BCryptPasswordEncoder affects startup time of server - spring-boot

Question

Why does the strength of the BCryptPasswordEncoder affect the startup time of the server? There are no hashes generated at startup, so I'm wondering why this does have any effect on the startup.
Of course, I understand that checking whether a password matches takes time, but at start up is strange.
Code looks like this:
#Bean
public BCryptPasswordEncoder passwordEncoder() {
return new BCryptPasswordEncoder(17); // Affects startup time tremendously
}
#Autowired
BCryptPasswordEncoder bcryptEncoder;
#Autowired
CustomUserDetailsService userDetailsService;
#Override
protected void configure(AuthenticationManagerBuilder auth) throws Exception {
auth.userDetailsService(userDetailsService)
.passwordEncoder(bcryptEncoder);
}

Answer 1

(Depending on your configuration)
Have a look at the spring DaoAuthenticationProvider
The following method is called at startup:
private void setPasswordEncoder(PasswordEncoder passwordEncoder) {
Assert.notNull(passwordEncoder, "passwordEncoder cannot be null");
this.userNotFoundEncodedPassword = passwordEncoder.encodePassword(
USER_NOT_FOUND_PASSWORD, null);
this.passwordEncoder = passwordEncoder;
}
This was introduced so the server has an encoded password to verify the password against from a username attempting to authenticate doesn't exist.
/**
* The plaintext password used to perform
* {#link PasswordEncoder#isPasswordValid(String, String, Object)} on when the user is
* not found to avoid SEC-2056.
*/
private static final String USER_NOT_FOUND_PASSWORD = "userNotFoundPassword";
Refer to:
https://github.com/spring-projects/spring-security/issues/2280
https://jira.spring.io/browse/SEC-2056
Note: If you set the strength to 17 and your server is taking an extra 5minutes to start, it will take your sever approximately 5minutes to verify each users password when they authenticate.

Answer 2

This issue is now resolved, so upgrade your Spring Boot/Spring Security if necessary.
Spring Security calls PasswordEncoder.matches() regardless of whether the user was found, so that hackers can't detect if the user existed or not by comparing response times. To create the dummy encrypted password or hash for non-existing users, Spring Security calls PasswordEncoder.encode("userNotFoundEncodedPassword") once and reuses the result. This call was previously made during startup but is now called lazily when first needed.

Answer 3

The constructor of BCryptPasswordEncoder is not doing anything on startup depending on the password strength :
public BCryptPasswordEncoder(int strength) {
this(strength, null);
}
public BCryptPasswordEncoder(int strength, SecureRandom random) {
if (strength != -1 && (strength < BCrypt.MIN_LOG_ROUNDS || strength > BCrypt.MAX_LOG_ROUNDS)) {
throw new IllegalArgumentException("Bad strength");
}
this.strength = strength;
this.random = random;
}
Having seen this, I dont think that only changing the strength parameter can increase startup time as described.
But when you actually use the encrypter, 'strength' will for sure impact the performance. So may be you are encrypting many passwords somewhere at startup ?