What are look-up secrets (nachschlagbare Geheimnisse), and what requirements must they meet?
A list of (alpha)numeric values exchanged in advance between the subject and the IdP, such as a tally sheet, a TAN block or recovery codes; to authenticate, the user enters one specific value from the list. They are a single-factor, possession-based authenticator.
| Property | Look-up secret |
|---|---|
| Type | Single-factor (SF) |
| Category | Possession-based (besitzabhängig) |
| Secret value | The list of (alpha)numeric values |
| Activation value | None |
| Authentication function | Selection of the requested value |
| Output value | One (alpha)numeric value |
| Credential | The chosen value (stored hashed by the verifier) |
Requirements:
- The values must be randomly generated with enough entropy (NIST: at least six digits or equivalent).
- Each value may usually be used only once.
- The number of attempts must be limited, so codes cannot be guessed online.
Why "something you have" although they are just numbers? Nobody memorises a list of twenty random codes; the secret is the printed sheet or saved file, and whoever holds it can authenticate. Today look-up secrets mostly serve as emergency or backup codes for when the main authenticator is lost; the TAN lists once used in online banking are their best-known historical form.
Go deeper:
NIST SP 800-63B-4: Look-up secrets — generation, single use, hashing and rate limiting.- Transaction authentication number (Wikipedia) — TAN lists, indexed TANs and why banks moved on.