Universally Composable Direct Anonymous Attestation

Direct Anonymous Attestation (DAA) is one of the most complex cryptographic algorithms that has been deployed in practice. In spite of this, and the long body of work on the subject, there is still no fully satisfactory security definition for DAA. This was already acknowledged by Bernard et al. (IJIC\u2713) who showed that in existing models even fully insecure protocols may be deemed secure. Bernard et al. therefore proposed an extensive set of security games, which however aimed only at a simplified setting, termed pre-DAA. In pre-DAA the host platform that runs the TPM is assumed to be trusted too. Consequently, their notion does not guarantee any security if the TPM is embedded in a potentially corrupt host, which is a significant restriction. In this paper, we give a comprehensive security definition for full DAA in the form of an ideal functionality in the Universal Composability model. Our definition considers the host and TPM to be individual entities that can be in different corruption states. None of the existing DAA schemes immediately satisfies our strong security notion, and we therefore also propose a realization that is based on a DAA scheme supported by the TPM 2.0 standard and rigorously prove it secure in our model

Anonymity and Rewards in Peer Rating Systems

When peers rate each other, they may choose to rate inaccurately in order to boost their own reputation or unfairly lower another’s. This could be successfully mitigated by having a reputation server incentivise accurate ratings with a reward. However, assigning rewards becomes a challenge when ratings are anonymous, since the reputation server cannot tell which peers to reward for rating accurately. To address this, we propose an anonymous peer rating system in which users can be rewarded for accurate ratings, and we formally define its model and security requirements. In our system ratings are rewarded in batches, so that users claiming their rewards only reveal they authored one in this batch of ratings. To ensure the anonymity set of rewarded users is not reduced, we also split the reputation server into two entities, the Rewarder, who knows which ratings are rewarded, and the Reputation Holder, who knows which users were rewarded. We give a provably secure construction satisfying all the security properties required. For our construction we use a modification of a Direct Anonymous Attestation scheme to ensure that peers can prove their own reputation when rating others, and that multiple feedback on the same subject can be detected. We then use Linkable Ring Signatures to enable peers to be rewarded for their accurate ratings, while still ensuring that ratings are anonymous. Our work results in a system which allows for accurate ratings to be rewarded, whilst still providing anonymity of ratings with respect to the central entities managing the system

With a Little Help from My Friends: Constructing Practical Anonymous Credentials

Anonymous credentials (ACs) are a powerful cryptographic tool for the secure use of digital services, when simultaneously aiming for strong privacy guarantees of users combined with strong authentication guarantees for providers of services. They allow users to selectively prove possession of attributes encoded in a credential without revealing any other meaningful information about themselves. While there is a significant body of research on AC systems, modern use-cases of ACs such as mobile applications come with various requirements not sufficiently considered so far. These include preventing the sharing of credentials and coping with resource constraints of the platforms (e.g., smart cards such as SIM cards in smartphones). Such aspects are typically out of scope of AC constructions, and, thus AC systems that can be considered entirely practical have been elusive so far. In this paper we address this problem by introducing and formalizing the notion of core/helper anonymous credentials (CHAC). The model considers a constrained core device (e.g., a SIM card) and a powerful helper device (e.g., a smartphone). The key idea is that the core device performs operations that do not depend on the size of the credential or the number of attributes, but at the same time the helper device is unable to use the credential without its help. We present a provably secure generic construction of CHACs using a combination of signatures with flexible public keys (SFPK) and the novel notion of aggregatable attribute-based equivalence class signatures (AAEQ) along with a concrete instantiation. The key characteristics of our scheme are that the size of showing tokens is independent of the number of attributes in the credential(s) and that the core device only needs to compute a single elliptic curve scalar multiplication, regardless of the number of attributes. We confirm the practical efficiency of our CHACs with an implementation of our scheme on a Multos smart card as the core and an Android smartphone as the helper device. A credential showing requires less than 500 ms on the smart card and around 200 ms on the smartphone (even for a credential with 1000 attributes)

YOLO YOSO: Fast and Simple Encryption and Secret Sharing in the YOSO Model

Achieving adaptive (or proactive) security in cryptographic protocols is notoriously difficult due to the adversary\u27s power to dynamically corrupt parties as the execution progresses. Inspired by the work of Benhamouda et al. in TCC 2020, Gentry et al. in CRYPTO 2021 introduced the YOSO (You Only Speak Once) model for constructing adaptively (or proactively) secure protocols in massively distributed settings (e.g. blockchains). In this model, instead of having all parties execute an entire protocol, smaller anonymous committees are randomly chosen to execute each individual round of the protocol. After playing their role, parties encrypt protocol messages towards the the next anonymous committee and erase their internal state before publishing their ciphertexts. However, a big challenge remains in realizing YOSO protocols: efficiently encrypting messages towards anonymous parties selected at random without learning their identities, while proving the encrypted messages are valid w.r.t. the protocol. In particular, the protocols of Benhamouda et al. and of Gentry et al. require showing ciphertexts contain valid shares of secret states. We propose concretely efficient methods for encrypting a protocol\u27s secret state towards a random anonymous committee. We start by proposing a very simple and efficient scheme for encrypting messages towards randomly and anonymously selected parties. We then show constructions of publicly verifiable secret (re-)sharing (PVSS) schemes with concretely efficient proofs of (re-)share validity that can be generically instantiated from encryption schemes with certain linear homomorphic properties. Finally, we show that our PVSS schemes can be efficiently realized from our encyption scheme

Critical Perspectives on Provable Security: Fifteen Years of Another Look Papers

We give an overview of our critiques of “proofs” of security and a guide to our papers on the subject that have appeared over the past decade and a half. We also provide numerous additional examples and a few updates and errata

Direct Anonymous Attestation with Optimal TPM Signing Efficiency

Direct Anonymous Attestation (DAA) is an anonymous signature scheme, which allows the Trusted Platform Module (TPM), a small chip embedded in a host computer, to attest to the state of the host system, while preserving the privacy of the user. DAA provides two signature modes: fully anonymous signatures and pseudonymous signatures. One main goal of designing DAA schemes is to reduce the TPM signing workload as much as possible, as the TPM has only limited resources. In an optimal DAA scheme, the signing workload on the TPM will be no more than that required for a normal signature like ECSchnorr. To date, no scheme has achieved the optimal signing efficiency for both signature modes. In this paper, we propose the first DAA scheme which achieves the optimal TPM signing efficiency for both signature modes. In this scheme, the TPM takes only a single exponentiation to generate a signature, and this single exponentiation can be pre-computed. Our scheme can be implemented using the existing TPM 2.0 commands, and thus is compatible with the TPM 2.0 specification. We benchmarked the TPM 2.0 commands needed for three DAA use cases on an Infineon TPM 2.0 chip, and also implemented the host signing and verification algorithm for our scheme on a laptop with 1.80GHz Intel Core i7-8550U CPU. Our experimental results show that our DAA scheme obtains a total signing time of about 144 ms for either of two signature modes (compared to an online signing time of about 65 ms). Based on our benchmark results for the pseudonymous signature mode, our scheme is roughly 2x (resp., 5x) faster than the existing DAA schemes supported by TPM 2.0 in terms of total (resp., online) signing efficiency. In addition, our DAA scheme supports selective attribute disclosure, which can satisfy more application require- ments. We also extend our DAA scheme to support signature-based revocation and to guarantee privacy against subverted TPMs. The two extended DAA schemes keep the TPM signing efficiency optimal for both of two signa- ture modes, and outperform existing related schemes in terms of signing performance