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

NAME         top

       crypt, crypt_r - password and data encryption

SYNOPSIS         top

       #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         top

       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

       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

       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         top

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

ERRORS         top

       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         top

       For an explanation of the terms used in this section, see

       │Interface Attribute     Value                │
       │crypt()   │ Thread safety │ MT-Unsafe race:crypt │
       │crypt_r() │ Thread safety │ MT-Safe              │

CONFORMING TO         top

       crypt(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.  crypt_r()
       is a GNU extension.

NOTES         top

   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 "$":


       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         top

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

COLOPHON         top

       This page is part of release 4.02 of the Linux man-pages
       project.  A description of the project, information about
       reporting bugs, and the latest version of this page, can be
       found at

                                 2015-08-08                         CRYPT(3)