des.doc

des的变形算法

                       DES.C, 3DES.C, AND DES486.ASM

     These are high-speed, 486-only implementations of the Data Encryption 
Standard (DES) for IBM pc clones running DOS. They may over-write the 
original files, or they may send the output to the same filenames on 
another path. Default is to over-write. 
     DES.EXE uses Stream Cipher mode with IV/Ciphertext Feedback, which 
helps hide patterns in repetitive data.
     3DES.EXE is similar, but uses three full rounds of DES with two 
passwords. The procedure is to encrypt with password 1, decrypt with 
password 2, re-encrypt with password 1. This procedure gives much higher 
security. You'll note that if password 1 is set the same as password 2, 
3DES is equivalent to regular DES.
     The assembly source is designed to be used with C. I used Turbo 
Assembler and Turbo C to develop this code.

     Usage: DES -c|d [-oPATH] [-h] [password] filespec.
     -c crypt (encrypt)
     -d decrypt
     -o output path (optional)
     -h hide password input (optional)
     For DES, but not for 3DES, password may be optionally entered on the 
command line.

     You must specify -c (for crypt) or -d (for decrypt).
     The default action of DES.EXE is to over-write the original file. To 
preserve the original, and make a new file of the same name on a new path, 
use the -o (output) option. For example, 'DES -c -od:\temp\ password 
myfile' will encrypt myfile to d:\temp\myfile. You can't change the name, 
only the path. THERE MUST BE NO SPACES BETWEEN "-o" AND THE PATH.
     Wildcards are permissible in the filespec. 'DES -d "Better Angels of 
our Nature" *.dbf' decrypts all .dbf files.

                            PASSWORD SELECTION:
     You have 4 options for passwords. 
     If you simply type in text, it will be used more or less verbatim. 
Characters after the first 8 will be combined with the first 8 bytes.
     If you type the character '#', followed by text, the password will be 
an MD5 hash of the text. For example, 'DES -c "#I love the lovely bully" 
myfile' will use the MD5 hash of 'I love the lovely bully' as the password. 
(Note: an MD5 hash is 16 bytes. DES simply combines the 1st 8 with the 2nd 
8 to form the 8-byte password.)
     It's also possible to use a file as a source for a password. To do 
this,  use the character '@' or '!' and a filename, possibly followed by a 
comma and an offset value, for the password. F'rinstance, 
"DES -c @hidstuff,1025 myfile" takes 8 bytes for the password from hidstuff 
starting at the 1025th byte. The offset is optional, and defaults to 0.
     If you use '!' instead of '@' as the first character, the 8 bytes 
taken from the file will be DES decrypted using a second password, for 
which you'll be prompted. The "-h" switch will hide the input for this 
password also. 
     Example: "DES -c -h !b:secrets *.dbf" will encrypt all .dbf files. 
You'll be prompted for an auxiliary password. Its input will be hidden, and 
it will be used to decrypt the first 8 bytes of the file b:secrets. Those 8 
decrypted bytes will be used as the password for the .dbf files. 
     Another example: "3DES -c myfile" will triple-encrypt myfile. If you 
enter !c:\command.com,102 at the password prompt, you'll be prompted for an 






auxiliary password. It'll be used to decrypt the 8 bytes at offset 102 in 
the file c:\command.com. Those 8 bytes will form the first password. Then 
you'll be prompted for the second password-- if you enter, for example, 
"Stiffen the sinews, conjure up the blood," that text will be mashed down 
to the 8 bytes that form the second password.
     For DES (not for 3DES), the password may be specified on the command 
line. If it isn't specified on the command line, you'll be prompted for it. 
If you want the password concealed on the terminal, use the "-h" option 
(for 'hide'), and asterisks will be echoed back instead.
     Ultimately, the password is reduced to 8 bytes. If your password 
doesn't come from a file, please feel free to specify a longer password: 
the extra characters are mashed into the first 8. Long passwords are 
recommended. 
     To specify a multi-word password on the command line, use quotes. E.g. 
DES -c "de tongues of de mans is be full of deceits" *.asp. If you let DES 
prompt for the password, don't use quotes. For example, 'DES -c 
"break up their drowsy grave" myfile' puts the password on the command 
line. If you just type 'DES -d myfile', when you're prompted for the 
password, don't type the quotes. Just type break up their drowsy grave.
     To help see what the password ends up looking like, use password.exe, 
which displays a password. Type "password password" or 
"password @filename<,offset>" or "password !filename<,offset>" or 
"password #password" to see what 8 bytes end up as your password. 
     You can type "DES" or "3DES" with no arguments at the dos prompt for 
help with password usage.

     The DES algorithm specifies how to encrypt or decrypt 8 bytes of data 
at a time. It's called a block cipher because it operates on a block of 
data. There are many ways to implement a block cipher in a program-- 
DES.EXE uses Stream Cipher Mode with IV/Ciphertext feedback. This kind of 
feedback gives Error Propogation-- that is, an error in any bit of the 
cryptotext cascades and propogates all the way to the end of the file, 
blasting the decryption. In addition, DES.EXE uses a second, internal 
password made from the filename. What all this means to you is this: if you 
encrypt a file, then change its name, it is unrecoverable even with the 
right password. To successfully decrypt it you have to rename it back to 
its original name first. For instance, to more securely encrypt a .pcx 
graphics file called graphic.pcx, first rename it to graphic.tif. Then use 
DES.EXE to encrypt that file, then rename it back to graphic.pcx. Anyone 
attacking that file without knowing to rename it, is outa luck. The only 
drawback is that you have to remember the changed name as well, or you're 
the one outa luck.

     To use this DES486 code in your C programs you need to: declare an 8-
byte block for your password and put your password in it. Then call 
schedule(password,keytbl) to set up the key schedule. Please note that for 
regular DES there is one keytable (called keytbl), but for 3DES there are 
two (en_keytbl and de_keytbl). Then you may encrypt_block() or 
decrypt_block() 8 bytes at a time in the supplied space unsigned char 
block[8]. You'll need to assemble des486.asm and table.asm and link them 
with your C code (being sure to use the /mx and /3 options). Use the small 
model.
     The password is the only space required by DES486 which it doesn't 
supply itself.
     The included makefile (DES.MAK) will do all this automatically. 
     If you are having trouble grokking the DES486.asm algorithms, have a 
look at maketabl.c, the program that precomputes the tables.







     My DES486.ASM owes a big debt of gratitude to Phil Karn, whose ideas 
on combining the E, S and P tables led to the biggest single speed 
increase.
     My knowledge of DES started with Al Stevens's programs in his DDJ 
columns of Sept. and Nov. 1990. (The Sept. version is busted)
     Thanks to Allen Wyatt for his asm module CPU, which returns the cpu 
type of the current machine.
     The timing code is lifted almost verbatim from IDEA.C in Phil 
Zimmerman's PGP21, which also supplied some of the stream-cypher algorithm.
     MD5 is the RSA Data Security, Inc. MD5 Message-Digest Algorithm. 
Please see the file "MD5.C" for licensing details.
     I can be reached as Steve Allen on CompuServe 73277,620 (internet: 
[e-mail]). Feel free to use this software in a non-
commercial environment, and to distribute it within the United States. 
Please be sure my name is associated with it.