1CRYPT(3)                   Linux Programmer's Manual                  CRYPT(3)
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NAME

6       crypt, crypt_r - password and data encryption
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SYNOPSIS

9       #define _XOPEN_SOURCE
10       #include <unistd.h>
11
12       char *crypt(const char *key, const char *salt);
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14       char *crypt_r(const char *key, const char *salt,
15                     struct crypt_data *data);
16
17       Link with -lcrypt.
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DESCRIPTION

20       crypt()  is  the password encryption function.  It is based on the Data
21       Encryption Standard algorithm with  variations  intended  (among  other
22       things) to discourage use of hardware implementations of a key search.
23
24       key is a user's typed password.
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26       salt is a two-character string chosen from the set [azAZ09./].  This
27       string is used to perturb the algorithm in one of 4096 different ways.
28
29       By taking the lowest 7 bits of each of the first  eight  characters  of
30       the  key, a 56-bit key is obtained.  This 56-bit key is used to encrypt
31       repeatedly a constant  string  (usually  a  string  consisting  of  all
32       zeros).   The returned value points to the encrypted password, a series
33       of 13 printable ASCII characters (the first  two  characters  represent
34       the salt itself).  The return value points to static data whose content
35       is overwritten by each call.
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37       Warning: The key space consists of 2**56 equal 7.2e16 possible  values.
38       Exhaustive searches of this key space are possible using massively par‐
39       allel computers.  Software, such as crack(1), is available  which  will
40       search  the  portion of this key space that is generally used by humans
41       for passwords.  Hence, password selection  should,  at  minimum,  avoid
42       common words and names.  The use of a passwd(1) program that checks for
43       crackable passwords during the selection process is recommended.
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45       The DES algorithm itself has a few quirks which make  the  use  of  the
46       crypt()  interface  a very poor choice for anything other than password
47       authentication.  If you are planning on using the crypt() interface for
48       a  cryptography project, don't do it: get a good book on encryption and
49       one of the widely available DES libraries.
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51       crypt_r() is a reentrant version of crypt().  The structure pointed  to
52       by  data  is  used  to  store  result data and bookkeeping information.
53       Other than allocating it, the only thing that the caller should do with
54       this  structure  is  to  set data->initialized to zero before the first
55       call to crypt_r().
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RETURN VALUE

58       On success, a pointer to the encrypted password is returned.  On error,
59       NULL is returned.
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ERRORS

62       ENOSYS The  crypt()  function  was not implemented, probably because of
63              U.S.A. export restrictions.
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CONFORMING TO

66       crypt(): SVr4, 4.3BSD, POSIX.1-2001.  crypt_r() is a GNU extension.
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NOTES

69   Glibc Notes
70       The glibc2 version of  this  function  supports  additional  encryption
71       algorithms.
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73       If  salt is a character string starting with the characters "$id$" fol‐
74       lowed by a string terminated by "$":
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76              $id$salt$encrypted
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78       then instead of using the DES machine,  id  identifies  the  encryption
79       method  used  and  this  then  determines  how the rest of the password
80       string is interpreted.  The following values of id are supported:
81
82              ID  | Method
83              ─────────────────────────────────────────────────────────
84              1   | MD5
85              2a  | Blowfish (not in mainline glibc; added in some
86                  | Linux distributions)
87              5   | SHA-256 (since glibc 2.7)
88              6   | SHA-512 (since glibc 2.7)
89
90       So   $5$salt$encrypted   is   an   SHA-256   encoded    password    and
91       $6$salt$encrypted is an SHA-512 encoded one.
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93       "salt" stands for the up to 16 characters following "$id$" in the salt.
94       The encrypted part of the password string is the actual computed  pass‐
95       word.  The size of this string is fixed:
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97       MD5     | 22 characters
98       SHA-256 | 43 characters
99       SHA-512 | 86 characters
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101       The  characters  in  "salt"  and  "encrypted"  are  drawn  from the set
102       [azAZ09./].  In the MD5 and SHA implementations the  entire  key  is
103       significant (instead of only the first 8 bytes in DES).
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SEE ALSO

106       login(1),    passwd(1),   encrypt(3),   getpass(3),   passwd(5),   fea‐
107       ture_test_macros(7)
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COLOPHON

110       This page is part of release 3.22 of the Linux  man-pages  project.   A
111       description  of  the project, and information about reporting bugs, can
112       be found at http://www.kernel.org/doc/man-pages/.
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116                                  2010-06-20                          CRYPT(3)
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