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Sybil-Resistant Pseudonymization and Pseudonym Change Without Trusted Third Parties Sybil-Resistant Pseudonymization and Pseudonym Change without Trusted Third Parties Martin Florian, Johannes Walter, Ingmar Baumgart Institute of Telematics Karlsruhe Institute of Technology (KIT) 76131 Karlsruhe, Germany {florian,baumgart}@kit.edu, [email protected] ABSTRACT grading the utility of novel systems and services. Addition- The issuing of pseudonyms is an established approach for ally, without limiting access in some form, sybil attacks [9] protecting the privacy of users while limiting access and pre- become possible where adversaries create large numbers of venting sybil attacks. To prevent pseudonym deanonymiza- fake virtual identities (sybils). This both enhances the po- tion through continuous observation and correlation, fre- tential magnitude of abuse and enables malicious users to quent and unlinkable pseudonym changes must be enabled. avoid blacklisting. Existing approaches for realizing sybil-resistant pseudonym- The issuing of unlinkable pseudonyms to users is a com- ization and pseudonym change (PPC) are either inherently mon solution to the challenge of hiding user identities while dependent on trusted third parties (TTPs) or involve signif- enabling access control and the effective protection against icant computation overhead at end-user devices. In this pa- sybil attacks. Additionally, unlinkable pseudonym changes per, we investigate a novel, TTP-independent approach to- must be made possible for reducing the linkability between wards sybil-resistant PPC. Our proposal is based on the use (potentially privacy-relevant) data samples originating from of cryptocurrency block chains as general-purpose, append- the same user. If frequent pseudonym changes are realized only bulletin boards. We present a general approach as well and an adversary successfully links one pseudonym to a user as BitNym, a specific design based on the unmodified Bitcoin (e.g., using side channels and correlation), only the data network. We discuss and propose TTP-independent mech- samples observed in connection with this pseudonym be- anisms for realizing sybil-free initial access control, pseudo- come linkable to the user. nym validation and pseudonym mixing. Evaluation results Established approaches for enabling sybil-resistant pseu- demonstrate the practical feasibility of our approach and donymization and pseudonym change (PPC) inherently re- show that anonymity sets encompassing nearly the complete quire a trusted third party (TTP) like a certification author- user population are easily achievable. ity for enforcing issuing criteria and preventing sybil attacks. Upon compromise of the TTP, large-scale sybil attacks be- come possible and the trustworthiness of issued pseudonyms Categories and Subject Descriptors is greatly reduced. Thus, centralized pseudonym issuers be- C.2.0 [Computer-Communication Networks]: General| come attractive targets for attacks, resulting in high opera- Security and protection; K.4.1 [Computers and Society]: tional costs for maintaining their security. Additionally, the Public Policy Issues|Privacy notion of universal trust anchors shared by all system par- ticipants is questionable when considering mobile users in a Keywords globally interconnected world. In this paper, as an alternative to assuming centralized pseudonym; sybil attack; blacklisting; block chain TTPs, we explore the use of distributed, non-malleable bul- letin boards as provided by cryptocurrency block chains. 1. INTRODUCTION More specifically, our contributions are the following: Privacy-preservation is becoming increasingly important • A novel approach towards TTP-free and abuse-resistant as more and more areas of life are benefiting from the ubiq- pseudonymization and pseudonym change (PPC). To uitous interconnection of humans and autonomous devices. the best of our knowledge, we are the first to propose a At the same time, allowing users (and their devices) to col- complete PPC system that both prevents sybil attacks laborate and use services anonymously can lead to abuse, de- and doesn't rely on a TTP for ensuring the correctness Permission to make digital or hard copies of all or part of this work for personal or and security of any of its operations. classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than the • A specific implementation of the approach - BitNym author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or - leveraging the existing Bitcoin [15] network without republish, to post on servers or to redistribute to lists, requires prior specific permission requiring any modifications to its underlying protocols. and/or a fee. Request permissions from [email protected]. WPES’15, October 12 2015, Denver, CO, USA • A prototype of the proposed system and an evaluation Copyright is held by the owner/author(s). Publication rights licensed to ACM. of our approach using simulations of user populations ACM 978-1-4503-3820-2/15/10 . $15.00 and pseudonym changing behavior. DOI: http://dx.doi.org/10.1145/2808138.2808145 2. RELATED WORK More specifically, we aim at realizing the following proper- Approaches based on blind signatures [12] and zero-know- ties: ledge proofs [6] have been proposed for enabling the central- 1. Unlinkability of pseudonyms to user identities and to ized issuing of unlinkable pseudonyms. Here, a TTP is also other pseudonyms by the same user. required for changing pseudonyms, as the old pseudonym must be marked invalid to prevent sybil attacks. Alterna- 2. Limitation to a fixed number of simultaneously active tive approaches like e-token-dispensers [5] or the issuing of pseudonyms per user. pools of pseudonyms with non-overlapping validity periods 3. Authenticity of the linking between a pseudonym and still inherently require a TTP for enforcing issuing criteria its holder. and ensuring that the issuing of pseudonyms is sybil-free. As an alternative to TTP-based access control, decentral- 4. Possibility of unlinkable pseudonym changes. ized approaches for preventing sybil attacks were proposed. Proof-of-resource schemes like CAPTCHA [20] and proof-of- 5. Possibility of blacklisting pseudonym holders. work [4] are not effective for systems involving end-users, as 6. Complete independence of any trusted third parties determined adversaries can amass multiple orders of magni- (TTPs) for realizing any of the preceding properties. tude more resources than regular users are ready to spend for continuously using a system [13]. Approaches based on Properties 1 to 5 have been widely discussed in the liter- the social graph between users [19] are more promising, but ature [16] and are shared by multiple existing and proposed either incompatible with privacy requirements or suited only systems. Their combination with property 6, on the other for scenarios in which nearly all users are continuously reach- hand, is, to the best of our knowledge, unique to our pro- able and active within a large-scale peer-to-peer network. posal. The usage of proof-of-burn, i.e., the provable destruction of a non-replenishable resource, was proposed for establish- 3.2 Decentralized append-only bulletin boards ing sybil-free and trustworthy online identities [11] using the Our strategy for avoiding the reliance on TTPs is to build Bitcoin cryptocurrency [15]. However, resulting identities upon recent results on realizing distributed consistency in are linkable to the originating users. Even if funded via highly adversarial environments. More specifically, we build anonymous sources, changing such identities is not possible upon decentralized cryptocurrency systems like Bitcoin [15], without repeating the significant initial investment. in which a globally consistent transaction log is collabora- In [10], Garman et al. propose a scheme for realizing de- tively maintained within a network of non-colluding peers. centralized anonymous credentials. As in our proposal, the Transactions are typically grouped in blocks, yielding a cryp- authors build upon cryptocurrency systems like Bitcoin for tographically secured block chain as the practical manifes- avoiding the dependence on TTPs. However, the proposal tation of the network consensus. In the following, we will is based on computation-intensive zero-knowledge proofs, also refer to Bitcoin-like networks as block chain networks. making it less suited for more resource-constrained devices, Transactions in block chain networks are composed of out- and requires a TTP during the initial setup phase. The chal- puts - the number of funds exiting the transaction in combi- lenge of initially issuing pseudonyms in a TTP-independent nation with a challenge that needs to be solved for spending and yet sybil-resistant manner is discussed only marginally. them - and inputs - references to preceding outputs in com- Also, the blacklisting of malicious users is not supported. bination with a valid solution. Typically, outputs contain To the best of our knowledge, we are the first to propose a the public part of an asymmetric cryptographic key pair complete pseudonymization system that is fully decentral- (respectively a hash thereof) and can be spent upon proving ized, resistant to sybil attacks and supports efficient and the possession of the corresponding secret key. In addition, unlinkable pseudonym changes as well as the blacklisting of the inclusion of arbitrary data in transactions
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