Märt Saarepera

Rational choice of security measures via multi-parameter attack trees

We present a simple risk-analysis based method for studying the security of institutions against rational (gain-oriented) attacks. Our method uses a certain refined form of attack-trees that are used to estimate the cost and the success probability of attacks. We use elementary game theory to decide whether the system under protection is a realistic target for gain-oriented attackers. Attacks are considered unlikely if their cost is not worth their benefits for the attackers. We also show how to decide whether the investments into security are economically justified. We outline the new method and show how it can be used in practice by going through a realistic example.

On Provably Secure Time-Stamping Schemes

It is almost a folklore-knowledge that hash-based time-stamping schemes are secure if the underlying hash function is collision-resistant but still no rigorous proofs have been published. We try to establish such proof and conclude that the existing security conditions are improper because they ignore precomputations by adversaries.After analyzing a simplistic patent filing scenario, we suggest a new security condition for time-stamping schemes that leads to a new security property of hash functions – chain-resistance. We observe that if the variety of possible shapes of hash-chains is polynomial (and the verification procedure is suitably improved), then the time-stamping scheme becomes provably secure, assuming that the underlying hash function is collision-resistant. Finally, we show that in some sense, the restrictions in the security definition are necessary – conventional black-box techniques are unable to prove that chain-resistance follows from collision-resistance.

Improving the Availability of Time-Stamping Services

We discuss the availability questions that arise when digital time stamps are used for preserving the evidentiary value of electronic documents. We analyze the time-stamping protocols known to date and point out some weaknesses that have not been addressed so far in scientific literature. Without addressing and solving them, any advantage of the linkage-based protocols over the hash-and-sign time-stamping would be questionable. We present several new techniques and protocols for improving the availability ofb oth the hash-and-sign and the linkage-based time-stamping services. We introduce fault-tolerant linking as a new concept to neutralize fault-sensitivity as the main weakness of linkage-based time-stamping.

Are the Current System Engineering Practices Sufficient to Meet Cyber Crime

During the last decades, we have witnessed an explosive growth of computer-technology and the Internet. Due to the growing role of computers and Internet in important business and state-related activities, investments to computer security and the security industry have also been growing fast. In spite of that, we also see the growing trend of cyber crime and losses due to security incidents. We predict that these three growing trends will continue in the future the main reasons being that: (1) as more and more assets will be connected to the Internet, the number of potential targets and stimuli for attackers grow; (2) fundamental (and hard to change) design decisions made in early development stages of todays Internet- and computer technology guarantee persistent technical vulnerabilities in Internet-based systems due to which attackers will always be one step ahead of defenders; (3) growing role of Chief Security Officers (CSOs) in organisations, who do not necessarily have to understand the detailed purpose and functionality of the system but whose duty is still to make the ITC system of the organisation secure. These reasons guarantee the continuous growth of the security industry but also the continuous growth of losses through cyber crime.