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NAME    [Toc]    [Back]

     crypt -- Trapdoor encryption

LIBRARY    [Toc]    [Back]

     Crypt Library (libcrypt, -lcrypt)

SYNOPSIS    [Toc]    [Back]

     #include <unistd.h>

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

     const char *

     crypt_set_format(const char *string);

DESCRIPTION    [Toc]    [Back]

     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
			Extended Format is used in interpreting both the key
			and the salt, as outlined below.
	   Modular	If it begins with the string ``$digit$'' then the Modular
 Crypt Format is used, as outlined below.
	   Traditional	If neither of the above is true, it assumes the Traditional
 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 nullpadded)
 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
     becomes the initial DES key.  For each additional group, the XOR of the
     encryption 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 (ie. 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 terminated
 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 format
 to something else, the built-in default format is used.  This is currently
 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 according
 to the supplied string.

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

RETURN VALUES    [Toc]    [Back]

     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.

SEE ALSO    [Toc]    [Back]

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

BUGS    [Toc]    [Back]

     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.

HISTORY    [Toc]    [Back]

     A rotor-based crypt() function appeared in Version 6 AT&T UNIX.  The current
 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.

AUTHORS    [Toc]    [Back]

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

FreeBSD 5.2.1		       January 19, 1997 		 FreeBSD 5.2.1
[ Back ]
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