Yacov Yacobi

A Key Escrow System with Warrant Bounds

We propose a key escrow system that permits warrants for the interception and decryption of communications for arbitrary time periods, and with either one or two communicating parties specified as the target. The system is simple and practical, and affords reasonable protection against misuse. We argue that use of such a system can produce both greater privacy protection and more effective law enforcement than we now enjoy.

On Privacy Homomorphisms (Extended Abstract)

An additive privacy homomorphism is an encryption function in which the decryption of a sum (or possibly some other operation) of ciphers is the sum of the corresponding messages. Rivest, Adleman, and Dertouzos have proposed four different additive privacy homomorphisms. In this paper, we show that two of them are insecure under a ciphertext only attack and the other two can be broken by a known plaintext attack. We also introduce the notion of an R -additive privacy homomorphism, which is essentially an additive privacy homomorphism in which only at most R messages need to be added together. We give an example of an R -additive privacy homomorphism that appears to be secure against a ciphertext only attack.

Secure Audio Teleconference

A number of alternative encryption techniques have been suggested for secure audio teleconferencing implementable on public switched network, in which the centralized facility, called bridge, does not hold any secret. The role of the bridge is to synchronously add simultaneous encrypted signals, modulo some known number, and then transmit the result to all the participants. Each terminal has a secret key, with which it can decrypt the above modular sum of encrypted signals to obtain the desired ordinary sum of cleartext signals. Secrecy of the systems is analyzed. Some of which are provably secure, assuming the existence of one way functions, and for the others we have partial cryptanalysis.We also present a N-party identification and signature systems, based on Fiat and Shamirs single party systems, and another N-party signature system based on discrete-log problem. Our systems have communication complexity 2N times that of the basic Fiat-Shamir systems (as compared to a factor of N2 in the direct application of the basic scheme to all pairs).

SuperForms: a security-enhanced smart electronic form management system

The initial design of SuperForms, an automated system for designing, filling out, routing, and tracking business forms, is described. SuperForms is shown to be able to: create high-quality screen images for blank forms from paper copies by scanning and image processing; fill out forms quickly and accurately using an interactive user interface that includes error checking, help menus, user profiles, and automated calculations; use public-key cryptography to append multiple digital signatures to forms; route and track forms through a network; add annotations; and to receive automatic reminders when action on forms is due. The security-enhanced features include user authentication and encryption for privacy.<<ETX>>

Attack on the Koyama-Ohta Identity Basedd Key Distribution Scheme

Koyama and Ohta proposed an identity based key distribution scheme. They considered three configurations: ring, complete graph, and star. The most practical configuration is the star which is used in teleconferencing. We show the Koyama-Ohta star scheme to be insecure. Specifically, we show that an active eavesdropper may cut one of the lines, and perform a bidirectional impersonation, thus establishing two separate keys. One with each side.