Cryptology - Unofficial St. Mary's College of California Web Site

8.6. PERFECT SECRECY 153
invent now” each night while relaxing on his sofa. 16
Put endless and senseless together and we have the
One-time Pad: 17 Pick any random keyword of length equal to the length of
your message. Treat it as the key word of a Vigenère cipher. Throw it away
after you use it (hence the name).
A properly used one-time pad is the only unbreakable cipher, or, in fancier
language, is holocryptic. Why is it unbreakable Consider the ciphertext UVAET.
What is the plaintext Using this ciphertext and the ciphertext only it is impossible
to tell. This is because for every 5 letter block of letters you can pick,
there is a (possibly non-nonsensical) 5 letter Vigenère keyword that will turn
your plaintext into UVAET. And unless you have some other knowledge, all are
equally possible. So it is impossible to tell what UVAET means.
This idea holds on a much larger scale. If you pick a keyword that is as
long as your message, make the keyword to be a random collection of letters,
and use the keyword exactly twice, once to encipher and once to decipher, then
there is no way that anyone can break the message. This was a favorite method
of Russian spies in the 1950’s. 18 It is also popular in movies, mainly because
the one-time pads were usually written on very small pieces of paper that we
hidden in false shoe bottoms, or inside fake cigarettes, fake nickels, etc.
As Friedman put it in his Encyclopedia Britannica article on cryptology
[Britt, pg 1059]
a letter-for-letter cipher system which employs, once and only once, a
keying sequence composed of characters or elements in a random and
entirely unpredictable sequence may be considered holocryptic, that is,
messages in such a systems cannot be read by indirect processes involving
cryptanalysis, but only by direct processes involving possession of the
key or keys, obtained either legitimately, by virtue of being among the
intended communicators, or by stealth.
Examples: Encipher and decipher using a one-time pad.
(1) Encipher holocryptic using the key SLMPQOSUCFC.
(2) Decipher HROJA OPMNZ using the key NEXFA LPLCV. 19
⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄
How much use do you think this system that allows perfect secrecy gets
The answer is almost none. Consider the problems you would have if you were
16 Among the things he did invent was a “Secret Signaling System” that was awarded U.S.
Patent 1,310,719. This was, more or less, a teletypewriter that performed Vigenére encryption.
17 Ciphers very similar to one-time pads were also discovered in Germany and Russia about
this same time [Bauer, page 144].
18 Unfortunately for them, the Russians made 9 copies of some of their one-time pads. Even
this small lapse was enough for the NSA to break these messages. (This information was only
recently declassified, and can be found in the “Verona Breaks” pages at the NSA website.)
19 (1) ZZXDS FQJVN E, (2) unreadable.