how to compare a password text with the bcrypt hashes?

Question

I have a use case in my application that should prevent the user from choosing one of their last 3 passwords while resetting their password. I'm using Angular for the front end and Spring Boot for the back end . In my scenario, the user passwords are stored as bcrypt hash.

How can I compare the password entered by the user with the last 3 stored bcrypt passwords?

When I run the following code snipped example,

BCryptPasswordEncoder b = new BCryptPasswordEncoder();

    for(int i =0;i<10;i++) {
        System.out.println(b.encode("passw0rd"));

    }

it generates the following bcrypt hashes. each hash is different which is reasonable because when I check the org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder, I can see the salt generated is random value.

$2a$10$tztZsPFZ.T.82Gl/VIuMt.RDjayTwuMLAkRkO9SB.rd92vHWKZmRm
$2a$10$yTHyWDmcCBq3OSPOxjj4TuW9qXYE31CU.fFlWxppii9AizL0lKMzO
$2a$10$Z6aVwg.FNq/2I4zmDjDOceT9ha0Ur/UKsCfdADLvNHiZpR7Sz53fC
$2a$10$yKDVeOUvfTQuTnCHGJp.LeURFcXK6JcHB6lrSgoX1pRjxXDoc8up.
$2a$10$ZuAL06GS7shHz.U/ywb2iuhv2Spubl7Xo4NZ7QOYw3cHWK7/7ZKcC
$2a$10$4T37YehBTmPWuN9j.ga2XeF9GHy6EWDhQS5Uc9bHvJTK8.xIm1coS
$2a$10$o/zxjGkArT7YdDkrk5Qer.oJbZAYpJW39iWAWFqbOhpTf3FmyfWRC
$2a$10$eo7yuuE2f7XqJL8Wjyz.F.xj78ltWuMS1P0O/I6X7iNPwdsWMVzu6
$2a$10$3ErH2GtZpYJGg1BhfgcO/uOt/L2wYg4RoO8.fNRam458WWdymdQLW
$2a$10$IksOJvL/a0ebl4R2/nbMQ.XmjNARIzNo8.aLXiTFs1Pxd06SsnOWa

Spring security configuration.

  @Configuration
    @Import(SecurityProblemSupport.class)
    @EnableWebSecurity
    @EnableGlobalMethodSecurity(prePostEnabled = true, securedEnabled = true)
    public class SecurityConfiguration extends WebSecurityConfigurerAdapter {

        @PostConstruct
        public void init() {
            try {
                authenticationManagerBuilder
                    .userDetailsService(userDetailsService)
                    .passwordEncoder(passwordEncoder());
            } catch (Exception e) {
                throw new BeanInitializationException("Security configuration failed", e);
            }
        }
       @Bean
        public PasswordEncoder passwordEncoder() {
            return new BCryptPasswordEncoder();
        }
    }

Answer 1

you can use matches method in BCryptPasswordEncoder, something like this:

b.matches("passw0rd", hash)

Answer 2

Actually I found my answer . I realized that I can use matches function in the class org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder.

System.out.println(b.matches("passw0rd", "$2a$10$tztZsPFZ.T.82Gl/VIuMt.RDjayTwuMLAkRkO9SB.rd92vHWKZmRm"));

Answer 3

Spring Security just reads the salt from previously generated hash and rehashes the input password again with same salt. And it compares both final hashes and obviously it will be same.

Example:

Password: test

Hash: $2a$10$nCgoWdqJwQs9prt7X5a/2eWLn88I8pon6iNat90u4rq4mHqtoPGQy

Hash has 3 segments each separated by $ symbol. 2a is version of the Bcrypt, 10 is the total rounds and nCgoWdqJwQs9prt7X5a/2e is the salt.

So spring security takes the password test and salt nCgoWdqJwQs9prt7X5a/2e and runs the hashing method. Obviously it generates the same hash as the password and salt matches.