Quantum string seal is insecure

Chau, H. F.

doi:10.1103/physreva.75.012327

Cited by 8 publications

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“…(Note that it was claimed in Ref. [18] that all QSS protocols are insecure. But as indicated later in Ref.…”

Section: Introductionmentioning

confidence: 95%

Application of Quantum Seal Theory on Quantum Repeaters

2009

2009 Symposium on Photonics and Optoelectronics

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It is revealed that quantum repeaters and quantum seals have a very close relationship, thus the theory in one field can be applied to the other. We give examples of novel results found by applying the existing theory of quantum seals on quantum repeater problem. I. INTRODUCTIONConsider that different users share a single quantum communication channel in multiuser transmission. Each user decodes the transmitted information from the channel, and passes the carrier to the subsequent user. In this case, they need a quantum repeater [1] in which the information can be decoded reliably, while the quantum state of the carrier should be optimally preserved. When quantum information is being transmitted, the information/disturbance trade-off was intensively studied in literature [4]-[7]. Optimal quantum repeaters were also proposed in Refs. [1], [8]-[10]. Nevertheless, in most cases of quantum communication, a user generally cares little about the exact quantum state of the carrier. Instead, he only wants to know the classical information encoded in this quantum state. For example, in the well-know quantum key distribution (QKD) problem [11]-[13], a classical bit 0 (1) can be encoded as the quantum state |0 or |+ (|1 or |− ), where |0 and |1 are the two orthogonal states of a qubit, and |± = (|0 ± |1 )/ √2. An eavesdropper is only interested in whether the classical content is 0 or 1. There is no need for him to distinguish |0 from |+ , or |1 from |− . Therefore it is more important to study the trade-off between classical information gained versus quantum disturbance in quantum cryptography, and search for optimal quantum repeaters for this purpose.On the other hand, the concept of quantum seals (QS) is a relatively less-known quantum cryptographic problem. Its goal can be summarized as follows. Alice, the owner of the secret data to be sealed, encodes the data with quantum states.

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“…(Note that it was claimed in Ref. [18] that all QSS protocols are insecure. But as indicated later in Ref.…”

Section: Introductionmentioning

confidence: 95%

Application of Quantum Seal Theory on Quantum Repeaters

2009

2009 Symposium on Photonics and Optoelectronics

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“…That is, the results in Ref. [1] corresponding to different ν values in fact shows that if the amount of information obtained by the attack measurement is optimized, the probability of escaping the detection will be trivial, or vice versa. In either case, Chau's strategy is not a successful attack.…”

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In a recent paper [1], Chau claimed that all quantum string seals are insecure. The core of Chau's attack strategy is the measurement

(see Eq.

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confidence: 99%

“…IV of Ref. [1], it was claimed that "the major loophole in He's proof of the security of his quantum string seal in Ref. [2] is that he incorrectly assumed that measuring all the qubits is the only method to obtain a significant portion of information of the sealed message".…”

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confidence: 99%

“…IV of Ref. [1], by fixing ν = 1/2, the attacker can * Electronic address: hegp@mail.sysu.edu.cn escapes the verifier's detection at least half of the time, so that all quantum seals are claimed to be insecure. But in this case, the above equation becomes…”

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Comment on “Quantum string seal is insecure”

2007

Phys. Rev. A

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In a recent paper [1], Chau claimed that all quantum string seals are insecure. The core of Chau's attack strategy is the measurement(see Eq. (29) of that reference). It was claimed that with this measurement, the attacker can obtain non-trivial information on the sealed string while escapes the verifier's detection with at least 50% chance. However, the paper concentrated only on the fidelity of the sealed state corresponding to the attacker's measurement, without providing a detailed evaluation on the amount of information obtained by the attacker. Here it will be shown that for a class of quantum string seal protocols including the one proposed by He [2], this amount of information is only trivial. Therefore in contrast to Chau's claim, quantum string seal can be unconditionally secure. In fact, the general proof on why Chau's attack strategy fails had already been well addressed in Ref. [3]. Briefly, consider a simple model of imperfect quantum string seal, in which the sealed state for the message i ′ is taken aswhere the notation is the same as that in Eq. (1) of Ref.[1]. Applying the measurement Q i0 on it yieldsThus the probability for the message i ′ to be decoded as i by the attacker iswhere ν is defined by Eq. (12) It means that any one of the N possible choices of the message i ′ has at least the probability 1/(2N ) to be decoded as message i, even if its content is completely irrelevant with i. In other words, whenever the attacker obtains a message i via the measurement strategy, there is at less a probability p i = i ′ 1/(2N ) = 1/2 that the original message can be anything, i. e., the amount of information he obtained is zero. Thus it can be seen that the attack strategy is useless. Though at half of the time it can escape the verifier's detection, the amount of information obtained on the sealed message is only trivial. Therefore Chau's claim that all quantum seals are insecure is wrong. Now it will be shown that the protocol proposed in Ref.[2] is indeed such a secure quantum string seal. In this protocol, to seal a stringThus by takingwhere f m (θ m ) is cos θ m (sin θ m ) if the m-th bit of the string j ′ equals to (does not equal to) that of the string i ′ , we can see that the protocol belongs to the class of quantum string seal described by Eq. (2). Therefore as shown above, it cannot be broken by Chau's attack strategy. In Sec. IV of Ref.[1], it was claimed that "the major loophole in He's proof of the security of his quantum string seal in Ref.[2] is that he incorrectly assumed that measuring all the qubits is the only method to obtain a significant portion of information of the sealed message". But this is obviously incorrect. In the paragraph before Eq. (5) of Ref. [2], it was clearly written that the general security proof starts as follows. Let H denotes the 2 n dimensional Hilbert space where the sealed state lives

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No-go bounds for quantum seals

Kimmel

Kolkowitz

2019

Phys. Rev. A

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We investigate the feasibility of quantum seals. A quantum seal is a state provided by Alice to Bob along with information which Bob can use to make a measurement, "break the seal," and read the classical message stored inside. There are two success criteria for a seal: the probability Bob can successfully read the message without any further information from Alice must be high, and if Alice asks for the state back from Bob, the probability Alice can tell if Bob broke the seal without permission must be high. We build on the work of [Chau, PRA 2007], which gave optimal bounds on these criteria, showing that they are mutually exclusive for high probability. We weaken the assumptions of this previous work by providing Bob with only a classical description of a prescribed measurement, rather than classical descriptions of the possible seal states. We show that this weakening does not affect the bounds but does simplify the analysis. We also prove upper and lower bounds on an alternative operational metric for measuring the success criteria.

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Quantum string seal is insecure

Cited by 8 publications

References 9 publications

Application of Quantum Seal Theory on Quantum Repeaters

Application of Quantum Seal Theory on Quantum Repeaters

Comment on “Quantum string seal is insecure”

No-go bounds for quantum seals

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