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NSA Technical Journal published "The Unbreakable Cipher" in
Spring 1961. <br><br>
<a href="http://www.nsa.gov/public_info/_files/tech_journals/The_Unbreakable_Cipher.pdf" eudora="autourl">
http://www.nsa.gov/public_info/_files/tech_journals/The_Unbreakable_Cipher.pdf<br>
<br>
</a>Excerpts:<br><br>
[Quote]<br><br>
David Kahn, "Lyen Otuu Wllwgh WI Etjown" pp. 71, 83, 84,
86,<br>
88 and 90 of the <i>New York Times Magazine </i>November 13, 1960<br>
says that an unbreakable cipher system can be made from one <br>
time key "that is absolutely random and never repeats."
...<br><br>
For each cipher system there is an upper bound to the amount of <br>
traffic it can protect against cryptanalytic attack. What is <br>
"cryptanalytic attack"? It is a process applied to cipher text
<br>
in order to extract information, especially information <br>
contained in the messages and intended to be kept secret. <br>
If some of the information is gotten by other means and this <br>
results in more being extracted from the cipher, this is (at<br>
least partially) a successful attack. If certain phrases can be <br>
recognized when they are present, this is successful cryptanalysis. <br>
If a priori probabilities on possible contents are altered by <br>
examination of the cipher, this is cryptanalytic progress. <br>
If in making trial decipherments it is possible to pick out <br>
the correct one then cryptanalysis is successful. ...<br><br>
Another example is that of Mr. Kahn, one-time key. Here the<br>
limit is quite clear; it is the amount of key on hand. The key
arrives<br>
in finite "messages," so there is only a finite amount on hand
at<br>
anyone time, and this limits the amount of traffic which can be sent<br>
securely. Of course another shipment of key raises this bound, but<br>
technically another cipher system is now in effect, for by my <br>
definition a cipher system is a message. A sequence of messages <br>
is a sequence of cipher systems, related perhaps, but not the same.
...<br><br>
[Answer to the question:] "Does there exist an unbreakable
cipher" <br>
would be this, "Every cipher is breakable, given enough traffic, and
<br>
every cipher is unbreakable, if the traffic volume is restricted<br>
enough."<br><br>
[End quote]<br><br>
Is this conclusion still valid? If so, what could be done to restrict
traffic<br>
volume to assure unbreakablility? And how to sufficiently test that.<br>
Presuming that NSA and cohorts have investigated this effect.</body>
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