CRYPT
Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
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NAME
crypt, crypt_r - password and data encryption
SYNOPSIS
#define _XOPEN_SOURCE /* See feature_test_macros(7) */
#include <unistd.h>
char *crypt(const char *key, const char *salt);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <crypt.h>
char *crypt_r(const char *key, const char *salt,
struct crypt_data *data);
Link with -lcrypt.
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: 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
- EINVAL
-
salt
has the wrong format.
- ENOSYS
-
The
crypt()
function was not implemented, probably because of U.S.A. export restrictions.
- EPERM
-
/proc/sys/crypto/fips_enabled
has a nonzero value,
and an attempt was made to use a weak encryption type, such as DES.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
| Interface | Attribute | Value
|
|
crypt()
| Thread safety | MT-Unsafe race:crypt
|
|
crypt_r()
| Thread safety | MT-Safe
|
CONFORMING TO
crypt():
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
crypt_r()
is a GNU extension.
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 optionally terminated by "$",
then the result has the form:
-
$id$salt$encrypted
id
identifies the encryption method used instead of DES 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) |
|
Thus, $5$salt$encrypted and $6$salt$encrypted
contain the password encrypted with, respectively, functions
based on SHA-256 and SHA-512.
"salt" stands for the up to 16 characters
following "$id$" in the salt.
The "encrypted"
part of the password string is the actual computed password.
The size of this string is fixed:
| MD5 | 22 characters |
|
| SHA-256 | 43 characters |
|
| SHA-512 | 86 characters |
|
The characters in "salt" and "encrypted" are drawn from the set
[a-zA-Z0-9./].
In the MD5 and SHA implementations the entire
key
is significant (instead of only the first
8 bytes in DES).
Since glibc 2.7,
the SHA-256 and SHA-512 implementations support a user-supplied number of
hashing rounds, defaulting to 5000.
If the "$id$" characters in the salt are
followed by "rounds=xxx$", where xxx is an integer, then the
result has the form
-
$id$rounds=yyy$salt$encrypted
where
yyy is the number of hashing rounds actually used.
The number of rounds actually used is 1000 if
xxx
is less than
1000, 999999999 if
xxx
is greater than 999999999, and
is equal to
xxx
otherwise.
SEE ALSO
login(1),
passwd(1),
encrypt(3),
getpass(3),
passwd(5)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- NOTES
-
- Glibc notes
-
- SEE ALSO
-
- COLOPHON
-
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|
- Running on 
-
:
: 