# What is an Identity Provider (IdP)?

> An identity provider is the system that authenticates users and issues proof of identity that other applications trust — the central sign-in behind SSO and federation.

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## Content

Instead of every application checking passwords itself, applications redirect users to the IdP, which verifies them and returns a token or assertion (SAML, OIDC, Kerberos tickets). The IdP concentrates authentication policy — MFA, conditional access, session rules — in one control point for many services.

### Key points

- **Critical-infrastructure protection:** Compromise of the IdP or its signing keys yields tokens for every connected application; harden, monitor, and restrict it accordingly.
- **Federation standards:** Prefer OIDC or SAML federation over local accounts per application so policy, offboarding, and MFA apply consistently.
- **Important limitation:** Centralization concentrates both control and risk. An IdP outage or compromise affects everything that trusts it, and it cannot see what users do inside each application after tokens are issued.

### Related terms

[Federated identity](<https://yellowcube.eu/glossary/federated-identity/>) · [Single sign-on (SSO)](<https://yellowcube.eu/glossary/single-sign-on/>) · [OpenID Connect (OIDC)](<https://yellowcube.eu/glossary/openid-connect/>) · [Security Assertion Markup Language (SAML)](<https://yellowcube.eu/glossary/security-assertion-markup-language/>) · [Authentication](<https://yellowcube.eu/glossary/authentication/>)

### Sources

[NIST SP 800-63C-4, Digital Identity Guidelines: Federation and Assertions](https://csrc.nist.gov/pubs/sp/800/63/c/4/final) · [IDPro, Body of Knowledge](https://bok.idpro.org/)

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