Challenging the Trustworthiness of PGP: Is the Web-of-Trust Tear-Proof?
The OpenPGP protocol provides a long time adopted and widespread tool for secure and authenticated asynchronous communications, as well as supplies data integrity and authenticity validation for software distribution. In this work, we analyze the Web-of-T
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stract. The OpenPGP protocol provides a long time adopted and widespread tool for secure and authenticated asynchronous communications, as well as supplies data integrity and authenticity validation for software distribution. In this work, we analyze the Web-of-Trust on which the OpenPGP public key authentication mechanism is based, and evaluate a threat model where its functionality can be jeopardized. Since the threat model is based on the viability of compromising an OpenPGP keypair, we performed an analysis of the state of health of the global OpenPGP key repository. Despite the detected amount of weak keypairs is rather low, our results show how, under reasonable assumptions, approximately 70 % of the Web-of-Trust strong set is potentially affected by the described threat. Finally, we propose viable mitigation strategies to cope with the highlighted threat.
Keywords: Web-of-Trust
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· WoT · OpenPGP · GPG · PGP
Introduction
The continuous increase in the size of computing systems, and the amount of data processed and exchanged by them calls for a widespread and trustworthy infrastructure for secure communications, encompassing both synchronous data transport and asynchronous messaging. Secure and endpoint-authenticated transport is nowadays provided by the Transport Layer Security (TLS) protocol [6], which is regarded as the most widespread solution when it comes to interactive communications between a server and a client. By contrast, the main workhorse in providing both confidentiality of the contents and sender authenticity, when it comes to secure e-mails, is the Open Pretty Good Privacy (OpenPGP) protocol [3]. The use of OpenPGP has been recently encouraged as a practical countermeasure to dragnet surveillance actions involving e-mail inspection. In particular the Free Software Foundation has promoted a campaign [29] to foster its use even among non technically-savvy users. Finally, the OpenPGP protocol is widely used to ensure data authentication and integrity of c Springer International Publishing Switzerland 2015 G. Pernul et al. (Eds.): ESORICS 2015, Part I, LNCS 9326, pp. 429–446, 2015. DOI: 10.1007/978-3-319-24174-6 22
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binary packages of both all the Debian and RedHat derived GNU/Linux distributions, and a significant number of other popular ones such as Arch, Slackware and Gentoo. Therefore, the authenticity of the software binaries installed on the overwhelming majority of GNU/Linux systems is provided by OpenPGP signatures. Since 2010, the official implementation of the OpenPGP protocol is available as a commercial technology by Symantec Corp., even if its source code is publicly available for peer review [28]. In addition to its employment as a solution to provide confidentiality and sender-authentication for e-mails, Symantec’s products also employ the same protocol for securing files and documents. The OpenPGP protocol, first defined in the RFC2440 [4] and then amended and extended in the RFC4880 [3] by the Internet Engineering Task Force (IETF), has its best known impleme
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