Abstract We show how fixed-unitary quantum encryption schemes can be attacked in a black-box setting. We use an efficient technique to invert a unitary transformation on a quantum computer to retrieve an encrypted secret quantum state $${|{\psi }\rangle }$$ | ψ ⟩ . This attack has a success rate of 100% and can be executed in constant time. We name a vulnerable scheme which security is fully broken by our attack and suggest how to improve the scheme to invalidate this attack. The proposed attack highlights the importance of carefully designing quantum encryption schemes to ensure their security against quantum adversaries, even in a black-box setting. We point to the faulty assumption and name a criterion for future quantum cipher design to prevent similar vulnerabilities.
Abstract We show how fixed-unitary quantum encryption schemes can be attacked in a black-box setting. We use an efficient technique to invert a unitary transformation on a quantum computer to retrieve an encrypted secret quantum state ∣ψ⟩ | ψ ⟩ . This attack has a success rate of 100% and can be executed in constant time. We name a vulnerable scheme which security is fully broken by our attack and suggest how to improve the scheme to invalidate this attack. The proposed attack highlights the importance of carefully designing quantum encryption schemes to ensure their security against quantum adversaries, even in a black-box setting. We point to the faulty assumption and name a criterion for future quantum cipher design to prevent similar vulnerabilities.
Abstract In this paper, we analyse the impact of the HHL quantum algorithm on stream ciphers in a black-box setting. We assume a black-box access to an oracle MS M S defining the output of the stream cipher. For a state k encapsulating the key material, the value MS⋅k M S · k is the keystream (k1′,k2′,...,kN′) ( k 1 ′ , k 2 ′ , . . . , k N ′ ) generated by some stream cipher S. We translate this scenario into the quantum setting and describe how the HHL algorithm could be used to attack this construction. Further, we give simple and verifiable criteria under which a black-box attack on stream ciphers with the HHL algorithm is not efficiently feasible. Usually, these criteria follow from already known design principles for symmetric ciphers and should apply to the ciphers used today. We complement the criteria with a simple test, which confirms the resistance of said cipher. Moreover, we use our technique to test the currently used stream ciphers: Trivium, HC-128, and Salsa20.
In the classical setting, public-key encryption requires randomness in order to be secure against a forward search attack, whereby an adversary compares the encryption of a guess of the secret message with the encryption of the actual secret message. We show that this is also true in the information-theoretic setting---where the public keys are quantum systems---by defining and giving an example of a forward search attack for any deterministic quantum-public-key bit-encryption scheme. However, unlike in the classical setting, we show that any such deterministic scheme can be used as a black box to build a randomized bit-encryption scheme that is no longer susceptible to this attack.
Encryption of data is fundamental to secure communication in the modern world. Beyond encryption of data lies obfuscation, i.e., encryption of functionality. It is well-known that the most powerful means of obfuscating classical programs, so-called ``black-box obfuscation',' is provably impossible [Barak et al '12]. However, several recent results have yielded candidate schemes that satisfy a definition weaker than black-box, and yet still have numerous applications. In this work, we initialize the rigorous study of obfuscating programs via quantum-mechanical means. We define notions of quantum obfuscation which encompass several natural variants. The input to the obfuscator can describe classical or quantum functionality, and the output can be a circuit description or a quantum state. The obfuscator can also satisfy one of a number of obfuscation conditions: black-box, information-theoretic black-box, indistinguishability, and best possible; the last two conditions come in three variants: perfect, statistical, and computational. We discuss many applications, including CPA-secure quantum encryption, quantum fully-homomorphic encryption, and public-key quantum money. We then prove several impossibility results, extending a number of foundational papers on classical obfuscation to the quantum setting. We prove that quantum black-box obfuscation is impossible in a setting where adversaries can possess more than one output of the obfuscator. In particular, generic transformation of quantum circuits into black-box-obfuscated quantum circuits is impossible. We also show that statistical indistinguishability obfuscation is impossible, up to an unlikely complexity-theoretic collapse. Our proofs involve a new tool: chosen-ciphertext-secure encryption of quantum data, which was recently shown to be possible assuming quantum-secure one-way functions exist [Alagic et al '16].
It is an important question to find constructions of quantum cryptographic protocols which rely on weaker computational assumptions than classical protocols. Recently, it has been shown that oblivious transfer and multi-party computation can be constructed from one-way functions, whereas this is impossible in the classical setting in a black-box way. In this work, we study the question of building quantum public-key encryption schemes from one-way functions and even weaker assumptions. Firstly, we revisit the definition of IND-CPA security to this setting. Then, we propose three schemes for quantum public-key encryption from one-way functions, pseudorandom function-like states with proof of deletion and pseudorandom function-like states, respectively.
The development of robust and secure quantum information cryptography is a critical area of research, addressing the vulnerabilities of current cryptographic methods. This paper introduces a novel approach to quantum information encryption based on the principles of topological quantum states. We propose a quantum information encryption scheme that leverages the inherent topological properties of quantum states to provide enhanced security and privacy. The core mechanism involves manipulating quantum states through topological transformations, creating complex and unpredictable states that are resistant to attacks. This research explores the potential of utilizing these topological structures to develop a fundamentally new paradigm for quantum data protection. The study focuses on the design of an encryption algorithm that offers improved resilience against various computational attacks compared to existing cryptographic techniques. We begin with a detailed theoretical framework, outlining the mathematical foundations underpinning the proposed scheme, and then present a practical implementation of the core algorithm. The paper concludes by discussing future research directions and potential applications of this innovative approach.
In a functional encryption (FE) scheme, the owner of the secret key can generate restricted decryption keys that allow users to learn specific functions of the encrypted messages and nothing else. In many known constructions of FE schemes, such a notion of security is guaranteed only for messages that are fixed ahead of time (i.e., before the adversary even interacts with the system). This is called selective security, which is too restrictive for many realistic applications. Achieving adaptive security (also called full security), where security is guaranteed even for messages that are adaptively chosen at any point in time, seems significantly more challenging. The handful of known fully-secure schemes are based on specifically tailored techniques that rely on strong assumptions (such as obfuscation assumptions or multilinear maps assumptions). In this paper we show that any sufficiently expressive selectively-secure FE scheme can be transformed into a fully secure one without introducing any additional assumptions. We present a direct black-box transformation, making novel use of hybrid encryption, a classical technique that was originally introduced for improving the efficiency of encryption schemes, combined with a new technique we call the Trojan Method. This method allows to embed a secret execution thread in the functional keys of the underlying