> For the complete documentation index, see [llms.txt](https://docs.tiun.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.tiun.io/reference/authentication/how-it-works.md). # How it works tiun identifies users without passwords or OAuth. Identity is established through **email** plus **one-time passcode (OTP)** verification, all handled by the SDK. *** ## Two entry points There are two ways a user can become authenticated: 1. **Through checkout** — authentication is built into the payment flow. The user enters their email (and completes OTP where needed) as part of checkout, so they're signed in the moment the purchase completes. 2. **Through dedicated login** — call `tiun.login()` to open the login overlay without starting a purchase. Useful for returning users who already have a subscription. *** ## Login flow * **Returning user.** They enter their email. Because a phone number is already linked to the account, the OTP is sent to their **phone via SMS**. * **New user.** They enter their **email and phone number**. The OTP is sent to the **phone** to verify and link it. * **Fallback.** If the SMS doesn't arrive, the overlay offers to resend the code to the **email** instead. After a successful flow, the user is available via `tiun.getUser()` — see [User object](/reference/authentication/user-object.md) for the shape and what each field means. *** ## Session persistence tiun manages sessions for you. On the same browser, returning visitors are recognized automatically. When the page loads and a session is restored, the `userChange` event fires with `event: 'init'` so your UI can align without extra calls. See [Subscriptions](/reference/checkout/subscriptions.md) for the full lifecycle. *** ## Logout Call `tiun.logout()` to clear the session on the current device. After logout, treat the user as unauthenticated until they sign in or complete checkout again. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation by asking a question. Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters: ``` GET https://docs.tiun.io/reference/authentication/how-it-works.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.