Password encoding with BCrypt in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this BCrypt password encoding snippet?

Consider the following Spring Boot code that encodes a password using BCryptPasswordEncoder. What will be the output type and general format of the encoded password string?

Spring Boot

import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;

public class PasswordEncoderTest {
    public static void main(String[] args) {
        BCryptPasswordEncoder encoder = new BCryptPasswordEncoder();
        String rawPassword = "mypassword";
        String encodedPassword = encoder.encode(rawPassword);
        System.out.println(encodedPassword);
    }
}

A A string starting with "$2a$" or "$2b$" followed by a 60-character hash

B A plain text password identical to the input string

C A numeric hash value representing the password

D A Base64 encoded string without any prefix

💡 Hint

Think about how BCrypt hashes are formatted and what the encoder outputs.

BCryptPasswordEncoder outputs a hashed password string that starts with a version prefix like "$2a$" or "$2b$" and is always 60 characters long. It is not plain text or numeric.

❓ state_output

intermediate

What is the result of matching a raw password against a BCrypt hash?

Given this code snippet, what will be the output of the match check?

Spring Boot

import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;

public class PasswordMatchTest {
    public static void main(String[] args) {
        BCryptPasswordEncoder encoder = new BCryptPasswordEncoder();
        String rawPassword = "secret123";
        String encodedPassword = encoder.encode(rawPassword);
        boolean matches = encoder.matches("secret123", encodedPassword);
        System.out.println(matches);
    }
}

A Runtime exception due to invalid password format

B true

C Compilation error due to missing imports

D false

💡 Hint

The matches method compares raw and encoded passwords correctly if used properly.

The matches method returns true if the raw password matches the encoded hash. Since the same raw password is checked, it returns true.

📝 Syntax

advanced

Which option correctly configures a BCryptPasswordEncoder bean in Spring Boot?

You want to create a BCryptPasswordEncoder bean in a Spring Boot configuration class. Which code snippet is syntactically correct and follows Spring Boot patterns?

A

@Bean
public void passwordEncoder() {
    new BCryptPasswordEncoder();
}

B

public BCryptPasswordEncoder passwordEncoder() {
    return new BCryptPasswordEncoder();
}

C

@Bean
public BCryptPasswordEncoder passwordEncoder() {
    BCryptPasswordEncoder encoder;
}

D

@Bean
public BCryptPasswordEncoder passwordEncoder() {
    return new BCryptPasswordEncoder();
}

💡 Hint

Remember that a bean method must return the bean instance and be annotated with @Bean.

Option D correctly defines a method annotated with @Bean that returns a new BCryptPasswordEncoder instance. Other options either miss @Bean, return type, or return statement.

🔧 Debug

advanced

Why does this password match check always return false?

Examine the code below. Why does the password match check always print false?

Spring Boot

import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;

public class PasswordCheck {
    public static void main(String[] args) {
        BCryptPasswordEncoder encoder = new BCryptPasswordEncoder();
        String rawPassword = "pass123";
        String encodedPassword = encoder.encode(rawPassword);
        boolean matches = encoder.matches(rawPassword, rawPassword);
        System.out.println(matches);
    }
}

A Because matches compares the raw password to the encoded password, but here it compares raw to raw

B Because rawPassword is null causing NullPointerException

C Because encode method returns null causing matches to fail

D Because BCryptPasswordEncoder is not initialized properly

💡 Hint

Check the arguments passed to the matches method carefully.

The matches method expects the first argument as raw password and the second as encoded password. Here both are raw, so it returns false.

🧠 Conceptual

expert

What is the main security benefit of using BCrypt for password encoding?

Why is BCrypt preferred over simple hashing algorithms like MD5 or SHA-1 for password encoding in Spring Boot applications?

A BCrypt hashes can be reversed to get the original password if needed

B BCrypt produces shorter hashes which save database space

C BCrypt automatically salts passwords and is computationally expensive, making brute-force attacks harder

D BCrypt uses symmetric encryption to protect passwords

💡 Hint

Think about how password hashing defends against attackers guessing passwords.

BCrypt adds a salt and is slow to compute, which makes it much harder for attackers to use brute-force or rainbow table attacks compared to fast hashes like MD5.

Practice

1. What is the main purpose of using BCryptPasswordEncoder in Spring Boot?

easy

A. To validate email addresses

B. To decode passwords back to plain text

C. To generate random passwords for users

D. To securely encode passwords before storing them

Solution

1. Step 1: Understand BCryptPasswordEncoder role

   BCryptPasswordEncoder is used to convert plain passwords into a secure encoded form.

2. Step 2: Identify correct purpose

   It does not decode or generate passwords, only encodes them securely.

3. Final Answer:

   To securely encode passwords before storing them -> Option D

4. Quick Check:

   Password encoding = Secure storage [OK]

Hint: BCrypt encodes, never decodes passwords [OK]

Common Mistakes:

• Thinking BCrypt can decode passwords
• Confusing encoding with password generation
• Using it for unrelated tasks like email validation

2. Which of the following is the correct way to create a BCryptPasswordEncoder instance in Spring Boot?

easy

A. BCryptPasswordEncoder encoder = BCryptPasswordEncoder();

B. BCryptPasswordEncoder encoder = new BCryptPasswordEncoder();

C. BCryptPasswordEncoder encoder = new BCryptPasswordEncoder.encode();

D. BCryptPasswordEncoder encoder = encode(new BCryptPasswordEncoder());

Solution

1. Step 1: Recall Java object creation syntax

   In Java, to create an object, use the new keyword followed by the constructor.

2. Step 2: Match correct syntax

   BCryptPasswordEncoder encoder = new BCryptPasswordEncoder(); correctly uses new BCryptPasswordEncoder(); to create an instance.

3. Final Answer:

   BCryptPasswordEncoder encoder = new BCryptPasswordEncoder(); -> Option B

4. Quick Check:

   Object creation = new + constructor [OK]

Hint: Use 'new' keyword to create objects in Java [OK]

Common Mistakes:

• Omitting 'new' keyword when creating objects
• Calling methods instead of constructors
• Incorrect method chaining in object creation

3. Given the following code snippet, what will be the output of matches method?

BCryptPasswordEncoder encoder = new BCryptPasswordEncoder();
String rawPassword = "mypassword";
String encodedPassword = encoder.encode(rawPassword);
boolean result = encoder.matches("mypassword", encodedPassword);
System.out.println(result);

medium

A. true

B. false

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand encode and matches methods

   The encode method creates a hashed password. The matches method checks if the raw password matches the encoded hash.

2. Step 2: Analyze the code flow

   The raw password "mypassword" is encoded, then matches compares the same raw password with the encoded one, so it returns true.

3. Final Answer:

   true -> Option A

4. Quick Check:

   matches(raw, encoded) = true if same password [OK]

Hint: matches() returns true if raw matches encoded password [OK]

Common Mistakes:

• Assuming encode returns plain text
• Thinking matches compares encoded strings directly
• Expecting false because encoded password looks different

4. Identify the error in the following Spring Boot code snippet for password encoding:

BCryptPasswordEncoder encoder;
String encoded = encoder.encode("secret");

medium

A. String type cannot hold encoded password

B. encode method does not exist in BCryptPasswordEncoder

C. encoder is not initialized before use

D. Missing import statement for BCryptPasswordEncoder

Solution

1. Step 1: Check variable initialization

   The variable encoder is declared but not assigned an instance before calling encode.

2. Step 2: Understand consequences

   Using an uninitialized object causes a NullPointerException at runtime.

3. Final Answer:

   encoder is not initialized before use -> Option C

4. Quick Check:

   Uninitialized objects cause runtime errors [OK]

Hint: Always initialize objects before calling methods [OK]

Common Mistakes:

• Forgetting to create new instance with 'new'
• Assuming declaration equals initialization
• Ignoring runtime NullPointerException

5. You want to store user passwords securely in your Spring Boot application. Which approach correctly uses BCryptPasswordEncoder to encode and verify passwords during login?

hard

A. Encode password on registration, store encoded; on login, use matches(rawPassword, storedEncodedPassword)

B. Store plain password; on login, encode input and compare with stored plain password

C. Encode password on registration, store encoded; on login, encode input and compare encoded strings directly

D. Encode password on registration, store encoded; on login, decode stored password and compare with input

Solution

1. Step 1: Understand secure password storage

   Passwords must be encoded before storing; plain text storage is insecure.

2. Step 2: Verify password correctly on login

   Use matches(rawPassword, storedEncodedPassword) to check if input matches stored hash without decoding.

3. Final Answer:

   Encode password on registration, store encoded; on login, use matches(rawPassword, storedEncodedPassword) -> Option A

4. Quick Check:

   Use matches() to verify passwords securely [OK]

Hint: Use matches() to check raw vs encoded passwords [OK]

Common Mistakes:

• Comparing encoded strings directly (they differ each time)
• Storing plain text passwords
• Trying to decode encoded passwords (not possible)