Sasa Radomirovic

Dr Sasa Radomirovic

Senior Lecturer in Secure Systems
+44 (0)1483 689305
05 BB 02


Areas of specialism

Formal Methods for Information Security; Formal Modeling and Verification of Cryptographic Protocols

University roles and responsibilities

  • Director of the GCHQ-accredited Information Security MSc


    Postgraduate research supervision



    Sven Hammann, Michael Crabb, Sasa Radomirovic, Ralf Sasse, David A. Basin: "I'm Surprised So Much Is Connected". CHI 2022: 620:1-620:13

    David A. Basin, Jannik Dreier, Sofia Giampietro, Sasa Radomirovic: Verifying Table-Based Elections. CCS 2021: 2632-2652

    David A. Basin, Sasa Radomirovic, Lara Schmid: Dispute Resolution in Voting. CSF 2020: 1-16

    Sven Hammann, Sasa Radomirovic, Ralf Sasse, David A. Basin: User Account Access Graphs. CCS 2019: 1405-1422

    David A. Basin, Jannik Dreier, Lucca Hirschi, Sasa Radomirovic, Ralf Sasse, Vincent Stettler:
    A Formal Analysis of 5G Authentication. CCS 2018: 1383-1396

    David A. Basin, Sasa Radomirovic, Lara Schmid: Modeling Human Errors in Security Protocols. CSF 2016: 325-340

    Barbara Kordy, Sjouke Mauw, Sasa Radomirovic, Patrick Schweitzer: Attack-defense trees. J. Log. Comput. 24(1): 55-87 (2014)

    Ton van Deursen, Sjouke Mauw, Sasa Radomirovic: mCarve: Carving Attributed Dump Sets. USENIX Security Symposium 2011


    David Basin, Jannik Dreier, Lucca Hirschi, Sasa Radomirovic, Ralf Sasse, Vincent Stettler (2018)A Formal Analysis of 5G Authentication

    Mobile communication networks connect much of the world's population. The security of users' calls, SMSs, and mobile data depends on the guarantees provided by the Authenticated Key Exchange protocols used. For the next-generation network (5G), the 3GPP group has standardized the 5G AKA protocol for this purpose. We provide the first comprehensive formal model of a protocol from the AKA family: 5G AKA. We also extract precise requirements from the 3GPP standards defining 5G and we identify missing security goals. Using the security protocol verification tool Tamarin, we conduct a full, systematic, security evaluation of the model with respect to the 5G security goals. Our automated analysis identifies the minimal security assumptions required for each security goal and we find that some critical security goals are not met, except under additional assumptions missing from the standard. Finally, we make explicit recommendations with provably secure fixes for the attacks and weaknesses we found.

    Jannik Dreier, Lucca Hirschi, SASA RADOMIROVIC, Ralf Sasse (2020)Verification of stateful cryptographic protocols with exclusive OR, In: Journal of computer security28(1)pp. 1-34 Ios Press

    In cryptographic protocols, in particular RFID protocols, exclusive-or (XOR) operations are common. Due to the inherent complexity of faithful models of XOR, there is only limited tool support for the verification of cryptographic protocols using XOR. In this paper, we improve the TAMARIN prover and its underlying theory to deal with an equational theory modeling XOR operations. The XOR theory can be combined with all equational theories previously supported, including user-defined equational theories. This makes TAMARIN the first verification tool for cryptographic protocols in the symbolic model to support simultaneously this large set of equational theories, protocols with global mutable state, an unbounded number of sessions, and complex security properties including observational equivalence. We demonstrate the effectiveness of our approach by analyzing several protocols that rely on XOR, in particular multiple RFID-protocols, where we can identify attacks as well as provide proofs.

    Sven Hammann, Michael Crabb, Sasa Radomirovic, Ralf Sasse, David Basin (2022)"I'm surprised so much is connected", In: CHI '22: Proceedings of the 2022 CHI Conference on Human Factors in Computing Systems Association for Computing Machinery (ACM)

    A person’s online security setup is tied to the security of their individual accounts. Some accounts are particularly critical as they provide access to other online services. For example, an email account can be used for external account recovery or to assist with single-sign-on. The connections between accounts are specific to each user’s setup and create unique security problems that are difficult to remedy by following generic security advice. In this paper, we develop a method to gather and analyze users’ online accounts systematically. We demonstrate this in a user study with 20 participants and obtain detailed insights on how users’ personal setup choices and behaviors affect their overall account security. We discuss concrete usability and privacy concerns that prevented our participants from improving their account security. Based on our findings, we provide recommendations for service providers and security experts to increase the adoption of security best practices.

    Sven Hammann, SASA RADOMIROVIC, Ralf Sasse, David Basin (2019)User Account Access Graphs

    The primary authentication method for a user account is rarely the only way to access that account. Accounts can often be accessed through other accounts, using recovery methods, password managers, or single sign-on. This increases each account's attack surface, giving rise to subtle security problems. These problems cannot be detected by considering each account in isolation, but require analyzing the links between a user's accounts. Furthermore, to accurately assess the security of accounts, the physical world must also be considered. For example, an attacker with access to a physical mailbox could obtain credentials sent by post. Despite the manifest importance of understanding these interrelationships and the security problems they entail, no prior methods exist to perform an analysis thereof in a precise way. To address this need, we introduce account access graphs, the first formalism that enables a comprehensive modeling and analysis of a user's entire setup, incorporating all connections between the user's accounts, devices, credentials, keys, and documents. Account access graphs support systematically identifying both security vulnerabilities and lockout risks in a user's accounts. We give analysis algorithms and illustrate their effectiveness in a case study, where we automatically detect significant weaknesses in a user's setup and suggest improvement options.

    David Basin, SASA RADOMIROVIC, Lara Schmid (2020)Dispute Resolution in Voting

    In voting, disputes arise when a voter claims that the voting authority is dishonest and did not correctly process his ballot while the authority claims to have followed the protocol. A dispute can be resolved if any third party can unambiguously determine who is right. We systematically characterize all relevant disputes for a generic, practically relevant, class of voting protocols. Based on our characterization, we propose a new definition of dispute resolution for voting that accounts for the possibility that both voters and the voting authority can make false claims and that voters may abstain from voting. A central aspect of our work is timeliness: a voter should possess the evidence required to resolve disputes no later than the election's end. We characterize what assumptions are necessary and sufficient for timeliness in terms of a communication topology for our voting protocol class. We formalize the dispute resolution properties and communication topologies symbolically. This provides the basis for verification of dispute resolution for a broad class of protocols. To demonstrate the utility of our model, we analyze a mixnet-based voting protocol and prove that it satisfies dispute resolution as well as verifiability and receipt-freeness. To prove our claims, we combine machine-checked proofs with traditional pen-and-paper proofs.

    David Basin, Jannik Dreier, Sofia Giampietro, Saša Radomirović (2021)Verifying Table-Based Elections, In: CCS '21: Proceedings of the 2021 ACM SIGSAC Conference on Computer and Communications Securitypp. 2632-2652 Association for Computing Machinery (ACM)

    Verifiability is a key requirement for electronic voting. However, the use of cryptographic techniques to achieve it usually requires specialist knowledge to understand; hence voters cannot easily assess the validity of such arguments themselves. To address this, solutions have been proposed using simple tables and checks, which require only simple verification steps with almost no cryptography. This simplicity comes at a cost: numerous verification checks must be made on the tables to ensure their correctness, raising the question whether the success of all the small verification steps entails the overall goal of end-to-end verifiability while preserving vote secrecy. Do the final results reflect the voters' will? Moreover, do the verification steps leak information about the voters' choices? In this paper, we provide mathematical foundations and an associated methodology for defining and proving verifiability and voter privacy for table-based election protocols. We apply them to three case studies: the Eperio protocol, Scantegrity, and Chaum's Random-Sample Election protocol. Our methodology helps us, in all three cases, identify previously unknown problems that allow an election authority to cheat and modify the election outcome. Furthermore, it helps us formulate and verify the corrected versions.