An authenticator is a means used to confirm a user's identity, that is, to perform digital authentication. A person authenticates to a computer system or application by demonstrating that he or she has possession and control of an authenticator. In the simplest case, the authenticator is a common password. Using the terminology of the NIST Digital Identity Guidelines, the party to be authenticated is called the claimant while the party verifying the identity of the claimant is called the verifier. When the claimant successfully demonstrates possession and control of one or more authenticators to the verifier through an established authentication protocol, the verifier is able to infer the claimant's identity. Classification Authenticators may be characterized in terms of secrets, factors, and physical forms. Authenticator secrets Every authenticator is associated with at least one secret that the claimant uses to demonstrate possession and control of the authenticator. Since an attacker could use this secret to impersonate the user, an authenticator secret must be protected from theft or loss. The type of secret is an important characteristic of the authenticator. There are three basic types of authenticator secret: a memorized secret and two types of cryptographic keys, either a symmetric key or a private key. Memorized secret A memorized secret is intended to be memorized by the user. A well-known example of a memorized secret is the common password, also called a passcode, a passphrase, or a personal identification number (PIN). An authenticator secret known to both the claimant and the verifier is called a shared secret. For example, a memorized secret may or may not be shared. A symmetric key is shared by definition. A private key is not shared. An important type of secret that is both memorized and shared is the password. In the special case of a password, the authenticator is the secret. Cryptographic key A cryptographic authenticator is one that uses a cryptographic key. Depending on the key material, a cryptographic authenticator may use symmetric-key cryptography or public-key cryptography. Both avoid memorized secrets, and in the case of public-key cryptography, there are no shared secrets as well, which is an important distinction. Examples of cryptographic authenticators include OATH authenticators and FIDO authenticators. By way of counterexample, a password authenticator is not a cryptographic authenticator. See the #Examples section for details. Symmetric key A symmetric key is a shared secret used to perform symmetric-key cryptography. The claimant stores