crypt(3)

1CRYPT(3)                       Library functions                      CRYPT(3)
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NAME

6       crypt - password and data encryption
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SYNOPSIS

9       #define _XOPEN_SOURCE
10       #include <unistd.h>
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12       char *crypt(const char *key, const char *salt);
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DESCRIPTION

15       crypt()  is  the password encryption function.  It is based on the Data
16       Encryption Standard algorithm with  variations  intended  (among  other
17       things) to discourage use of hardware implementations of a key search.
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19       key is a user's typed password.
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21       salt is a two-character string chosen from the set [a–zA–Z0–9./].  This
22       string is used to perturb the algorithm in one of 4096 different ways.
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24       By taking the lowest 7 bits of each of the first  eight  characters  of
25       the  key, a 56-bit key is obtained.  This 56-bit key is used to encrypt
26       repeatedly a constant  string  (usually  a  string  consisting  of  all
27       zeros).   The returned value points to the encrypted password, a series
28       of 13 printable ASCII characters (the first  two  characters  represent
29       the salt itself).  The return value points to static data whose content
30       is overwritten by each call.
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32       Warning: The key space consists of 2**56 equal 7.2e16 possible  values.
33       Exhaustive searches of this key space are possible using massively par‐
34       allel computers.  Software, such as crack(1), is available  which  will
35       search  the  portion of this key space that is generally used by humans
36       for passwords.  Hence, password selection  should,  at  minimum,  avoid
37       common words and names.  The use of a passwd(1) program that checks for
38       crackable passwords during the selection process is recommended.
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40       The DES algorithm itself has a few quirks which make  the  use  of  the
41       crypt(3)  interface a very poor choice for anything other than password
42       authentication.  If you are planning on using  the  crypt(3)  interface
43       for  a cryptography project, don't do it: get a good book on encryption
44       and one of the widely available DES libraries.
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RETURN VALUE

47       A pointer to the encrypted password is returned.   On  error,  NULL  is
48       returned.
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ERRORS

51       ENOSYS The  crypt()  function  was not implemented, probably because of
52              U.S.A. export restrictions.
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GNU EXTENSION

55       The glibc2 version of this function has the following  additional  fea‐
56       tures.   If  salt is a character string starting with the three charac‐
57       ters "$1$" followed by at most eight characters, and optionally  termi‐
58       nated  by  "$",  then instead of using the DES machine, the glibc crypt
59       function uses an MD5-based algorithm,  and  outputs  up  to  34  bytes,
60       namely  "$1$<salt>$<encoded>",  where  "<salt>"  stands for the up to 8
61       characters following "$1$" in the salt, and "<encoded>" is a further 22
62       characters.   The characters in "<salt>" and "<encoded>" are drawn from
63       the set [a–zA–Z0–9./].  The entire key is significant here (instead  of
64       only the first 8 bytes).
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66       Programs using this function must be linked with -lcrypt.
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CONFORMING TO

69       SVr4, 4.3BSD, POSIX.1-2001
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