Authentication is successfully validating that an entity you are communicating with is actually who they claim to be. The entity typically demonstrates who they are by showing they possess a secret that only they could know.

The entity is not necessarily proving their identity, they could be proving they are a member of a larger group of identities or possess a certain attribute.

Authentication techniques are usually classified as belong to one of three categories (called factors): something the entity knows (password or key), something the entity has (a token or smartcard), or something the entity is (biometrics).

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