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SSH overhead.

Hi,
When I send a packet of 100 bytes (say) using ssh, how much overhead bytes
can I expect for ssh to add to my packet.
I am assuming that I use public key encryption with following options:
cipher: aes128-cbc
mesage-digest: hmac-md5
compression: none
I know the crude technique of finding out how much overhead will each
cipher+message digest algorithm will add. But I would like to know a more
elegant technique (if any such exists).


Is the following correct:
Application (100 bytes) --> ssh(??) --> TCP ((20 bytes basic + 12 bytes
options = 32 bytes) --> IP (20 bytes) --> ethernet (14 bytes) --> packet
sent over the wire.



-- 
Regards
Shashank
http://mia.ece.uic.edu/~papers


0
shashank8 (15)
6/26/2003 10:33:35 PM
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Read the SSH protocol specs and find out, in the same way that you
researched the other protocols at hand to find out their overheads. 

-- 
  Richard Silverman
  res@qoxp.net

0
res49 (1410)
6/27/2003 12:34:17 AM
cool way of saying you don't know - eh?

Richard E Silverman wrote:
> 
> Read the SSH protocol specs and find out, in the same way that you
> researched the other protocols at hand to find out their overheads.
> 
> --
>   Richard Silverman
>   res@qoxp.net
0
dawbins (2)
6/27/2003 1:16:35 AM
>>>>> "dawbins" == dawbins  <dawbins@n0n0.com> writes:

    dawbins> cool way of saying you don't know - eh?

From the depth of confusion illustrated by your recent posts, you are
hardly one to talk.  In any event, the OP wrote:

> I know the crude technique of finding out how much overhead will each
> cipher+message digest algorithm will add. But I would like to know a more
> elegant technique (if any such exists).

So, I answered this question.

-- 
  Richard Silverman
  res@qoxp.net

0
res49 (1410)
6/27/2003 3:28:25 PM
This is what i found out, but will appreciate a second opinion.

> When I send a packet of 100 bytes (say) using ssh, how much overhead bytes
> can I expect for ssh to add to my packet.
> I am assuming that I use public key encryption with following options:
> cipher: aes128-cbc
> mesage-digest: hmac-md5
> compression: none
> I know the crude technique of finding out how much overhead will each
> cipher+message digest algorithm will add. But I would like to know a more
> elegant technique (if any such exists).

The answere to the above questions is:
It depends:
The cipher algorithm that one uses to encrypt the 100 bytes of data will
produce upto 100 bytes of encrypted output, depending on
  a> the amount of randomness in the data
  b>whether the encryption algorithm is CBC, CFB, ECB or OFB.

I found that when i provide random input, the encrypted output has same 100
bytes,
but when the input 100 bytes has the same character (say, A), the encrypted
output was much less than 100 bytes (around 48 - 50 bytes).

The hash function (hmac-md5) will add 16 bytes to the above encrypted data.

Does the above make sense?
Will love to hear some comments
Shank


>
>
> Is the following correct:
> Application (100 bytes) --> ssh(??) --> TCP ((20 bytes basic + 12 bytes
> options = 32 bytes) --> IP (20 bytes) --> ethernet (14 bytes) --> packet
> sent over the wire.













>
>
>
> -- 
> Regards
> Shashank
> http://mia.ece.uic.edu/~papers
>
>


0
shashank8 (15)
6/27/2003 8:02:12 PM
Mm, my comment was meant to be tongue in cheek, but could have used a
smiley after it -- sorry if you were offended.

> Seriously - great book on ssh and I'm looking forward to your 
> ipsec/freeswan 2.00 book.

In fact, I've been thinking about this.  We'll see.

-- 
  Richard Silverman
  res@qoxp.net

0
res49 (1410)
6/30/2003 3:22:06 PM
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