What is a look-up secret?

A look-up secret is a list of random, single-use alphanumeric values exchanged in advance between subject and IdP, with enough entropy, used as a second factor in MFA or as emergency or backup codes.

Examples: tally sheets, TAN lists, recovery codes. They must be stored securely since anyone holding the list can use it.

Go deeper:

• NIST SP 800-63B-4 Authentication and Authenticator Management — every authenticator type defined and graded by assurance level.