des_read_password, des_read_2password, des_string_to_key,
des_string_to_2key, des_read_pw_string, des_random_key,
des_set_key, des_key_sched, des_ecb_encrypt,
des_3ecb_encrypt, des_cbc_encrypt, des_3cbc_encrypt,
des_pcbc_encrypt, des_cfb_encrypt, des_ofb_encrypt,
des_cbc_cksum, des_quad_cksum, des_enc_read,
des_enc_write, des_set_odd_parity, des_is_weak_key, crypt
- (non USA) DES encryption
unsigned char *input;
unsigned char *output;
unsigned char *input,*output;
extern int des_rw_mode;
This library contains a fast implementation of the DES
There are two phases to the use of DES encryption. The
first is the generation of a des_key_schedule from a key,
the second is the actual encryption. A des key is of type
des_cblock. This type is made from 8 characters with odd
parity. The least significant bit in the character is the
parity bit. The key schedule is an expanded form of the
key; it is used to speed the encryption process.
des_read_password writes the string specified by prompt to
the standard output, turns off echo and reads an input
string from standard input until terminated with a newline.
If verify is non-zero, it prompts and reads the
input again and verifies that both entered passwords are
the same. The entered string is converted into a des key
by using the des_string_to_key routine. The new key is
placed in the des_cblock that was passed (by reference) to
the routine. If there were no errors, des_read_password
returns 0, -1 is returned if there was a terminal error
and 1 is returned for any other error.
des_read_2password operates in the same way as
des_read_password except that it generates 2 keys by using
the des_string_to_2key function.
des_read_pw_string is called by des_read_password to read
and verify a string from a terminal device. The string is
returned in buf. The size of buf is passed to the routine
via the length parameter.
des_string_to_key converts a string into a valid des key.
des_string_to_2key converts a string into 2 valid des
keys. This routine is best suited for use to generate
keys for use with des_3ecb_encrypt.
des_random_key returns a random key that is made of a combination
of process id, time and an increasing counter.
Before a des key can be used it is converted into a
des_key_schedule via the des_set_key routine. If the
des_check_key flag is non-zero, des_set_key will check
that the key passed is of odd parity and is not a weak or
semi-weak key. If the parity is wrong, then -1 is
returned. If the key is a weak key, then -2 is returned.
If an error is returned, the key schedule is not generated.
des_key_sched is another name for the des_set_key function.
The following routines mostly operate on an input and output
stream of des_cblock's.
des_ecb_encrypt is the basic DES encryption routine that
encrypts or decrypts a single 8-byte des_cblock in elec-
tronic code book mode. It always transforms the input
data, pointed to by input, into the output data, pointed
to by the output argument. If the encrypt argument is
non-zero (DES_ENCRYPT), the input (cleartext) is encrypted
in to the output (ciphertext) using the key_schedule specified
by the schedule argument, previously set via
des_set_key. If encrypt is zero (DES_DECRYPT), the input
(now ciphertext) is decrypted into the output (now cleartext).
Input and output may overlap. No meaningful value
des_3ecb_encrypt encrypts/decrypts the input block by
using triple ecb DES encryption. This involves encrypting
the input with ks1, decryption with the key schedule ks2,
and then encryption with the first again. This routine
greatly reduces the chances of brute force breaking of DES
and has the advantage of if ks1 and ks2 are the same, it
is equivalent to just encryption using ecb mode and ks1 as
des_cbc_encrypt encrypts/decrypts using the cipher-block-
chaining mode of DES. If the encrypt argument is nonzero,
the routine cipher-block-chain encrypts the cleartext
data pointed to by the input argument into the ciphertext
pointed to by the output argument, using the key
schedule provided by the schedule argument, and initialisation
vector provided by the ivec argument. If the
length argument is not an integral multiple of eight
bytes, the last block is copied to a temporary area and
zero filled. The output is always an integral multiple of
eight bytes. To make multiple cbc encrypt calls on a
large amount of data appear to be one des_cbc_encrypt
call, the ivec of subsequent calls should be the last 8
bytes of the output.
des_3cbc_encrypt encrypts/decrypts the input block by
using triple cbc DES encryption. This involves encrypting
the input with key schedule ks1, decryption with the key
schedule ks2, and then encryption with the first again. 2
initialisation vectors are required, ivec1 and ivec2.
Unlike des_cbc_encrypt, these initialisation vectors are
modified by the subroutine. This routine greatly reduces
the chances of brute force breaking of DES and has the
advantage of if ks1 and ks2 are the same, it is equivalent
to just encryption using cbc mode and ks1 as the key.
des_pcbc_encrypt encrypt/decrypts using a modified block
chaining mode. It provides better error propagation characteristics
than cbc encryption.
des_cfb_encrypt encrypt/decrypts using cipher feedback
mode. This method takes an array of characters as input
and outputs an array of characters. It does not require
any padding to 8 character groups. Note: the ivec variable
is changed and the new changed value needs to be
passed to the next call to this function. Since this
function runs a complete DES ecb encryption per numbits,
this function is only suggested for use when sending small
numbers of characters.
des_ofb_encrypt encrypts using output feedback mode. This
method takes an array of characters as input and outputs
an array of characters. It does not require any padding
to 8 character groups. Note: the ivec variable is changed
and the new changed value needs to be passed to the next
call to this function. Since this function runs a complete
DES ecb encryption per numbits, this function is
only suggested for use when sending small numbers of characters.
des_cbc_cksum produces an 8 byte checksum based on the
input stream (via cbc encryption). The last 4 bytes of
the checksum is returned and the complete 8 bytes is
placed in output.
des_quad_cksum returns a 4 byte checksum from the input
bytes. The algorithm can be iterated over the input,
depending on out_count, 1, 2, 3 or 4 times. If output is
non-NULL, the 8 bytes generated by each pass are written
des_enc_write is used to write len bytes to file descriptor
fd from buffer buf. The data is encrypted via
pcbc_encrypt (default) using sched for the key and iv as a
starting vector. The actual data sent down fd consists of
4 bytes (in network byte order) containing the length of
the following encrypted data. The encrypted data then
follows, padded with random data out to a multiple of 8
des_enc_read is used to read len bytes from file descriptor
fd into buffer buf. The data being read from fd is
assumed to have come from des_enc_write and is decrypted
using sched for the key schedule and iv for the initial
vector. The des_enc_read/des_enc_write pair can be used
to read/write to files, pipes and sockets. I have used
them in implementing a version of rlogin in which all data
des_rw_mode is used to specify the encryption mode to use
with des_enc_read and des_end_write. If set to
DES_PCBC_MODE (the default), des_pcbc_encrypt is used. If
set to DES_CBC_MODE des_cbc_encrypt is used. These two
routines and the variable are not part of the normal MIT
des_set_odd_parity sets the parity of the passed key to
odd. This routine is not part of the standard MIT
des_is_weak_key returns 1 if the passed key is a weak key
(pick again :-), 0 if it is ok. This routine is not part
of the standard MIT library.
crypt is a replacement for the normal system crypt. It is
much faster than the system crypt.
The encryption routines have been tested on 16bit, 32bit
and 64bit machines of various endian and even works under
If you think this manual is sparse, read the des_crypt(3)
manual from the MIT kerberos (or bones outside of the USA)
des_cfb_encrypt and des_ofb_encrypt operates on input of 8
bits. What this means is that if you set numbits to 12,
and length to 2, the first 12 bits will come from the 1st
input byte and the low half of the second input byte. The
second 12 bits will have the low 8 bits taken from the 3rd
input byte and the top 4 bits taken from the 4th input
byte. The same holds for output. This function has been
implemented this way because most people will be using a
multiple of 8 and because once you get into pulling input
bytes apart things get ugly!
des_read_pw_string is the most machine/OS dependent function
and normally generates the most problems when porting
des_string_to_key is probably different from the MIT version
since there are lots of fun ways to implement one-way
encryption of a text string.
The routines are optimised for 32 bit machines and so are
not efficient on IBM PCs.
NOTE: extensive work has been done on this library since
this document was originally written. Please try to read
des.doc from the libdes distribution since it is far more
up to date and documents more of the functions. Libdes is
now also being shipped as part of SSLeay, a general cryptographic
library that amongst other things implements
Netscape's SSL protocol. The most recent version can be
found in SSLeay distributions.
Eric Young (firstname.lastname@example.org or email@example.com)
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