Combined hashes can be used for i.e. converting already existing
password hashes to bcrypt. While this will not provide the same security
a pure bcrypt hash provides, it will still be significantly more secure
than a standard salted md5.
A combined hash will look as follows:
$H\2y${salted_for_H_prefix}${salt_+_settings_for_2y_prefix}${hash}
The prefixes are seperated by a backslash. Individual settings (which
can include either just the salt or a salt and possible additional settings)
are seperated by dollar signs. As backslashes and dollar signs are not
allowed in hashes or salts, they will be used for seperating the settings
from the salt.
Here is an example of a password hash:
$H\2a$9zv1uIaq1$10\1ff4640409fb96a449c1fO$/oN1O0cdUmFSMZT3UZKrgAyalhnt1LC
The 'H' prefix stands for the salted md5 implementation of phpBB 3.0.
Its settings will be parsed as 9zv1uIaq1 resulting in a hash for the check
as follows:
$H$9zv1uIaq1{hash}
Since the password is used for hashing, the {hash} can be left blank and
will basically be filled by the hashing algorithm. The {hash} will then be
used as password for the next hashing algorithm. In this case that would be
the bcrypt algorithm. The settings are set to 10\1ff4640409fb96a449c1fO which
will be transformed to 10$1ff4640409fb96a449c1fO resulting in a hash like
this for the bcrypt hashing function:
$2a$10$1ff4640409fb96a449c1fO{hash}
The {hash} will again be basically filled by the hashing algorithm.
Afterwards, the {hash} will be extracted from the returned hash and put at
the end of the already known hash settings:
$H\2a$9zv1uIaq1$10\1ff4640409fb96a449c1fO$
If the password is correct, the combined hash will of course be the same
as the stored one.
PHPBB3-11610
