NAME
crypt — Trapdoor encryption
LIBRARY
Crypt Library (libcrypt, −lcrypt)
SYNOPSIS
#include <unistd.h>
char *
crypt(const char *key, const char *salt);
const char *
crypt_get_format(void);
int
crypt_set_format(const char *string);
DESCRIPTION
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 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 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 (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 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:
$4$thesalt$rest
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
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
login(1), passwd(1), auth_getval(3), cipher(3), getpass(3), auth.conf(5), passwd(5)
BUGS
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
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
Originally written by David Burren <davidb [AT] werj.au>, later additions and changes by Poul-Henning Kamp, Mark R V Murray, Michael Bretterklieber, Kris Kennaway, Brian Feldman, Paul Herman and Niels Provos.
BSD January 19, 1997 BSD