Rectangular algebras as tree recognizers

We consider finite rectangular algebras of finite type as tree recognizers. The type is represented by a ranked alphabet Σ. We determine the varieties of finite rectangular Σ-algebras and show that they form a Boolean lattice in which the atoms are minimal varieties of finite Σ-algebras consisting of projection algebras. We also describe the corresponding varieties of Σ-tree languages and compare them with some other varieties studied in the literature. Moreover, we establish the solidity properties of these varieties of finite algebras and tree languages. Rectangular algebras have been previously studied by R. Pöschel and M. Reichel (1993), and we make use of some of their results

Two content protection schemes for digital items

Modern techniques make digital articles easy to copy and manipulate. Content protection systems aim at protecting the rights of producers and distributors. These mostly rely on data encryption, digital watermarking, and special-purpose devices. In this paper, we describe two content protection schemes, both of which make use of tamper-resistant devices and devicer dependent decryption keys. One of the schemes uses a modified El Gamal system, in the other one we combine watermarking with encryption