【作者】 杨秋华；

【导师】 袁坚；

【作者基本信息】 清华大学 ， 信息与通信工程， 2011， 硕士

【摘要】 随着无线网络的广泛应用，保证数据安全传输的同时，提高无线传输的有效性是一个越来越受关注的问题。环境的开放性使无线网络易受攻击，而信道的突发误码则影响无线加密传输的性能。针对无线环境中的加密对传输性能的影响，本文深入分析了分组加密传输的误码扩散比率和输出误码分布，并且对一种时空混沌流密码算法的性能进行了有效评估。首先，针对非理想条件下的无线分组加密传输过程，本文提出了一种误码扩散比率的分析方法。现有研究对误码扩散的分析基于两个假设条件（加密算法满足严格雪崩标准，传输信道为二进制对称）过于理想。本文利用加密算法的依赖关系矩阵元素的平均值表征算法的雪崩强度，用马尔科夫模型分析无线信道，推导出非理想条件时系统的误码扩散比率的定量结果。分析表明，系统输出误码扩散比率随着算法雪崩强度增强而增大。当传输信道中错误分布不均匀时，系统的误码扩散比率小于理想假设条件下的比率值。其次，通过分析无线加密传输的输出分组内部和分组之间的统计依赖关系，本文刻画了无线加密系统的输出误码分布。对输出误码特征的详细了解有助于设计优良的纠错编码，提高传输的有效性。由于无线信道的随机性和加密算法的记忆性，无线加密系统的输出误码具有一定的分布特点。现有研究只是对加密系统输出误码的一阶统计值进行描述。本文推导得出加密信道的转移概率，这个转移概率可以在分组的级别上表征系统的输出误码分布。最后，为了对流密码的加解密速度进行有效评估，本文对一种时空混沌流密码算法Gemstone进行了性能测试。在实际应用之前须验证流密码的可用性，流密码本身不扩散传输信道中的错误，然而加解密过程会增加系统传输延时。本文利用eSTREAM平台对Gemstone算法的性能进行测试，结果表明Gemstone的各项速度指标与国际上成熟的流密码算法性能接近，具有良好的可用性。此外，本文还展示了搭建的一个演示平台，展示了Gemstone算法的加解密过程和各项性能指标。更多还原

【Abstract】 With the proliferation of wireless networks, how to keep both secrecyand effectiveness of wireless transmission is a problem which gains more andmore considerations. The wireless network is open to intruders, andencryption will lower the throughput of wireless transmission. To explore theexact impact of cipher on the throughput, in this thesis we study errorcharacterizations of wireless block ciphered transmission. Furthermore, theencryption and decryption speeds of a spatiotemporal stream cipher are tested.First, we proposed a method to analyze error expansion rate of wirelessblock-ciphered cryptosystem under practical conditions. The related workswhich have theoretically deduced error expansion formulas of block cipheredencryption in each operation mode are based on two assumptions: the cipheralgorithm satisfying strict avalanche criterion (SAC), and the transmissionchannel being binary symmetric. However, the conditions are not necessarilymet in practical transmissions. In this paper, the influence of these twoassumptions on the output error expansion rates is considered. The mean ofdependence matrix elements is calculated to denote block cipher’s avalanchestrength and Markov model is developed to characterize Rayleigh channel.Quantitative results of the rate are derived when conditions are not exactlymet. It shows that the error expansion rate increases with the ciphers’avalanche strength, and is smaller when cipher-texts are transmitted in thechannel which is not steady.Second, the output error distribution of the wireless encryptedtransmission is explored in this thesis. Channel coding before encryption anddecoding after decryption are employed to deal with errors in decryptedciphertext. Since error patterns in decrypted plaintext affect code design, amore profound understanding of error characteristics can help develop apowerful error correcting code, thus to enhance the transmission’s throughput. Previous works on the throughput of encryption system focus on the system’saverage output BER, which is the first order statistics of the output errors. Inthis thesis, we studied output error features of block ciphered cryptosystemsin ECB, CBC and CFB modes, and defined the three modes of encryptedchannels. Due to channel randomness and memory of block cipher, outputerrors tend to be in bursts. By analyzing this bursty property, we obtained aregular conditional probability between bits within and across output blocks.Based on these statistical results, we derived transition probabilities of theencrypted channels, which can characterize the output error features ofcryptosystems from a block perspective.In the last part of this thesis, the performance of a spatiotemporal chaoticstream cipher, Gemstone, is tested. Stream cipher is immune to output errorexpansion, while encryption and decryption processes can increasetransmission delay. Therefore, we tested encryption and decryption speed ofthis cipher using eSTREAM platform. The test results show that the speed ofGemstone is comparable with widely used cipher such as AES. From speedperspective, our stream cipher Gemstone has good availability. Furthermore,we have developed a demo platform to show the encryption and decryptionprocess of Gemstone.更多还原