Output Error Analysis Of Wireless Block Ciphered Transmission And Performance Test For A Stream Cipher

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.