to a website or other server online, you usually reveal your own IP address. This doesn't necessarily reveal either your identity (it's hard to map an IP address to a real address or a particular computer). An IP address can give away some information about you, however, such as your rough location or the name of your Internet Service Provider. Services like Tor let you hide your IP address, which helps give you anonymity online.


Metadata (or "data about data") is data that describes a piece of information, apart from the information itself. So the content of a message is not metadata, but who sent it, when, where from, and to whom, are all examples of metadata. Legal systems often protect content more than metadata: for instance, in the United States, law enforcement needs a warrant to listen to a person's telephone calls, but claims the right to obtain the list of who you have called far more easily. However, metadata can often reveal a great deal, and will often need to be protected as carefully as the data it describes.


In computer security, a threat is a potential event that could undermine your efforts to defend your data. Threats can be intentional (conceived by attackers), or they could be accidental (you might leave your computer turned on and unguarded).


A way of thinking about the sorts of protection you want for your data so you can decide which potentional threats you are going to take seriously. It's impossible to protect against every kind of trick or adversary, so you should concentrate on which people might want your data, what they might want from it, and how they might get it. Coming up with a set of possible threats you plan to protect against is called threat modeling or assessing your risks.

Zero day

A flaw in a piece of software or hardware that was previously unknown to the maker of the product. Until the manufacturers become aware of the flaw and fixes it, attackers can use it for their own purposes.