CRYPT(3) Linux Programmer's Manual CRYPT(3)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.
DESCRIPTIONcrypt() 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 par‐
allel 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 TOcrypt(): 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 "$id$" fol‐
lowed 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 ALSOlogin(1), passwd(1), encrypt(3), getpass(3), passwd(5)COLOPHON
This page is part of release 4.14 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/.
2017-09-15 CRYPT(3)
