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
functions.
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
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 current 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
encryption library. Programs linked against the
crypt() interface may be exported from the U.S.A.
only if they use crypt() solely for authentication
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>