[cryptography] The Unbreakable Cipher

Jonathan Katz jkatz at cs.umd.edu
Wed Sep 25 12:40:12 EDT 2013

On Wed, Sep 25, 2013 at 10:11 AM, John Young <jya at pipeline.com> wrote:

>  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."  ...

I'm not sure why this was news in 1961; Shannon had this observation a
decade earlier and the one-time pad predates that.

> [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?

"Every cipher is breakable, given enough traffic": in principle, yes, as
long as the traffic (formally, the entropy of the traffic) is larger than
the keylength.

"Every cipher is unbreakable, if the traffic volume is restricted enough":
not true; the cipher that ignores the key and outputs the message in the
clear is not secure for any non-zero traffic. On the other hand, the
one-time pad is secure as long as the traffic is less than the keylength.

> 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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