基于ASP.NET的汉字验证码的研究与实现

网站为了防止用户利用机器人自动注册、登录、灌水,都采用了验证码技术.通常的验证码有数字验证码、图文验证码和汉字验证码.汉字验证码比较难于识别,使用汉字验证码对网站的安全防范更高.本文介绍了汉字验证码的设计与实现.     

基于Java的中文验证码生成器的设计与实现

通过对生成随机数技术、汉字编码的区位码和机内码技术进行深入的分析和研究,给出了中文验证码生成器的设计思路及生成中文验证码的流程图;最后又使用Java语言实现了中文验证码生成器,并进行了测试,结果表明该生成器易于复用和推广。     

一种新型图像验证码系统

验证码是一种防止自动化脚本程序对网站进行攻击的重要技术。当前主流网站采用的验证码多是基于文本模式的,这种模式的验证码将一系列文本字符通过旋转扭曲叠加的方式进行变形,然后再添加复杂干扰背景图片等方式使自动化程序难以辨识。然而,这种文本验证码要么过于简单,很容易被脚本采用光学字符识别（OCR）或者机器学习的方式所识别,要么由于过于复杂甚至连真正的用户都难以正确的识别。观察到文本验证码的这种安全性和可用性方面存在的矛盾,本文提出了一种基于物体识别的新型图像验证码系统。该系统通过将若干物体图片附加到随机的背景图片中并让用户识别的方式来辨别自动脚本和真正的用户。     

基于字符特征的文本验证码破解

文中使用基于字符特征识别方法对基于文本图像的验证码进行破解研究。以某银行的验证码为例,介绍基于文本图像验证码的识别和破解过程。具体分为验证码预处理、字符提取和字符识别3个阶段。在识别阶段,分别使用颜色填充法、三线条法、全面扫描法的字符特征识别方法,在准确率和速度上都取得了理想的效果,对验证码的识别正确率达78%。文中所提字符特征识别和验证码破解方法,同样适用于其他非粘连的基于文本图像的验证码。     

Android设备获取web服务端验证码技术的研究

传统的使用session记录验证码的方法存在缺陷:如果客户端无法使用cookie,则无法正确的进行验证。故文章提出了一种通过Mysql数据库来记录验证码的方法。首先web服务器收到客户端索要验证码的请求后发送验证码,并将验证码和请求中的标识符作为一组数据通过JDBC(Java Data Base Connectivity)记录在Mysql数据库中。然后等web服务器收到客户端提交的验证码后,依照提交请求中的标识符到Mysql数据库中找到对应的验证码,拿此验证码和提交的验证码进行匹配。测试结果表明:此方法有效地解决了使用session记录用户信息的弊端,而且还具有数据保存时间灵活,数据方便于管理,服务器内存占用减少的优点。     

高噪声环境下语音数字化降噪技术的研究

本文通过研究语音信号与噪声信号来探讨语音端点检测技术和语音增强技术,并取得了显著的成果。传统的研究方式是通过使用某种算法来实现语音增强,但是经过不断的实践与探索总结发现,在高噪声环境下实现语音增强技术,需要结合多种方式才能有效的实现。     

数字签名算法MD5的FPGA高速实现

信息安全在网络发展中非常重要。信息验证是验证信息来源的基本技术。常用的信息验证码是使用单向散列函数生成验证码。使用FPGA高速实现MD5认证算法,协处理器用Xilinx公司的Virtex系列FPGA,以PCI卡形式处理认证服务。     

基于Web的DoS攻击与防范策略研究

为了防范基于Web的 DoS 攻击,验证码技术得到了广泛应用.分析了Web上实施DoS攻击的基本原理,讨论了图片验证码的实现技术以及图片验证码的重要安全性指标--图片识别率问题.论证了运用基于验证码的表单提交技术,该技术能够有效地防范针对数据交互这一薄弱环节所实施的DoS攻击.最后对实现方法的进一步完善提出了建议.     

数字签名算法SHA-1的FPGA高速实现

随着网络的迅速发展，信息安全越来越重要，信息认证是验证收到信息来源和内容的基本技术。常用的信息验证码是使用单向散列函数生成验证码，安全散列算法SHA-1使用在是因特网协议安全性(IPSec)标准中。在设计中使用FPGA高速实现SHA-1认证算法，以PCI卡形式处理认证服务。     

隐式验证码的设计与实现

随着互联网行业的蓬勃发展以及对信息安全重视程度的提高,文本、图像、声音等验证码已应用到我们生活的各个角落。其中,文本验证码以其简洁、不依赖第三方辅助设备得到最广泛应用。为增加机器识别难度,文本验证码设计越发趋于复杂,影响了使用便捷性,导致用户对其存在褒贬不一。本文在回顾验证码发展及现状的基础上,结合文本验证码设计原理,转换思路,给出一种隐式验证码的设计方法,在确保机器识别难度基础上,有效降低用户登录复杂性。实践表明,该方法实用性强,在实际使用中取得了良好效果。     

Impact of entropic coding on the time delays associated with time-encoded speech (TES) systems

Transmission delays in time-encoded speech systems may be reduced an order of magnitude by using suboptimum bounded entropic codes in place of constant-length codes. Experimental evidence of this is offered.     

基于多相扩频码的MC-CDMA信号峰平比抑制方法研究

降低信号峰平比(PAPR)是多载波码分多址(MC-CDMA)系统的关键技术之一。该文推导了基于正交互补扩频码的MC-CDMA信号PAPR关系式;基于多相脉压码构造了可用于MC-CDMA系统的多相脉压扩频码集合;证明了多相脉压扩频码即为正交互补扩频码。仿真结果表明,各种输入信息调制方式下,大部分多相脉压扩频码比常用扩频码能进一步降低信号的PAPR,其中多电平调制方式下多相脉压扩频码的PAPR性能更优;在不同用户数情况下,多相脉压扩频码都是MC-CDMA系统扩频码的一个很好选择。     

基于GMM-UBM模型的语言辨识研究

与说话人识别、连续语音识别相比,自动语言辨识是一个相对较新的研究,而且是一项较难的课题。本文给出了一种基于GMM-UBM模型的语言辨识系统,并利用OGI-TS电话语音库对算法的性能进行了测试,然后给出了实验结果。实验结果表明,该算法也是进行语言辨识的一种有效方法。     

Speech transmission using rate-compatible trellis codes andembedded source coding

This paper presents bandwidth-efficient speech transmission systems using rate-compatible channel coders and variable bitrate embedded source coders. Rate-compatible punctured convolutional codes (RCPC) are often used to provide unequal error protection (UEP) via progressive bit puncturing. RCPC codes are well suited for constellations for which Euclidean and Hamming distances are equivalent (BPSK and 4-PSK). This paper introduces rate-compatible punctured trellis codes (RCPT) where rate compatibility and UEP are provided via progressive puncturing of symbols in a trellis. RCPT codes constitute a special class of codes designed to maximize residual Euclidean distances (RED) after symbol puncturing. They can be designed for any constellation, allowing for higher throughput than when restricted to using 4-PSK. We apply RCPC and RCPT to two embedded source coders: a perceptual subband coder and the ITU embedded ADPCM G.727 standard. Different operating modes with distinct source/channel bit allocation and UEP are defined. Each mode is optimal for a certain range of AWGN channel SNRs. Performance results using an 8-PSK constellation clearly illustrate the wide range of channel conditions at which the adaptive scheme using RCPT can operate. For an 8-PSK constellation, RCPT codes are compared to RCPC with bit interleaved coded modulation codes (RCPC-BICM). We also compare performance to RCPC codes used with a 4-PSK constellation     

Real-number convolutional codes for speech-like quasi-stationary sources (Corresp.)

A quasi-stationary source is one which is stationary over short periods, but changes occasionally to a new mode of stationarity; a typical example is the speech source. Convolutional codes are designed for the speech source for use with search algorithms such as the Viterbi, stack, andM-algorithms; the design is first by heuristic means and then by an empirical optimization. These codes have the same generating structure as the usual binary convolutional codes, but employ ordinary arithmetic. It is found by experiment that, at rate 2 bits/sample (which allows telephone quality speech), such codes need not have constraint length longer than five or six, which implies a generating circuit of about 1000 states. Encoding noise becomes white and uncorrelated with simple code searching; this shows that a short fixed code can successfully decorrelate waveforms from different source modes.     

Speech coding technology for ATM networks

A type of speech coding for asynchronous transfer mode (ATM) is described. Cell processing, which improves service quality, is taken into account. Missing-cell recovery methods are discussed, and the distinctive features of missing-cell recovery methods used with low-bit-rate coding are examined. An example of the speech quality obtained using speech coding techniques in the ATM networks is described. The performance levels for increasing cell loss are compared for various speech coding methods, in combination with methods for dividing coded speech signals into cells and discarding cells. Representative feasible network applications of coding technologies are considered     

The performance of rate-compatible punctured convolutional codesfor digital mobile radio

The unequal error protection capabilities of convolutional codes belonging to the family of rate-compatible punctured convolutional codes (RCPC codes) are studied. The performance of these codes is analyzed and simulated for the first fading Rice and Rayleigh channels with differentially coherent four-phase modulation (4-DPSK). To mitigate the effect of fading, interleavers are designed for these unequal error protection codes, with the interleaving performed over one or two blocks of 256 channel bits. These codes are decoded by means of the Viterbi algorithm using both soft symbol decisions and channel state information. For reference, the performance of these codes on a Gaussian channel with coherent binary phase-shift keying (2-CPSK) is presented. A number of examples are provided to show that it is possible to accommodate widely different error protection levels within short information blocks. Unequal error protection codes for a subband speech coder are studied in detail. A detailed study of the effect of the code and channel parameters such as the encoder memory, the code rate, interleaver depth, fading bandwidth, and the contrasting performance of hard and soft decisions on the received symbols is provided     

Subband speech coding and matched convolutional channel coding formobile radio channels

The effects of digital transmission errors on a family of variable-rate embedded subband speech coders (SBC) are analyzed in detail. It is shown that there is a difference in error sensitivity of four orders of magnitude between the most and the least sensitive bits of the speech coder. As a result, a family of rate-compatible punctured convolutional codes with flexible unequal error protection capabilities have been matched to the speech coder. These codes are optimally decoded with the Viterbi algorithm. Among the results, analysis and informal listening tests show that with a 4-level unequal error protection scheme transmission of 12 kb/s speech is possible with very little degradation in quality over a 16 kb/s channel with an average bit error rate (BER) of 2×10^-2 at a vehicle speed of 60 m.p.h. and with interleaving over two 16 ms speech frames     

The `stretched'-Golay and other codes for high-SNR finite-delayquantization of the Gaussian source at 1/2 bit per sample

Error-correcting codes are considered as codebooks for high-performance vector quantization (VQ) of the IID Gaussian source at fractional bit rate. A family of good rate-one-half codes is introduced: the stretched Golay codes. The performance of these codes is compared to other good block codes, trellis-coded quantization, and other techniques. The stretched Golay codes are shown to outperform previously published results for block lengths 32, 40, 56, and 64. The good performance, together with fast decoding make these techniques attractive for applications such as low-bit-rate coding of speech     

Trellis source codes designed by conjugate gradient optimization

Time-invariant trellis codes for stationary, ergodic, discrete-time sources are designed by unconstrained, nonlinear optimization of the performance in a simulated source encoding with the Viterbi algorithm. A nonderivative conjugate directions algorithm and a conjugate gradient algorithm with restarts are applied to design low-constraint-length, unit-rate, binary codes for the memoryless Gaussian source. The latter algorithm is also used to design codes for the memoryless Laplacian source and a third-order autoregressive model for speech. Good codes are tabulated and compared to other known results on a performance versus complexity basis. Those for the Gaussian source are tested in a joint (tandem) trellis-coding system with known convolutional channel codes