Re: understanding ssh

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• In reply to: Sven Müller: "understanding ssh"
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Date: Sun, 11 Jan 2004 23:49:25 -0800
To: Sven Müller <sven@hitnet.rwth-aachen.de>

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Hello Sven,

SM> But I´m not sure at all and hope anyone can help me.

SM> My problems belong to the transport-layer:

SM> Starting a ssh session, the client and server handle out which public key
SM> authentication is to use (RSA/DES).
SM> After that, the server sends in this example the DES public key.

There are two such keys - I prefer to call them "static" and
"dynamic".

SM> The client compares this key with the key saved in file "known_hosts".

Static is constant for the host and is the one used for the
known_hosts comparison.

Dynamic is a key that changes once per hour, greatly mitigating the
effect of somebody stealing the static key.
(They have to do mitm not just sniff)

SM> If the entry matches, client and server handle out which symmetric
SM> algorithm should be used for encrypting data.

This does counter what you said two places below, saying 3des.
My memory is a bit flaky.

SM> After that, ssh uses diffie-hellman to create a key for encrypting data
SM> with 3DES.

At that point, a session key is created from random data, as is proper
for symmetrical ciphers. I believe it is the client doing this, as the
client has the servers public keys: The session key is then encrypted
with the two public keys and sent to the server.

SM> Now, the sending data will be encrypted with 3DES.

SM> Authentication can start now.

SM> Is that correct?

Yes. Part of the point is that encryption is up and running before
authentication starts.

SM> I read something about randomised sessions. What is the meaning of
SM> this? Does it belongs to creating keys with DH?
SM> What about the sessions to differ between ssh connections?

What you read about was what I explained above - session keys that are
randomly created, and different from session to session. They are used
for the actual data.

SM> And what about the session integrity provided through sha1 or md5 hash
SM> algorithms? Where do I find the md5 checksum? When will it happen?

I believe that once the encryption is up and running, each packet will
have a checksum in it - inside the encryption. The purpose of this is
that if sombody insert garbage packets into the stream, those packets
won't be accepted.

SM> What about ssh1, does it only use RSA keys? Are they created like DES
SM> when installing ssh or when creating a ssh session.

This makes me believe you don't quite understand symmetric/asymmetric.

Or: I cannot understand what you want to know.

As far as ssh1 goes, there is only rsa keys as far as asymmetrical
goes, but the pair of keys, static and dynamic, are both there. As far
as symmetrical goes, there is little relevant difference.

SM> Does the symmetric key change after one hour?

I believe the session key can be set to expire after the first of
(1) a period of time (e.g. 1 hour)
(2) an amount of traffic (e.g. 10 Mb)
Otherwise it stays in place until the session closes. I *might* be
thinking of some other software though...

- --
Best regards,
 Marius mailto:mahuja@c2i.net

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