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NAME

       crypt, crypt_r - password and data encryption

SYNOPSIS

       #define _XOPEN_SOURCE       /* See feature_test_macros(7) */
       #include <unistd.h>

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

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <crypt.h>

       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 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.

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

       EINVAL salt has the wrong format.

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

       EPERM  /proc/sys/crypto/fips_enabled has a nonzero  value,  and  an  attempt  was  made  to  use  a  weak
              encryption type, such as DES.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────┬───────────────┬──────────────────────┐
       │InterfaceAttributeValue                │
       ├──────────┼───────────────┼──────────────────────┤
       │crypt()   │ Thread safety │ MT-Unsafe race:crypt │
       ├──────────┼───────────────┼──────────────────────┤
       │crypt_r() │ Thread safety │ MT-Safe              │
       └──────────┴───────────────┴──────────────────────┘

CONFORMING TO

       crypt(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.  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)

COLOPHON

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                                                   2015-08-08                                           CRYPT(3)