rsync hashing / collision handling?

Andrew Pennebaker andrew.pennebaker at
Tue Oct 2 14:58:04 MDT 2012

Based on the published rsync algorithm
it appears to filter definitely different blocks from possibly unchanged
blocks using weak hashing, then applying strong hashing to double-check the
rest of the file for true differences. MD4 and MD5 do experience
collisions, so isn't rsync fast at the risk of being inaccurate? If I can
prove that the strong algorithms rsync uses have collisions, can you prove
that rsync avoids corner cases where a file block was changed but even the
strong hash algorithm misses it?

Have recent versions of rsync considered using a more robust implementation
of file change monitoring, like Dropbox
<> does?
It looks fast on my machine.


Andrew Pennebaker
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