Ubuntu Manpage: crypt, crypt_r - password and data encryption

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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

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

ATTRIBUTES

   Multithreading (see pthreads(7))
       The crypt() function is not thread-safe.

       The crypt_r() function is thread-safe.

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).

COLOPHON

       This page is part of release 3.54 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/.

                                                   2013-06-21                                           CRYPT(3)