One key requirement for many cryptograhic schemes is the generation of random numbers. Sequences of random numbers are used at several stages of a standard cryptographic protocol. One simple example is a Vernam cipher, where a string of random numbers is added to message string to generate encrypted code. C = M ⊕ K. It has been mathematically shown that this simple scheme is unbreakable if key K is as long as M and is used only once. The security of a cryptosystem shall not be based on keeping the algorithm secret but solely on keeping the key secret. The security of a random number generator (RNG) is related to the difficulty of predicting its future sequence values from past values. The quality and unpredictability of secret data is critical to securing communication by modern cryptographic techniques. The generation of such data for cryptographic purposes typically requires an unpredictable physical source of random data. We studied a chaotic circuit which consisted of an inductor, capacitance, diode and thus used for the BB84 protocol. We have studied both pseudo random and true random number generators and evaluated them through various tests like frequency, correlation, NIST etc.

No abstract

Atomic response to a probe beam can be tailored, by creating coherences between atomic levels with help of another beam. Changing parameters of the control beam will change the nature of coherences and hence the nature of atomic response as well. Such change can depend upon intensity of both probe and control beams, in a nonlinear fashion. We present a situation where this nonlinearity in dependence can be precisely controlled, as to obtain different variations as desired. We also present a detailed analysis of how this nonlinear dependency arises and show that this is an interesting effect of several Coherent Population Trap(CPT) states that exist and a competition among them to trap atomic population in.

We have simulated atmospheric effects such as fog and smoke in laboratory environment to simulate depolarisation due to atmospheric effects during a free space optical communication. This has been used to study noise in two components of quaternary encoding for polarization shift keying. Individual components of a Quaternary encoding, such as vertical and horizontal as well as 45• and 135• , are tested separately and indicates that the depolarization effects are different for these two situation. However, due to a differential method used to extract information bits, the protocol shows extremely low bit error rates. The information obtained is useful during deployment of a fully functional Quaternary encoded PolSK scheme in free space.

Abstract:A practical scheme for measurement-device-independent polarization shift keying using two state polarization encoding is presented. Most of the previous work on optical free space laser communications through the atmosphere was concentrated on intensity modulated systems. However, polarization modulated systems may be more appropriate for such communication links, because the polarization seems to be the most stable characteristic of a laser beam while propagating through the atmosphere. Thus, a detailed comparison between intensity and polarization modulated systems is of big interest. The system used the big and powerful LabVIEW handling data and showing function to carry out a real-time processing, analysis and display. When two computers run LabVIEW at the same time, real-time date send and receive between computers by the interface of Virtual instrument, which can realize multimachine wireless data transmission and reading, in order to complete remote data. Introduction:Quantum key distribution (QKD) enables two remote parties to securely exchange cryptographic keys [1,2]. The security of QKD protocols has been proven in literature [3][4][5]. Meanwhile, a lot of efforts have been made to achieve the security of QKD with realistic devices [6]. Various device imperfections should be examined before security proofs can be applied to practical scenarios. Our interest is working on bb84 protocol that requires four state polarizations. Here we did two state of polarization based communication.