Storage security rules in Firebase - Time & Space Complexity

Time Complexity: Storage security rules

O(n)

Understanding Time Complexity

We want to understand how the time to check storage security rules changes as more files or requests happen.

How does the system handle more requests and how long does each check take?

Scenario Under Consideration

Analyze the time complexity of the following security rule checks.

rules_version = '2';
service firebase.storage {
  match /b/{bucket}/o {
    match /{allPaths=**} {
      allow read, write: if request.auth != null && request.auth.uid == resource.metadata.ownerId;
    }
  }
}

This rule allows read and write only if the user is logged in and owns the file.

Identify Repeating Operations

Identify the API calls, resource provisioning, data transfers that repeat.

• Primary operation: Checking the security rule for each file access request.
• How many times: Once per file read or write request.

How Execution Grows With Input

Each request triggers one rule check, regardless of total files stored.

Input Size (n)	Approx. API Calls/Operations
10	10 rule checks
100	100 rule checks
1000	1000 rule checks

Pattern observation: The time grows linearly with the number of requests, not with total files stored.

Final Time Complexity

Time Complexity: O(n)

This means the time to check rules grows directly with the number of file access requests.

Common Mistake

[X] Wrong: "The rule check time depends on how many files are stored in total."

[OK] Correct: Each check only looks at the single file requested, so total stored files do not affect check time.

Interview Connect

Understanding how security rules scale helps you design systems that stay fast and safe as they grow.

Self-Check

"What if the rule checked multiple files at once? How would the time complexity change?"

Practice

1. What is the main purpose of Firebase Storage security rules?

easy

A. To control who can read or write files in storage

B. To speed up file uploads

C. To increase storage space automatically

D. To backup files to another server

Solution

1. Step 1: Understand the role of security rules

   Security rules define who can access files and what actions they can perform.

2. Step 2: Identify the main function

   The main function is to control read and write permissions to protect data.

3. Final Answer:

   To control who can read or write files in storage -> Option A

4. Quick Check:

   Security rules = control access [OK]

Hint: Security rules control access permissions only [OK]

Common Mistakes:

• Thinking rules speed up uploads
• Believing rules increase storage size
• Confusing rules with backup features

2. Which of the following is the correct syntax to allow only authenticated users to read files in Firebase Storage rules?

easy

A. allow read: if request.auth != null;

B. allow read: if user.authenticated == true;

C. allow read: if auth.user != null;

D. allow read: if request.user.authenticated;

Solution

1. Step 1: Check Firebase Storage rules syntax

   The correct way to check authentication is using request.auth != null.

2. Step 2: Compare options

   Only allow read: if request.auth != null; uses the correct Firebase syntax for authentication check.

3. Final Answer:

   allow read: if request.auth != null; -> Option A

4. Quick Check:

   Authentication check = request.auth != null [OK]

Hint: Use request.auth to check user authentication [OK]

Common Mistakes:

• Using incorrect variable names like user.authenticated
• Missing the 'request.' prefix
• Using wrong boolean checks

3. Given the following Firebase Storage rule snippet, what happens when an unauthenticated user tries to read a file?

service firebase.storage {
  match /b/{bucket}/o {
    match /files/{fileId} {
      allow read: if request.auth != null;
      allow write: if false;
    }
  }
}

medium

A. The read request causes a syntax error

B. The read request is allowed because write is false

C. The read request is allowed because no write permission is needed

D. The read request is denied because the user is not authenticated

Solution

1. Step 1: Analyze the read rule condition

   The rule allows read only if request.auth != null, meaning user must be authenticated.

2. Step 2: Consider unauthenticated user

   An unauthenticated user has request.auth == null, so the condition fails and read is denied.

3. Final Answer:

   The read request is denied because the user is not authenticated -> Option D

4. Quick Check:

   Unauthenticated read = denied [OK]

Hint: Read allowed only if request.auth is not null [OK]

Common Mistakes:

• Ignoring the read condition and assuming access
• Confusing read and write permissions
• Thinking false write affects read

4. Identify the error in this Firebase Storage rule that aims to allow users to write only to their own folder:

service firebase.storage {
  match /b/{bucket}/o {
    match /users/{userId}/{allPaths=**} {
      allow write: if request.auth.uid == userId;
    }
  }
}

medium

A. The variable userId is not defined in the path

B. The wildcard {allPaths=**} is incorrectly used

C. The rule does not allow read access

D. The condition should use request.auth.userId instead of request.auth.uid

Solution

1. Step 1: Check path variables

   The path correctly defines {userId} and {allPaths=**} wildcards.

2. Step 2: Review condition and permissions

   The condition correctly compares request.auth.uid with userId for write permission.

3. Step 3: Identify missing read permission

   The rule only allows write, no read permission is granted, which may block legitimate reads.

4. Final Answer:

   The rule does not allow read access -> Option C

5. Quick Check:

   Missing read permission = problem [OK]

Hint: Check if both read and write permissions are set [OK]

Common Mistakes:

• Assuming write permission covers read
• Confusing request.auth.uid with userId variable
• Misusing wildcard syntax

5. You want to allow users to upload images only to their own folder and only if the file size is less than 2MB. Which Firebase Storage rule correctly enforces this?

service firebase.storage {
  match /b/{bucket}/o {
    match /users/{userId}/{fileName} {
      allow write: if request.auth.uid == userId && request.resource.size < 2 * 1024 * 1024;
      allow read: if request.auth.uid == userId;
    }
  }
}

hard

A. The file size check should use resource.size instead of request.resource.size

B. This rule correctly enforces user ownership and file size limit

C. The rule should allow write without checking user ID

D. The size limit should be checked on the existing file, not request.resource

Solution

1. Step 1: Verify user ownership condition

   The rule checks request.auth.uid == userId to ensure users write only to their folder.

2. Step 2: Verify file size condition

   The rule uses request.resource.size < 2 * 1024 * 1024 to limit uploads to less than 2MB.

3. Step 3: Confirm read permission

   Read is allowed only for the authenticated user matching userId.

4. Final Answer:

   This rule correctly enforces user ownership and file size limit -> Option B

5. Quick Check:

   User ID and size check = correct [OK]

Hint: Use request.resource.size for new file size check [OK]

Common Mistakes:

• Using resource.size instead of request.resource.size
• Not checking user ID for write
• Checking size on existing file instead of upload