Rahul Jain

We present a robust and composable device-independent (DI) quantum protocol between two parties for oblivious transfer (OT) using Magic Square devices in the bounded storage model [DFR`07, DFSS08] in which the (honest and cheating) devices and parties have no long- term quantum memory. After a fixed constant (real-world) time interval, referred to as DELAY, the quantum states decohere completely. The adversary (cheating party), with full control over the devices, is allowed joint (non-IID) quantum operations on the devices, and there are no time and space complexity bounds placed on its powers. The running time of the honest parties is polylog(λ) (where λ is the security parameter). Our protocol has negligible (in λ) correctness and security errors and can be implemented in the NISQ (Noisy Intermediate Scale Quantum) era. By robustness, we mean that our protocol is correct even when devices are slightly off (by a small constant) from their ideal specification. This is an important property since small manufacturing errors in the real-world devices are inevitable. Our protocol is sequentially composable and, hence, can be used as a building block to construct larger protocols (including DI bit-commitment and DI secure multi-party computation) while still preserving correctness and security guarantees. None of the known DI protocols for OT in the literature are robust and secure against joint quantum attacks. This was a major open question in device-independent two-party distrustful cryptography, which we resolve. We prove a parallel repetition theorem for a certain class of entangled games with a hybrid (quantum-classical) strategy to show the security of our protocol. The hybrid strategy helps to incorporate DELAY in our protocol. This parallel repetition theorem is a main technical contribution of our work. Since our games use hybrid strategies and the inputs to our games are not independent, we use a novel combination of ideas from previous works showing parallel rep- etition of classical games [Raz95, Hol07], quantum games [JPY14, JMS20, JK22], and anchored games [BVY17, JK21]. Although we present security proof for protocols in the bounded storage model with no long-term quantum memory (after DELAY), we state (without further justification) that we can extend our results, along the lines of [JK22] and [DFR`07], to incorporate linear (in the number of devices) long term quantum memory and linear leakage between the devices.

One-way state generators (OWSG) [MY22a] are natural quantum analogs to classical one-way functions. We show that O(n/log(n))-copy OWSGs (n represents the input length) are equivalent to poly(n)-copy OWSGs and to quantum commitments. Since known results show that o(n/log(n))-copy OWSGs cannot imply commitments [CGG`23], this shows that O(n/log(n))-copy OWSGs are the weakest OWSGs from which we can get commitments (and hence much of quantum cryptography). Our construction follows along the lines of Håstad, Impagliazzo, Levin and Luby [HILL99], who obtained classical pseudorandom generators (PRG) from classical one-way functions (OWF), however with crucial modifications. Our construction, when applied to the classical case, provides an alternative to the construction provided by [HILL99]. Since we do not argue conditioned on the output of the one-way function, our construction and analysis are arguably simpler and may be of independent interest.

We show an area law in the mutual information for the maximally-mixed state Ω in the ground space of general Hamiltonians, which is independent of the underlying ground space degeneracy. Our result assumes the existence of a `good' approximation to the ground state projector (a good AGSP), a crucial ingredient in former area-law proofs. Such approximations have been explicitly derived for 1D gapped local Hamiltonians and 2D frustration-free and locally-gapped local Hamiltonians. As a corollary, we show that in 1D gapped local Hamiltonians, for any eps>0 and any bi-partition Lcup L^c of the system, beginalign* I^eps_max(L:L^c)_Ømega łe bigO łog (|L|) + łog(1/eps), endalign* where |L| represents the number of sites in L and I^eps_max(L:L^c)_Ømega represents the eps-emphsmoothed maximum mutual information with respect to the L:L^c partition in Ω. From this bound we then conclude I(L:L^c)_Ømega łe bigOłog(|L|) – an area law for the mutual information in 1D systems with a logarithmic correction. In addition, we show that Ω can be approximated up to an eps in trace norm with a state of Schmidt rank of at most poly(|L|/eps). Similar corollaries are derived for the mutual information of 2D frustration-free locally-gapped local Hamiltonians.

Non-malleable codes are fundamental objects at the intersection of cryptography and coding theory. These codes provide security guarantees even in settings where error correction and detection are impossible, and have found applications to several other cryptographic tasks. Roughly speaking, a non-malleable code for a family of tampering functions guarantees that no adversary can tamper (using functions from this family) the encoding of a given message into the encoding of a related distinct message. We focus on the split-state tampering model, one of the strongest and most well-studied adversarial tampering models. In this model, a codeword is split into two parts which are stored in physically distant servers, and the adversary can then independently tamper with each part using arbitrary functions. Previous works on non-malleable codes in the split-state tampering model only considered the encoding of classical messages. Furthermore, until the recent work by Aggarwal, Boddu, and Jain (arXiv 2022), adversaries with quantum capabilities and shared entanglement had not been considered, and it is a priori not clear whether previous coding schemes remain secure in this model. In this work, we introduce the notion of split-state non-malleable codes for quantum messages secure against quantum adversaries with shared entanglement. We construct explicit codes in this model by relying on a recent quantum-secure 2-source non-malleable randomness encoder by Batra, Boddu, and Jain [BBJ23], arguments from Aggarwal, Boddu and Jain [ABJ22] and with use of unitary 2-designs. 1) More precisely, we construct the first efficiently encodable and decodable split-state non- malleable code for quantum messages (while preserving entanglement with external sys- tems) achieving security against quantum adversaries having shared entanglement with codeword length n, any message length at most $n^\Omega(1)$, and error $2^{-n^{\Omega(1)}}$. 2) For the case of uniform quantum message, we provide the first constant rate (rate 1/11) non-malleable code (while preserving entanglement with external systems) achieving code- word length n and error $2^{-n^{\Omega(1)}}$. .

“Non-Malleable Randomness Encoder” (NMRE) was introduced by Kanukurthi, Obbattu, and Sekar [KOS18] as a useful cryptographic primitive helpful in the construction of non- malleable codes. To the best of our knowledge, their construction is not known to be quantum secure. We provide a construction of a first rate-$1/2$, $2$-split, quantum secure NMRE and use this in a black-box manner, to construct for the first time the following: 1. rate $1/11$, $3$-split, quantum non-malleable code, 2. rate $1/3$, $3$-split, quantum secure non-malleable code, 3. rate $1/5$, $2$-split, quantum secure non-malleable code.

We present robust and composable device-independent quantum protocols for oblivious transfer (OT) and bit commitment (BC) using Magic Square devices. We assume there is no long-term quantum memory, that is, after a finite time interval, referred to as extbf DELAY, the states stored in the devices decohere. By robustness, which is a highlight of our protocols, we mean that the protocols are correct and secure even when devices are slightly off from their ideal specifications (the \emph{faulty but non-malicious} regime). This is an important property, since in the real world, devices would certainly have small manufacturing errors and cannot be expected to be ideal. To the best of our understanding and knowledge, none of the known DI protocols for OT and BC in the literature are robust; they can not guarantee correctness in the faulty but non-malicious regime. Our protocols are sequentially composable and hence, can be used as building blocks to construct larger protocols, while still preserving security guarantees.

Multi-source-extractors are functions that extract uniform randomness from multiple (weak) sources of randomness. With the advent of quantum computers, it is natural to investigate the security of multi-source-extractors against adversaries with quantum side-information on the sources of randomness (potentially generated using quantum entanglement). Quantum multi- source-extractors were considered by Kasher and Kempe (for the quantum-independent- adversary and the quantum-bounded-storage-adversary), Chung, Li, and Wu (for the general- entangled-adversary), and Arnon-Friedman, Portmann, and Scholz (for the quantum-Markov- adversary). In this work, we propose two new models of adversaries, the quantum-measurement-adversary (qm-adv) and the quantum-communication-adversary (qc-adv). qm-adv generates side-information post-measurement outcomes and qc-adv generates side-information using a communication protocol. We show that: 1. qm-adv is the strongest adversary among all the known adversaries, in the sense that the side-information of all other adversaries can be generated by qm-adv. 2. The (generalized) inner-product function (in fact a general class of two-wise independent functions) continue to work as a good extractor against qm-adv (with matching parameters as that of Chor and Goldreich against classical-adversaries). 3. A non-malleable extractor proposed by Li (against classical-adversaries) continues to be secure against quantum side-information. A non-malleable extractor (nm-ext) for two sources (X, Y) is an extractor such that nm-ext(X, Y) is uniform and independent of nm-ext(X, Y')YY', where Y' is not equal to Y and Y' is generated by the adversary using Y and the side-information on X. 4. A modification (not needing any local uniform randomness) of the Dodis and Wich's protocol for privacy-amplification is secure against active quantum adversaries. This strengthens on a recent result due to Aggarwal, Chung, Lin, and Vidick which uses local uniform randomness. 5. As a byproduct, we reproduce the quantum communication complexity lower bound for the (generalized) inner-product function via different proof techniques.

We show new bounds on the quantum communication cost of single-shot entanglementassisted one-way quantum communication protocols for the quantum state redistribution task and for the sub-tasks quantum state splitting and quantum state merging. Our bounds are tighter than previously known best bounds for the latter two sub-tasks. A key technical tool that we use is a convex-split lemma which may be of independent interest. This differs from other achievability bounds for quantum state redistribution and quantum state merging , which use decoupling by application of random unitary. Convex-split lemma is based on the fact that in a one-way quantum communication protocol, measurement by Alice leads to an ensemble of states on registers owned by Bob and Referee, convex combination of which is the original mixed state shared between Bob and Referee. Through convex-split lemma, we design one such ensemble of states and use it to construct a protocol for the task of quantum state redistribution.