SSH1 vs. SSH2 - compression level

Dan Kaminsky dan at
Thu Nov 13 04:13:19 EST 2003

Markus Friedl wrote:

>On Fri, Nov 07, 2003 at 11:32:00AM +1100, Darren Tucker wrote:
>>[0] Ignoring implementation-specific extensions, eg "zlib-1 at"
>>through "zlib-9 at" or something.
>yes, but i don't think it's worth the trouble.
>openssh-unix-dev mailing list
>openssh-unix-dev at
Hmmm.  Here's a neat trick, which we can only do because the compression 
implementations tend to be stable:

If client supports compression, receive some amount of compressed data 
from them and recompress it ourselves, looking for the closest match 
between how large the client's version of the data is vs. how large ours 
is.  If level 9 yields 50 bytes and level 1 yields 120 bytes, and we 
received a 120 byte compressed message, output all further messages at 
level 1.

We can even create a sample message (of SSH type IGNORE) with content 
specifically tuned to result in different sizes for different 
compression levels.  This would allow us to improve the consistency of 
the results.

It'd be one heck of a stunt :)


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