Ubuntu Manpage: crypt, crypt_r - password and data encryption

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).

       ┌──────────┬───────────────┬──────────────────────┐
       │Interface │ Attribute     │ Value                │
       ├──────────┼───────────────┼──────────────────────┤
       │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).

COLOPHON

       This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.

                                            2015-08-08                                   CRYPT(3)