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



NAME
       crypt, crypt_r - password and data encryption

SYNOPSIS
       #define _XOPEN_SOURCE
       #include <unistd.h>

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

       char *crypt_r(const char *key, const char *salt,
                     struct crypt_data *data);

       Link with -lcrypt.

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: The key space consists of 2**56 equal 7.2e16 possible values.  Exhaustive searches of this key space are  possi-
       ble  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 rec-
       ommended.

       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.

       crypt_r() is a reentrant version of crypt().  The structure pointed to by data is used to store result data and bookkeep-
       ing  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().

RETURN VALUE
       On success, a pointer to the encrypted password is returned.  On error, NULL is returned.

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

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

NOTES
   Glibc Notes
       The glibc2 version of this function supports additional encryption algorithms.

       If salt is a character string starting with the characters "$id$" followed by a string terminated by "$":

              $id$salt$encrypted

       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:

              ID  | Method
              ---------------------------------------------------------
              1   | MD5
              2a  | Blowfish (not in mainline glibc; added in some
                  | Linux distributions)
              5   | SHA-256 (since glibc 2.7)
              6   | SHA-512 (since glibc 2.7)

       So $5$salt$encrypted is an SHA-256 encoded password and $6$salt$encrypted is an SHA-512 encoded one.

       "salt" 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:

       MD5     | 22 characters
       SHA-256 | 43 characters
       SHA-512 | 86 characters

       The characters 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).

SEE ALSO
       login(1), passwd(1), encrypt(3), getpass(3), passwd(5), feature_test_macros(7)

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



                                                           2010-06-20                                                   CRYPT(3)