Computer Laboratory

Security Group

1997 seminars

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9 December 16:15Attacks on pay-tv access control systems / Markus Kuhn, University of Cambridge

Room TP4, Computer Laboratory

Subscription financed pay-TV channels such as BSkyB scramble their broadcast signal and provide their subscribers with special decoders to prevent unauthorized free access. Modern smartcard-based pay-TV access control systems like VideoCrypt are the first large scale consumer application of both cryptography and tamper resistant processors. Their security aspects have been scrutinized in the past five years by both professional pirate decoder manufacturers and amateur hackers. Professional pirates have developed reverse engineering skills previously only assumed to be available to major governments or corporations, while undergraduate students have found surprisingly simple and cheap ways to evade cryptographic protection schemes. There are valuable lessons to be learned for the design of future large-scale cryptographic applications.

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2 December 16:15Fair and blind certification of knowledge / Wenbo Mao, Hewlett-Packard Laboratories

Room TP4, Computer Laboratory

We propose a new notion of fair and blind certification of knowledge. Blindly certified knowledge has a verifiable structure which can only be constructed by its exclusive owner with the help of a certification authority (CA). Verification of knowledge possession will include a simple check on the structure to convince a verifier of proper certification of the knowledge in addition to its exclusive belonging to the owner (prover). Unlike a blind signature, in blind knowledge certification no visible signature of the CA is available to the verifier and thus different sessions in which the same knowledge is used can easily be made unlinkable. As a result, a single blindly certified knowledge can be re-used polynomially many times without linking to the anonymous user. To prevent anonymity misuse with impunity, we also add fairness to the unlinkable anonymity by escrowing the knowledge to an off-line third party.

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25 November 16:15A secure inter-hospital image reporting tool / Ed Somer, United Medical and Dental Schools

Room TP4, Computer Laboratory

A system has been developed that allows the rapid, secure and private transmission of Nuclear Medicine and Positron Emission Tomography (PET) image data with associated patient files between a number of hospital sites facilitating clinical collaboration and accelerated education of Nuclear Medicine physicians.

Participating departments maintain an mSQL database populated with DICOM objects generated from Interfile or proprietary data. The database contains patient and study information and references to image data which may, or may not, be in DICOM format. An internet navigator is used to query the database and Multipurpose Mail Extension (MIME) typing enables the navigator to download a specific image, and launch a viewer appropriate to the data format and viewing platform. The clinicians can then report the images and submit a text report via either fax or over the internet. The protocols are independent of the network infrastructure linking the hospital sites and data transfer has been secured through the Secure Socket Layer (SSL). ATM networking has made the establishment of a dial-up telemedicine virtual LAN practical offering enhanced security.

Nuclear Medicine and PET departments on three hospital sites are currently involved in this project and the scaleable nature of this solution makes further expansion practical.

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18 November 16:15The discrete logarithm problem on elliptic curves / Nigel Smart, Hewlett-Packard Laboratories

Room TP4, Computer Laboratory

In recent years the use of elliptic curves in cryptography has become something of a "hot topic". This is because the discrete logarithm problem on elliptic curves is in general harder than the associated problem in the more traditional finite fields. In this talk I shall outline how one should choose one's elliptic curve by showing how certain attacks work. In particular I shall concentrate on the recent attack on "trace one curves".

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28 October 16:15Dynamic separation of duties in the clark-wilson model: shifting trust in the application back into the tcb / Simon Foley, University College, Cork

Room TP4, Computer Laboratory

The Clark-Wilson security model may be used for systems where security is enforced across both the operating system and the application systems. Under this model, a secure system may be viewed as a certified application running on top of a trusted computing base (TCB). Certifying an application corresponds to arguing (to a degree) its correctness; the TCB is expected to have undergone some sort of security evaluation. A variety of existing implementation models, for example multilevel security, have been shown capable of upholding the Clark-Wilson TCB requirements.

We argue that, given an evaluated TCB, an application designer should try to minimize the amount of security critical code that is contained within the application and rely on the TCB to enforce security wherever possible. Under the Clark-Wilson model, the TCB is expected to support (enforce) static segregation of duties. However it appears that dynamic segregation of duty must be implemented within the application itself.

In this talk I will describe a framework in which dynamic Clark-Wilson style segregation of duty policies can be expressed and supported by the TCB. I will also describe how these policies can be enforced under Unix and Multilevel TCBs.

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21 October 16:15Inductive analysis of the internet protocol tls / Larry Paulson, University of Cambridge

Room TP4, Computer Laboratory

Internet browsers use security protocols to protect confidential messages. An inductive analysis of TLS (a descendant of SSL 3.0) has been performed using the theorem prover Isabelle. Proofs are based on higher-order logic and make no assumptions concerning beliefs or finiteness. All the obvious security goals can be proved; session resumption appears to be secure even if old session keys have been compromised. The analysis suggests modest changes to simplify the protocol.

TLS, even at an abstract level, is much more complicated than most protocols that researchers have verified. Session keys are negotiated rather than distributed, some messages are optional, and others may be sent at various times. The resources needed to verify TLS are modest: the inductive approach scales up.

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14 October 16:15Edifact security and the public key infrastructure / Peter Landrock, Cryptomathic and Aarhus University

Room TP4, Computer Laboratory

The purpose of EDI is to process business data in an automated way. This used to be handled on a bilateral basis between contracting parties using leased lines etc., but when parties without an initial contract do business over the Internet and/or X.400, it is absolutely vital to secure the interchanges by what we call security services: non-repudiation of origin/receipt and confidentiality are the obvious choices.

Back in the early 90's, the UN Security Joint Working Group came up with a number of proposals for the integration of these services at the EDIFACT syntax level, thus making them independent of the transport mechanism in use. This implied that the EDI translators would handle security, and in an automated fashion. The next step was to bring the supporting public key infrastructure with communicating CAs, LRAs and Directories into play, and a new EDIFACT message, KEYMAN, was designed to handle this.

The EDIFACT certificate thus derived was far more business minded than the original X.509 certificate in its design. The PKI and the underlying business model will be described, and we will explain how to avoid blacklists. We believe this model is the right one to take forward in electronic commerce, and this is exactly what we are doing in large pilots such as SEMPER, BOLERO, DYP, and ELSME.

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3 October 10:00Pachyderm: keeping your email on the web / Michael Schroeder, Digital Systems Research Center

Room TP4, Computer Laboratory

Pachyderm is an experimental email system exhibiting the following properties:

The Platform is the Web: all interaction with Pachyderm is through a web browser; you can create, read and browse your email from any web-connected computer.

Location Independence: there is no state locked in particular client computers; you can move among client computers at will, and your email state will still be available to you.

Bandwidth Tolerance: Pachyderm is designed to tolerate a wide range of connectivity bandwidths, from high-speed local area networks to dial-up modems.

Easy Data Retrieval: all access to your email is based on queries on a full-text index; you can find the right message from among tens of thousands without any need for manual classification schemes such as folders.

The talk will outline the rationale for Pachyderm and describe the structure of the system.

NOTE: the time is nonstandard for a security seminar.

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29 September 11:15Composable and emergent security properties / Heather Hinton, Ryerson Polytechnic University

Room TP4, Computer Laboratory

Emergent behaviours are those that result from interaction between the behaviour of the components of a composite system. We show that they play a role in the composite system's security properties: they may give rise to vulnerabilities directly, or result in the non-composability of security properties.

Using an emergent properties analysis, we can identify which aspects of component behaviour lead to undesirable emergent behaviour. This may enable us to strengthen individual systems so that desired properties compose. We can also use this approach to identify, a priori, when non-composable properties will be violated within a composite system.

We have shown how to apply this approach to several toy examples and are currently using it to analyse a Network Reference Monitor.

NOTE: the time is nonstandard for a security seminar.

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25 September 9:30Security analysis of rsa-type cryptosystems / Marc Joye, Katholieke Universiteit Leuven

Discussion Room, Computer Laboratory

In 1978, Rivest, Shamir and Adleman introduced the public-key cryptosystem RSA. Thereafter, it was extended to Lucas sequences and elliptic curves. In this talk, we will analyse the security of these cryptosystems in given contexts. In particular, some major known attacks against RSA-type systems will be reviewed. We will also see how these attacks can be avoided.

NOTE: the time and the place are nonstandard for a security seminar.

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29 August 16:00Visual cryptography with polarisation / Eli Biham, Technion, Haifa, Israel.

Room TP4, Computer Laboratory

Visual cryptography was introduced by Naor and Shamir as a way to allow fast visual decryption of graphic objects. No decryption device is required; instead decryption is done by fitting slides together. Several schemes were suggested which allow users to share secret pictures (and text) in an information theoretically secure way, so that deciphering is easy if all the shares are given, but it is impossible if one of them is missing. The drawbacks of all the existing methods are the exponentially small contrast of the deciphered picture as the number of shares increases, and the reduction in quality due to pixels' being represented by many smaller (black and white) pixels.

In this talk we suggest new visual cryptographic schemes based on light polarization which are better than the optimal existing schemes. Then we present an ultimate scheme which does not subdivide pixels, and in which the contrast is independent of the number of shares.

Joint work with Ayal Itzkovitz

NOTE: the time is nonstandard for a security seminar.

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7 August State reachability techniques in the formal verification of cryptographic protocols: state of the art and open issues / Catherine A. Meadows, Naval Research Laboratory

Room TP4, Computer Laboratory

NOTE: the time is nonstandard for a security seminar.

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10 July 15:00Security and types in the java virtual machine / Mart�n Abadi, DEC Systems Research Center

Room TP4, Computer Laboratory

Java is typically compiled into an intermediate language, which we call JVML. The Java Virtual Machine interprets JVML code. Because mobile JVML code is not always trusted, a bytecode verifier enforces static constraints that prevent various dynamic errors. Given the importance of the bytecode verifier to security, its current descriptions are inadequate. We consider the use of typing rules to describe the bytecode verifier because they are more precise than prose, clearer than code, and easier to reason about than either. We explore the viability of this approach by developing a sound type system for a subset of JVML. This subset, despite its small size, is interesting because it includes JVML subroutines, a source of substantial difficulty for the bytecode verifier. (Joint work with Raymie Stata.)

NOTE: the time is nonstandard for a security seminar.

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8 July 16:15A new paradigm for massively parallel random search / Adi Shamir, Weizmann Institute of Science, Israel

Room TP4, Computer Laboratory

The problem of optimizing combinatorial problems or breaking cryptographic codes led to several novel paradigms for carrying out such a massively parallel random search, including quantum and DNA computers. In this talk, the speaker will propose a new paradigm, which is based on a simple and easy to implement idea.

The speaker will use some props to demonstrate the new paradigm in real time. It's accessible to everyone, even though some familiarity with the structure of DES like schemes helps to motivate the research.

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18 June 16:15Abstractions for mobile computation / Luca Cardelli, DEC Systems Research Center

Hopkinson lecture theatre, Computer Laboratory

There are two distinct areas of work in mobility: "mobile computing", concerning computation that is carried out in mobile devices, and "mobile computation", concerning mobile code that moves between devices. These distinctions are destined to vanish. We aim to describe all aspects of mobility within a single framework that encompasses mobile agents, the ambients where agents interact and the mobility of the ambients themselves.

The main difficulty with mobile computation is not in mobility per se, but in the crossing of administrative domains. Mobile programs must be equipped to navigate a hierarchy of domains, at every step obtaining authorization to move further. Therefore, at the most fundamental level we need to capture notions of locations, of mobility and of authorization to move.

We identify "mobile ambients" as a fundamental abstraction that generalizes both dynamic agents and the static domains they must cross. From a formal point of view we develop a simple but computationally powerful calculus that directly embodies domains and mobility (and little else). The calculus forms the basis of a small-language/ Java-library. We demonstrate the expressiveness of the approach by a series of examples, including showing how a notion

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20 May 16:15Secure transfer of trust / Carl Ellison, CyberCash Inc.

Room TP4, Computer Laboratory

For a decade, people have viewed the binding of names to keys as the only problem. I will argue that names are almost meaningless in the context of the global Internet, and that we need rather to transfer authorisation securely from one entity to another. I will discuss two new attempts to do this, namely SDSI and SPKI.

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24 March 14:15Steganography and copyright marking / Ross Anderson, University of Cambridge

Hopkinson lecture theatre, Computer Laboratory

One of the fastest growing areas of security research is steganography - hiding information in other information. One example is hiding encrypted copyright marks in digital images; the same ideas can be applied to other problems such as annotation and indexing, and to other kinds of object such as digital audio.

Research in this subject is highly interdisciplinary, and a number of people with backgrounds in graphics, signals processing and statistics have expressed interest in getting involved. I will therefore be giving a brief tutorial on the subject and outlining where the interesting areas of research appear to be.

NOTE: the time and location are nonstandard for a security seminar.

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18 March The impact of dynamic linking on java security / Drew Dean, Princeton University

Room TP4, Computer Laboratory

We survey some of the major security flaws found in Java-enabled web browsers from Sun, Netscape, and Microsoft over the last 15 months. While numerous issues have been found throughout the system, the worst problems come from type safety failures in the implementations that allow an attacker to run arbitrary machine code. Several of the type safety failures can be traced to dynamic linking. We examine a formal model of dynamic linking, and find some necessary conditions for safety.

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11 March Programming goofs that will hose your system security / Alec Muffett, Sun Microsystems

Room TP4, Computer Laboratory

This seminar is an illustrated and light-hearted introduction to vectors of insecurity within modern computer systems, focussing especially on Unix-like operating systems (i.e. don't expect the speaker to bang on about viruses for very long) and how it seems that the same problems come up time and again in new guises.

The presentation should be suitable for programmers, C/S students, and systems administrators of all grades, especially those with a programming bent, and will try to educate the audience away from some of the more grotesque mistakes of systems programming.

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4 March Non-repudiation / Mike Roe, University of Cambridge

Room TP4, Computer Laboratory

The invention of public-key cryptography led to the idea of non-repudiation protocols, which are intended to enable the resolution of disputes about pervious protocol exchanges. Later work on non-repudiation has shown that the means by which cryptographic keys are managed is just as important as the cryptographic algorithm, and public-key cryptography is neither necessary nor sufficient.

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25 February The composition of security properties / Aris Zakinthinos, University of Cambridge

Room TP4, Computer Laboratory

The ability to design and construct complex systems that enforce a security property presupposes an understanding of security properties themselves, as well as the security properties of a system that is composed from secure components. This talk will present a general theory of possibilistic security properties and of system composition for such properties.

It has been demonstrated that security properties do not fit into the saftey/liveness framework defined by Alpern and Schneider. That is, a security property can not be expressed as a property of a trace. (A trace is an ordered stream of events that can occur at the inputs and outputs of a component). However, we demonstrate that security properties can be expressed as a predicate over the set of all traces of a component that are consistent with a given low-level view of a trace.

The issue of composition with feedback has been the focus of much research, we demonstrate that the problem with feedback composition is related to the synchronization of the communications events between the various components. This allows us to provide necessary and sufficient conditions for determining when feedback composition will fail for Generalized Noninterference and for all properties stronger than Generalized Noninterference.

An understanding of what is a security property allows us to provide a method of constructing a system that satisfies a desired security property. This analysis yields a condition that can be used to determine how a property may emerge under composition.

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21 February 16:00The sdsi public-key infrastructure / Butler Lampson, Microsoft Corporation

Room TP4, Computer Laboratory

SDSI is a new distributed security infrastructure, joint work by Butler Lampson and Ron Rivest. It has a simple public-key infrastructure that emphasizes linked local name spaces rather than a hierarchical global name space. SDSI makes it easy to define groups and issue group-membership certificates. Groups provide simple, clear terminology for defining access control lists and security policies.

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18 February Trust / Perri 6, Demos

Room TP4, Computer Laboratory

Unless, as citizens and consumers, we believe that we can trust the government and business organisations that keep personal information about us, to treat that information confidentially and to use it only in ways that we consider to be in our interest, the ``information society'' will be a conflict-ridden, expensive and litigious affair.

In this presentation, I want to explore how we can understand better what determines trust in general. I then go on to look more specifically at how trust in connection with privacy, and I will conclude by setting out some strategies for policy in the fields of data protection, trusted third parties and independence of subject access to buttress trust and trustworthiness in the digital age.

11 February Electronic copyright management - the way ahead / Alastair Kelman, Barrister

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7 February The breaking of the german lorenz world war 2 cypher: max newman's contribution and colossus. / Tony Sale, President, The Bletchley Park Trust

To mark the centenary of the birth of M.H.A. Newman (1897-1984), St John's College are hosting a talk by Tony Sale, President of the Bletchley Park Trust.

Tony Sale is widely known for his achievement in constructing a working replica of the wartime Colossus computer.

The centenary will also be marked by an exhibition about Max Newman's life and work, to be held in the College Library from February 7th until Easter.

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4 February Security based on error correcting codes and its application to pay-tv / Jean-Bernard Fischer, Thomson Consumer Electronics, France

Room TP4, Computer Laboratory

We build an original cryptographic toolbox based on error-correcting codes. Having studied the difficulty of the syndrome decoding problem, we define a one-way function and a general setting for its use. Our results allow us to prove the security of Stern's authentication protocol SD; we also construct a provably secure pseudo-random generator and a very efficient and versatile keyed one-way function. These algorithms are used to provide end-to-end security for an analog pay-tv system using smart cards, similar to VideoCrypt.

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28 January Mechanised proofs for a recursive authentication protocol / Larry Paulson, University of Cambridge

Room TP4, Computer Laboratory

A novel protocol has been formally analyzed using the prover Isabelle/HOL, following the inductive approach described in the speaker's earlier work. A single run of the protocol delivers session keys to any number of agents, allowing neighbours to perform mutual authentication. The basic security theorem states that session keys are correctly delivered to adjacent pairs of honest agents, even if other agents in the chain are compromised. The complexity of the protocol caused modest difficulties in the specification and proofs, but symmetries in the protocol reduced the number of separate theorems to prove.