RFE: OpenSSH Support for PKCS11 Funneling to PAM for Kerberos/PKINIT

Wed Dec 19 20:50:15 AEDT 2018

As one of the dev of it, I can also recommend Caml Crush, a PKCS#11
filtering proxy [1] as a solution that works well under both tcp and unix
domain sockets, it is packaged for Debian.
I have tested this approach in the past, redirecting the server socket
through SSH and using our client library through the tunnel worked fine.

Cheers,

Thomas

[1]: https://github.com/caml-pkcs11/caml-crush

On Wed, 19 Dec 2018, 06:32 Alon Bar-Lev <alon.barlev at gmail.com wrote:

> On Wed, Dec 19, 2018 at 3:03 AM mailto428496 <mailto628496 at cox.net> wrote:
> >
> > Alon,
> >
> >
> > On 12/18/2018 06:52 PM, Alon Bar-Lev wrote:
> > > OK... So you have an issue...
> > >
> > > First, you need to delegate your smartcard to remote machine, probably
> > > using unix socket redirection managed by openssh. This can be done in
> > > many levels...
> > > 1. Delegate USB device, this will enable only exclusive usage of the
> > > smartcard by remote machine.
> > > 2. Delegate PC/SC, this will enable sharing the reader between local
> > > and remote machines, rdesktop is using this method.
>
> Check this[1] out, openssh now supports redirecting unix domain socket.
>
> [1]
> https://ludovicrousseau.blogspot.com/2010/11/pcsc-client-and-server-on-two-different.html
>
> > > 3. Delegate PKCS#11, this is the preferred method, however, there is
> > > no maintained solution to do so.
> > > 4. Delegate the ssh-agent and implement a minimal PKCS#11 provider on
> > > top to support PKINIT requirements.
> > > 5. If your card is gpg supported, use gpg-agent as ssh-gent and
> > > delegate gpg-agent to remote and use scute[1] as PKCS#11 provider,
> > > however, scute is unmaintained.
> >
> > I agree that number 3 would be preferred.  My hope was that maybe this
> > would be something that the OpenSSH group would be interested in
> > implementing / supporting as a future feature, but it sounds like there
> > aren't currently a lot of people waiting in line (Or it could be a
> > feature that a good number of people would like but just don't realize
> > it ;) (not everyone realizes/considers that PKINIT is possible for
> > smartcard auth, at least based on my observations).  I could imagine
> > this being of interest to anyone with a kerberos/AD infrastructure and
> > using smartcards (which is probably a good number) and even if they are
> > not using kerberos tickets for auth (not everyone is) but still have AD
> > and want to better centralize control of SSH smartcard auth.
>
> This belongs to device remote access and not directly to openssh. The
> softhsm[1] project or the  p11-kit[2] to implement remote access to
> PKCS#11, as it is the best software based PKCS#11 implementation. What
> you seek is a PKCS#11 proxy module that serialized everything via a
> socket and a stub that reads socket and call a real PKCS#11
> implementation.
>
> Please look if [3] helps.
>
> [1] https://www.opendnssec.org/softhsm/
> [2] https://p11-glue.github.io/p11-glue/p11-kit.html
> [3] https://p11-glue.github.io/p11-glue/p11-kit/manual/remoting.html
>
> >
> > > The problem is that non of these methods have a good solution... But
> > > once you have done that, you can use PKINIT at remote to access your
> > > local smartcard.
> > >
> > > If you choose to implement minimal PKCS#11 on top of ssh-agent you
> > > should use file based X.509 certificate to perform the signature.
> > > Actually, it once supported that when Roumen Petrov and I worked on
> > > the PKCS#11 implementation for OpenSSH which was not merged
> > > eventually, now I am unsure if the agent is patched to allow that[2],
> > > you can check this out maybe the patched agent enables you to perform
> > > access the X.509 certificate as well, so you can implement a nice
> > > provider on top of the patched ssh-agent.
> >
> > Having it work with the agent would be nice too.
> >
> > Anyway, I wanted to toss this out there.  Such functionality would
> > certainly help us a lot but I know that there would need to be
> > sufficient interest in order for such a thing to get into the mainstream.
> >
> >
> > Jim
> >
> > >
> > > On Wed, Dec 19, 2018 at 1:31 AM mailto428496 <mailto628496 at cox.net>
> wrote:
> > >> Alon,
> > >>
> > >> I should have provided more background.  You are assuming that I could
> > >> perform the PKINIT prior to connecting to the SSH server.  In this
> case
> > >> (and others) there is an interest in not exposing the kerberos servers
> > >> to the world and thus someone connecting remotely would not be able to
> > >> obtain a TGT or do a PKINIT.  The goal would be for SSH to handle all
> > >> the auth and only after connecting to the SSH gateway server, and
> doing
> > >> a PKINIT as part of the process, then the user would have access to
> > >> kerberos and could obtain a TGT.
> > >>
> > >>
> > >> Jim
> > >>
> > >>
> > >> On 12/18/2018 06:18 PM, Alon Bar-Lev wrote:
> > >>> Hi,
> > >>>
> > >>> Maybe I am wrong, but I believe you did not get it right.
> > >>>
> > >>> You should use PKCS#11 to perform PKINIT in order to authenticate
> > >>> against the KDC to acquire TGT.
> > >>> Then ssh can use the TGT in order to issue ticket to access remote
> > >>> sshd using GSSAPI KEX.
> > >>>
> > >>> If you like to use pam_krb5 locally on your system to issue the TGT,
> > >>> do it... it yet another method to have TGT in your user context. The
> > >>> ssh command will use the TGT (or available keytab) to interact with
> > >>> sshd, without requiring any special pam module at the remote side.
> > >>>
> > >>> You can delegate your TGT using forwarded TGT into the remote machine
> > >>> if you need to jump additional hope.
> > >>>
> > >>> In other words, kerberos is SSO technology, the PK is used at
> > >>> authentication phase only and if smartcards are being used this phase
> > >>> is performed on local machine, once TGT is available, the remaining
> of
> > >>> the interaction is kerberos only.
> > >>>
> > >>> Regards,
> > >>> Alon
> > >>>
> > >>> On Wed, Dec 19, 2018 at 1:10 AM mailto428496 <mailto628496 at cox.net>
> wrote:
> > >>>> I know OpenSSH currently supports PKCS11 devices (such as
> smartcards)
> > >>>> for publickey authentication, but I would love to see PKCS11
> extended
> > >>>> further.  It is currently possible to perform PKCS11 certificate
> > >>>> authentication, via pam_krb5.so (on Linux at least and likely
> something
> > >>>> similar on other *NIX) which allows smartcard auth to a Kerberos
> > >>>> (including AD) server, where a TGT can also be granted.  How
> difficult
> > >>>> would it be to add functionality to OpenSSH so that it can funnel
> PKCS11
> > >>>> certs from SSH client to server and on to PAM where it could be
> used by
> > >>>> Kerberos/PKINIT?  My thought is that this is at least part way there
> > >>>> with the current PKCS11 support but I won't claim to be an expert
> > >>>> regarding the internals of what would be needed.  I would think
> that a
> > >>>> number of places using smartcards (I currently work for a gov agency
> > >>>> that uses smartcards) would find this approach to have additional
> > >>>> security and management features (given real-time validation
> against a
> > >>>> kerberos/AD server) over using publickey auth (based on PKCS11) and
> also
> > >>>> having the added benefit of granting a TGT on sign-in, enabling SSO
> > >>>> (GSSAPI) to additional backend servers.
> > >>>>
> > >>>> What are thoughts on this functionality being added to OpenSSH?  Am
> I
> > >>>> the first to suggest such a thing?
> > >>>>
> > >>>>
> > >>>> Jim
> > >>>>
> > >>>> _______________________________________________
> > >>>> openssh-unix-dev mailing list
> > >>>> openssh-unix-dev at mindrot.org
> > >>>> https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev
> >
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