Name
crypt, crypt_r — password and data encryption
Synopsis
#define _XOPEN_SOURCE #include <unistd.h>
char *crypt( |
const char *key, |
const char *salt); |
char *crypt_r( |
const char *key, |
| const char *salt, | |
struct crypt_data *data); |
![[Note]](../stylesheet/note.png) |
Note |
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Link with |
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 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]](../stylesheet/warning.png) |
Warning |
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The key space consists of 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() interface a
very poor choice for anything other than password
authentication. If you are planning on using the crypt() interface for a cryptography
project, don't do it: get a good book on encryption and one
of the widely available DES libraries.
crypt_r() is a reentrant
version of crypt(). The
structure pointed to by data is used to store result
data and bookkeeping information. Other than allocating it,
the only thing that the caller should do with this structure
is to set data->initialized to zero
before the first call to crypt_r().
RETURN VALUE
On success, 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.
NOTES
Glibc Notes
The glibc2 version of this function supports additional encryption algorithms.
If salt is a
character string starting with the characters
"$id$" followed by a string
terminated by "$":
$
id$salt$encrypted
then instead of using the DES machine, id identifies the encryption method used
and this then determines how the rest of the password
string is interpreted. The following values of id are supported:
ID Method 1 MD5 2a Blowfish (not in mainline glibc; added in some Linux distributions) 5 SHA-256 (since glibc 2.7) 6 SHA-512 (since glibc 2.7)
So $5$salt$encrypted is an SHA-256 encoded password
and $6$salt$encrypted is an SHA-512 encoded one.
"salt" stands
for the up to 16 characters following "$id$" in the salt. The encrypted part of
the password string is the actual computed password. The
size of this string is fixed:
| MD5 | 22 characters |
| SHA-256 | 43 characters |
| SHA-512 | 86 characters |
The characters in "salt" and "encrypted" are drawn from the set
[a–zA–Z0–9./].
In the MD5 and SHA implementations the entire key is significant (instead
of only the first 8 bytes in DES).
COLOPHON
This page is part of release 3.25 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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Michael Haardt (michaelcantor.informatik.rwth.aachen.de) Sat Sep 3 22:00:30 MET DST 1994 This is free documentation; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. The GNU General Public License's references to "object code" and "executables" are to be interpreted as the output of any document formatting or typesetting system, including intermediate and printed output. This manual is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this manual; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111, USA. Sun Feb 19 21:32:25 1995, faithcs.unc.edu edited details away TO DO: This manual page should go more into detail how DES is perturbed, which string will be encrypted, and what determines the repetition factor. Is a simple repetition using ECB used, or something more advanced? I hope the presented explanations are at least better than nothing, but by no means enough. added _XOPEN_SOURCE, aeb, 970705 added GNU MD5 stuff, aeb, 011223 |