Secure storage for credentials in Flutter - Build, Publish & Deploy

Build & Publish - Secure storage for credentials

Performance Impact

Using secure storage for credentials involves encryption and decryption operations. These operations are fast and usually do not affect the app's frame rate, which should remain at 60fps or higher for smooth UI. Memory usage is minimal since only small pieces of data like tokens or passwords are stored. Battery impact is negligible because secure storage accesses are infrequent and lightweight.

Optimization Tips

• Cache credentials in memory after reading once to avoid repeated secure storage access.
• Perform secure storage operations asynchronously to prevent blocking the UI thread.
• Use platform-specific secure storage plugins like flutter_secure_storage that leverage native encryption APIs.
• Minimize the frequency of reads/writes by batching updates when possible.

App Size and Startup Time

Adding secure storage libraries like flutter_secure_storage adds a small increase to app bundle size, typically under 1MB. This is considered small and does not significantly affect app startup time. The secure storage plugin uses native platform APIs, so it does not add heavy dependencies.

iOS vs Android Differences

• iOS: Uses Keychain services for secure storage, which is highly secure and integrated with the OS.
• Android: Uses EncryptedSharedPreferences or Keystore system for secure storage.
• Flutter plugins abstract these differences, but behavior and security guarantees depend on platform implementations.
• On Android, some older devices may have limited hardware-backed security.

Store Review Guidelines

• Apple App Store: Ensure you comply with user privacy guidelines, including secure handling of credentials and user consent if collecting sensitive data.
• Google Play Store: Follow data safety policies, clearly disclose data usage, and use secure storage for sensitive information.
• Do not store passwords in plain text or insecure storage.
• Use encryption and platform secure storage APIs as required by both stores.

Self-Check Question

Your app takes 5 seconds to load this screen that reads credentials from secure storage. What's likely wrong?

• Secure storage access is done synchronously on the main thread, blocking UI rendering.
• Repeated reads from secure storage without caching cause delays.
• Heavy computation or encryption done on the UI thread instead of asynchronously.

Key Result

Using secure storage for credentials in Flutter adds minimal performance overhead and bundle size. Optimize by caching and async access. iOS uses Keychain, Android uses Keystore. Follow store privacy and security guidelines to pass reviews.

Practice

1. What is the main purpose of using FlutterSecureStorage in a Flutter app?

easy

A. To improve app performance by caching data

B. To securely store sensitive data like passwords on the device

C. To create UI animations

D. To manage app navigation between screens

Solution

1. Step 1: Understand the role of secure storage

   Secure storage is designed to keep sensitive information safe on the device, such as passwords or tokens.

2. Step 2: Identify what FlutterSecureStorage does

   This package provides a way to write, read, and delete data securely, protecting it from unauthorized access.

3. Final Answer:

   To securely store sensitive data like passwords on the device -> Option B

4. Quick Check:

   Secure storage = safe data storage [OK]

Hint: Secure storage is for sensitive info, not UI or navigation [OK]

Common Mistakes:

• Confusing secure storage with caching
• Thinking it manages UI or navigation
• Assuming it speeds up app performance

2. Which of the following is the correct way to write a value using FlutterSecureStorage?

easy

A. storage.writeValue('token', 'abc123');

B. storage.write('token', 'abc123');

C. await storage.write(key: 'token', value: 'abc123');

D. await storage.save(key: 'token', value: 'abc123');

Solution

1. Step 1: Recall the method signature for writing data

   The correct method is write with named parameters key and value, and it returns a Future, so it should be awaited.

2. Step 2: Check each option for syntax correctness

   await storage.write(key: 'token', value: 'abc123'); uses await storage.write(key: 'token', value: 'abc123'); which is correct. Others either miss await, use wrong method names, or wrong parameter style.

3. Final Answer:

   await storage.write(key: 'token', value: 'abc123'); -> Option C

4. Quick Check:

   Write method with await and named params = await storage.write(key: 'token', value: 'abc123'); [OK]

Hint: Use await and named parameters key and value [OK]

Common Mistakes:

• Omitting await for async calls
• Using wrong method names like save or writeValue
• Passing parameters without names

3. Given this code snippet, what will be printed?

final storage = FlutterSecureStorage();
await storage.write(key: 'user', value: 'Alice');
String? name = await storage.read(key: 'user');
print(name);

medium

A. Alice

B. Error: key not found

C. FlutterSecureStorage instance

D. null

Solution

1. Step 1: Analyze the write operation

   The code writes the value 'Alice' with the key 'user' into secure storage.

2. Step 2: Analyze the read operation and print

   Reading the key 'user' returns the stored value 'Alice', which is then printed.

3. Final Answer:

   Alice -> Option A

4. Quick Check:

   Write then read same key returns stored value [OK]

Hint: Read returns stored value if key exists [OK]

Common Mistakes:

• Assuming read returns null without checking
• Confusing instance print with value print
• Expecting error if key was just written

4. What is wrong with this code snippet?

final storage = FlutterSecureStorage();
storage.write(key: 'pass', value: '1234');
String pass = storage.read(key: 'pass');
print(pass);

medium

A. Wrong method name for writing

B. Cannot print a String variable

C. Key 'pass' is invalid

D. Missing await for async calls

Solution

1. Step 1: Identify async methods usage

   Both write and read are asynchronous and return Futures, so they must be awaited.

2. Step 2: Check variable assignment and print

   Assigning String pass = storage.read(...) without await causes a type mismatch and print will show Future instance, not the value.

3. Final Answer:

   Missing await for async calls -> Option D

4. Quick Check:

   Async methods need await to get actual values [OK]

Hint: Always await async storage methods [OK]

Common Mistakes:

• Not awaiting async calls
• Assigning Future to String directly
• Ignoring async nature of storage methods

5. You want to securely store a user's credentials and then delete them when they log out. Which sequence of FlutterSecureStorage calls is correct?

hard

A. await storage.write(key: 'user', value: 'Bob'); await storage.delete(key: 'user');

B. storage.write(key: 'user', value: 'Bob'); storage.remove(key: 'user');

C. await storage.save(key: 'user', value: 'Bob'); await storage.deleteValue(key: 'user');

D. storage.set(key: 'user', value: 'Bob'); storage.delete(key: 'user');

Solution

1. Step 1: Identify correct method names and async usage

   The correct methods are write to store and delete to remove data. Both return Futures and must be awaited.

2. Step 2: Check each option for correctness

   await storage.write(key: 'user', value: 'Bob'); await storage.delete(key: 'user'); uses correct method names and awaits both calls. Other options use wrong method names or omit await.

3. Final Answer:

   await storage.write(key: 'user', value: 'Bob'); await storage.delete(key: 'user'); -> Option A

4. Quick Check:

   Use write and delete with await for secure storage [OK]

Hint: Use write and delete methods with await [OK]

Common Mistakes:

• Using wrong method names like save or remove
• Not awaiting async calls
• Mixing synchronous and async calls