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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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crypt(3) Library Functions Manual crypt(3)
crypt, crypt_r - password hashing
Password hashing library (libcrypt, -lcrypt)
#include <unistd.h>
char *crypt(const char *key, const char *salt);
#include <crypt.h>
char *crypt_r(const char *key, const char *salt,
struct crypt_data *restrict data);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
crypt():
Since glibc 2.28:
_DEFAULT_SOURCE
glibc 2.27 and earlier:
_XOPEN_SOURCE
crypt_r():
_GNU_SOURCE
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.
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 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.
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().
On success, a pointer to the hashed password is returned. On
error, NULL is returned.
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 hashing type, such as DES.
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 │
└─────────────────────────┴───────────────┴──────────────────────┘
crypt()
POSIX.1-2008.
crypt_r()
GNU.
crypt()
POSIX.1-2001, SVr4, 4.3BSD.
Availability in glibc
The crypt(), encrypt(3), 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. When recompiling applications in such
distributions, the programmer must detect if _XOPEN_CRYPT is not
available and include <crypt.h> for the function prototypes;
otherwise libxcrypt is an ABI-compatible drop-in replacement.
Features in glibc
The glibc version of this function supports additional hashing
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$hashed
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 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$hashed and $6$salt$hashed contain the password
hashed with, respectively, functions based on SHA-256 and SHA-512.
"salt" stands for the up to 16 characters following "$id$" in the
salt. The "hashed" part of the password string is the actual
computed password. The size of this string is fixed:
MD5 22 characters
SHA-256 43 characters
SHA-512 86 characters
The characters in "salt" and "hashed" are drawn from the set
[a-zA-Z0-9./]. In the MD5 and SHA implementations the entire key
is significant (instead of only the first 8 bytes in DES).
Since glibc 2.7, the SHA-256 and SHA-512 implementations support a
user-supplied number of hashing rounds, defaulting to 5000. If
the "$id$" characters in the salt are followed by "rounds=xxx$",
where xxx is an integer, then the result has the form
$id$rounds=yyy$salt$hashed
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.
login(1), passwd(1), encrypt(3), getpass(3), passwd(5)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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2025-02-02. If you discover any rendering problems in this HTML
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man-pages@man7.org
Linux man-pages 6.10 2024-07-23 crypt(3)
Pages that refer to this page: des_crypt(3), encrypt(3), getpass(3), key_setsecret(3), gshadow(5), passwd(5), passwd(5@@shadow-utils), shadow(5), slapd.conf(5), slapd-config(5), groupmod(8), pam_unix(8), pam_userdb(8), slappasswd(8), useradd(8), usermod(8)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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