crypt(3) BSD Library Functions Manual crypt(3)
NAME
crypt, encrypt, setkey -- DES encryption
SYNOPSIS
#include <unistd.h>
char *
crypt(const char *key, const char *salt);
void
encrypt(char *block, int edflag);
#include <stdlib.h>
void
setkey(const char *key);
DESCRIPTION
The crypt() function performs password encryption, based on the NBS Data
Encryption Standard (DES). Additional code has been added to deter key
search attempts. The first argument to crypt() is a null-terminated
string, typically a user's typed password. The second is in one of two
forms: if it begins with an underscore (``_''), an extended format is
used in interpreting both the key and the salt value, as outlined below.
Extended crypt:
The key is divided into groups of 8 characters (the last group is null-
padded) and the low-order 7 bits of each 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.
Traditional crypt:
The first 8 bytes of the key are null-padded, and the low-order 7 bits of
each character is used to form the 56-bit DES key.
The salt is a 2-character array of the ASCII-encoded salt. Thus, only 12
bits of salt are used. count is set to 25.
Algorithm:
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.
The functions, encrypt() and setkey() provide access to the DES algorithm
itself. setkey() is passed a 64-byte array of binary values (numeric 0
or 1). A 56-bit key is extracted from this array by dividing the array
into groups of 8 and ignoring the last bit in each group. That bit is
reserved for a byte parity check by DES, but is ignored by these func-
tions.
The block argument to encrypt() is also a 64-byte array of binary values.
If the value of edflag is 0, block is encrypted; otherwise, it is
decrypted. The result is returned in the original array block, after
using the key specified by setkey() to process it.
The function crypt() returns a pointer to the encrypted value on success,
and NULL on failure.
The crypt() and setkey() functions all manipulate the same key space.
SEE ALSO
login(1), passwd(1), getpass(3), compat(5), passwd(5)
LEGACY SYNOPSIS
#include <unistd.h>
int
encrypt(char *block, int edflag);
The function encrypt() returns 0 on success and 1 on failure.
void
setkey(const char *key);
The include file <unistd.h> is necessary and sufficient for the setkey()
function.
BUGS
The crypt() function returns a pointer to static data, and subsequent
calls to crypt() will modify the same object.
HISTORY
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.
This library (FreeSec 1.0) was developed outside the United States of
America as an unencumbered replacement for the U.S.-only libcrypt encryp-
tion library. Programs linked against the crypt() interface may be
exported from the U.S.A. only if they use crypt() solely for authentica-
tion purposes and avoid use of the other programmer interfaces listed
above. Special care has been taken in the library so that programs which
only use the crypt() interface do not pull in the other components.
AUTHOR
David Burren <davidb@werj.com.au>
FreeSec 1.0 March 9, 1994 FreeSec 1.0
Mac OS X 10.8 - Generated Sun Aug 26 16:07:35 CDT 2012