We demonstrate theoretically and experimentally that secure communication using intermediateenergy (mesoscopic) coherent states is possible. Our scheme is different from previous quantum cryptographic schemes in that a short secret key is explicitly used and in which quantum noise hides both the bit and the key. This encryption scheme can be optically amplified. New avenues are open to secure communications at high speeds in fiber-optic or free-space channels.For the encryption of data with perfect secrecy [1] that cannot be broken with any advance in technology, one may in principle employ one-time pad with secret key obtained by the BB84 [2] quantum cryptographic technique for key expansion. Such an approach is possible [3], however, it is slow and inefficient because the key length needs to be as long as the data, and it also requires a nearly ideal quantum communication line that is difficult to obtain in long distance commercial systems such as the Internet core. On the other hand, for both military and commercial applications, there are great demands for secret communications that are fast and secure but not necessarily perfectly secure. (There are many practical issues, human as well machine based, that would make theoretical perfect security in specific models not so important in real life [4]). In the following, a new scheme based on ideas similar to those of Ref.[5] is described for secure data encryption that can be operated at optical speeds with conventional optical technology, and a prototype experimental implementation is presented. In this scheme, a short secret key is classically extended and then used to encrypt data in a way that the quantum noise of the coherent states protects both the data and the key.The following line of reasoning describes the ideas [6] that led to the development of our present kind of quantum cryptographic schemes. One crucial element for obtaining security in BB84 involves the detection of small intrusion on weak signals, which is difficult to achieve in a network environment. This problem would be alleviated if quantum signal sets of higher energy are selected for different bit values by a secret key shared between the sender (Alice) and the receiver (Bob). It is important to remember that some shared secret key is needed in BB84 for message authentication during protocol execution. The resulting scheme is acceptable as key expansion if the new key is secure even if the shared secret key is known to the attacker after the user communications are completed. When a secret key is used to identify the signal set, it would be a secret CDMA (Code Division Multiple Access) scheme classically, which does not allow key expansion because the user and the attacker have the same observation. We would discuss elsewhere how a corresponding KCQ (Keyed CDMA in Quantum Noise) scheme can be used to obtain key expansion in the quantum case. In this paper, we are concerned with the use of KCQ for data encryption.There are two basic problems with classical encryption that does not employ...

We demonstrate theoretically and experimentally that secure communication using intermediate-energy (mesoscopic) coherent states is possible. Our scheme is different from previous quantum cryptographic schemes in that a short secret key is explicitly used and in which quantum noise hides both the bit and the key. This encryption scheme allows optical amplification. New avenues are open to secure communications at high speeds in fiber-optic or free-space channels.

We demonstrate high-rate randomized data-encryption through optical fibers using the inherent quantummeasurement noise of coherent states of light. Specifically, we demonstrate 650 Mbit/ s data encryption through a 10 Gbit/ s data-bearing, in-line amplified 200-km-long line. In our protocol, legitimate users ͑who share a short secret key͒ communicate using an M-ry signal set while an attacker ͑who does not share the secret key͒ is forced to contend with the fundamental and irreducible quantum-measurement noise of coherent states. Implementations of our protocol using both polarization-encoded signal sets as well as polarization-insensitive phase-keyed signal sets are experimentally and theoretically evaluated. Different from the performance criteria for the cryptographic objective of key generation ͑quantum key-generation͒, one possible set of performance criteria for the cryptographic objective of data encryption is established and carefully considered.

We review the notion of a classical random cipher and its advantages. We sharpen the usual description of random ciphers to a particular mathematical characterization suggested by the salient feature responsible for their increased security. We describe a concrete system known as ␣ and show that it is equivalent to a random cipher in which the required randomization is affected by coherent-state quantum noise. We describe the currently known security features of ␣ and similar systems, including lower bounds on the unicity distances against ciphertext-only and known-plaintext attacks. We show how ␣ used in conjunction with any standard stream cipher such as the Advanced Encryption Standard provides an additional, qualitatively different layer of security from physical encryption against known-plaintext attacks on the key. We refute some claims in the literature that ␣ is equivalent to a nonrandom stream cipher.