() 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 crypt
() is a
-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 (‘
a number then a different algorithm is used depending on the number. At the
’ chooses Blowfish hashing;
see below for more information.
() function is provided to
simplify checking a user's password. If both the hash and the password are the
empty string, authentication is a success. Otherwise, the password is hashed
and compared to the provided hash. If the hash is NULL, authentication will
always fail, but a default amount of work is performed to simulate the hashing
operation. A successful match will return 0. A failure will return -1 and set
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
The Blowfish version of crypt has 128 bits of
in order to make building dictionaries
of common passwords space consuming. The initial state of the Blowfish cipher
is expanded using the salt
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
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
is set to 25.
introduces disorder in the DES
algorithm in one of 16777216 or 4096 possible ways (i.e., with 24 or 12 bits:
if bit i
is set, then bits
swapped in the DES E-box output).
The DES key is used to encrypt a 64-bit constant using
iterations of DES. The value returned
-terminated string, 20 or 13 bytes
(plus NUL) in length, consisting of the
followed by the encoded 64-bit
The functions encrypt
() provide access to the DES algorithm
() 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
() is also a 64-byte array of binary
values. If the value of flag
is encrypted otherwise it is decrypted.
The result is returned in the original array
after using the key specified by
() 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
() encrypts (or decrypts if
is negative) the 64-bits stored in the
8 characters at in
iterations of DES and stores the 64-bit
result in the 8 characters at out
be the same as in
specifies perturbations to the DES E-box
output as described above.
() functions all manipulate the
same key space.
The function crypt
() returns a pointer to the
encrypted value on success, and
failure. The functions setkey
() return 0 on success and 1 on
A rotor-based crypt
() function appeared in
Version 3 AT&T UNIX
. The current style
() first appeared in
Version 7 AT&T UNIX
wrote the original DES functions.
() function returns a pointer to static
data, and subsequent calls to crypt
() will 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