CRYPT(3) OpenBSD Programmer's Manual CRYPT(3)
NAME
crypt, setkey, encrypt, des_setkey, des_cipher - DES encryption
SYNOPSIS
#include <unistd.h>
char
*crypt(const char *key, const char *setting);
int
setkey(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);
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 argu-
ment to crypt() is a null-terminated string, typically a user's typed
password. The second is in one of three forms: if it begins with an un-
derscore (``_'') then an extended format is used in interpreting both the
key and the setting, as outlined below. If it begins with an string char-
acter (``$'') and a number then a different algorithm is used depending
on the number. At the moment a ``$1'' chooses MD5 hashing and a ``$2''
chooses Blowfish hashing, see below for more information.
Extended crypt:
The key is divided into groups of 8 characters (the last group is null-
padded) and the low-order 7 bits of each character (56 bits per group)
are used to form the DES key as follows: the first group of 56 bits be-
comes the initial DES key. For each additional group, the XOR of the en-
cryption of the current DES key with itself and the group bits becomes
the next DES key.
The setting is a 9-character array consisting of an underscore followed
by 4 bytes of iteration count and 4 bytes of salt. These are encoded as
printable characters, 6 bits per character, least significant character
first. The values 0 to 63 are encoded as ``./0-9A-Za-z''. This allows
24 bits for both count and salt.
MD5 crypt:
For MD5 crypt the version number, salt and the hashed password are sepa-
rated by the ``$'' 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$hsKqOjrFRRN6K32OWkCBf1''.
The whole MD5 password string is passed as setting for interpretation.
Blowfish crypt:
The Blowfish version of crypt has 128 bits of salt in order to make
building dictionaries of common passwords space consuming. The initial
state of the Blowfish cipher is expanded using the salt and the password
repeating the process a variable number of rounds, which is encoded in
the password string. The maximum password length is 72. The final Blow-
fish password entry is created by encrypting the string ``OrpheanBehold-
erScryDoubt'' with the Blowfish state 64 times.
The version number, the logarithm of the number of rounds and the con-
catenation of salt and hashed password are separated by the ``$'' charac-
ter. An encoded ``8'' would specify 256 rounds. A valid Blowfish pass-
word looks like this:
``$2a$12$eIAq8PR8sIUnJ1HaohxX2O9x9Qlm2vK97LJ5dsXdmB.eXF42qjchC''.
The whole Blowfish password string is passed as setting for interpreta-
tion.
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 (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 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 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.
The crypt(), setkey() and des_setkey() functions all manipulate the same
key space.
SEE ALSO
login(1), passwd(1), blowfish(3), getpass(3), md5(3), passwd(5)
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 ex-
ported 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>
OpenBSD 2.6 March 9, 1994 3