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

Christopher R. Redinger redinger at usm.maine.edu
Mon Oct 27 11:11:31 EST 1997


On Monday, October 27, 1997 10:28 AM, Chris Grantham 
[SMTP:casper at optima.mme.wsu.edu] wrote:
> 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 ;->  ).

Here's the technical explanation from RSA on what a one-time pad is. I 
think this pretty much answers your above three questions. Of course 
now you're going to have questions like, what are "stream ciphers?" :) 
I would recommend perusing RSA's website http:www.rsa.com and 
specifically reading through their FAQ for a nice introduction to 
cryptography.

A one-time pad, sometimes called the Vernam cipher, uses a string of 
bits that is generated completely at random. The keystream is the same 
length as the plaintext message and the random string is combined using 
bitwise exclusive-or with the plaintext to produce the ciphertext. S  
ince the entire keystream is random, an opponent with infinite 
computational resources can only guess the plaintext if he sees the 
ciphertext. Such a cipher is said to offer perfect secrecy and the 
analysis of the one-time pad is seen as one of the cornerstones of 
modern cryptography.

While the one-time pad saw use during wartime, over diplomatic channels 
requiring exceptionally high security, the fact that the secret key 
(which can be used only once) is as long as the message introduces 
severe key-management problems. While perfectly secure, the one-time 
pad is impractical.

Stream ciphers were developed as an approximation to the action of the 
one-time pad, and while contemporary stream ciphers are unable to 
provide the satisfying theoretical security of the one-time pad, they 
are at least practical.

Be seeing you,
	Christopher
"To iterate is human, to recurse is divine"

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