#man crypt

NAME
crypt - password and data encryption

SYNOPSIS
#define _XOPEN_SOURCE
#include <unistd.h>

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

DESCRIPTION
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–zA–Z0–9./]. This
string is used to perturb the algorithm in one of 4096 different ways.

By taking the lowest 7 bits of each of the first eight characters 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 f 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 2**56 equal 7.2e16 possible values.
Exhaustive searches of this key space are possible using massively par-
allel 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.

RETURN VALUE
A pointer to the encrypted password is returned. On error, NULL is
returned.

ERRORS
ENOSYS The crypt function was not implemented, probably because of
U.S.A. export restrictions.

GNU EXTENSION
The glibc2 version of this function has the following additional fea-
tures. If salt is a character string starting with the three charac-
ters "$1$" followed by at most eight characters, and optionally termi-
nated by "$", then instead of using the DES machine, the glibc crypt
function uses an MD5-based algorithm, and outputs up to 34 bytes,
namely "$1$<string>$", where "<string>" stands for the up to 8 charac-
ters following "$1$" in the salt, followed by 22 bytes chosen from the
set [a–zA–Z0–9./]. The entire key is significant here (instead of only
the first 8 bytes).

Programs using this function must be linked with -lcrypt.

CONFORMING TO
SVID, X/OPEN, BSD 4.3, POSIX 1003.1-2001

SEE ALSO
login(1), passwd(1), encrypt(3), getpass(3), passwd(5)