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Research activity in 1997

This research is concerned with public key certification in distributed environments, proposing a certification scheme to exchange documents that are digitally signed. Certification is the weakest point of cryptographic and authentication systems based on public keys. Thus, it is the real condition for security in every kind of network transaction that has to be private and associated in an unambiguous way to a unique user. In fact, if the public key is associated to a person different from the legitimate owner, the system is compromised, and it is possible to read reserved messages and to prepare false digital signatures. Certification avoids wrong associations of public keys. However, certification brings about a complex set of problems, made even more difficult by the possibility that the legitimate key pair owner loses his private key. For this reason the certification is at the base of almost all computer network security systems, and is treated explicitly by standards such as X.509. Our system, proposed in collaboration with the University of Cambridge, includes the implementation of a separate authority dedicated to delete the public key certificates that are no longer valid. Log file records are bound in a chain of hash values, so that they may not be deleted by the authority in an undetected way. The system has been implemented and a series of experiments is starting to integrate the above certification system with a few browser and mail services available on the market, both in Unix and Windows NT environments. Through this structure it will be possible to obtain a number of secure services that require signed and/or encrypted documents to be sent.

In this research we studied PGP (Pretty Good Privacy), the most popular mail encryption and signature tool available worldwide. Within the context of two undergraduate theses, that were supervised by the security group, a public domain program, named "ACT", was implemented. This tool is compatible with PGP (versions prior to 5.0), but solves some of its weaknesses.

The research was about the study of the mechanism to generate digital signatures based on public key algorithms. In collaboration with the University of Cambridge we made a new and alternative proposal to generate digital signatures based only on hash function chains. This mechanism turns out to be more efficient than the traditional approaches, and moreover it does not involve all the problems connected to the restrictions imposed by some countries about the export of encryption software.

The research is concerned with authentication and privacy in the World Wide Web. The security of the transactions is guaranteed using public key systems, together with the certification scheme described above. The implementation is based on the original HTTP protocol and on commercial browsers. It is obtained through Java applets associated to the client, that silently transform the base HTTP transaction into an exchange of encrypted and authenticated information. The actual communication is not carried out at the HTTP level, but between application processes that are added for this task, both on the client host and on the server. The idea is an alternative w.r.t. the SHTTP protocol, that modifies the standard HTTP format, and also w.r.t. SSL, that works at the level of the communication software. The advantage is a quicker integration with the pre-existent tools. We are finishing the implementation of a prototype that will be also used to address some of the aspects we study in the context of public key certification.

Another aspect of our research is related to the important problem of password checking. User passwords tend to be weak, and checking at the time of password creation or change is highly recommended. Although there are more sophisticated authentication systems - like those based on dedicated hardware (for instance a smartcard) or those asking for an answer to a "challenge" – the most common systems are based on passwords. This as a consequence of their semplicity: they can be integrated in every environment without dedicated hardware and with well known user interfaces. In order to prevent bad password choices, the user passwrod is usually confronted with a dictionary, with implementation difficulties that are related to checking time and especially space to store the dictionary. We have developed a proactive password checking method that can lead to very high dictionary compression, with low error rates, under one per cent. On the basis of the classification alghorithm, the password checker ProCheck has been implemented as a patch for the Unix command passwd. ProCheck is freely distributed from http://maga.di.unito.it, and versions for SunOS, Ultrix and OSF1 are available. The research is described in [4].

Within the same context, the research group is carrying out an extensive research to detect unauthorized accesses through the observation of user behavior. For several months, a few volunteers were monitored during their normal work at the computer. Parameters such as log-in time, typing speed, executed commands, and so on, were gathered and stored for every user. A model of each user was then built from these parameters and used to classify further connections as "legal" or "illegal" (i.e. not originated by the owner of the account used in the connection). A 90% accuracy was achieved, on the average, in the classification of new connections within the first ten minutes of the log-in session [6].

[2] F. Bergadano, B. Crispo, and M. Lomas. Strong Authentication and Privacy with Standard Browsers. Journal of Computer Security, 1997.

[3] B. Crispo and M. Lomas. A Certification Scheme for Electronic Commerce. In Proc. Security Protocols Int. Workshop. LNCS 1189, pages 19-32, Springer-Verlag, 1997.

[4] F.Bergadano, B.Crispo, and G.Ruffo. Proactive Password Checking with Decision Trees. In Proc. od ACM Computer and Communication Security Conference, pages 67-76, 1997.

[5] R. Anderson, F. Bergadano, B. Crispo, J. Lee, C. Manifavas, and R. Needham. A New Family of Authentication Protocols. Technical Report, 1997. (Submitted for publication).

[6] F. Bergadano, D. Gunetti and G. Ruffo. Online Anomaly Detection in a Real Network Environment. Technical Report, 1997. (Submitted for publication).