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intermediate Phase · Authentication

Password Hashing

Hash passwords with bcrypt, scrypt, or Argon2 - never store plaintext.

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Password Hashing

Why Hash?

Plaintext storage: DB leak = all passwords compromised
Hashed storage: DB leak = attacker gets hashes (hard to reverse)

Hashing Algorithms

Algorithm Use Case Status
MD5 Passwords DEPRECATED
SHA-256 Passwords Weak
BCrypt Passwords Recommended
Argon2 Passwords Best
PBKDF2 Passwords Good

BCrypt Example (Spring Security)

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

// Hash
String hash = passwordEncoder.encode("rawPassword");

// Verify
boolean matches = passwordEncoder.matches("rawPassword", hash);

Salt

BCrypt auto-generates a unique salt per password. Two identical passwords produce different hashes.

Password Best Practices

Policy Requirements

  • Minimum 8 characters
  • Mix of character types
  • Check against breached passwords
  • No personal information

Storage

  • Use bcrypt/scrypt/Argon2
  • Unique salt per password
  • Appropriate cost factor
  • Never store plaintext

Reset Flow

  1. Verify user identity
  2. Generate single-use token
  3. Send via secure channel
  4. Token expires in 1 hour
  5. Invalidate all existing sessions

Key Points

  • Understanding Password Hashing is essential for production systems
  • Always consider scalability and maintainability
  • Test thoroughly before deploying to production
  • Monitor performance and set up alerting

Common Patterns

  1. Validation: Always validate input at the boundary
  2. Error Handling: Use structured error responses
  3. Logging: Log key events for debugging
  4. Testing: Unit, integration, and load tests
  5. Documentation: Keep docs updated with code changes

Practice Problems

0 / 3 solved
Implement Password Hashing

Design and implement a solution for Password Hashing in a backend system. Consider scalability, error handling, and production readiness.

Solution 
// Password Hashing implementation
// Key aspects: validation, error handling, logging, testing

public class PasswordHashing {
    // Production-ready implementation
}
Password Hashing Edge Cases

Identify and handle edge cases for Password Hashing. What happens under high load, with invalid input, or during failures?

Solution 
// Edge case handling:
// 1. Null/empty input -> validation
// 2. High load -> rate limiting, queuing
// 3. Failures -> retries, circuit breaker
// 4. Concurrent access -> locks, idempotency
Password Hashing Testing Strategy

Write a testing strategy for Password Hashing. Include unit tests, integration tests, and performance tests.

Solution 
// Test plan:
// - Unit: 80% coverage target
// - Integration: API contracts
// - Performance: latency, throughput
// - Chaos: failure injection

Quiz

1. Which algorithm is recommended for passwords?

Question 1 options

2. What is a salt?

Question 2 options

3. What is a common mistake when implementing Password Hashing?

Question 3 options

Flashcards

Question

Recommended password algorithm?

Click to reveal answer

Answer

BCrypt or Argon2

Question

What is a salt?

Click to reveal answer

Answer

Random data added before hashing

Question

Password Hashing best practices

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Answer

Follow SOLID principles, write clean code, test thoroughly, document decisions, and monitor in production.

Revision Notes

Key Takeaways

  • 1. Never store plaintext passwords
  • 2. Use BCrypt or Argon2
  • 3. BCrypt auto-adds unique salt
  • 4. Verify with passwordEncoder.matches()

Interview Tips

  • • Know why hashing matters
  • • Explain salt purpose

Cheat Sheet

Password Hashing

  • Never store plaintext
  • Use BCrypt or Argon2
  • Salt: unique per password
  • Verify: encoder.matches(raw, hash)