ADR-0016: MFA is a user preference; workloads opt in. Close NK-WP-0037.

- The operator's Vergabe recipient journey completed after the KeyCape
  fresh-login fix: sign-in, callback and account confirmation.
- ADR-0016 records MFA as user preference by default, workload-requested
  step-up for all or part of a feature set, and an unchanged IAM v0.3 floor.
- NK-WP-0042 (proposed) plans the step-up contract and user journey.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 299762@bnt-lap001
Assistant-Session: d3d3cea1-869c-44f1-be2a-3d6d3550e72e

docs/adr/ADR-0016-mfa-user-preference-and-workload-step-up.md

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+---
+id: NK-ADR-0016
+type: architecture-decision-record
+title: "MFA Is a User Preference by Default; Workloads Opt In to Requiring It"
+status: accepted
+owner: net-kingdom
+revision: "1"
+decided: "2026-09-23"
+last_reviewed: "2026-09-23"
+review_interval: 12m
+---
+
+# ADR-0016 - MFA Is a User Preference by Default; Workloads Opt In to Requiring It
+
+**Status:** Accepted
+**Date:** 2026-09-23
+**Deciders:** Bernd Worsch, Claude
+
+## Context
+
+The attended Vergabe recipient journey on 2026-09-23 (NK-WP-0036-T05,
+NK-WP-0037-T02) completed sign-in and account confirmation without MFA.
+KeyCape policy P06 (`key-cape/docs/optional-mfa.md`, deployed 2026-09-14 for
+`vergabe-demo-company` and `user-engine-portal`) allowed this. Under P06, AAL1
+is permitted when privacyIDEA authoritatively reports no active factor, and
+AAL2 is required once one exists.
+
+NK-WP-0037 had been written on 2026-09-12 as "provider-required MFA". The
+operator decided against requiring MFA for Vergabe Teilnahme now. The
+enrollment and recovery journeys (user-engine U06, factor recovery) have not
+yet been accepted from a user's perspective, and requiring MFA before they
+work pushes users into support paths.
+
+## Decision
+
+1. **Default: MFA is the user's preference.** For ordinary human sign-in to a
+   workload, MFA is offered but not required. A user who activates a factor
+   is held to it: later sign-ins require AAL2, and a prior AAL1 session does
+   not satisfy it (P06 semantics).
+2. **Workloads opt in to requiring MFA,** for all of their features or for
+   some of them. A workload states the requirement explicitly, either per
+   client registration or per request as a step-up (an explicit AAL2 request
+   at the moment a protected feature is used). An explicit workload
+   requirement always wins over a user's preference. Authorization decisions
+   read it from the normalized `assurance.level` claim (IAM Profile v0.3),
+   never from a provider-native claim.
+3. **The IAM Profile v0.3 floor is unchanged.** Privileged, destructive,
+   platform-root, secret, credential-vending and emergency flows require AAL2
+   or stronger, and administration requires MFA even where ordinary sign-in
+   does not. This ADR relaxes nothing below that floor.
+4. **Usability comes before a mandatory requirement.** The enrollment,
+   re-enrollment and recovery journeys must be accepted from a user's
+   perspective before any workload's default sign-in is switched to
+   required MFA.
+
+## Consequences
+
+- Vergabe Teilnahme keeps P06 optional MFA. NK-WP-0037's "provider-required
+  MFA" wording is superseded.
+- A workload that needs MFA for part of its feature set needs a step-up path:
+  it requests AAL2 when a protected feature is used, and KeyCape answers with
+  an MFA challenge that its existing session cannot bypass. The KeyCape and
+  application contract for that step-up is follow-up work (NK-WP-0042,
+  proposed).
+- The user-engine enrollment journey (U06) and factor recovery are
+  prerequisites for any workload making MFA mandatory by default.
+- A per-client `mfaRequired: false` remains unconditional AAL1 and is not a
+  preference mechanism. Use P06 `mfaOptional` for preference-based clients.
+
+## Related
+
+- IAM Profile v0.3 (Human Interactive Flow; Assurance Evidence)
+- `key-cape/docs/optional-mfa.md` (P06)
+- NK-WP-0036, NK-WP-0037, NK-WP-0041
+- user-engine account journeys U06 (voluntary authenticator activation)