Secure storage for credentials in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Securely storing and retrieving a password

Given the Flutter code below using flutter_secure_storage, what will be the output when retrieving the stored password after saving it?

Flutter

final storage = FlutterSecureStorage();
await storage.write(key: 'password', value: 'mySecret123');
String? pwd = await storage.read(key: 'password');
print(pwd);

A null

B mySecret123

C Throws a runtime error

D Empty string

💡 Hint

Think about what the read method returns after a successful write.

The write method stores the value securely. The read method returns the stored value as a string if it exists, so it prints 'mySecret123'.

❓ lifecycle

intermediate

When to clear secure storage on logout?

In a Flutter app using flutter_secure_storage for credentials, when is the best time to clear stored credentials to keep the app secure?

A When the user logs out

B When the app starts

C Only when the app is uninstalled

D Never clear stored credentials

💡 Hint

Think about user privacy and security after ending a session.

Clearing credentials on logout ensures no sensitive data remains accessible if someone else uses the device.

📝 Syntax

advanced

Correct syntax for writing to secure storage

Which option shows the correct syntax to write a username 'user123' to flutter_secure_storage with key 'username'?

Flutter

final storage = FlutterSecureStorage();

A await storage.write(key: 'username', value: 'user123');

B storage.write('username', 'user123');

C storage.write(key='username', value='user123')

D await storage.set('username', 'user123');

💡 Hint

Check the method name and parameter names carefully.

The write method requires named parameters key and value and returns a Future, so await is needed.

🔧 Debug

advanced

Why does reading from secure storage return null?

A developer writes this code but reading the key 'token' returns null. What is the most likely cause?

Flutter

final storage = FlutterSecureStorage();
await storage.write(key: 'Token', value: 'abc123');
String? token = await storage.read(key: 'token');
print(token);

A The write method failed silently

B flutter_secure_storage does not support string values

C The read method requires a callback

D Key names are case-sensitive, so 'Token' and 'token' differ

💡 Hint

Check the exact spelling and casing of keys used.

flutter_secure_storage treats keys as case-sensitive strings, so 'Token' and 'token' are different keys.

🧠 Conceptual

expert

Why use flutter_secure_storage over SharedPreferences for credentials?

Which reason best explains why flutter_secure_storage is preferred for storing user credentials instead of SharedPreferences?

A flutter_secure_storage stores data in plain text for easy access

B SharedPreferences is faster and more secure

C flutter_secure_storage encrypts data and stores it securely on device hardware

D SharedPreferences automatically clears data on app close

💡 Hint

Think about data encryption and protection on mobile devices.

flutter_secure_storage uses platform-specific secure storage mechanisms with encryption, unlike SharedPreferences which stores plain text.

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