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

Alon Bar-Lev alon.barlev at gmail.com
Wed Dec 19 17:27:35 AEDT 2018

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

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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