− password and data encryption
_XOPEN_SOURCE /* See feature_test_macros(7) */
*crypt(const char *key, const char
_GNU_SOURCE /* See feature_test_macros(7) */
*crypt_r(const char *key, const char
struct crypt_data *data);
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.
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.
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.
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.
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 encrypted password is returned. On error,
NULL is returned.
salt has the wrong
The crypt() function was
not implemented, probably because of U.S.A. export
/proc/sys/crypto/fips_enabled has a nonzero
value, and an attempt was made to use a weak encryption
type, such as DES.
explanation of the terms used in this section, see
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD. crypt_r()
is a GNU extension.
The glibc2 version of this function supports additional
is a character string starting with the characters
"$id$" followed by a string terminated by
then instead of
using the DES machine, id identifies the encryption
method used and this then determines how the rest of the
password string is interpreted. The following values of
id are supported:
$5$salt$encrypted is an SHA-256 encoded
password and $6$salt$encrypted is an SHA-512
stands for the up to 16 characters following
"$id$" in the salt. The encrypted part of
the password string is the actual computed password. The
size of this string is fixed:
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).
passwd(1), encrypt(3), getpass(3),
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