From: gdallal@pearl.tufts.edu (Jerry Dallal)
Newsgroups: sci.crypt
Subject: Algorithm for VCR Plus+ codes:  thanks
Date: 28 Sep 92 22:14:00 GMT
Organization: Tufts University - Medford, MA
News-Software: VAX/VMS VNEWS 1.41


            Thanks   to  those  who  responded  to  my   request   for 
        information  regarding the algorithm for creating VCR+  codes.  
        Many  respondents  asked  me  to  forward  or  summarize   any 
        information I might receive.  

            Special thanks go to  those  who pointed me to Shirriff K, 
        Welch C,   Kinsman A,   "Decoding  a  VCR  Controller   Code," 
        Cryptologia,  16 (#3, July 1992), 227-234.   (I'd thank you by 
        name,   but I wasn't  sure you'd want  it posted to the  net!) 
        Thinking this  to  be  what-you're-not-likely-to-find-in-your-
        typical-college-library  I  first  checked  the  MIT   library 
        catalog,   but they don't subscribe to the journal.   However, 
        *Tufts*  does.  (Ever notice how sometimes you're surprised by 
        what goes on in your own backyard?) 

            The  abstract  of the paper reads:   "The VCR Plus+  is  a 
        remote control for programming video cassette recorders.    It 
        uses  an  encrypted  1  to 8 digit  number  that  encodes  the 
        channel,   start  time,  length,  and day of the month of  the 
        television  show  to be recorded.    This  paper  describes  a 
        procedure for decoding the 1 to 6 digit codes." 

            The  Acknowledgements  section reads:   "We would like  to 
        thank  the participants in the cryptography discussion on  the 
        Usenet  computer  network  for  their  comments.    We   would 
        especially  like  to thank the Electronic Frontier  Foundation 
        for their legal assistance."

            A footnote to the name VCR Plus+  reads:  "VCR Plus+   and 
        PlusCode   are   trademarks   of   the   Gemstar   Development 
        Corporation.   Gemstar claims copyright and patent  protection 
        on  the  PlusCode numbers and software to create  them.    The 
        decoding process in this paper is described for  cryptographic 
        research purposes only."

            There is enought information  to turn the algorithm around 
        for encoding as well as decoding.  (I've done it; everything I 
        needed  was in SWK.)   For example,  they include the encoding 
        multiplier for no-carry multiplication along with the decoding 
        multiplier, rather than leave it  to readers to figure out for 
        themselves.  The paper includes some pseudocode procedures for 
        the  decoding  process.    Two by-hand  sample  decodings  are 
        included, too:  In May, 1991, 3316 is shown to correspond to a 
        2 hour show on channel 4 at 9pm on May 10th;  in March,  1992, 
        21362 is shown to correspond to a 30 minute show on channel 24 
        at 7:30 pm on March 11th.  

            There  are two minor typos.   In the second example,   the 
        mapped top is written 28 (decimal); it should be 66.  However, 
        it is also written 66 (binary) and used that way in subsequent 
        calculations.  Also, in figure 1, the binary representation of 
        'channel'  is given as c5c4c3c2c1;  it should be c5c4c3c2c1c0.  
        However, it is described as a 6 bit binary number in the text. 

            I've  checked  the TV Guide and the number of  shows  that 
        have 7 or 8 digit codes seems quite small.   They all seem  to 
        have odd starting times, except for our local channel 68.  The 
        odd starting times shouldn't be a problem,  though.   Just use 
        the nearest hour or  half-hour starting time and press the add 
        time button or adjust the duration accordingly.    Thus,   the 
        SWK results should meet most needs. 

            Enjoy! 

            P.S.   In  my first encoder,  I  reversed the SWK  map  by 
        mapping  all 1,  2,  or 3 digit numbers until I found the  one 
        that mapped appropriately.  However,  I pass along a hint that 
        was sent to me:   The map is easily inverted.  (Again, I don't 
        credit the source because the message was sent privately,  but 
        I am happy to acknowledge him/her publicly if he/she desires.) 

            P.P.S.  Now that these programs are available, what can be 
        done with them?   I'd give mine away, source and all (What can 
        I tell you?   I'm an academic.  I'm not into it for the money, 
        just for the love of learning.  And, in this case, the ability 
        to use my VCR Plus+ when I can't find the TV Guide!),  but I'm 
        not eager to have Gemstar's attorneys at my door. 
Newsgroups: sci.crypt