linuxperlubuntuuser-administration

What value of salt should I give to crypt to create a new user on Linux?

I am writing a Perl script which will create a new user (on Ubuntu).

It will need a step along the lines of

$encrypted_password = crypt ($password, $salt);
system ("useradd -d $home -s /bin/bash -g $group -p $encrypted_password $name");

What should the value of $salt be? Examples on the Internet seem to use arbitrary values, but if the encrypted password is going to be tested against what the user enters, then the kernel needs to hash the input with the same salt in order to pass the comparison.

This website claims the salt is encoded in the output of crypt, but that is apparently not true.

In Perl the output of

print crypt("foo", "aa");
print crypt("foo", "aabbcc");
print crypt("foo", "aa1kjhg23gh43jhgk32kh325423g");
print crypt("foo", "abbbcc");

is

aaKNIEDOaueR6
aaKNIEDOaueR6
aaKNIEDOaueR6
abQ9KY.KfrYrc

Aside from there being identical hashes from different salts, which is suspicious, it seems only the first two characters of the salt are used. This does not make sense from a security point of view. Also the output is not in the format as claimed in the link above.

So what value of salt should I use when encrypting a password for useradd?

Solution

  • All the information about crypt is in perldoc -f crypt.

    Here is the part that answers your question:

    When choosing a new salt create a random two character string whose characters come from the set [./0-9A-Za-z] (like join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64] ). This set of characters is just a recommendation; the characters allowed in the salt depend solely on your system's crypt library, and Perl can't restrict what salts crypt() accepts.

    I hope this helps.