<br />

This guide builds on the [learn the core syntax of the Firebase Security Rules language](https://firebase.google.com/docs/storage/security/core-syntax) guide
to show how to add conditions to your Firebase Security Rules for Cloud Storage.

The primary building block of Cloud Storage Security Rules is the **condition** . A
condition is a boolean expression that determines whether a particular operation
should be allowed or denied. For basic rules, using `true` and `false` literals
as conditions works perfectly well. But the Firebase Security Rules for Cloud Storage
language gives you ways to write more complex conditions that can:

- Check user authentication
- Validate incoming data

## Authentication

Firebase Security Rules for Cloud Storage integrates with Firebase Authentication to provide
powerful user based authentication to Cloud Storage. This allows for
granular access control based on claims of a Firebase Authentication token.

When an authenticated user performs a request against Cloud Storage,
the `request.auth` variable is populated with the user's `uid`
(`request.auth.uid`) as well as the claims of the Firebase Authentication JWT
(`request.auth.token`).

Additionally, when using custom authentication, additional claims are surfaced
in the `request.auth.token` field.

When an unauthenticated user performs a request, the `request.auth` variable is
`null`.

Using this data, there are several common ways to use authentication to secure
files:

- Public: ignore `request.auth`
- Authenticated private: check that `request.auth` is not `null`
- User private: check that `request.auth.uid` equals a path `uid`
- Group private: check the custom token's claims to match a chosen claim, or read the file metadata to see if a metadata field exists

### Public

Any rule that doesn't consider the `request.auth` context can be considered a
`public` rule, since it doesn't consider the authentication context of the user.
These rules can be useful for surfacing public data such as game assets, sound
files, or other static content.  

```gdscript
// Anyone to read a public image if the file is less than 100kB
// Anyone can upload a public file ending in '.txt'
match /public/{imageId} {
  allow read: if resource.size < 100 * 1024;
  allow write: if imageId.matches(".*\\.txt");
}
```

### Authenticated private

In certain cases, you may want data to be viewable by all authenticated users of
your application, but not by unauthenticated users. Since the `request.auth`
variable is `null` for all unauthenticated users, all you have to do is check
the `request.auth` variable exists in order to require authentication:  

```scilab
// Require authentication on all internal image reads
match /internal/{imageId} {
  allow read: if request.auth != null;
}
```

### User private

By far the most common use case for `request.auth` will be to provide individual
users with granular permissions on their files: from uploading profile pictures
to reading private documents.

Since files in Cloud Storage have a full "path" to the file, all it takes
to make a file controlled by a user is a piece of unique, user identifying
information in the filename prefix (such as the user's `uid`) that can be
checked when the rule is evaluated:  

```gdscript
// Only a user can upload their profile picture, but anyone can view it
match /users/{userId}/profilePicture.png {
  allow read;
  allow write: if request.auth.uid == userId;
}
```

### Group private

Another equally common use case will be to allow group permissions on an object,
such as allowing several team members to collaborate on a shared document. There
are several approaches to doing this:

- Mint a Firebase Authentication [custom token](https://firebase.google.com/docs/auth/admin/create-custom-tokens) that contains additional information about a group member (such as a group ID)
- Include group information (such as a group ID or list of authorized `uid`s) in the [file metadata](https://firebase.google.com/docs/storage/web/file-metadata)

Once this data is stored in the token or file metadata, it can be referenced
from within a rule:  

```mysql
// Allow reads if the group ID in your token matches the file metadata's `owner` property
// Allow writes if the group ID is in the user's custom token
match /files/{groupId}/{fileName} {
  allow read: if resource.metadata.owner == request.auth.token.groupId;
  allow write: if request.auth.token.groupId == groupId;
}
```

## Request Evaluation

Uploads, downloads, metadata changes, and deletes are evaluated using the
`request` sent to Cloud Storage. In addition to the user's unique ID and
the Firebase Authentication payload in the `request.auth` object as described above,
the `request` variable contains the file path where the request is being
performed, the time when the request is received, and the new `resource` value
if the request is a write.

The `request` object also contains the user's unique ID and the
Firebase Authentication payload in the `request.auth` object, which will be
explained further in the [User-Based Security](https://firebase.google.com/docs/storage/security/rules-conditions#authentication)
section of the docs.

A full list of properties in the `request` object is available below:

| Property | Type | Description |
|---|---|---|
| `auth` | map\<string, string\> | When a user is logged in, provides `uid`, the user's unique ID, and `token`, a map of Firebase Authentication JWT claims. Otherwise, it will be `null`. |
| `params` | map\<string, string\> | Map containing the query parameters of the request. |
| `path` | path | A `path` representing the path the request is being performed at. |
| `resource` | map\<string, string\> | The new resource value, present only on `write` requests. |
| `time` | timestamp | A timestamp representing the server time the request is evaluated at. |

| **Note:** Detailed documentation for these properties is available in the [Firebase Security Rules for Cloud Storage API
| reference](https://firebase.google.com/docs/reference/security/storage#request).

## Resource Evaluation

When evaluating rules, you may also want to evaluate the metadata of the file
being uploaded, downloaded, modified, or deleted. This allows you to create
complex and powerful rules that do things like only allow files with certain
content types to be uploaded, or only files greater than a certain size to be
deleted.

Firebase Security Rules for Cloud Storage provides file metadata in the `resource`
object, which contains key/value pairs of the metadata surfaced in a
Cloud Storage object. These properties can be inspected on `read` or
`write` requests to ensure data integrity.

On `write` requests (such as uploads, metadata updates, and deletes), in
addition to the `resource` object, which contains file metadata for the file
that currently exists at the request path, you also have the ability to use the
`request.resource` object, which contains a subset of the file metadata to be
written if the write is allowed. You can use these two values to ensure data
integrity or enforce application constraints such as file type or size.

A full list of properties in the `resource` object is available below:

| Property | Type | Description |
|---|---|---|
| `name` | string | The full name of the object |
| `bucket` | string | The name of the bucket this object resides in. |
| `generation` | int | The [Google Cloud Storage object generation](https://cloud.google.com/storage/docs/object-versioning) of this object. |
| `metageneration` | int | The [Google Cloud Storage object metageneration](https://cloud.google.com/storage/docs/object-versioning) of this object. |
| `size` | int | The size of the object in bytes. |
| `timeCreated` | timestamp | A timestamp representing the time an object was created. |
| `updated` | timestamp | A timestamp representing the time an object was last updated. |
| `md5Hash` | string | An MD5 hash of the object. |
| `crc32c` | string | A crc32c hash of the object. |
| `etag` | string | The etag associated with this object. |
| `contentDisposition` | string | The content disposition associated with this object. |
| `contentEncoding` | string | The content encoding associated with this object. |
| `contentLanguage` | string | The content language associated with this object. |
| `contentType` | string | The content type associated with this object. |
| `metadata` | map\<string, string\> | Key/value pairs of additional, developer specified custom metadata. |

`request.resource` contains all of these with the exception of `generation`,
`metageneration`, `etag`, `timeCreated`, and `updated`.
| **Note:** Detailed documentation for these properties is available in the [Firebase Security Rules for Cloud Storage API
| reference](https://firebase.google.com/docs/reference/security/storage#resource).

## Enhance with Cloud Firestore

You can access documents in Cloud Firestore to evaluate other authorization
criteria.

Using the `firestore.get()` and `firestore.exists()` functions, your security
rules can evaluate incoming requests against documents in Cloud Firestore.
The `firestore.get()` and `firestore.exists()` functions both expect fully
specified document paths. When using variables to construct paths for
`firestore.get()` and `firestore.exists()`, you need to explicitly escape
variables using the `$(variable)` syntax.
| **Note:** Like any other access to Cloud Firestore documents, reads of documents from Storage Security Rules count towards your project's Firestore quota and billing.
| **Warning:** Storage Security Rules can only access documents from the default Cloud Firestore database when [multiple databases](https://firebase.google.com/docs/firestore/manage-databases) are active.

In the example below, we see a rule that restricts read access to files to those
users who are members of particular clubs.  

```text
service firebase.storage {
  match /b/{bucket}/o {
    match /users/{club}/files/{fileId} {
      allow read: if club in
        firestore.get(/databases/(default)/documents/users/$(request.auth.id)).data.memberships
    }
  }
}
```
In the next example, only a user's friends can see their photos.  

```text
service firebase.storage {
  match /b/{bucket}/o {
    match /users/{userId}/photos/{fileId} {
      allow read: if
        firestore.exists(/databases/(default)/documents/users/$(userId)/friends/$(request.auth.id))
    }
  }
}
```

Once you create and save your first Cloud Storage Security Rules that use these Cloud Firestore
functions, you'll be prompted in the Firebase console or Firebase CLI to
enable permissions to connect the two products.

You can disable the feature by removing an IAM role, as described in
[Manage and deploy Firebase Security Rules](https://firebase.google.com/docs/rules/manage-deploy#manage_permissions_for_cross-service).

## Validate data

Firebase Security Rules for Cloud Storage can also be used for data validation, including
validating file name and path as well as file metadata properties such as
`contentType` and `size`.  

```gdscript
service firebase.storage {
  match /b/{bucket}/o {
    match /images/{imageId} {
      // Only allow uploads of any image file that's less than 5MB
      allow write: if request.resource.size < 5 * 1024 * 1024
                   && request.resource.contentType.matches('image/.*');
    }
  }
}
```

## Custom functions

As your Firebase Security Rules become more complex, you may want to wrap sets of
conditions in functions that you can reuse across your ruleset. Security rules
support custom functions. The syntax for custom functions is a bit like JavaScript,
but Firebase Security Rules functions are written in a domain-specific language
that has some important limitations:

- Functions can contain only a single `return` statement. They cannot contain any additional logic. For example, they cannot execute loops or call external services.
- Functions can automatically access functions and variables from the scope in which they are defined. For example, a function defined within the `service firebase.storage` scope has access to the `resource` variable, and for Cloud Firestore only, built-in functions such as `get()` and `exists()`.
- Functions may call other functions but may not recurse. The total call stack depth is limited to 10.
- In version `rules2`, functions can define variables using the `let` keyword. Functions can have any number of let bindings, but must end with a return statement.

A function is defined with the `function` keyword and takes zero or more
arguments. For example, you may want to combine the two types of conditions used
in the examples above into a single function:  

    service firebase.storage {
      match /b/{bucket}/o {
        // True if the user is signed in or the requested data is 'public'
        function signedInOrPublic() {
          return request.auth.uid != null || resource.data.visibility == 'public';
        }
        match /images/{imageId} {
          allow read, write: if signedInOrPublic();
        }
        match /mp3s/{mp3Ids} {
          allow read: if signedInOrPublic();
        }
      }
    }

Using functions in your Firebase Security Rules makes them more maintainable as the complexity
of your rules grows.

## Next steps

After this discussion of conditions, you've got a more sophisticated
understanding of Rules and are ready to:

**Learn how to handle core use cases, and learn the workflow for developing,
testing and deploying Rules:**

- Write rules that address [common scenarios](https://firebase.google.com/docs/rules/basics).
- Build on your knowledge by reviewing situations where you must [spot and avoid insecure Rules](https://firebase.google.com/docs/rules/insecure-rules).
- Test rules using the [Cloud Storage emulator and dedicated Security Rules test library](https://firebase.google.com/docs/rules/emulator-setup).
- Review the methods available for [deploying Security Rules](https://firebase.google.com/docs/rules/manage-deploy).