From: Christian Wettergren <cwe@Csli.Stanford.EDU>
Date: Mon, 4 Sep 95 16:14:32 PDT
To: cypherpunks@toad.com
Subject: Re: Emergency File Wipe Algorithim
In-Reply-To: <Pine.SUN.3.91.950904184522.20203A-100000@access5.digex.net>
Message-ID: <199509042314.QAA18360@Csli.Stanford.EDU>
MIME-Version: 1.0
Content-Type: text/plain



Someone proposed that one could wipe the memory before power-down, for
example during 1 second or something like that. Unfortunately, that
wont help, unless I misread the paper. It is effectively the same as
if the key had been stored in the cell for 1 second less, nothing else.

The only way I can see how to avoid generating "imprints" of more or
less static data is to make them non-static. Start circulating them
around.

One way that springs to mind for keys are to do something like
inverting the meaning of the key every x milliseconds. Like this; 

     /* pseudo code */
     char master_key[KEYSIZE];
     int  meaning = ZEROS;

     void encryption(char *input, char *output); /* implicit master_key */
     int using_key = FALSE;

     main() {
       input_from_keyboard(master_key);
       timer(100 ms, flipem()); /* calls flipem every 20 ms */

       main_loop(); /* occansionally using encryption() */
     }

     void flipem() {

       if (using_key) /* risk of never being able to flipem() */
         return;
   
       /* some kind of semaphored section */
       using_key = TRUE;
       master_key = inverse(master_key);
       meaning = (!meaning);
       using_key = FALSE;

     }

     void encryption(char *input, char *output) {

      char real_key[KEYSIZE]; /* must be on stack */

      copy_key(real_key, master_key);
      if (meaning == ONES)
        invert(real_key);           /* recovering real content */
      encrypt(input, output, real_key);

      write_random_key(real_key); /* so "real" key doesn't become 
                                     imprinted as well. */
     }

Do don't care about the plaintext in the above. Nor stack content vrey
much. Nor about coding style.