Available: Multi-threaded AES-CTR Cipher

Chris Rapier rapier at psc.edu
Wed Jan 30 05:47:54 EST 2008

On multiple core systems OpenSSH is limited to using a single core for 
all operations. On these systems this can result in a transfer being 
processor bound even though additional CPU resources exist. In order to 
  open up this bottleneck we've developed a multi-threaded version of 
the AES-CTR cipher. Unlike CBC mode, since there is no dependency 
between cipher blocks in CTR mode we parallelize cipher block operations 
among multiple threads. Furthermore, since the AES_encrypt operations do 
not depend on the data to be ciphered we pre-generate the effective 
keystream. The main thread still handles the packetization, MAC 
computation, and associated I/O but the computationally expensive 
AES_encrypt operations are offloaded to one or more additional cores. In 
our tests this resulted in a performance improvement of up to 125% on 
systems that were previously CPU bound. In fact, utilizing less than 
four cores we were able to achieve near line rate on a GigE LAN 
connection with 128, 192, and 256-bit AES.

More details on the implementation can be found at 
starting at slide 30. Results can be found on slide 46.

As the resulting cipher stream is indistinguishable from the original 
single-threaded implementation of AES-CTR there are no known issues with 
backward compatibility.

This patch should be thought of as experimental at this point. While it 
has performed well in test environments it is not yet, to our knowledge, 
deployed in critical production environments and the threading can 
impose a performance penalty on single core systems (but only when using 
AES-CTR). We're still exploring methods to have single-threaded and 
multi-threaded implementations of CTR mode exist side by side.

The patch itself can be found at
or more specifically

Additionally, this patch will apply on top of the HPN-SSH12v20 patch. It 
will, within a week or so, be incorporated into the HPN suite of patches 
as HPN13. If you have any problems applying the patch please let us know.

Any comments, suggestions, or critiques you may have are welcome and 

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