In this paper we discuss the processes in the Shannon cipher system with
discrete memoryless source and a guessing wiretapper. The wiretapper observes a
cryptogram of N-vector of ciphered messages in the public channel and tries
to guess successively the vector of messages within given distortion level
Ξ and small probability of error less than exp{βNE} with positive
reliability index E. The security of the system is measured by the expected
number of guesses which wiretapper needs for the approximate reconstruction of
the vector of source messages. The distortion, the reliability criteria and the
possibility of upper limiting the number of guesses extend the approach studied
by Merhav and Arikan. A single-letter characterization is given for the region
of pairs (RLβ,R) (of the rate RLβ of the maximum number of guesses L(N)
and the rate R of the average number of guesses) in dependence on key rate
RKβ, distortion level Ξ and reliability E.Comment: 14 pages, 3 figures, Submitted to IEEE Transactions on Information
Theor