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crypt(3) - crypt, crypt - password and data encryption - man 3 crypt

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CRYPT(3)                       Library functions                      CRYPT(3)

       crypt - password and data encryption

       #define _XOPEN_SOURCE
       #include <unistd.h>

       char *crypt(const char *key, const char *salt);

       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(3,n) chosen from the set(7,n,1 builtins) [a-zA-Z0-9./].  This
       string(3,n)  is used to perturb the algorithm in(1,8) 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(3,n)  (usually  a  string(3,n)  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(1,5)(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(3)  interface a very poor choice for anything other than password
       authentication.  If you are planning on using  the  crypt(3)  interface
       for  a cryptography project, don't do it: get a good book on encryption
       and one of the widely available DES libraries.

       A pointer to the encrypted password is returned.   On  error(8,n),  NULL  is

       ENOSYS The  crypt  function  was  not  implemented, probably because of
              U.S.A. export restrictions.

       The glibc2 version(1,3,5) of this function has the following  additional  fea-
       tures.   If  salt is a character string(3,n) starting with the three charac-
       ters "$1$" followed by at most eight characters, and optionally  termi-
       nated  by  "$",  then instead of using the DES machine, the glibc crypt
       function uses an MD5-based algorithm,  and  outputs  up  to  34  bytes,
       namely  "$1$<string(3,n)>$", where "<string(3,n)>" stands for the up to 8 charac-
       ters following "$1$" in(1,8) the salt, followed by 22 bytes chosen from  the
       set(7,n,1 builtins) [a-zA-Z0-9./].  The entire key is significant here (instead of only
       the first 8 bytes).

       Programs using this function must be linked with -lcrypt.

       SVID, X/OPEN, BSD 4.3, POSIX 1003.1-2001

       login(1,3,5)(1), passwd(1,5)(1), encrypt(3), getpass(3), passwd(1,5)(5)

                                  2001-12-23                          CRYPT(3)

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