[Twisted-Python] conch problem with ecdsa-sha2-nistp256 host key?

[Glyph Lefkowitz]

> On Dec 2, 2016, at 12:19 AM, Glyph Lefkowitz <glyph at twistedmatrix.com> wrote:
> 
> 
>> On Dec 1, 2016, at 7:01 PM, Mark Williams <markrwilliams at gmail.com <mailto:markrwilliams at gmail.com>> wrote:
>> 
>> On Thu, Dec 01, 2016 at 05:11:37PM -0800, Craig Rodrigues wrote:
>>> Hi,
>>> 
>>> I filed this bug:
>>> https://twistedmatrix.com/trac/ticket/8931 <https://twistedmatrix.com/trac/ticket/8931>
>>> 
>>> At least for me, conch fails to parse a host key created by OpenSSH
>>> in ~/.ssh/known_hosts
>>> which is of type ecdsa-sha2-nistp256.
>>> 
>>> Anyone have an idea as to how to fix this?
>>> 
>> 
>> As usual you've found a fantastically interesting issue.
>> 
>> This is conch, the client, right?  I'm guessing so because
>> ~/.ssh/known_hosts contains the servers the ssh client trusts.
>> (Specifically, among other things it contains a hostname and that
>> host's sshd server's public key fingerprint).
>> 
>> If it is conch, the-client, then deleting the offending entry from
>> ~/.ssh/known_hosts and getting a new one makes sense.  That's because
>> sshd usually generates a couple of different keys in case clients
>> don't support the latest and greatest technology.
>> 
>> I think deleting the entry in ~/.ssh/known_hosts allows conch to ask
>> the server for a different key that it *can* understand.  You should
>> be able to find out which server key conch negotiated by doing thing
>> following after deleting the offending ~/.ssh/known_hosts entry:
>> 
>> (<hostname is the problematic OS X server>)
>> $ ssh-keygen -H -F <hostname> | awk '{ print $NF }'
>> dGhpcyBpcyB2ZXJ5IHZlcnkgdmVyeSB2ZXJ5IHZlcnkgbG9uZyBob3N0IGtleQ==
>> 
>> Then on the OS X server, grep for that in /etc/ssh/*.pub
>> 
>> I bet the key negotiated by conch is not an ECDSA key but rather an
>> RSA key.  If this is all the case, then I think you've found a key
>> that LibreSSL supports but your client's libssl (which conch calls
>> into via cryptography) does not.  What version of libssl do you have?
>> 
>> If any of this is helpful or relevant I'll ask more questions in the
>> ticket.
> 
> I think there might be a regression in 16.6.0.
> 
> For every version up to 16.6.0, I can do 'conch twistedmatrix.com <http://twistedmatrix.com/>' in a shell and it works fine.
> 
> On 16.6.0, I get:
> 
> Connection to twistedmatrix.com <http://twistedmatrix.com/> closed.
> conch: exiting with error [Failure instance: Traceback (failure with no frames): <class 'twisted.conch.error.ConchError'>: ('bad host key', 9)
> ]
> 
> instead.
> 
> Worth noting: the keys I have for twistedmatrix.com <http://twistedmatrix.com/> are RSA keys.
> 
> What did we add recently that changes key parsing?

The offending commit is 8164d89104a453947215b9296e8b406f15e63252.  Clearly something went wrong when introducing ECDSA parsing.

-glyph

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