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Subject: Re: SSH2 host key length and hash compute

Re: SSH2 host key length and hash compute

From: Kalpesh Parekh <kalpesh.ork2_at_gmail.com>
Date: Mon, 8 Jul 2013 08:48:24 -0700

First, apologies for not replying inline to the emails.

After your suggestions Alex, here is what I could determine.

I printed out the hex code for the session key for both RSA and DSA types
in my setup.

Here are the first few bytes:
(RSA) 00 00 00 07 73 73 68 2D 72 73 61 00 00 00 01 23 00 00 00 81

(DSA) 00 00 00 07 73 73 68 2D 64 73 73 00 00 00 81

As you mentioned, the first four bytes indicate, the length of the
identifier (7 in this case)
73 73 68 2D 72 73 61=ssh-rsa
73 73 68 2D 64 73 73=ssh-dsa

The next four bytes for RSA indicates some sort of value. In my case it is
#. For DSA, it indicates (length of the key + 1) verified through
ssh-keygen -l command.
For RSA, the last four bytes printed above indicate (length of the key + 1).

My next target is to decode the signature blobs. I feel like I am close and
can close this if any one can help.

On Thu, Jul 4, 2013 at 3:07 AM, Kalpesh Parekh <kalpesh.ork2_at_gmail.com>wrote:

> Thanks Alex,
>
> So far it has been helpful.
>
> So based on the RFC, I understand that the host key received from the
> server has following components
> K_S || f || s
>
> I need to parse the K_S part for RSA key. How do I do that.
>
> Also, how can I implement a generic logic for getting the bit strength
> from the host key.
>
>

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Received on 2013-07-08

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