crypt
is the password encryption function. It is based on the Data Encryption
Standard algorithm with variations intended (among other things) to
discourage use of hardware implementations of a key search.
key
is a user's typed password.
salt
is a two-character string chosen from the set
[a\(enzA\(enZ0\(en9./]. This string is used to
perturb the algorithm in one of 4096 different ways.
By taking the lowest 7 bit of each character of the
key ,
a 56-bit key is obtained. This 56-bit key is used to encrypt repeatedly a
constant string (usually a string consisting of all zeros). The returned
value points to the encrypted password, a series of 13 printable ASCII
characters (the first two characters represent the salt itself). The
return value points to static data whose content is overwritten by each
call.
Warning: The key space consists of
t 2\s-2\u56\s0\d
n 2**56
equal 7.2e16 possible values. Exhaustive searches of this key space are
possible using massively parallel computers. Software, such as
crack (1),
is available which will search the portion of this key space that is
generally used by humans for passwords. Hence, password selection should,
at minimum, avoid common words and names. The use of a
passwd (1)
program that checks for crackable passwords during the selection process is
recommended.
The DES algorithm itself has a few quirks which make the use of the
crypt (3)
interface a very poor choice for anything other than password
authentication. If you are planning on using the
crypt (3)
interface for a cryptography project, don't do it: get a good book on
encryption and one of the widely available DES libraries.