phpBB3 uses a complex hashing algorithm in order to prevent someone from determining the plaintext value of a password. phpBB2, however, used a much simpler and less secure md5 algorithm to store passwords. This is one of the many reasons why we have decided to no longer support the phpBB2 software. Because hashes cannot be reversed, phpBB3 is set to convert phpBB2 hashes to the new phpBB3 standard during the first user login. Those users who registered while phpBB.com used phpBB2 and did not login on the new phpBB3 board continue to have their password hashes stored in the old format. Passwords stored in the old format are much less secure than those stored in the new format. The attackers have been focusing purely on the passwords stored in the old format.