the effects of a spy

[Nicholas Bohm]

Perry E. Metzger wrote:
> "Steven M. Bellovin" <smb at cs.columbia.edu> writes:
> 
>>Bruce Schneier's newsletter Cryptogram has the following fascinating 
>>link: http://www.fas.org/irp/eprint/heath.pdf
>>It's the story of effects of a single spy who betrayed keys and 
>>encryptor designs.

[...]

> One intriguing question that I was left with after reading the whole
> thing was not mentioned in the document at all. One portion of the
> NSA's role is to break other people's codes. However, we also have to
> assume that equipment would fall into "the wrong people's hands" at
> intervals, as happened with the Pueblo incident. If properly designed,
> the compromise of such equipment won't reveal communications, but
> there is no way to prevent it from revealing methods, which could then
> be exploited by an opponent to secure their own communications.
> 
> Does the tension between securing one's own communications and
> breaking an opponents communications sometimes drive the use of COMSEC
> gear that may be "too close to the edge" for comfort, for fear of
> revealing too much about more secure methods? If so, does the public
> revelation of Suite B mean that the NSA has decided it prefers to keep
> communications secure to breaking opposition communications?

Of historical interest on this question there is useful material in
"Between Silk and Cyanide" by Leo Marks.

Marks was responsible for ciphers used during WWII by SOE for
communications with agents in German occupied Europe.  He describes an
episode when he was visited by people from Bletchley Park who were
concerned that he was equipping agents with ciphers that (he deduced)
were too strong for Bletchley Park to attack if they should fall into
German hands and come into use by them.

It is understandable, particularly during the Battle of the Atlantic,
that UK priorities should have been to maintain the availability of
breaks into enemy traffic even at the risk of hazarding communications
with agents.  (If Britain had failed in the Atlantic the war in the west
would have been over.  If SOE failed, there were no short-term
consequences of similar seriousness.)

The preservation of secrecy about those breaks for nearly thirty years
after the end of the war suggests that those priorities may have become
ossified, which may in turn account for excessive governmental anxieties
over the spread of strong cryptography.  Any change in these priorities
would be of great interest.

Nicholas Bohm
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