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Abstract. An attribute-based signature (ABS) is a special digital signature created using a dynamic set of issued attributes. For instance, a doctor can sign a medical statement with his name, medical license number and medical speciality. These attributes can be verified along with the signature by any verifier with the correct public keys of the respective attribute issuers. This functionality not only makes ABS a much more flexible alternative to the standard PKI-based signatures, but also offers the ability to create privacy-preserving signatures. However, none of the ABS constructions presented in the literature is practical or easily realizable. In fact, to the best of our knowledge, there is currently no ABS implementation used in practice anywhere. This is why we put forward a new ABS technique based on the IRMA attribute-based authentication. IRMA already has an efficient and practical smart-card implementation, and an experimental smart-phone implementation too. They are currently used in several pilot projects. In this paper, we propose an ABS scheme based on the existing IRMA technology, extending the currently available IRMA devices with ABS functionality. We study the practical issues that arise due to the introduction of the signature functionality to an existing attribute-based authentication scheme, and we propose possible cryptographic and infrastructural solutions. We also discuss use cases and implementation aspects.

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Users’ devices, e.g., smartphones or laptops, are typically incapable of securely storing and processing cryptographic keys.We present Tandem, a novel set of protocols for securing cryptographic keys with support from a central server. Tandem uses one-time-use key-share tokens to preserve users’ privacy with respect to a malicious central server. Additionally, Tandem enables users to block their keys if they lose their device, and it enables the server to limit how often an adversary can use an unblocked key. We prove Tandem’s security and privacy properties, apply Tandem to attributebased credentials, and implement a Tandem proof of concept to show that it causes little overhead.

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