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.