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CRYPT(3)		 BSD Library Functions Manual		      CRYPT(3)

     crypt -- Trapdoor encryption

     Crypt Library (libcrypt, -lcrypt)

     #include <unistd.h>

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

     const char	*

     crypt_set_format(const char *string);

     The crypt() function performs password hashing with additional code added
     to	deter key search attempts.  Different algorithms can be	used to	in the
     hash.  Currently these include the	NBS Data Encryption Standard (DES),
     MD5 hash, NT-Hash (compatible with	Microsoft's NT scheme) and Blowfish.
     The algorithm used	will depend upon the format of the Salt	(following the
     Modular Crypt Format (MCF)), if DES and/or	Blowfish is installed or not,
     and whether crypt_set_format() has	been called to change the default.

     The first argument	to crypt is the	data to	hash (usually a	password), in
     a null-terminated string.	The second is the salt,	in one of three	forms:

	   Extended	If it begins with an underscore	("_") then the DES Ex-
			tended Format is used in interpreting both the key and
			the salt, as outlined below.
	   Modular	If it begins with the string "$digit$" then the	Modu-
			lar Crypt Format is used, as outlined below.
	   Traditional	If neither of the above	is true, it assumes the	Tradi-
			tional Format, using the entire	string as the salt (or
			the first portion).

     All routines are designed to be time-consuming.  A	brief test on a
     Pentium 166/MMX shows the DES crypt to do approximately 2640 crypts a CPU
     second and	MD5 to do about	62 crypts a CPU	second.

   DES Extended	Format:
     The key is	divided	into groups of 8 characters (the last group is null-
     padded) and the low-order 7 bits of each character	(56 bits per group)
     are used to form the DES key as follows: the first	group of 56 bits be-
     comes the initial DES key.	 For each additional group, the	XOR of the en-
     cryption of the current DES key with itself and the group bits becomes
     the next DES key.

     The salt is a 9-character array consisting	of an underscore followed by 4
     bytes of iteration	count and 4 bytes of salt.  These are encoded as
     printable characters, 6 bits per character, least significant character
     first.  The values	0 to 63	are encoded as ``./0-9A-Za-z''.	 This allows
     24	bits for both count and	salt.

     The salt introduces disorder in the DES algorithm in one of 16777216 or
     4096 possible ways	(i.e., with 24 or 12 bits: if bit i of the salt	is
     set, then bits i and i+24 are swapped in the DES E-box output).

     The DES key is used to encrypt a 64-bit constant using count iterations
     of	DES.  The value	returned is a null-terminated string, 20 or 13 bytes
     (plus null) in length, consisting of the salt followed by the encoded
     64-bit encryption.

   Modular crypt:
     If	the salt begins	with the string	$digit$	then the Modular Crypt Format
     is	used.  The digit represents which algorithm is used in encryption.
     Following the token is the	actual salt to use in the encryption.  The
     length of the salt	is limited to 8	characters--because the	length of the
     returned output is	also limited (_PASSWORD_LEN).  The salt	must be	termi-
     nated with	the end	of the string (NULL) or	a dollar sign.	Any characters
     after the dollar sign are ignored.

     Currently supported algorithms are:

	   1.	MD5
	   2.	Blowfish
	   3.	NT-Hash

     Other crypt formats may be	easily added.  An example salt would be:


   Traditional crypt:
     The algorithm used	will depend upon whether crypt_set_format() has	been
     called and	whether	a global default format	has been specified.  Unless a
     global default has	been specified or crypt_set_format() has set the for-
     mat to something else, the	built-in default format	is used.  This is cur-
     rently DES	if it is available, or MD5 if not.

     How the salt is used will depend upon the algorithm for the hash.	For
     best results, specify at least two	characters of salt.

     The crypt_get_format() function returns a constant	string that represents
     the name of the algorithm currently used.	Valid values are `des',	`blf',
     `md5' and `nth'.

     The crypt_set_format() function sets the default encoding format accord-
     ing to the	supplied string.

     The global	default	format can be set using	the /etc/auth.conf file	using
     the crypt_default property.

     The crypt() function returns a pointer to the encrypted value on success,
     and NULL on failure.  Note: this is not a standard	behaviour, AT&T
     crypt() will always return	a pointer to a string.

     The crypt_set_format() function will return 1 if the supplied encoding
     format was	valid.	Otherwise, a value of 0	is returned.

     login(1), passwd(1), auth_getval(3), getpass(3), auth.conf(5), passwd(5)

     A rotor-based crypt() function appeared in	Version	6 AT&T UNIX.  The cur-
     rent style	crypt()	first appeared in Version 7 AT&T UNIX.

     The DES section of	the code (FreeSec 1.0) was developed outside the
     United States of America as an unencumbered replacement for the U.S.-only
     NetBSD libcrypt encryption	library.

     Originally	written	by David Burren	<>, later additions
     and changes by Poul-Henning Kamp, Mark R V	Murray,	Michael
     Bretterklieber, Kris Kennaway, Brian Feldman, Paul	Herman and Niels

     The crypt() function returns a pointer to static data, and	subsequent
     calls to crypt() will modify the same data.  Likewise, crypt_set_format()
     modifies static data.

     The NT-hash scheme	does not use a salt, and is not	hard for a competent
     attacker to break.	 Its use is not	recommended.

BSD			       January 19, 1997				   BSD


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