NAME
crypt
, setkey
,
encrypt
, des_setkey
,
des_cipher
, bcrypt_gensalt
,
bcrypt
, md5crypt
—
DES encryption
SYNOPSIS
#include <pwd.h>
#include <unistd.h>
char *
crypt
(const
char *key, const char
*setting);
int
setkey
(const
char *key);
int
encrypt
(char
*block, int
flag);
int
des_setkey
(const
char *key);
int
des_cipher
(const
char *in, char
*out, int32_t salt,
int count);
char *
bcrypt_gensalt
(u_int8_t
log_rounds);
char *
bcrypt
(const
char *key, const char
*salt);
char *
md5crypt
(const
char *key, const char
*salt);
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
and to use stronger hashing algorithms.
The first argument to
crypt
() is
a NUL
-terminated string, typically a user's typed
password. The second is in one of three forms: if it begins with an
underscore (‘_
’) then an extended
format is used in interpreting both the key and the setting, as outlined
below. If it begins with a string character
(‘$
’) and a number then a different
algorithm is used depending on the number. At the moment a
‘$1
’ chooses MD5 hashing and a
‘$2
’ chooses Blowfish hashing; see
below for more information.
Extended crypt
The key is divided into groups of 8 characters (the last group is null-padded) and the low-order 7 bits of each character (56 bits per group) are used to form the DES key as follows: the first group of 56 bits becomes the initial DES key. For each additional group, the XOR of the encryption of the current DES key with itself and the group bits becomes the next DES key.
The setting is a 9-character array consisting of an underscore followed by 4 bytes of iteration count and 4 bytes of salt. These are encoded as printable characters, 6 bits per character, least significant character first. The values 0 to 63 are encoded as “./0-9A-Za-z”. This allows 24 bits for both count and salt.
MD5 crypt
For MD5 crypt the version number, salt and
the hashed password are separated by the
‘$
’ character. The maximum length of a
password is limited by the length counter of the MD5 context, which is about
2**64. A valid MD5 password entry looks like this:
“$1$caeiHQwX$hsKqOjrFRRN6K32OWkCBf1”.
The whole MD5 password string is passed as setting for interpretation.
Blowfish crypt
The Blowfish version of crypt has 128 bits of salt in order to make building dictionaries of common passwords space consuming. The initial state of the Blowfish cipher is expanded using the salt and the password repeating the process a variable number of rounds, which is encoded in the password string. The maximum password length is 72. The final Blowfish password entry is created by encrypting the string
“OrpheanBeholderScryDoubt”
with the Blowfish state 64 times.
The version number, the logarithm of the number of rounds and the
concatenation of salt and hashed password are separated by the
‘$
’ character. An encoded
‘8’ would specify 256 rounds. A valid Blowfish password looks
like this:
“$2a$12$eIAq8PR8sIUnJ1HaohxX2O9x9Qlm2vK97LJ5dsXdmB.eXF42qjchC”.
The whole Blowfish password string is passed as setting for interpretation.
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 setting is a 2-character array of the ASCII-encoded salt. Thus only 12 bits of salt are used. count is set to 25.
DES Algorithm
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
NUL
-terminated string, 20 or 13 bytes (plus NUL) in
length, consisting of the setting followed by the
encoded 64-bit encryption.
The functions
encrypt
(),
setkey
(),
des_setkey
(), and
des_cipher
() 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
flag 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 argument to
des_setkey
()
is a character array of length 8. The least significant bit (the parity bit)
in each character is ignored, and the remaining bits are concatenated to
form a 56-bit key. The function
des_cipher
()
encrypts (or decrypts if count is negative) the
64-bits stored in the 8 characters at in using
abs(3) of count iterations of DES and stores
the 64-bit result in the 8 characters at out (which
may be the same as in). The salt
specifies perturbations to the DES E-box output as described above.
The
crypt
(),
setkey
(),
and des_setkey
() functions all manipulate the same
key space.
RETURN VALUES
The function crypt
() returns a pointer to
the encrypted value on success, and NULL
on failure.
The functions setkey
(),
encrypt
(), des_setkey
(), and
des_cipher
() return 0 on success and 1 on
failure.
SEE ALSO
login(1), passwd(1), blowfish(3), getpass(3), md5(3), passwd(5)
HISTORY
A rotor-based crypt
() function appeared in
Version 3 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.
AUTHORS
David Burren ⟨davidb@werj.com.au⟩
BUGS
The crypt
() function returns a pointer to
static data, and subsequent calls to crypt
() will
modify the same object.