Chapter 139: Password Hashing in Java

Password hashing turns a secret into a one-way, slow, salted string safe to store in a database. Adaptive functions (Argon2id, bcrypt, scrypt, PBKDF2) let you raise cost as hardware gets faster.

Never encrypt passwords reversibly; never compare plaintext in logs or exceptions.

1. Topic title

Password hashing: per-password salt, high iteration/memory cost, upgrade path when algorithms age

2. What it means

Salt — unique random bytes stored with the hash—defeats rainbow tables.

Pepper — secret not in DB—optional extra defense; key management harder than salt alone.

Spring Security ships PasswordEncoder with BCryptPasswordEncoder and DelegatingPasswordEncoder to support multiple formats during migrations.

3. Why it is used

Database leaks should not immediately reveal user passwords reused elsewhere.

Compliance frameworks expect modern algorithms and rotation policies.

4. Mental sketch

Hashing is pulverizing grain into flour—you cannot reconstruct the original kernel, but you can bake (verify) by pulverizing a candidate the same way and comparing dust.

5. Java code example

import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;
 
public class HashDemo {
    public static void main(String[] args) {
        BCryptPasswordEncoder enc = new BCryptPasswordEncoder();
        String raw = "correct horse battery staple";
        String hash = enc.encode(raw);
        System.out.println("matches=" + enc.matches(raw, hash));
        System.out.println("wrong=" + enc.matches("guess", hash));
    }
}

Requires spring-security-crypto on the classpath—often already present with Spring Security.

6. Explanation of code

encode generates a new salt each call—hashes differ for the same password—normal.

7. Common mistakes

MD5/SHA256 once on passwords—too fast; GPUs break them.

Logging User.password DTOs—DTO reviews must exclude secrets.

8. Best practices

Rehash on login when work factor increases—transparent upgrades.

Prefer Argon2id when policy allows; bcrypt remains acceptable with strong cost parameters.

9. Small practice task

Benchmark BCryptPasswordEncoder with strength 10 vs 12` on your laptop—pick the slowest acceptable p99 login latency.

Beginner tip

“We hash client-side” does not remove server-side hashing—treat client output as a password equivalent anyone on the wire could replay.