PB173 - Tématický vývoj aplikací v C/C++
(podzim 2014)
Skupina: Aplikovaná kryptografie a bezpečné programování
https://is.muni.cz/auth/el/1433/podzim2013/PB173/index.qwarp?fakulta=143
3;obdobi=5983;predmet=734514;prejit=2957738;
Petr Švenda svenda@fi.muni.cz
Konzultace: A406, Monday 15-15:50
PB173
Optimizing crypto
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 (Gladman)
• 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
– pre-computed key stream (if mode supports)
• key stream in advance, then simple xor
PB173
Parallelization of operations
• Speedup by parallel execution
• Purpose build hardware
– cryptographic coprocessors
– e.g., fast modulo exponentiation
• Using multiple CPU cores
– multiple threads running
– http://msdn.microsoft.com/en-us/library/69644x60%28v=VS.80%29.aspx
– use so-called worker threads AfxBeginThread(mywork)
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
PB173 5/17
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)
PB173 6/17
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
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 never repeat with a same
key
PB173
Counter (CTR) mode for encryption
http://www.mindspring.com/~dmcgrew/ctr-security.pdf
PB173
Practical assignment
• Create robust packet protection between clients
– packet encryption and authentication
– reasonable key distribution (session keys, no fixed
master key only)
• Speed-up encryption of data packets between
two clients with CTR mode
– pre-compute key stream (e.g., 100MB in RAM array)
– use parallel threads to prepare key stream
• number of available cores is parameter for function
• at least one thread required ;)
PB173
Practical assignment
• Document performance gains
– speed before and after the optimization
– account correctly for key stream pre-computation
• Write tests for continuous transmission of large
data between clients
– (Used also or performance documentation)