(I had only sent this to the 'other' mailing list and not here - my appologies to those who get this for a second time. -Giff) On Mon, 13 Oct 1997 semprini@theschool.com wrote:

Hello. I just posted an encryption program on the web. It uses a key- generated pseudorandom stream of characters that get XORed with the plaintext to produce the cyphertext. For decryption, the same stream is run against the cyphertext. (Such is the nature of the XOR operation.) You can download the program at:

http://www.brigadoon.com/~semprini/3dmx

I am interested in other people's views, comments, questions, etc. on this program. Any questions, comments, etc. can be sent to me at "semprini@theschool.com".

Thanks,

Dylan

From my reading of the code (assuming that the code I read was, in fact,

From the RNG, a 4 dimensional array (called a 'cube') is created with elements 0..15 for each of the four sides. The contents are filled from

I have downloaded the program and read over it's contents (the .frm files). It's Visual Basic, which I am familiar with. the code which does the encrypting) - the key supplied is not used - or at least not directly. The user apparantly supplies three keys which are padded with 'random' stuff each to get them to 32 bytes in length. Then these are placed into a special array where each bit gets its own position and various scrambing is done 'randomly' on the data. This is dependent on the key values and what bits are on or off at a given time. During this process, the 'seed' for the RNG is reset to some unknown value (I believe the effective range for the seed is 0..65535) the entire time the bits are processed. Then, amazingly, those bits are not used again. the RNG with a value 0..255. Then during the encryption process of a file, the program will choose four random numbers 0..15 to use as indexes into the cube. The number found there is XORed with the datafile byte to get the encrypted/decrypted answer. *whew* Hope you followed that, I'm trying to be complete and concise. The upshot seems to be (and I haven't written any code to try this out yet): This method fails to the chosen plaintext attack where I ask the opponent to encrypt a large file of (say) all zero bytes. The encrypted result I should then be able to XOR directly against the target file to get the original contents. A less efficient but more satisfing method is to simply try all seed values for the RNG. Even if I don't know the RNG process, if the seed values are only 0..65535 (as I believe they are), trying all possible decryptions by this manner will give me the correct answer. If I know something about the file (i.e. being English text) I should have no problem finding the right answer programmatically. If I am incorrect about my reading of the program or its operation, please let me know. -Giff