Parts rewritten and updated by Bill
Swingle.
Every user on a UNIX® system has a password
associated with their account. It seems obvious that these passwords need to be known
only to the user and the actual operating system. In order to keep these passwords
secret, they are encrypted with what is known as a “one-way hash”, that is,
they can only be easily encrypted but not decrypted. In other words, what we told you a
moment ago was obvious is not even true: the operating system itself does not really know the password. It only knows
the encrypted form of the password.
The only way to get the “plain-text” password is by a brute force search of
the space of possible passwords.
Unfortunately the only secure way to encrypt passwords when UNIX came into being was based on DES, the Data Encryption
Standard. This was not such a problem for users resident in the US, but since the source
code for DES could not be exported outside the US, FreeBSD had to find a way to both
comply with US law and retain compatibility with all the other UNIX variants that still used DES.
The solution was to divide up the encryption libraries so that US users could install
the DES libraries and use DES but international users still had an encryption method that
could be exported abroad. This is how FreeBSD came to use MD5 as its default encryption
method. MD5 is believed to be more secure than DES, so installing DES is offered
primarily for compatibility reasons.
Currently the library supports DES, MD5 and Blowfish hash functions. By default
FreeBSD uses MD5 to encrypt passwords.
It is pretty easy to identify which encryption method FreeBSD is set up to use.
Examining the encrypted passwords in the /etc/master.passwd
file is one way. Passwords encrypted with the MD5 hash are longer than those encrypted
with the DES hash and also begin with the characters $1$.
Passwords starting with $2a$ are encrypted with the Blowfish
hash function. DES password strings do not have any particular identifying
characteristics, but they are shorter than MD5 passwords, and are coded in a 64-character
alphabet which does not include the $ character, so a relatively
short string which does not begin with a dollar sign is very likely a DES password.
The password format used for new passwords is controlled by the passwd_format login capability in /etc/login.conf, which takes values of des, md5 or blf.
See the login.conf(5) manual
page for more information about login capabilities.
