One Time Pad (was: Re: [rc5] The DOS client appears at last!)

Seth Dillingham seth at snet.net
Mon Oct 27 11:06:47 EST 1997


Chris Grantham (casper at optima.mme.wsu.edu), on 10/27/97 10:27 AM, wrote 
the following:

>Hmm. Never (No.. ever) is a strong word, and one that is frequently over
>used. Maybe you could explain to the laymen (such as myself) on the
>group what OTP is, and why it is completely secure (and maybe why we
>aren't all using it for encryption ;->  ).

If OTP is what I think it is, then it can't be used for general purpose 
privacy.

What I'm thinking of (can't remember any other names for it) requires a 
pad (e.g. "book") of code rules, each of which are only used once.

This requires that the parties using the pads have agreed tot his 
beforehand. When you use the first page in your pad to encode a message, 
I must use the first page in my pad to decode the message. Then the first 
page is discarded.

The second page in the pad is completely different, so a crack attempt 
has nothing to compare.

Obviously, this doesn't work at all with general purpose stuff, and 
requires careful organizastion to work at all.

I think I've got this right, but I've pulled this from the deepest 
recesses of my memory, from back in highschool when I read a book on 
cryptography.

The only other part of this, which I'm even less sure of, is what's on 
the pad. I think in its most basic incarnation, each page of the pad is a 
different 'translation table' for keywords and 'general words' to be used 
when encrypting a message. If I'm wrong about this, of course, I'm going 
to look like a fool, but here's what I remember (this is just a silly, 
overly-simplified example):

ENGLISH   |    CODE
-------------------
a         |    0001
better    |    0002
encryption|    0003
is        |    0004
it        |    0005
message   |    0006
method    |    0007
move      |    0008
secret    |    0009
the       |    0010
time      |    0011
to        |    0012


The secret message is: it's time to move to a better encryption method.

becomes

00100009000600040005000400110012000800120001000200030007

You can let your computer spin on that baby for as long as you want. As 
long as you don't get a copy of the single page from my pad, you'll never 
figure it out. Next time I write a code, all of the cypher keys change, 
so there's nothing to compare it with.

So, Christopher Thompson, do I look stupid, or did I get this pretty 
close?

Of course, this can be combined with other encoding methods to make it 
more secure, if that's necessary... I'm not sure if it is. The weakness 
of this method is the pads themselves... if they're copied by whoever 
you're trying to hide your messages from, then they're not encoded at 
all, really.

I'm not sure how the computerized version of this process would work, 
though I'm fairly certain it would similar to what I've described here. 



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