Documents can be verified as being created on ink or paper readily available at the time of the item's implied creation. In general, it relies on the facts that creating a forgery indistinguishable from a genuine artifact requires expert knowledge, that mistakes are easily made, and that the amount of effort required to do so is considerably greater than the amount of profit that can be gained from the forgery.
In art and antiques, certificates are of great importance for authenticating an object of interest and value.
Some antiques are accompanied by certificates attesting to their authenticity.
Signed sports memorabilia is usually accompanied by a certificate of authenticity.
A vendor selling branded items implies authenticity, while he or she may not have evidence that every step in the supply chain was authenticated.
Centralized authority-based trust relationships back most secure internet communication through known public certificate authorities; decentralized peer-based trust, also known as a web of trust, is used for personal services such as email or files (pretty good privacy, GNU Privacy Guard) and trust is established by known individuals signing each other's cryptographic key at Key signing parties, for instance.
The ways in which someone may be authenticated fall into three categories, based on what are known as the factors of authentication: something the user knows, something the user has, and something the user is.
Consumer goods such as pharmaceuticals, perfume, fashion clothing can use all three forms of authentication to prevent counterfeit goods from taking advantage of a popular brand's reputation (damaging the brand owner's sales and reputation).
As mentioned above, having an item for sale in a reputable store implicitly attests to it being genuine, the first type of authentication.
In computer science, verifying a person's identity is often required to allow access to confidential data or systems.
Authentication can be considered to be of three types: The first type of authentication is accepting proof of identity given by a credible person who has first-hand evidence that the identity is genuine.
Certificates can, however, also be forged, and the authentication of these poses a problem.