Today's smartphone app stores are full of apps with diverse features. Many developers use third-party libraries to reduce the development time and cost, but developers often ignore the security problems of third-party libraries. A major security problem introduced by third-party libraries is that a third-party library has the same permissions as the apps, calledhost-appshereafter, that use it. According to previous research, having the same permissions as its host apps, a third-party library could have unauthorized access to user data, which poses a serious threat to app users. Therefore, how to prevent third-party libraries from abusing permissions has become an important issue. To solve this problem, this paper proposes a Dynamic Permission Control mechanism, calledDynamic Permission ControllerorDPChereafter, for app developers to prohibit third-party libraries from abusing host apps’ dangerous permissions. DPC modifies the permission control mechanism of Android framework to make apps have a more flexible permission management mechanism when they are running. DPC provides new APIs which allows an app to dynamically disable a granted dangerous permission before invoking an API of a third-party library and restore the dangerous permission after completing the API. Hence, DPC protects user's privacy by blocking unauthorized access from third-party libraries. Meanwhile, without the requirement that an app developer needs to know the detail of third-party libraries, the app still can use APIs of third-party libraries safely. Experimental results show that DPC works with many popular apps downloaded from Google Play well and DPC prohibits a third-party library from having the same dangerous permissions that its host apps have. Hence, unlike previous solutions, DPC does not have compatibility problems. The overhead introduced by DPC on an emulator and Nexus 7 are 1.8 and 0.3 percent respectively.

Abstract The reputation system is used to display the reputation of entities based on the ratings or appraisals given by users who have used or purchased those entities. Web service providers supply various reputation systems for their users. However, these promising reputation systems face some challenges. First, even though different persons have different preference andvalues(values are a person's beliefs about what things are good or bad), these systems still give the same rating to the same entity for all users. Second, they may be greatly influenced by Sybil attacks. In this paper, we propose a reputation system, called values‐based reputation system (VRS), to solve the aforementioned problems. VRS customizes the rating of an entity for each user based on the ratings of the entities provided by other users who have similarvaluesor preference to the user. Experimental results on 256 users show that compared with existing reputation systems VRS is more robust to Sybil attacks and provides a recommendation rating that is closer to the rating given by the user after it used the related entity. Copyright © 2016 John Wiley & Sons, Ltd.

Email spam is a critical problem to the Internet for a long time. The average amount of spam mail reached 72.1% of all email traffic in the world in 2012. The greatest threat to the email service providers was the spam mail sent from botnet, because the spam mail sent from botnet was accounting for more than 78% in 2011; therefore appeared many anti-spam solutions and techniques that were focus on the botnet. Owing to these anti-spam techniques, botnet spam is not effective as before. Spammers are finding new way to send the spam mail. One of the effective methods is using compromised accounts (or bot accounts) to send the spam mail because compromised accounts have good reputation IP addresses and compromised accounts send the spam mail with complete SMTP implemented server, such as Gmail, Yahoo!Mail, and Microsoft Live Mail. The spam mail send form compromised accounts are very difficult to be detected by any anti-spam techniques. Hence, we focus on the features spammers cannot easily hide. According to our research we find that normal users usually do not reply to the spam mail. Moreover, our empirical analysis reveals that the compromised account actually have low reply rate. We develop a system called “Hawkeye” that can find the compromised accounts effectively by checking the account's reply rate.

A fast-flux service network (FFSN) uses dynamic DNS to map a dynamic domain, called fast-flux domain (FF domain), to various IP addresses and uses flux bots to redirect network traffic. Due to its powerful capability to conceal the hosts hidden behind the flux bots, FFSNs are widely adopted by attackers to cover various scams. Although diverse promising solutions have been proposed to detect FF domains, they face the same problem-different countermeasures could be used to bypass their detection. Hence, it becomes a critical issue to develop a new detection solution. According to our survey, unlike normal network services that use dynamic DNS to balance the workloads of their hosts, FFSNs utilize dynamic DNS to hide important bots. As a result, the response time of subsequent requests to an FF domain becomes more fluctuating. Based on the response time differences, this paper develops a new metric, Fast-Flux Score (FF-Score), to detect FF domains. Our system, called fast-flux domain detector (FFDD), is used on a computer that could be an end host or an IDS. A user with a set of unknown URLs, which may be obtained from spam or social networks, can simply determine whether they are benign domains or fast-flux ones using FFDD. Experimental results show that FFDD can accurately detect FF domains with only a 0.3% false positive rate and a 2% false negative rate. It takes less than 20 min for FFDD to determine whether a domain is an FF domain. In addition, FFDD is a lightweight stand-alone system; hence, it does not require special support from an ISP or any other network service.

A fundamental principle of local interaction of radio waves with a refractive spherical medium is formulated and illustrated using the radio occultation (RO) method of remote sensing of the atmosphere and the ionosphere of the Earth and the planets. In accordance with this principle, the main contribution to variations of the amplitude and the phase of radio waves propagating through a medium makes a neighborhood of a tangential point, where the gradient of the refractive index is perpendicular to the radio wave trajectory. A necessary and sufficient condition (a criterion) is established to detect the displacement of the tangential point from the radio ray perigee using analysis of the RO experimental data. This criterion is applied to the identification and the location of layers in the atmosphere and the ionosphere by the use of Global Positioning System RO data. RO data from the CHAllenge Minisatellite Payload (CHAMP) are used to validate the criterion introduced when significant variations of the amplitude and the phase of the RO signals are observed at the RO ray perigee altitudes below 80 km. The detected criterion opens a new avenue in terms of measuring the altitude and the slope of the atmospheric and ionospheric layers. This is important for the location determination of the wind shear and the direction of internal wave propagation in the lower ionosphere and possibly in the atmosphere. The new criterion provides an improved estimation of the altitude and the location of the ionospheric plasma layers compared with the backpropagation radio-holographic method previously used.

Ray tracing is an important tool for the operation of GPS L-band frequency signal propagation; in particular GPS RO techniques where accurate and near real-time results are often required. Ray tracing techniques are commonly used for calculating the path of an electromagnetic signal in a medium specified by a position dependent refractive index, such as the Earth's atmosphere. In this study three dimensional numerical ray tracing techniques are used to simulate GPS L-band signals received by the LEO satellites and primarily focusing on regions of increased electron density gradients such as the equatorial anomaly. The down range refractive gradients on the GPS to LEO signal paths will be investigated. The ionosphere is a birefringent medium due to the presence of the Earth's magnetic field. The effects of the Earth's magnetic field on the transmitted signals and the paths of the ordinary and extraordinary signals are investigated.

The Australian Low Earth Orbit (LEO) microsatellite, FedSat (named to commemorate the centenary of the Australian Federation in 2001), was launched into orbit on December 14, 2002 from the Tanegashima Space Centre, Japan. A Global Positioning System (GPS) receiver was one of the instruments onboard. The received GPS signals can be used to investigate the ionospheric electron density and the atmosphere below FedSat's orbiting altitude, using radio occultation (RO) techniques. The RO technique developed involves a simplified form of the Abel transform using the slant total electron content (STEC) determined from radio signals that traverse below FedSat's orbiting altitude. Electron density profiles from the GPS RO data, recorded by the GPS receiver onboard the FedSat satellite, are determined for the first time. The technique combined with simultaneous occultation density profile extraction from different LEO satellites and satellite navigation systems has the potential to image near real-time 3-D structures of the ionospheric electron density.

The ionosphere and atmosphere may have significant impacts on the high-stable navigational signals of the Global Positioning System (GPS) in the communication link satellite to satellite. The classification of the different types of the ionospheric impact on the phase and amplitude of the GPS signals at altitudes of 40-90 km is introduced using the CHAllenging Minisatellite Payload (CHAMP) radio occultation (RO) data. An analytical model is elaborated for the description of the radio wave propagation in the stratified ionosphere and atmosphere. The propagation medium consists of sectors having the spherically symmetric distributions of refractivity. The newly developed model presents analytical expressions for the phase path and refractive attenuation of radio waves. The model explains significant amplitude and phase variations at altitudes of 40-90 km of the RO ray perigee associated with the influence of the inclined ionospheric layers. An innovative eikonal acceleration technique is described and applied to the identification and location of the inclined ionospheric layers using the comparative analysis of the amplitude and phase variations of the RO signals.

GPS radio occultation, (RO) is an emerging and robust space-based earth observation system, with the potential for atmospheric profiling and meteorological applications. GPS RO requires GPS receivers onboard Low Earth Orbit (LEO) satellites to measure the radio signals from GPS satellites so that the atmospheric profiles of parameters such as temperature, pressure and water vapour can be obtained via a complicated atmospheric retrieval process. This research focuses on the ionospheric correction using the Radio Occultation Processing Package (ROPP) to investigate the effect from ionosphere for GPS radio occultation in the Australia region. The MSISE-90 model with the statistical optimization method produced the best results for altitudes greater than 40 km. The influence from the ionosphere can be removed using the generic Lc method which produced the best results for altitudes less than 40 km.

Positioning accuracy by the Global Positioning System (GPS) is of great concern in a variety of research tasks. It is limited due to error sources such as ionospheric effect, orbital uncertainty, antenna phase center variation, signal multipath, and tropospheric influence. In this study, the tropospheric influence, primarily due to water vapor inhomogeneity, on GPS positioning height is investigated. The data collected by the GPS receivers along with co-located surface meteorological instruments in 2003 are utilized. The GPS receivers are established as continuous operating reference stations by the Ministry of the Interior (MOI), Central Weather Bureau (CWB), and Industrial Technology Research Institute (ITRI) of Taiwan, and International GNSS Service (IGS). The total number of GPS receivers is 21. The surface meteorological measurements include temperature, pressure, and humidity. They are introduced to GPS data processing with 24 tropospheric parameters for the station heights, which are compared with those obtained without a priori knowledge of surface meteorological measurements. The results suggest that surface meteorological measurements have an expected impact on the GPS height. The daily correction maximum with the meteorological effect may be as large as 9.3 mm for the cases of concern.