Venona not all decrypted?

[Steven M. Bellovin]

Have a look at http://www.nsa.gov/publications/publi00039.cfm .  The 
one-time pad was used to superencrypt a codebook; two different 
codebooks were used.  Most of the successful decryptions were done by 
1952; there was some additional help from a partial codebook recovered 
in 1953.  Here's the key section of that monograph:

		The Translations and KGB Cryptographic Systems

	The VENONA translations from 1942 to 1943 messages occasionally
	are fragmentary and difficult to understand. The code itself
	was complex and difficult to exploit using pure analytic
	techniques. Moreover, the broad contextual sweep of the
	content of these messages vastly complicated the difficulty
	of reading these KGB systems.

	The cryptographic systems used by the KGBís First Chief
	Directorate involved a codebook in which words and phrases
	were represented by numbers. These numbers were then further
	enciphered by the addition of random number groups, additives
	taken from a so-called one-time pad. A one-time pad comprised
	pages of random numbers, copies of which were used by the
	sender and receiver of a message to add and remove an extra
	layer of encipherment. One-time pads used properly only
	once are unbreakable; however, the KGB?s cryptographic
	material manufacturing center in the Soviet Union apparently
	reused some of the pages from one-time pads. This provided
	Arlington Hall with an opening. Very few of the 1942 KGB
	messages could be solved because there was very little
	duplication of one-time pad pages in those messages. The
	situation was more favorable in 1943, even more so in 1944,
	and the success rate improved accordingly. In order to
	break into the system successfully, Arlington Hall analysts
	had to first identify strip off the layer of additive in
	order to attack the underlying code. These two levels of
	encryption caused immense difficulty in exploiting the
	codebook, and many code groups were, therefore, never
	recovered. The KGB messages from 1942 through 1943 and into
	1944, as well as from earlier years, were based on one
	codebook version. The 1944 to 1945 messages were based on
	a new codebook.

Given that intelligence scrutiny of the intercepts continued until 1980,
I doubt there's any more to recover.  That said, the NSA admits of the
possibility:

	There are still gaps of two different types in the translated
	messages, as indicated by the words "unrecovered" or
	"unrecoverable." The phrase "unrecovered" meant that the
	underlying Russian text in theory could be obtained, but the
	cryptanalysts did not have sufficient text to do so.
	"Unrecoverable," on the other hand, indicates passages
	unaffected by the Soviet misuse of their own system which
	therefore could never be solved by cryptanalysts 

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