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According to the docs :
Implementation of PasswordEncoder that uses the BCrypt strong hashing function. Clients can optionally supply a "strength" (a.k.a. log rounds in BCrypt) and a SecureRandom instance. The larger the strength parameter the more work will have to be done (exponentially) to hash the passwords. The default value is 10.
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Class BCryptPasswordEncoder Clients can optionally supply a "strength" (a.k.a. log rounds in BCrypt) and a SecureRandom instance. The larger the strength parameter the more work will have to be done (exponentially) to hash the passwords. The default value is 10.
The problem is that every encryption of it is different every time, so it can never match the one in the database. This is normal bcrypt behavior. bcrypt returns a different hash each time because it incorporates a different random value into the hash. This is known as a "salt".
Bcrypt is highly secure because a slat is used to protect against rainbow table attacks and it is an adaptive hashing function with the cryptographic chunk that can be increased when computer hardware gets faster that means the hashing function can be configured to run slower which also means it can slow down attacks.
bcrypt has a maximum length input length of 72 bytes for most implementations. To protect against this issue, a maximum password length of 72 bytes (or less if the implementation in use has smaller limits) should be enforced when using bcrypt.
The strength is translated to iterations. For strength x there will be 2x iterations. Implementations are assumed to use unsigned 32-bit integer, where the maximum value is 4294967295. If x is larger than 31, 2x is bigger than this maximum value and an overflow would occur.
In practice, the Java implementation in Spring Security actually uses a 64-bit long since integers are signed in Java (maximum of int is 231-1).
A strength of 31 or close thereof is very slow and not usable anyway.
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