From: Francis Cianfrocca <garbagecat10@...>
Date: 2006-07-31T20:58:37+09:00
Subject: Re: How to md5 a file?

Timothy Goddard wrote:
> Notice that using MD5 is significantly slower than normal string
> comparison. This also demonstrates that there are few performance gains
> between 10KB buffers and 100KB buffers, indicating that somewhere in
> the 10K range would be a good buffer size for the memory/performance
> tradeoff.
> 

I notice that MD5-generation is not twice as time-consuming as string 
comparison. In fact, it's only a little more time-consuming, which was 
an interesting surprise until I checked the source code and realized 
that Ruby uses the C reference implementation to compute MD5.

Comparing strings is obviously the better choice for doing one-off 
comparisons that won't be repeated. But for applications like 
cache-management or public email systems, where you're going to be 
comparing many times against the same chunk of bits, it makes more sense 
to store an MD5. That way, subsequent trials only have to compute one 
hash, not two.

Someone upthread suggested using SHA1 instead of MD5 for this purpose. I 
haven't done the comparison in Ruby, but in C implementations, SHA1 is 
just slightly slower than MD5, not enough to matter. And Ruby's SHA1 
implementation is also in C.

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