path: root/man3/crypt.3

'\" t
.\" Michael Haardt (michael@cantor.informatik.rwth.aachen.de)
.\"     Sat Sep  3 22:00:30 MET DST 1994
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.\"
.\" 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 2023-07-20 "Linux man-pages 6.05.01"
.SH NAME
crypt, crypt_r \- password hashing
.SH LIBRARY
Password hashing library
.RI ( libcrypt ", " \-lcrypt )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "char *crypt(const char *" key ", const char *" salt );
.PP
.B #include <crypt.h>
.PP
.BI "char *crypt_r(const char *" key ", const char *" salt ,
.BI "              struct crypt_data *restrict " data );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR crypt ():
.nf
    Since glibc 2.28:
        _DEFAULT_SOURCE
    glibc 2.27 and earlier:
        _XOPEN_SOURCE
.fi
.PP
.BR crypt_r ():
.nf
    _GNU_SOURCE
.fi
.SH DESCRIPTION
.BR crypt ()
is the password hashing 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\-zA\-Z0\-9./\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 hashed 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.
.PP
.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 hashed password is returned.
On error, NULL is returned.
.SH ERRORS
.TP
.B EINVAL
.I salt
has the wrong format.
.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.
.TP
.B EPERM
.I /proc/sys/crypto/fips_enabled
has a nonzero value,
and an attempt was made to use a weak hashing type, such as DES.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR crypt ()
T}	Thread safety	MT-Unsafe race:crypt
T{
.na
.nh
.BR crypt_r ()
T}	Thread safety	MT-Safe
.TE
.sp 1
.SH STANDARDS
.TP
.BR crypt ()
POSIX.1-2008.
.TP
.BR crypt_r ()
GNU.
.SH HISTORY
.TP
.BR crypt ()
POSIX.1-2001, SVr4, 4.3BSD.
.SS Availability in glibc
The
.BR crypt (),
.BR encrypt (3),
and
.BR setkey (3)
functions are part of the POSIX.1-2008 XSI Options Group for Encryption
and are optional.
If the interfaces are not available, then the symbolic constant
.B _XOPEN_CRYPT
is either not defined,
or it is defined to \-1 and availability can be checked at run time with
.BR sysconf (3).
This may be the case if the downstream distribution has switched from glibc
crypt to
.IR libxcrypt .
When recompiling applications in such distributions,
the programmer must detect if
.B _XOPEN_CRYPT
is not available and include
.I <crypt.h>
for the function prototypes;
otherwise
.I libxcrypt
is an ABI-compatible drop-in replacement.
.SH NOTES
.SS Features in glibc
The glibc version of this function supports additional
hashing algorithms.
.PP
If
.I salt
is a character string starting with the characters "$\fIid\fP$"
followed by a string optionally terminated by "$",
then the result has the form:
.RS
.PP
$\fIid\fP$\fIsalt\fP$\fIhashed\fP
.RE
.PP
.I id
identifies the hashing method used instead of DES and this
then determines how the rest of the password string is interpreted.
The following values of
.I id
are supported:
.RS
.TS
lb lb
l lx.
ID	Method
_
1	MD5
2a	T{
Blowfish (not in mainline glibc; added in some
Linux distributions)
T}
.\" 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
.PP
Thus, $5$\fIsalt\fP$\fIhashed\fP and $6$\fIsalt\fP$\fIhashed\fP
contain the password hashed with, respectively, functions
based on SHA-256 and SHA-512.
.PP
"\fIsalt\fP" stands for the up to 16 characters
following "$\fIid\fP$" in the salt.
The "\fIhashed\fP"
part of the password string is the actual computed password.
The size of this string is fixed:
.RS
.TS
lb l.
MD5	22 characters
SHA-256	43 characters
SHA-512	86 characters
.TE
.RE
.PP
The characters in "\fIsalt\fP" and "\fIhashed\fP" are drawn from the set
[\fBa\-zA\-Z0\-9./\fP].
In the MD5 and SHA implementations the entire
.I key
is significant (instead of only the first
8 bytes in DES).
.PP
Since glibc 2.7,
.\" glibc commit 9425cb9eea6a62fc21d99aafe8a60f752b934b05
the SHA-256 and SHA-512 implementations support a user-supplied number of
hashing rounds, defaulting to 5000.
If the "$\fIid\fP$" characters in the salt are
followed by "rounds=\fIxxx\fP$", where \fIxxx\fP is an integer, then the
result has the form
.RS
.PP
$\fIid\fP$\fIrounds=yyy\fP$\fIsalt\fP$\fIhashed\fP
.RE
.PP
where \fIyyy\fP is the number of hashing rounds actually used.
The number of rounds actually used is 1000 if
.I xxx
is less than
1000, 999999999 if
.I xxx
is greater than 999999999, and
is equal to
.I xxx
otherwise.
.SH SEE ALSO
.BR login (1),
.BR passwd (1),
.BR encrypt (3),
.BR getpass (3),
.BR passwd (5)