More timings for what they're worth. First, IDEA, 128-bit key, 4-byte encryption: KeyStream Size # KeyStreams Elapsed Time Rate -------------- ------------ ------------ ---- 4194304 1 2.697752 1.482716 2097152 2 2.554216 1.566038 1048576 4 2.557143 1.564246 524288 8 2.553800 1.566293 262144 16 2.545676 1.571292 131072 32 2.541943 1.573599 65536 64 2.529572 1.581295 32768 128 2.531937 1.579818 16384 256 2.524743 1.584320 8192 512 2.529972 1.581045 4096 1024 2.512121 1.592280 2048 2048 2.514471 1.590792 1024 4096 2.509619 1.593867 512 8192 2.511240 1.592839 256 16384 2.513205 1.591593 128 32768 2.521096 1.586612 64 65536 2.538900 1.575485 32 131072 2.556545 1.564612 16 262144 2.605195 1.535394 8 524288 2.682644 1.491066 RC4, 128-bit key, 4-byte encryption: (this is using RC4 in the same way as DES or IDEA are used, which is sorta odd... going to direct RC4 encryption shaves another 5-10 100ths off the times) KeyStream Size # KeyStreams Elapsed Time Rate -------------- ------------ ------------ ---- 4194304 1 0.677363 5.905253 2097152 2 0.536031 7.462255 1048576 4 0.536748 7.452287 524288 8 0.540009 7.407284 262144 16 0.522846 7.650436 131072 32 0.526067 7.603594 65536 64 0.508882 7.860369 32768 128 0.580653 6.888796 16384 256 0.497530 8.039716 8192 512 0.495569 8.071530 4096 1024 0.484818 8.250519 2048 2048 0.489014 8.179725 1024 4096 0.484513 8.255713 512 8192 0.485179 8.244380 256 16384 0.483978 8.264838 128 32768 0.487116 8.211596 64 65536 0.493723 8.101708 32 131072 0.507106 7.887897 16 262144 0.536722 7.452648 8 524288 0.573686 6.972456 Blowfish (128-bit key, 4-byte encryption): KeyStream Size # KeyStreams Elapsed Time Rate -------------- ------------ ------------ ---- 4194304 1 1.244634 3.213796 2097152 2 1.101053 3.632886 1048576 4 1.103253 3.625642 524288 8 1.101482 3.631471 262144 16 1.094061 3.656103 131072 32 1.096509 3.647941 65536 64 1.088593 3.674468 32768 128 1.077802 3.711257 16384 256 1.074708 3.721941 8192 512 1.067988 3.745360 4096 1024 1.057290 3.783257 2048 2048 1.053547 3.796698 1024 4096 1.058261 3.779786 512 8192 1.054496 3.793281 256 16384 1.058237 3.779872 128 32768 1.064963 3.755999 64 65536 1.077552 3.712118 32 131072 1.108137 3.609662 16 262144 1.195494 3.345897 8 524288 1.240991 3.223230 Again, 4k seems as good a number as any. All of these were compiled with gcc -O3 -fomit-frame-pointer -mv8