International Association
for Cryptologic Research

User privacy is becoming increasingly important in our digital society. Yet, many applications face legal requirements or regulations that prohibit unconditional anonymity guarantees, e.g., in electronic payments where surveillance is mandated to investigate suspected crimes. As a result, many systems have no effective privacy protections at all, or have backdoors, e.g., stored at the operator side of the system, that can be used by authorities to disclose a user’s private information (e.g., lawful interception). The problem with such backdoors is that they also enable silent mass surveillance within the system. To prevent such misuse, various approaches have been suggested which limit possible abuse or ensure it can be detected. Many works consider auditability of surveillance actions but do not enforce that traces are left when backdoors are retrieved. A notable exception which offers retrospective and silent surveillance is the recent work on misuse-resistant surveillance by Green et al. (EUROCRYPT’21). However, their approach relies on extractable witness encryption, which is a very strong primitive with no known efficient and secure implementations. In this work, we develop a building block for auditable surveillance. In our protocol, backdoors or escrow secrets of users are protected in multiple ways: (1) Backdoors are short-term and user-specific; (2) they are shared between trustworthy parties to avoid a single point of failure; and (3) backdoor access is given conditionally. Moreover (4) there are audit trails and public statistics for every (granted) backdoor request; and (5) surveillance remains silent, i.e., users do not know they are surveilled. Concretely, we present an abstract UC-functionality which can be used to augment applications with auditable surveillance capabilities. Our realization makes use of threshold encryption to protect user secrets, and is concretely built in a blockchain context with committee-based YOSO MPC. As a consequence, the committee can verify that the conditions for backdoor access are given, e.g., that law enforcement is in possession of a valid surveillance warrant (via a zero-knowledge proof). Moreover, access leaves an audit trail on the ledger, which allows an auditor to retrospectively examine surveillance decisions. As a toy example, we present an Auditably Sender-Traceable Encryption scheme, a PKE scheme where the sender can be deanonymized by law enforcement. We observe and solve problems posed by retrospective surveillance via a special non-interactive non-committing encryption scheme which allows zero-knowledge proofs over message, sender identity and (escrow) secrets.

Onion routing (OR) protocols are a crucial tool for providing anonymous internet communication. An OR protocol enables a user to anonymously send requests to a server. A fundamental problem of OR protocols is how to deal with replies: ideally, we would want the server to be able to send a reply back to the anonymous user without knowing or disclosing the user's identity. Existing OR protocols do allow for such replies, but do not provably protect the payload (i.e., message) of replies against manipulation. Kuhn et al. (IEEE S&P 2020) show that such manipulations can in fact be leveraged to break anonymity of the whole protocol. In this work, we close this gap and provide the first framework and protocols for OR with protected replies. We define security in the sense of an ideal functionality in the universal composability model, and provide corresponding (less complex) game-based security notions for the individual properties. We also provide two secure instantiations of our framework: one based on updatable encryption, and one based on succinct non-interactive arguments (SNARGs) to authenticate payloads both in requests and replies. In both cases, our central technical handle is an implicit authentication of the transmitted payload data, as opposed to an explicit, but insufficient authentication (with MACs) in previous solutions. Our results exhibit a new and surprising application of updatable encryption outside of long-term data storage.

An updatable encryption scheme allows a data host to update ciphertexts of a client from an old to a new key, given so-called update tokens from the client. Rotation of the encryption key is a common requirement in practice in order to mitigate the impact of key compromises over time. There are two incarnations of updatable encryption: One is ciphertext-dependent, i.e. the data owner has to (partially) download all of his data and derive a dedicated token per ciphertext. Everspaugh et al. (CRYPTO’17) proposed CCA and CTXT secure schemes in this setting. The other, more convenient variant is ciphertext-independent, i.e., it allows a single token to update all ciphertexts. However, so far, the broader functionality of tokens in this setting comes at the price of considerably weaker security: the existing schemes by Boneh et al. (CRYPTO’13) and Lehmann and Tackmann (EUROCRYPT’18) only achieve CPA security and provide no integrity protection. Arguably, when targeting the scenario of outsourcing data to an untrusted host, plaintext integrity should be a minimal security requirement. Otherwise, the data host may alter or inject ciphertexts arbitrarily. Indeed, the schemes from BLMR13 and LT18 suffer from this weakness, and even EPRS17 only provides integrity against adversaries which cannot arbitrarily inject ciphertexts. In this work, we provide the first ciphertext-independent updatable encryption schemes with security beyond CPA, in particular providing strong integrity protection. Our constructions and security proofs of updatable encryption schemes are surprisingly modular. We give a generic transformation that allows key-rotation and confidentiality/integrity of the scheme to be treated almost separately, i.e., security of the updatable scheme is derived from simple properties of its static building blocks. An interesting side effect of our generic approach is that it immediately implies the unlinkability of ciphertext updates that was introduced as an essential additional property of updatable encryption by EPRS17 and LT18.

In this work, we settle the relations among a variety of security notions related to non-malleability and CCA-security that have been proposed for commitment schemes in the literature. Interestingly, all our separations follow from two generic transformations. Given two appropriate security notions X and Y from the class of security notions we compare, these transformations take a commitment scheme that fulfills notion X and output a commitment scheme that still fulfills notion X but not notion Y.Using these transformations, we are able to show that some of the known relations for public-key encryption do not carry over to commitments. In particular, we show that, surprisingly, parallel non-malleability and parallel CCA-security are not equivalent for commitment schemes. This stands in contrast to the situation for public-key encryption where these two notions are equivalent as shown by Bellare et al. at CRYPTO ‘99.