NAME
crypt - password and data encryption
SYNOPSIS
#define _XOPEN_SOURCE
#include <unistd.h>
I char *crypt(const char * key , const char * salt );
Link with -lcrypt.
DESCRIPTION
R 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
[azAZ09./].
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
R 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
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.
RETURN VALUE
A pointer to the encrypted password is returned.
On error, NULL is returned.
ERRORS
ENOSYS
The
R crypt ()
function was not implemented, probably because of U.S.A. export restrictions.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001
NOTES
Glibc Notes
The glibc2 version of this function has the following additional features.
If
salt
is a character string starting with the three characters "$1$"
followed by at most eight characters, and optionally terminated by "$",
then instead of using the DES machine, the glibc crypt function
uses an MD5-based algorithm, and outputs up to 34 bytes,
namely "$1$<salt>$<encoded>", where
"<salt>" stands for the up to 8 characters following "$1$" in the salt,
and "<encoded>" is a further 22 characters.
The characters in "<salt>" and "<encoded>" are drawn from the set
[
azAZ09./].
The entire
key
is significant here (instead of only the first
8 bytes).
SEE ALSO
ČLÁNKY DO MAILU