[cryptography] The Unbreakable Cipher

Natanael natanael.l at gmail.com
Wed Sep 25 10:32:51 EDT 2013


For your question: Session keys and key rotation?
Den 25 sep 2013 16:11 skrev "John Young" <jya at pipeline.com>:

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