# ADR-001: Use OAuth 2.0 for Authentication

## Context

We need to choose an authentication protocol for our user authentication service. The system must support:
- Multiple client types (web, mobile, API)
- Third-party integrations
- Industry standard security practices

See [OAuth 2.0 RFC](https://tools.ietf.org/html/rfc6749) for specification details.

## Decision

We will use OAuth 2.0 as our authentication protocol with the following grant types:
- Authorization Code for web applications
- PKCE for mobile applications
- Client Credentials for service-to-service

## Consequences

**Positive:**
- Industry standard with extensive library support
- Flexible grant types for different use cases
- Well-understood security model

**Negative:**
- More complex than basic session-based auth
- Requires token management infrastructure
- Learning curve for developers new to OAuth 2.0

## Status

Accepted - Implementation in progress
