Quantum continuous variables [1] are being explored [2,3,4,5,6,7,8,9,10,11,12,13,14] as an alternative means to implement quantum key distribution, which is usually based on single photon counting [15]. The former approach is potentially advantageous because it should enable higher key distribution rates. Here we propose and experimentally demonstrate a quantum key distribution protocol based on the transmission of gaussian-modulated coherent states (consisting of laser pulses containing a few hundred photons) and shot-noise-limited homodyne detection; squeezed or entangled beams are not required [13]. Complete secret key extraction is achieved using a reverse reconciliation [14] technique followed by privacy amplification. The reverse reconciliation technique is in principle secure for any value of the line transmission, against gaussian individual attacks based on entanglement and quantum memories. Our table-top experiment yields a net key transmission rate of about 1.7 megabits per second for a loss-free line, and 75 kilobits per second for a line with losses of 3.1 dB. We anticipate that the scheme should remain effective for lines with higher losses, particularly because the present limitations are essentially technical, so that significant margin for improvement is available on both the hardware and software.

Abstract. In this paper we prove that the sponge construction introduced in [4] is indifferentiable from a random oracle when being used with a random transformation or a random permutation and discuss its implications. To our knowledge, this is the first time indifferentiability has been shown for a construction calling a random permutation (instead of an ideal compression function or ideal block cipher) and for a construction generating outputs of any length (instead of a fixed length).

A continuous key distribution scheme is proposed that relies on a pair of canonically conjugate quantum variables. It allows two remote parties to share a secret Gaussian key by encoding it into one of the two quadrature components of a single-mode electromagnetic field. The resulting quantum cryptographic information vs disturbance tradeoff is investigated for an individual attack based on the optimal continuous cloning machine. It is shown that the information gained by the eavesdropper then simply equals the information lost by the receiver.PACS numbers: 03.67. Dd, 03.65.Bz, 89.70.+c Quantum cryptography-or, more precisely, quantum key distribution-is a technique that allows two remote parties to share a secret chain of random bits (a secret key) that can be used for exchanging encrypted information [1][2][3]. The security of this process fundamentally relies on the Heisenberg uncertainty principle, or on the fact that any measurement of incompatible variables inevitably affects the state of a quantum system. Any leak of information to an eavesdropper necessarily induces a disturbance of the system, which is, in principle, detectable by the authorized receiver.In most quantum cryptosystems proposed so far, a single photon (or, in practice, a weak coherent state with an average photon number lower than one) is used to carry each bit of the key. Mathematically, the security is based on the use of a pair of non-commuting observables such as the x-and z-projections of a spin-1/2 particle, σ x and σ z , whose eigenstates are used to encode the key. The sender (Alice) randomly chooses to encode the key using either σ z (0 is encoded as | ↑ and 1 as | ↓ ) or σ x (0 is encoded as 2 −1/2 (| ↑ + | ↓ ) and 1 as 2 −1/2 (| ↑ − | ↓ )), the choice of the basis being disclosed only after the receiver (Bob) has measured the photon. This guarantees that an eavesdropper (Eve) cannot read the key without corrupting the transmission. Such a procedure, known as BB84 [1], is at the heart of most of the quantum cryptographic schemes that have been experimentally demonstrated in the past few years, which are based either on the polarization (e. g. [4,5]) or the optical phase (e. g. [6]) of single photons. An alternative scheme, realized experimentally only a year ago [7-9], can also be used based on a pair of polarization-entangled photons instead of single photons [10]. It is, however, fundamentally equivalent to BB84 (see [11]) and it again relies on the algebra of spin-1/2 particles.Recently, it has been shown that another protocol for quantum key distribution can be devised based on continuous variables, where squeezed coherent light modes are used to carry the key [12][13][14]. In these techniques, one exploits a pair of (continuous) canonical variables such as the two quadratures X 1 and X 2 of the amplitude of a mode of the electromagnetic field, which be-have just as position and momentum. The uncertainty relation ∆X 1 ∆X 2 ≥ 1/4 then implies than Eve cannot read both quadrature components without degrading the state. Even ...

Abstract. This paper proposes a novel construction, called duplex, closely related to the sponge construction, that accepts message blocks to be hashed and-at no extra cost-provides digests on the input blocks received so far. It can be proven equivalent to a cascade of sponge functions and hence inherits its security against single-stage generic a acks. The main application proposed here is an authenticated encryption mode based on the duplex construction. This mode is efficient, namely, enciphering and authenticating together require only a single call to the underlying permutation per block, and is readily usable in, e.g., key wrapping. Furthermore, it is the first mode of this kind to be directly based on a permutation instead of a block cipher and to natively support intermediate tags. The duplex construction can be used to efficiently realize other modes, such as a reseedable pseudo-random bit sequence generators and a sponge variant that overwrites part of the state with the input block rather than to XOR it in.

Quantum cryptography (or quantum key distribution) is a state-of-the-art technique that exploits properties of quantum mechanics to guarantee the secure exchange of secret keys. This 2006 text introduces the principles and techniques of quantum cryptography, setting it in the wider context of cryptography and security, with specific focus on secret-key distillation. The book starts with an overview chapter, progressing to classical cryptography, information theory (classical and quantum), and applications of quantum cryptography. The discussion moves to secret-key distillation, privacy amplification and reconciliation techniques, concluding with the security principles of quantum cryptography. The author explains the physical implementation and security of these systems, enabling engineers to gauge the suitability of quantum cryptography for securing transmission in their particular application. With its blend of fundamental theory, implementation techniques, and details of recent protocols, this book will be of interest to graduate students, researchers, and practitioners in electrical engineering, physics, and computer science.