term% cat index.txt DES(2) System Calls Manual DES(2)
NAME
setupDESstate, des_key_setup, block_cipher, desCBCencrypt, desCBCde‐
crypt, desECBencrypt, desECBdecrypt, des3CBCencrypt, des3CBCdecrypt,
des3ECBencrypt, des3ECBdecrypt, key_setup, des56to64, des64to56, se‐
tupDES3state, triple_block_cipher - single and triple digital encryp‐
tion standard
SYNOPSIS
#include <u.h>
#include <libc.h>
#include <mp.h>
#include <libsec.h>
void des_key_setup(uchar key[8], ulong schedule[32])
void block_cipher(ulong *schedule, uchar *data, int decrypting)
void setupDESstate(DESstate *s, uchar key[8], uchar *ivec)
void desCBCencrypt(uchar *p, int len, DESstate *s)
void desCBCdecrypt(uchar *p, int len, DESstate *s)
void desECBencrypt(uchar *p, int len, DESstate *s)
void desECBdecrypt(uchar *p, int len, DESstate *s)
void triple_block_cipher(ulong expanded_key[3][32], uchar text[8], int
ende)
void setupDES3state(DES3state *s, uchar key[3][8], uchar *ivec)
void des3CBCencrypt(uchar *p, int len, DES3state *s)
void des3CBCdecrypt(uchar *p, int len, DES3state *s)
void des3ECBencrypt(uchar *p, int len, DES3state *s)
void des3ECBdecrypt(uchar *p, int len, DES3state *s)
void key_setup(uchar[7], ulong[32])
void des56to64(uchar *k56, uchar *k64)
void des64to56(uchar *k64, uchar *k56)
DESCRIPTION
The Digital Encryption Standard (DES) is a shared-key or symmetric en‐
cryption algorithm using either a 56-bit key for single DES or three
56-bit keys for triple DES. The keys are encoded into 64 bits where
every eight bit is parity.
The basic DES function, block_cipher, works on a block of 8 bytes, con‐
verting them in place. It takes a key schedule, a pointer to the
block, and a flag indicating encrypting (0) or decrypting (1). The key
schedule is created from the key using des_key_setup.
Since it is a bit awkward, block_cipher is rarely called directly. In‐
stead, one normally uses routines that encrypt larger buffers of data
and which may chain the encryption state from one buffer to the next.
These routines keep track of the state of the encryption using a
DESstate structure that contains the key schedule and any chained
state. SetupDESstate sets up the DESstate structure using the key and
an 8-byte initialization vector.
Electronic code book, using desECBencrypt and desECBdecrypt, is the
less secure mode. The encryption of each 8 bytes does not depend on
the encryption of any other. Hence the encryption is a substitution
cipher using 64 bit characters.
Cipher block chaining mode, using desCBCencrypt and desCBCdecrypt, is
more secure. Every block encrypted depends on the initialization vec‐
tor and all blocks encrypted before it.
For both CBC and ECB modes, a stream of data can be encrypted as multi‐
ple buffers. However, all buffers except the last must be a multiple
of 8 bytes to ensure successful decryption of the stream.
There are equivalent triple-DES (DES3-EDE) functions for each of the
DES functions.
In the past, Plan 9 used a 56-bit or 7-byte format for DES keys. To be
compatible with the rest of the world, we've abandoned this format.
There are two functions, des56to64 and des64to56, to convert back and
forth between the two formats. Also a key schedule can be set up from
the 7-byte format using key_setup.
SOURCE
/sys/src/libsec
SEE ALSO
mp(2), aes(2), blowfish(2), dsa(2), elgamal(2), rc4(2), rsa(2),
sechash(2), prime(2), rand(2)
Breaking DES, Electronic Frontier Foundation, O'Reilly, 1998
BUGS
Single DES can be realistically broken by brute-force; its 56-bit key
is just too short. It should not be used in new code, which should
probably use aes(2) instead, or at least triple DES.
DES(2)