Nanjing University of Science and Technology

About: Nanjing University of Science and Technology is a education organization based out in Nanjing, China. It is known for research contribution in the topics: Catalysis & Computer science. The organization has 31581 authors who have published 36390 publications receiving 525474 citations. The organization is also known as: Nánjīng Lǐgōng Dàxué & Nánlǐgōng.

Attacks and design of image recognition CAPTCHAs

Abstract: We systematically study the design of image recognition CAPTCHAs (IRCs) in this paper. We first review and examine all existing IRCs schemes and evaluate each scheme against the practical requirements in CAPTCHA applications, particularly in large-scale real-life applications such as Gmail and Hotmail. Then we present a security analysis of the representative schemes we have identified. For the schemes that remain unbroken, we present our novel attacks. For the schemes for which known attacks are available, we propose a theoretical explanation why those schemes have failed. Next, we provide a simple but novel framework for guiding the design of robust IRCs. Then we propose an innovative IRC called Cortcha that is scalable to meet the requirements of large-scale applications. It relies on recognizing objects by exploiting the surrounding context, a task that humans can perform well but computers cannot. An infinite number of types of objects can be used to generate challenges, which can effectively disable the learning process in machine learning attacks. Cortcha does not require the images in its image database to be labeled. Image collection and CAPTCHA generation can be fully automated. Our usability studies indicate that, compared with Google's text CAPTCHA, Cortcha allows a slightly higher human accuracy rate but on average takes more time to solve a challenge.

A compact single layer monopulse microstrip antenna array

Abstract: A compact single layer monopulse microstrip patch antenna array for the application of monopulse radar has been designed, manufactured and tested. The array, the monopulse comparator and the feed network are placed on the same layer, resulting in a very compact structure. The limitation of this structure is discussed. The effects of spurious radiation and blockage caused by the comparator on the sidelobe level are estimated. A space mapping (SM) technique is applied to design microstrip subarray. The bandwidth (VSWR<2) of the antenna is 5.6%, the operating frequency is between 13.85-15.1 GHz and the designed central frequency is 14.25 GHz. Our measurements show that, both for the E plane and for the H plane, the sidelobe levels of the sum pattern are less than -17 dB, and the null depths of the difference pattern are less than -30 dB. The maximum gain at the operating frequencies is 24.5 dBi.