Name
crypt — password and data encryption
Synopsis
#define _XOPEN_SOURCE #include <unistd.h>
char
*crypt( |
const char * | key, |
| const char * | salt); |
![[Note]](../stylesheet/note.png) |
Note |
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Link with |
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]](../stylesheet/warning.png) |
Warning |
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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.
ERRORS
- ENOSYS
-
The
crypt() function was not implemented, probably because of U.S.A. export restrictions.
NOTES
Glibc Notes
The glibc2 version of this function has the following
additional features. If salt is a character string
starting with the three characters "$1$" followed by at
most eight characters, and optionally terminated by "$",
then instead of using the DES machine, the glibc crypt
function uses an MD5-based algorithm, and outputs up to 34
bytes, namely "$1$<salt>$<encoded>", where
"<salt>" stands for the up to 8 characters following
"$1$" in the salt, and "<encoded>" is a further 22
characters. The characters in "<salt>" and
"<encoded>" are drawn from the set [a–zA–Z0–9./].
The entire key is
significant here (instead of only the first 8 bytes).