crypt, crypt_r - password and data encryption
#define _XOPEN_SOURCE /* See feature_test_macros(7) */
char *crypt(const char *key, const char *salt);
#define _GNU_SOURCE /* See feature_test_macros(7) */
char *crypt_r(const char *key, const char *salt,
struct crypt_data *data);
Link with -lcrypt
() 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.
is a user's typed password.
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
, 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
() 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.
() 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
On success, a pointer to the encrypted password is returned. On error, NULL is
- salt has the wrong format.
- The crypt() function was not implemented, probably because of
U.S.A. export restrictions.
- /proc/sys/crypto/fips_enabled has a nonzero value, and an attempt
was made to use a weak encryption type, such as DES.
For an explanation of the terms used in this section, see attributes(7)
(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD. crypt_r
() is a
Availability in glibc¶
, and 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 _XOPEN_CRYPT
is either not defined, or it is defined to -1 and availability can be checked
at run time with sysconf(3)
. This may be the case if the downstream
distribution has switched from glibc crypt to libxcrypt
recompiling applications in such distributions, the programmer must detect if
is not available and include <crypt.h>
the function prototypes; otherwise libxcrypt
is an ABI-compatible
Features in glibc¶
The glibc version of this function supports additional encryption algorithms.
is a character string starting with the characters "$
$" followed by a string optionally terminated by "$",
then the result has the form:
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
|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
contain the password encrypted with, respectively, functions based on SHA-256
" stands for the up to 16 characters following "$
$" in the salt. The " encrypted
" part of the
password string is the actual computed password. The size of this string is
|MD5 | 22 characters
|SHA-256 | 43 characters
|SHA-512 | 86 characters
The characters in " salt
" and "encrypted
drawn from the set [ a-zA-Z0-9./
]. In the MD5 and SHA implementations
the entire key
is significant (instead of only the first 8 bytes in
Since glibc 2.7, the SHA-256 and SHA-512 implementations support a user-supplied
number of hashing rounds, defaulting to 5000. If the "$ id
characters in the salt are followed by "rounds= xxx
is an integer, then the result has the form
is the number of hashing rounds actually used. The number of
rounds actually used is 1000 if xxx
is less than 1000, 999999999 if
is greater than 999999999, and is equal to xxx
This page is part of release 4.16 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at