Re: SSH overhead.

From: Shashank Khanvilkar (shashank_at_evl.uic.edu)
Date: 06/27/03 

Date: Fri, 27 Jun 2003 15:02:12 -0500

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
>
>

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