Python crypt module -- what's the correct use of salts?

Question

First, context: I'm trying to create a command-line-based tool (Linux) that requires login. Accounts on this tool have nothing to do with system-level accounts -- none of this looks at /etc/passwd.

I am planning to store user accounts in a text file using the same format (roughly) as /etc/passwd.

Despite not using the system-level password files, using crypt seemed to be a good practice to use, as opposed to storing passwords in cleartext. (While crypt is certainly better than storing passwords in cleartext, I'm open to other ways of doing this.)

My crypt knowledge is based on this: https://docs.python.org/2/library/crypt.html

The documentation seems to ask for something that isn't possible: "it is recommended to use the full crypted password as salt when checking for a password."

Huh? If I'm creating the crypted password (as in, when creating a user record) how can I use the crypted password as a salt? It doesn't exist yet. (I'm assuming that you must use the same salt for creating and checking a password.)

I've tried using the plaintext password as a salt. This does work, but has two problems; one easily overcome, and one serious:

1) The first two letters of the plaintext password are included in the crypted password. You can fix this by not writing the first two characters to the file:

user_record = '%s:%s:%s' % (user_name, crypted_pw[2:], user_type)

2) By using the plaintext password as the salt, you would seem to be reducing the amount of entropy in the system. Possibly I'm misunderstanding the purpose of the salt.

The best practice I've been able to derive is to use the first two characters from the username as the salt. Would this be appropriate, or is there something I've missed that makes that a bad move?

My understanding of a salt is that it prevents pre-computing password hashes from a dictionary. I could use a standard salt for all passwords (such as my initials, "JS,") but that seems to be less of a burden for an attacker than using two characters from each user's username.

Answer 1

Python's crypt() is a wrapper for the system's crypt() function. From the Linux crypt() man page:

char *crypt(const char *key, const char *salt);

key is a user’s typed password.
salt is a two-character string chosen from the set [a–zA–Z0–9./]. 
This string is used to perturb the algorithm in one of 4096 
different ways.

Emphasis is on "two-character string". Now, if you look at crypt()'s behavior in Python:

>>> crypt.crypt("Hello", "World")
'Wo5pEi/H5/mxU'
>>> crypt.crypt("Hello", "ABCDE")
'AB/uOsC7P93EI'

you discover that the first two characters of the result always coincide with the first two characters of the original salt, which indeed form the true two-character-salt itself. That is, the result of crypt() has the form 2char-salt + encrypted-pass. Hence, there is no difference in the result if instead of passing the two-character-salt or the original many-characters-salt you pass the whole encrypted password.

Note: the set [a–zA–Z0–9./] contains 64 characters, and 64*64=4096. Here's how two characters relate to "4096 different ways".