harvey/sys/man/2/des

.TH DES 2
.SH NAME
setupDESstate, des_key_setup, block_cipher, desCBCencrypt, desCBCdecrypt, desECBencrypt, desECBdecrypt, des3CBCencrypt, des3CBCdecrypt, des3ECBencrypt, des3ECBdecrypt, key_setup, des56to64, des64to56, setupDES3state, triple_block_cipher  - single and triple digital encryption standard
.SH SYNOPSIS
.B #include <u.h>
.br
.B #include <libc.h>
.br
.B #include <mp.h>
.br
.B #include <libsec.h>
.PP
.B
void	des_key_setup(uchar key[8], ulong schedule[32])
.PP
.B
void	block_cipher(ulong *schedule, uchar *data, int decrypting)
.PP
.B
void	setupDESstate(DESstate *s, uchar key[8], uchar *ivec)
.PP
.B
void	desCBCencrypt(uchar *p, int len, DESstate *s)
.PP
.B
void	desCBCdecrypt(uchar *p, int len, DESstate *s)
.PP
.B
void	desECBencrypt(uchar *p, int len, DESstate *s)
.PP
.B
void	desECBdecrypt(uchar *p, int len, DESstate *s)
.PP
.in +0.5i
.ti -0.5i
.B
void	triple_block_cipher(ulong expanded_key[3][32], uchar text[8], int ende)
.PP
.B
void	setupDES3state(DES3state *s, uchar key[3][8], uchar *ivec)
.PP
.B
void	des3CBCencrypt(uchar *p, int len, DES3state *s)
.PP
.B
void	des3CBCdecrypt(uchar *p, int len, DES3state *s)
.PP
.B
void	des3ECBencrypt(uchar *p, int len, DES3state *s)
.PP
.B
void	des3ECBdecrypt(uchar *p, int len, DES3state *s)
.PP
.B
void	key_setup(uchar[7], ulong[32])
.PP
.B
void	des56to64(uchar *k56, uchar *k64)
.PP
.B
void	des64to56(uchar *k64, uchar *k56)
.SH DESCRIPTION
The Digital Encryption Standard (DES)
is a shared-key or symmetric encryption 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. 
.PP
The basic DES function,
.IR block_cipher ,
works on a block of 8 bytes, converting 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
.IR des_key_setup .
.PP
Since it is a bit awkward,
.I block_cipher
is rarely called directly.  Instead, 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
.B DESstate
structure that contains the key schedule and any chained
state.
.I SetupDESstate
sets up the
.B DESstate
structure using the key and an 8-byte initialization vector.
.PP
Electronic code book, using
.I desECBencrypt
and 
.IR 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.
.PP
Cipher block chaining mode, using
.I desCBCencrypt
and
.IR desCBCdecrypt ,
is more secure.  Every block encrypted depends on the initialization
vector and all blocks encrypted before it.
.PP
For both CBC and ECB modes, a stream of data can be encrypted as
multiple buffers.  However, all buffers except the last must
be a multiple of 8 bytes to ensure successful decryption of
the stream.
.PP
There are equivalent triple-DES (DES3-EDE) functions for each of the
DES functions.
.PP
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,
.I des56to64
and
.IR des64to56 ,
to convert back and forth between the two formats.
Also a key schedule can be set up from the 7-byte format using
.IR key_setup .
.SH SOURCE
.B /sys/src/libsec
.SH SEE ALSO
.IR mp (2),
.IR aes (2),
.IR blowfish (2),
.IR dsa (2),
.IR elgamal (2),
.IR rc4 (2),
.IR rsa (2),
.IR sechash (2),
.IR prime (2),
.IR rand (2)
.br
.IR "Breaking DES" ,
Electronic Frontier Foundation,
O'Reilly, 1998
.SH 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
.IR aes (2)
instead, or at least triple DES.