> ## Documentation Index
> Fetch the complete documentation index at: https://docs.adcontextprotocol.org/llms.txt
> Use this file to discover all available pages before exploring further.

# Principals & Security

A **Principal** represents a distinct client or buyer in AdCP. Sales agents use principals to identify who is making requests and enforce data isolation between different buyers.

## Authentication

All requests must be authenticated using a bearer token in the standard `Authorization` header:

```
Authorization: Bearer <your_token>
```

The server validates this token and associates it with a specific `principal_id`. All subsequent operations within that request are scoped to that authenticated principal.

See [Authentication](/dist/docs/2.5.3/reference/authentication) for details on obtaining credentials and authentication methods.

### The Principal Model

A Principal is defined by:

* **`principal_id`** (string): A unique identifier for the client (e.g., `"acme_corp"`).
* **`platform_mappings`** (object): Maps the `principal_id` to identifiers in ad serving platforms (e.g., `{"gam_advertiser_id": 12345}`).

## Data Isolation

Authentication provides the foundation for strict data isolation. Sales agents **MUST** enforce the following rules:

1. When an object like a `MediaBuy` is created, it **MUST** be permanently associated with the `principal_id` from the authenticated request context.
2. For any subsequent request to read or modify that object, the server **MUST** verify that the `principal_id` from the request matches the `principal_id` stored with the object.
3. If the IDs do not match, the server **MUST** return a permission denied error.

This model ensures that one principal can never view or modify another principal's data, as they will not possess the correct bearer token to do so. Passing a `principal_id` in the request body is not required or respected; the identity is based solely on the validated token.

## Security Requirements

### Required Security Measures

Sales agent implementations **MUST**:

* Validate bearer tokens on every authenticated request
* Enforce principal-based data isolation
* Use TLS for all communications
* Log authentication failures for security monitoring

### Recommended Security Measures

Sales agent implementations **SHOULD**:

* Implement rate limiting per principal
* Support token expiration and refresh
* Provide audit logging for compliance
* Support IP allowlisting for high-security principals