What are authentication means (Authentifizierungsmittel), and on which factors can they rest?

Authentication means are information or processes used to authenticate a subject; they rest on knowledge (password, PIN), possession (token with private key, electronic passport), a property of the subject (biometrics) or, rarely, behaviour (dynamic signature pattern).

* The factor families an authentication means can rest on. *

A means can use a single factor (SFA) or combine several (MFA), for example password plus a one-time code sent to the phone. The standard models the output value as a function: output = f(secret, [input value], [activation data]), where the secret is a private key or password, the optional input a challenge or nonce, and the optional activation data a PIN or biometric. Authentication means are issued by the credential service. Synonyms: authentication factor, authentication feature.

Go deeper:

• NIST SP 800-63B-4 Authentication and Authenticator Management — every authenticator type defined and graded by assurance level.
• Multi-factor authentication (Wikipedia) — knowledge, possession and inherence factors and why combining them helps.