idea.doc

强大的加解密算法

IDEA v2.2
        IDEA encryption for dos.

IDEA stands for International Data Encryption Algorithm
        it was invented by Xuejia Lai and James Massey in Switzerland
        its a private key block algorithm, and is thought to be 
        very secure.

This manifestation of the algorithm is adapted pretty much wholesale from 
Dr. Dobbs Journal #208 December 1993.  I simplified it, and made it
useable on dos files.

the algorithm operates on 64-bit data blocks, divided into 4 16-bit 
subblocks.  for dos files, it packs two characters into a 16-bit word
and treats 4 such words as the subblocks. since not all files have the right
length for this, a header is used to tell idea's decrypter how long the file
will be. thus encrypted files are a few bytes longer than they started. they
are of course put back to normal when decrypted.

the key is a 128-bits.  This program uses a simplified key, in that it takes
up to 8 characters and casts each one into a 16-bit word, by shifting it left
8 bits, and OR'ing it with the previous character (0 for the first character), 
making 8 16-bit subblocks of the key, which is no where near as complex a key 
is possible. this undoubtedly affects the security of the algorithm, but it 
should still be good for most purposes.  A future version will have an option
for a more complex key.  of course, source is included so feel free to write 
a better keying function yourself.

the .exe file i include here is a simple demonstration program.  Source code
is included and the program is written as a sort of "toolbox"---you can
use the algorithm through the 3 functions provided as a sort of "front-end": 
encode, decode, & clean-up.  actually, you can easily change the way it works 
too if you like--the algorithm is a bit obtuse but the usage is real easy.


usage is "idea <filename.ext> [e|d[w]] key"
        [e|d[w]] stands for either an "e" or a "d", instructing whether to  
                e-ncode or d-ecode, plus an optional "w", which tells
                IDEA to overwrite the original file with the encrypted
                data. leaving "w" off creates a new file called 
                "filename.enc" where "filename" is the same as the input file.
                overwriting is more secure--it destroys old data--but may
                not be what you want.
        key is a word up to 8 characters BUT NOT MORE!!! extra characters
                will be truncated offa there.

its not my code so don't send money or anything. (the IDEA algorithm is in 
fact patented, but it can be freely used and distributed for non-commercial 
use.  The patent is held by Ascom-Tech AG in switzerland, tel: [phone]) 
if you want to contact me, i'm on the internet as "[e-mail]".

Please note that use of this program is done with no insurance from me that 
it will work or anything----use it at your own risk. i've bug checked it
and everything and it looks good, but i'm not gonna be responsible for 
failure.  While we're on this topic, please do not forget your key!!!
a brute-force attack on this algorithm "requires 2^128 encryptions to
recover the key.....a billion chips testing a billion keys a second would 
take 10^13 years to break IDEA"*----a very secure algorithm (at least against
brute force attacks).





* Dr. Dobbs Journal #208 p.54