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Kildall has developed data propagation algorithms for code optimization m a general lattice theoretic framework. In another directmn, Hecht and Ullman gave a strong upper bound on the number of iterations required for propagation algorithms when the data is represented by bit vectors and depth-first ordering of the flow graph is used The present paper combines the ideas of these two papers by considering conditions under whmh the bound of Hecht and Ullman applies to the depth-first veremn of Klldall's general data propagation algorithm. It is shown that the following condition is necessary and sufficient Let f and g be any two functions which could be associated with blocks of a flow graph, let x be an arbitrary lattice element, and let 0 be the lattice zero Then (*) (Vf,g,x) [fg(0) > g(0)A f(x) /~ x] Then it is shown that several of the particular instances of the techniques Kildall found useful do not meet condition (*)

A new cryptosystem that is suitable for database encryption is presented. The system has the important property of having subkeys that allow the encryption and decryption of fields within a record. The system is based on the Chinese Remainder Theorem.

Considered here, for a particular model of databases in which only information about relatively large sets of records can be obtained, is the question of whether one can from statistical information obtain information about individuals. Under the assumption that the data in the database is taken from arbitrary integers, it is shown that essentially nothing can be inferred. It is also shown that when the values are known to be imprecise in some fixed range, one can often deduce the values of individual records.

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