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CentOS 7.0 - man page for crypt_r (centos section 3)

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

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).

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

COLOPHON
       This  page  is  part of release 3.53 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)


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