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
> 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
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
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,
"To iterate is human, to recurse is divine"
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