char *key, const char
char *in, char
*out, int32_t salt,
char *key, const char
char *key, const char
function performs password hashing 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
NUL-terminated string, typically a user's typed
password. The second is in one of three forms: if it begins with an
_’) 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.
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.
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:
The whole MD5 password string is passed as setting for interpretation.
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
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
The whole Blowfish password string is passed as setting for interpretation.
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.
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.
des_cipher() provide access to the DES algorithm
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
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
to process it.
The argument to
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
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.
des_setkey() functions all manipulate the same
crypt() returns a pointer to
the encrypted value on success, and
NULL on failure.
des_cipher() return 0 on success and 1 on
encrypt(1), login(1), passwd(1), blowfish(3), getpass(3), md5(3), passwd(5)
crypt() function appeared in
Version 3 AT&T UNIX. The current style
crypt() first appeared in
Version 7 AT&T UNIX.
David Burren <firstname.lastname@example.org> wrote the original DES functions.
crypt() function returns a pointer to
static data, and subsequent calls to
modify the same object.
With DES hashing, passwords containing the byte 0x80 use less key entropy than other passwords. This is an implementation bug, not a bug in the DES cipher.