Subversion Repositories planix.SVN

.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.