Dr. Dobb's Journal - October 2007 - (Page 31)

• Test1. Diffuser A should run at least two times. • Test2. The diffusion layer can be omitted and the cipher will pass. • Test3. Diffuser B should run at least one time. • Test4. Diffuser A should run at least two times. Plaintext Tweak Key Drive Sector Key Diffuser A Diffuser B What we learned was that you can achieve maximum performance (without sacrificing the statistical properties of AESCBC + Elephant diffuser) when running Diffuser A for two times and Diffuser B for one time. AES-CBC Ciphertext Figure 1: Overview of AES-CBC + Elephant diffuser. AES-ECB + Elephant Diffuser With the widespread use of dual-core processors, speeding up encryption using parallelization is possible. With this in mind, we replaced the AES-CBC layer (which is, by definition, sequential) with the AES-ECB layer (which can easily be parallelized). Figure 2 shows the modified cipher. We only need 128-bits as a sector key to be XORed with the plaintext. Note that we added a block counter to the drive sector key before it is XORed with the plaintext. This counter has the value of 0 for the first plaintext and is incremented by 1 for each plaintext within the block (until it reaches the value of 31). This counter was added on to the original design to make each plaintext dependent on its position within the block. When applying the four tests, the results show that if we use the current implementation of the diffuser layer, the modified cipher passes all the tests. With this in mind, we then examined whether we could reduce the number of runs of Diffuser A and Diffuser B, thereby increasing performance. Our results were: • Test1. Diffuser A should run at least two times. • Test2. The diffusion layer can be omitted and the cipher will pass it. • Test3. Diffuser B should run at least two times. • Test4. Diffuser A should run at least two times. From the results, we determined that you can achieve maximum performance without sacrificing the statistical properties of the AES- Plaintext Tweak Key Drive Sector Key Diffuser A Diffuser B AES-CBC Ciphertext Counter Figure 2: Overview of AES-ECB + Elephant diffuser. ECB + Elephant diffuser when running both Diffuser A and Diffuser B, each for two times. Performance Analysis We studied the performance of the AES-CBC + Elephant diffuser (the original implementation and the one recommended here for maximum performance) and the proposed AES-ECB + Elephant diffuser on a single processor, then estimated their performance on a dual-core processor. The measurements we report are processor clock cycles on a PIV 3-GHz processor running Windows Vista. The programming environment is Microsoft VC++. Single Processor Our optimized implementation for Diffuser A and Diffuser B (using a loop unrolling mechanism) shows that: • 4560 clock cycles are required for the current implementation of the diffusion layer. October 2007 l www.ddj.com l Dr. Dobb’s Journal 31 http://www.dtsearch.com http://www.dtsearch.com http://www.dtsearch.com http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - October 2007

Cover Contents Hmmmm Alia Vox Developer Diaries Developer’s Notebook AI: It’s OK Again! Conversations Visual Cryptography and Bit-Plane Complexity Segmentation Inside the Windows Vista Disk Encryption Algorithm Memory-Aware Components Software and the Core Description Process Logging In C++ Effective Concurrency The Agile Edge Swaine’s Flames

Dr. Dobb's Journal - October 2007

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