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PB173 - Tématický vývoj aplikací v C/C++ (jaro 2016)
Skupina: Aplikovaná kryptografie a bezpečné programování
•Petr Švenda svenda@fi.muni.cz
•Konzultace: A.406, Pondělí 15-15:50

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OPTIMIZING CRYPTO
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PB173

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PB173
Optimizing crypto
•Clever tricks both on design and implementation
–optimization of both algorithm and mode used
–see aestab.h and aesopt.h for example
–https://github.com/BrianGladman/AES
•Possibility for pre-computation
–code itself: macros, templates, static arrays
–pre-computed tables
•AES optimized with large tables
•table lookup only implementation (AES/DES)
•see http://cr.yp.to/aes-speed.html (Bernstein, Schwabe)
–pre-computed key stream (if mode supports)
•key stream in advance, then simple xor

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Parallelization of operations
•Speedup by parallel execution
•Purpose build hardware
–cryptographic coprocessors
–e.g., fast modulo exponentiation
•Using multiple CPU cores
–multiple threads running
–Worker thread
•AfxBeginThread(mywork), http://msdn.microsoft.com/en-us/library/69644x60%28v=VS.80%29.aspx
•QThread, https://doc.qt.io/qt-5/qthread.html
–When working with threads, don’t forget to synchronize
•CreateMutex, Qmutex…
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Mode of cipher usage - ECB
•ECB (Electronic Code Book mode)
–used for block ciphers
–processing of one block does not influence others
•Main problem
–same data with same key result in same ciphertext
–attacker can build code book
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Mode of cipher usage - CBC
•CBC (Cipher Back Chaining mode)
–used for block ciphers
–previous cipher block is xored with next plaintext
–prefer before ECB
•Problem
–Initialization vector must be somehow shared
–(random first block)
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PB173
Parallelization of modes
•Assume that algorithm itself is sufficiently optimized
•Algorithm is used in some mode
–e.g., block encryption modes (ECB, CBC…)
•We need parallelizable modes!
–CBC encryption is not parallelizable
•(decryption is – why?)
•Counter (CTR) mode
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PB173
Counter (CTR) mode for encryption
•Mode approved by NIST (US standardization)
–http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
•Designed for confidentiality with parallelization and pre-computation in mind
•Key stream is produced by iterated encryption of the incremental counter
–counter is incremented for each new block
–key stream is then xored to message
–key stream(== counter) must not repeat with same key

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PB173
Counter (CTR) mode for encryption
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ctrmode
http://www.mindspring.com/~dmcgrew/ctr-security.pdf

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PB173 | Sockets, Optimization

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PB173
Practical assignment
•Speed-up encryption of data packets between two clients with CTR mode
–Divide packet into multiple parts
–Use parallel threads to protect parts of data packet
•number of available cores is parameter for function
•(at least one thread required ;))
•Document performance gains
–speed before and after the optimization (can you increase speed linearly?)
–What is length of packet for which multiple threads brings speedup benefit? (overhead with running
threads)

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Submissions, deadlines
•Upload application source codes as single zip file into IS Homework vault (Crypto - 8. homework
(Threads))
•DEADLINE 25.4. 12:00
–0-10 points assigned
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PB173 | Network