[-SPAM-] Re: Can you keep a secret? This encrypted drive can...

[Brian Gladman]

Jon Callas wrote:
> I just ran a speed test on my laptop. Here are some relevant excerpts:
> 
> Cipher    Key Size  Block Size  Enc KB/sec  Dec KB/sec
> --------  --------  ----------  ----------  ----------
> IDEA      128 bits   8 bytes      24032.09    24030.66
> 3DES      192 bits   8 bytes      10387.67    10399.30
> CAST5     128 bits   8 bytes      29331.17    29459.49
> Twofish   256 bits  16 bytes      20233.63    19185.82
> AES-128   128 bits  16 bytes      44100.23    46266.98
> AES-192   192 bits  16 bytes      39731.33    41228.87
> AES-256   256 bits  16 bytes      36017.95    37302.43
> Blowfish  128 bits   8 bytes      35347.34    38311.22
> 
> Comparing AES-128 and AES-256, encrypt speed is 1.2243959x and decrypt
> is 1.2403208x. So that makes my
> lick-your-finger-and-stick-it-in-the-wind rule of thumb of 20% slower
> okay. I'll try to say 20-25% in the future.
> 
> Of course, though, implementation matters a lot. I'm running a PPC-32
> machine. You'll get different answers on an ia32, and different ones an
> AMD64.

For AES the round function and key scheduling cost per round are
basically the same for both AES-128 and AES-256.  In consequence I would
expect the speed ratio to be close to the ratio of the number of rounds,
which is 14 / 10 or 40%.

My own figures on AMD64 are 1.35 for encryption and 1.39 for decryption.
And on a P4 they are 1.36 and 1.38 respectively. These are hence close
to the expected 40% figure.

This suggests to me that a figure around 20% would apply in applications
in which about half the time is spent in encryption and half in other
higher level activities.

Can I hence assume that your benchmark is being run at application level
rather than algorithm level?  If not why is the ratio only 22% on the
PPC-32?

   Brian Gladman

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