.\" Michael Haardt (michael@cantor.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, faith@cs.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
.\"
.TH CRYPT 3 2010-06-20 "" "Linux Programmer's Manual"
.SH NAME
crypt, crypt_r \- password and data encryption
.SH SYNOPSIS
.nf
.B #define _XOPEN_SOURCE
.br
.B #include <unistd.h>
.sp
.BI "char *crypt(const char *" key ", const char *" salt );
.sp
.BI "char *crypt_r(const char *" key ", const char *" salt ,
.BI "              struct crypt_data *" data );
.fi
.sp
Link with \fI\-lcrypt\fP.
.SH DESCRIPTION
.BR 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.
.PP
.I key
is a user's typed password.
.PP
.I salt
is a two-character string chosen from the set
[\fBa\fP\(en\fBzA\fP\(en\fBZ0\fP\(en\fB9./\fP].
This string is used to
perturb the algorithm in one of 4096 different ways.
.PP
By taking the lowest 7 bits of each of the first eight characters of the
.IR 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.
.PP
Warning: The key space consists of
.if t 2\s-2\u56\s0\d
.if n 2**56
equal 7.2e16 possible values.
Exhaustive searches of this key space are
possible using massively parallel computers.
Software, such as
.BR 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
.BR passwd (1)
program that checks for crackable passwords during the selection process is
recommended.
.PP
The DES algorithm itself has a few quirks which make the use of the
.BR crypt ()
interface a very poor choice for anything other than password
authentication.
If you are planning on using the
.BR crypt ()
interface for a cryptography project, don't do it: get a good book on
encryption and one of the widely available DES libraries.

.BR crypt_r ()
is a reentrant version of
.BR crypt ().
The structure pointed to by
.I 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
.I data->initialized
to zero before the first call to
.BR crypt_r ().
.SH "RETURN VALUE"
On success, a pointer to the encrypted password is returned.
On error, NULL is returned.
.SH ERRORS
.TP
.B ENOSYS
The
.BR crypt ()
function was not implemented, probably because of U.S.A. export restrictions.
.\" This level of detail is not necessary in this man page. . .
.\" .PP
.\" When encrypting a plain text P using DES with the key K results in the
.\" encrypted text C, then the complementary plain text P' being encrypted
.\" using the complementary key K' will result in the complementary encrypted
.\" text C'.
.\" .PP
.\" Weak keys are keys which stay invariant under the DES key transformation.
.\" The four known weak keys 0101010101010101, fefefefefefefefe,
.\" 1f1f1f1f0e0e0e0e and e0e0e0e0f1f1f1f1 must be avoided.
.\" .PP
.\" There are six known half weak key pairs, which keys lead to the same
.\" encrypted data.  Keys which are part of such key clusters should be
.\" avoided.
.\" Sorry, I could not find out what they are.
.\""
.\" .PP
.\" Heavily redundant data causes trouble with DES encryption, when used in the
.\" .I codebook
.\" mode that
.\" .BR crypt ()
.\" implements.  The
.\" .BR crypt ()
.\" interface should be used only for its intended purpose of password
.\" verification, and should not be used as part of a data encryption tool.
.\" .PP
.\" The first and last three output bits of the fourth S-box can be
.\" represented as function of their input bits.  Empiric studies have
.\" shown that S-boxes partially compute the same output for similar input.
.\" It is suspected that this may contain a back door which could allow the
.\" NSA to decrypt DES encrypted data.
.\" .PP
.\" Making encrypted data computed using crypt() publicly available has
.\" to be considered insecure for the given reasons.
.SH "CONFORMING TO"
.BR crypt ():
SVr4, 4.3BSD, POSIX.1-2001.
.BR crypt_r ()
is a GNU extension.
.SH NOTES
.SS Glibc Notes
The glibc2 version of this function supports additional
encryption algorithms.

If
.I salt
is a character string starting with the characters "$\fIid\fP$"
followed by a string terminated by "$":
.RS

$\fIid\fP$\fIsalt\fP$\fIencrypted\fP

.RE
then instead of using the DES machine,
.I id
identifies the encryption method used and this then determines how the rest
of the password string is interpreted.
The following values of
.I id
are supported:
.RS
.TS
l l.
ID  | Method
_
1   | MD5
2a  | Blowfish (not in mainline glibc; added in some
    | Linux distributions)
.\" openSUSE has Blowfish, but AFAICS, this option is not supported
.\" natively by glibc -- mtk, Jul 08
.\"
.\" md5 | Sun MD5
.\" glibc doesn't appear to natively support Sun MD5; I don't know
.\" if any distros add the support.
5   | SHA-256 (since glibc 2.7)
6   | SHA-512 (since glibc 2.7)
.TE
.RE

So $5$\fIsalt\fP$\fIencrypted\fP is an SHA-256 encoded
password and $6$\fIsalt\fP$\fIencrypted\fP is an
SHA-512 encoded one.

"\fIsalt\fP" stands for the up to 16 characters
following "$\fIid\fP$" in the salt.
The encrypted part of the password string is the actual computed password.
The size of this string is fixed:
.TS
l l.
MD5     | 22 characters
SHA-256 | 43 characters
SHA-512 | 86 characters
.TE

The characters in "\fIsalt\fP" and "\fIencrypted\fP" are drawn from the set
[\fBa\fP\(en\fBzA\fP\(en\fBZ0\fP\(en\fB9./\fP].
In the MD5 and SHA implementations the entire
.I key
is significant (instead of only the first
8 bytes in DES).
.SH "SEE ALSO"
.BR login (1),
.BR passwd (1),
.BR encrypt (3),
.BR getpass (3),
.BR passwd (5),
.BR feature_test_macros (7)
.SH COLOPHON
This page is part of release 3.22 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
