Non-Interactive Blind Signatures for Random Messages

Blind signatures allow a signer to issue signatures on messages chosen by the signature recipient. The main property is that the recipient\u27s message is hidden from the signer. There are many applications, including Chaum\u27s e-cash system and Privacy Pass, where no special distribution of the signed message is required, and the message can be random. Interestingly, existing notions do not consider this practical use case separately. In this paper, we show that constraining the recipient\u27s choice over the message distribution spawns a surprising new primitive that improves the well-established state-of-the-art. We formalize this concept by introducing the notion of non-interactive blind signatures (${\sf NIBS}$). Informally, the signer can create a presignature with a specific recipient in mind, identifiable via a public key. The recipient can use her secret key to finalize it and receive a blind signature on a random message determined by the finalization process. The key idea is that online interaction between the signer and recipient is unnecessary. We show an efficient instantiation of ${\sf NIBS}$ in the random oracle model from signatures on equivalence classes. The exciting part is that, in this case, for the recipient\u27s public key, we can use preexisting keys for Schnorr, ECDSA signatures, El-Gamal encryption scheme, or even the Diffie-Hellman key exchange. Reusing preexisting public keys allows us to distribute anonymous tokens similarly to cryptocurrency airdropping. Additional contributions include tagged non-interactive blind signatures (${\sf TNIBS}$) and their efficient instantiation. A generic construction in the random oracle or common reference string model based on verifiable random functions, standard signatures, and non-interactive proof systems

Security Analysis of ePrint Report 2016/500 Efficient Identity-Based Encryption and Public-Key Signature from Trapdoor Subgroups

In this short report we analyse the security of three schemes proposed by J. H. Park et al. in Efficient Identity-Based Encryption and Public-Key Signature from Trapdoor Subgroups . The schemes make use of trapdoor subgroups of \ZZ_n^* and are secure under new assumptions called $q$-Trapdoor Subgroup Diffie-Hellman (TSDH) and $q$-Trapdoor Subgroup Exponent Inversion (TSEI). We show that given several secret keys in case of IBE or several signatures in case of PKS, one can easily extract the trapdoor and break security of the proposed schemes

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)

MLCapsule: Guarded Offline Deployment of Machine Learning as a Service

With the widespread use of machine learning (ML) techniques, ML as a service has become increasingly popular. In this setting, an ML model resides on a server and users can query it with their data via an API. However, if the user's input is sensitive, sending it to the server is undesirable and sometimes even legally not possible. Equally, the service provider does not want to share the model by sending it to the client for protecting its intellectual property and pay-per-query business model. In this paper, we propose MLCapsule, a guarded offline deployment of machine learning as a service. MLCapsule executes the model locally on the user's side and therefore the data never leaves the client. Meanwhile, MLCapsule offers the service provider the same level of control and security of its model as the commonly used server-side execution. In addition, MLCapsule is applicable to offline applications that require local execution. Beyond protecting against direct model access, we couple the secure offline deployment with defenses against advanced attacks on machine learning models such as model stealing, reverse engineering, and membership inference

Rai-Choo! Evolving Blind Signatures to the Next Level

Blind signatures are a fundamental tool for privacy-preserving applications. Known constructions of concurrently secure blind signature schemes either are prohibitively inefficient or rely on non-standard assumptions, even in the random oracle model. A recent line of work (ASIACRYPT `21, CRYPTO `22) initiated the study of concretely efficient schemes based on well-understood assumptions in the random oracle model. However, these schemes still have several major drawbacks: 1) The signer is required to keep state; 2) The computation grows linearly with the number of signing interactions, making the schemes impractical; 3) The schemes require at least five moves of interaction. In this paper, we introduce a blind signature scheme that eliminates all of the above drawbacks at the same time. Namely, we show a round-optimal, concretely efficient, concurrently secure, and stateless blind signature scheme in which communication and computation are independent of the number of signing interactions. Our construction also naturally generalizes to the partially blind signature setting. Our scheme is based on the CDH assumption in the asymmetric pairing setting and can be instantiated using a standard BLS curve. We obtain signature and communication sizes of 9KB and 36KB, respectively. To further improve the efficiency of our scheme, we show how to obtain a scheme with better amortized communication efficiency. Our approach batches the issuing of signatures for multiple messages

simTPM: User-centric TPM for Mobile Devices

Trusted Platform Modules are valuable building blocks for security solutions and have also been recognized as beneficial for security on mobile platforms, like smartphones and tablets. However, strict space, cost, and power constraints of mobile devices prohibit an implementation as dedicated on-board chip and the incumbent implementations are software TPMs protected by Trusted Execution Environments. In this paper, we present simTPM, an alternative implementation of a mobile TPM based on the SIM card available in mobile platforms. We solve the technical challenge of implementing a TPM2.0 in the resource-constrained SIM card environment and integrate our simTPM into the secure boot chain of the ARM Trusted Firmware on a HiKey960 reference board. Most notably, we address the challenge of how a removable TPM can be bound to the host device’s root of trust for measurement. As such, our solution not only provides a mobile TPM that avoids additional hardware while using a dedicated, strongly protected environment, but also offers promising synergies with co-existing TEE-based TPMs. In particular, simTPM offers a user-centric trusted module. Using performance benchmarks, we show that our simTPM has competitive speed with a reported TEE-based TPM and a hardware-based TPM

Token meets Wallet: Formalizing Privacy and Revocation for FIDO2

The FIDO2 standard is a widely-used class of challenge-response type protocols that allows to authenticate to an online service using a hardware token. Barbosa et al. (CRYPTO `21) provided the first formal security model and analysis for the FIDO2 standard. However, their model has two shortcomings: (1) It does not include privacy, one of the key features claimed by FIDO2. (2) It only covers tokens that store {all secret keys locally}. In contrast, due to limited memory, most existing FIDO2 tokens either derive all secret keys from a common seed or store keys on the server (the latter approach is also known as {key wrapping}). In this paper, we revisit the security of the WebAuthn component of FIDO2 as implemented in practice. Our contributions are as follows. (1) We adapt the model of Barbosa et al. so as to capture authentication tokens using key derivation or key wrapping. (2) We provide the {first formal definition of privacy for the WebAuthn component of FIDO2}. We then prove the privacy of this component in common FIDO2 token implementations if the underlying building blocks are chosen appropriately. (3) We address the unsolved problem of {global key revocation} in FIDO2. To this end, we introduce and analyze a simple revocation procedure that builds on the popular BIP32 standard used in cryptocurrency wallets and can efficiently be implemented with existing FIDO2 servers

On the Security of Rate-limited Privacy Pass

The privacy pass protocol allows users to redeem anonymously issued cryptographic tokens instead of solving annoying CAPTCHAs. The issuing authority verifies the credibility of the user, who can later use the pass while browsing the web using an anonymous or virtual private network. Hendrickson et al. proposed an IETF draft (privacypass-rate-limit-tokens-00) for a rate-limiting version of the privacy pass protocol, also called rate-limited Privacy Pass (RlP). Introducing a new actor called a mediator makes both versions inherently different. The mediator applies access policies to rate-limit users’ access to the service while, at the same time, should be oblivious to the website/origin the user is trying to access. In this paper, we formally define the rate-limited Privacy Pass protocol and propose a game-based security model to capture the informal security notions introduced by Hendrickson et al.. We show a construction from simple building blocks that fulfills our security definitions and even allows for a post-quantum secure instantiation. Interestingly, the instantiation proposed in the IETF draft is a specific case of our construction. Thus, we can reuse the security arguments for the generic construction and show that the version used in practice is secure