# NIST SP 800-63-4

## Source Type

Government guideline. NIST Special Publication 800-63-4, Digital Identity
Guidelines (2025).

## Domain

Identity assurance, authentication assurance, federation assurance, and
identity lifecycle.

## Why This Source Matters

NIST identity guidance separates identity proofing, authentication assurance,
federation assurance, and lifecycle management.

NIST 800-63 provides the most explicit assurance-level vocabulary for
separating how strongly an identity is bound to a person, how strongly
authentication occurred, and how federation preserves or degrades assurance.

## Key Concepts

- **IAL (Identity Assurance Level)**: confidence that a subscriber is who they
  claim to be (IAL1–IAL3).
- **AAL (Authenticator Assurance Level)**: confidence in authentication
  mechanism strength (AAL1–AAL3).
- **FAL (Federation Assurance Level)**: confidence in federation protocol and
  assertion protection (FAL1–FAL3).
- **Subscriber**: party enrolled with a CSP (credential service provider).
- **Credential Service Provider (CSP)**: issues credentials and performs
  identity proofing.
- **Relying Party (RP)**: depends on CSP assertions.
- **Identity Provider (IdP) / Asserting Party (AP)**: federates authentication
  to RPs.
- **Verifier**: entity confirming claimant possession of authenticator.
- **Binding**: association between subscriber, identity, and authenticator.
- **Identity proofing**: collection and validation of evidence about a person.
- **Authenticator**: something the subscriber possesses/controls for auth.
- **Federation assertion**: signed statement from IdP to RP about
  authentication and attributes.

## Relevant Terminology

| Term | Source meaning |
| --- | --- |
| Subscriber | Enrolled party at CSP; not necessarily named "user." |
| CSP | Provider performing proofing and credential issuance. |
| RP | Consumer of identity/authentication assertions. |
| IAL | Identity proofing strength level. |
| AAL | Authentication event strength level. |
| FAL | Federation protocol/assertion protection level. |
| Claimant | Party attempting authentication. |
| Verifier | Confirms authenticator use. |
| Binding | Link between subscriber identity and authenticator. |
| Supervised remote proofing | IAL2+ proofing with human or tech supervision. |

## Modeling Assumptions

- **Identity assurance, authentication, and federation are separable**
  dimensions with independent levels.
- **Subscriber is the enrolled entity**, not the legal person directly; binding
  connects them.
- **Proofing evidence matters** and should be retained per policy.
- **Federation may preserve or reduce assurance** depending on FAL and
  assertion contents.
- **Lifecycle includes enrollment, binding, maintenance, and termination.**
- **Pseudonymous enrollment is allowed** at IAL1 without real-name binding.
- **Agency/customer relationship** is outside the technical model but affects
  policy.

## Identity-Canon Implications

- NIST **Subscriber** maps to **Account** or enrolled **Identity Record**
  bound to **Natural Person** at higher IAL.
- **IAL** maps to **Assurance Level** on Identity Record / Person binding.
- **AAL** maps to **Assurance Level** on authentication event / Credential use.
- **FAL** maps to **Assurance Level** on **Trust Relationship** or federation
  assertion.
- **Binding** maps to **Synonymity Assertion** or **Identifier Binding**
  between subscriber, person, and authenticator.
- **Identity proofing evidence** maps to **Evidence Source**.
- Reinforces **P7** (synonymity as assertion) and **P8** (preserve evidence).
- Supports S12 (weak match insufficient for IAL2+), S13 (strong link with
  verification), S06 (family/guardian proofing).

## Terminology Conflicts

- **Subscriber vs. User**: NIST subscriber is enrolled party; apps say user.
- **Identity vs. Subscriber**: NIST separates identity proofing from
  subscriber record.
- **Credential vs. Authenticator**: NIST distinguishes credential (issued)
  from authenticator (possessed); products conflate.
- **IAL vs. Account trust**: assurance on person binding ≠ account permissions.
- **Federation vs. Synonymity**: federation assertion ≠ same-person claim
  across systems.

## Candidate Canonical Mappings

| NIST concept | Candidate canonical concept |
| --- | --- |
| Subscriber | Account / enrolled Identity Record |
| CSP / IdP | Issuer Scope + Trust Relationship |
| RP | Relying party Scope |
| IAL | Assurance Level (identity proofing) |
| AAL | Assurance Level (authentication) |
| FAL | Assurance Level (federation) |
| Authenticator | Credential |
| Binding | Identifier Binding / Synonymity Assertion |
| Proofing evidence | Evidence Source |
| Federation assertion | Claim + Credential (signed) |
| Claimant | Actor attempting authentication (projection) |

## Open Questions

- Should IAL/AAL/FAL be a unified Assurance Level vocabulary or three
  orthogonal dimensions in canon?
- How should pseudonymous IAL1 subscribers map when no Natural Person binding
  exists?
- Does guardian-assisted proofing for minors warrant a distinct Relationship
  type with assurance caps?
- Should CSP subscriber ID be a Scoped Identifier under CSP Scope?

## References

- NIST SP 800-63-4 — https://pages.nist.gov/800-63-4/
- NIST SP 800-63A (Enrollment and Identity Proofing) — https://pages.nist.gov/800-63-4/sp800-63A.html
- NIST SP 800-63B (Authentication and Lifecycle) — https://pages.nist.gov/800-63-4/sp800-63B.html
- NIST SP 800-63C (Federation and Assertions) — https://pages.nist.gov/800-63-4/sp800-63C.html