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V4/Kerberos PKINIT


Name: V4/Kerberos PKINIT
Ticket: #TODO
Target version: 4.5.0
Author: Ab
Incomplete.png Pending review
Last updated: 2017-09-1 by Pvoborni

Overview

Kerberos authentication framework allows to obtain Kerberos tickets based on a number of different mechanisms. Standard approach implies use of a secret shared by both a Kerberos KDC and a Kerberos client.

RFC 4556 describes a way to allow initial authentication in Kerberos protocol to happen with the help of Public Key Cryptography (PKINIT).

This feature page describes PKINIT implementation in FreeIPA.

Use Cases

Following use cases are defined for the scope of this FreeIPA feature:

  • As an administrator, I want to enable use of PKINIT for Kerberos authentication
    • By default, FreeIPA CA should issue certificates for the KDCs
    • Externally provided certificates should be accepted as well when deploying PKINIT configuration
  • As an administrator, I want to allow users to issue certificates that can be associated with their Kerberos identities
  • As an administrator, I want to define exact criteria for PKI certificates to be accepted for PKINIT purpose by the Kerberos KDC.
  • As an administrator, I want to define exact criteria for PKI certificates to be chosen for PKINIT purpose by Kerberos clients.
  • As a user, I want to be able to manage public key certificates associated with my Kerberos identity, regardless which CA did issue them.
  • As a user, I want to use Anonymous PKINIT feature to obtain an initial ticket granting ticket (TGT) to allow FAST exchange with two factor authentication without access to a different Kerberos principal.

Design

MIT Kerberos KDC and a Kerberos client library already support PKINIT for initial authentication. The scope of supported functionality is defined in the manual page for `krb5.conf` in the section PKINIT Options.

From Kerberos perspective, there are two separate actors: a Kerberos client has to choose which PKI certificate to use for initial Kerberos authentication, and a Kerberos KDC has to decide whether a PKINIT authentication request with the selected PKI certificate can be accepted to represent operations on behalf of the claimed Kerberos identity.

PKINIT initial authentication is implemented as a pre-authentication scheme in Kerberos protocol. Briefly, RFC 4556 document defines the following extensions to RFC 4120:

  1. The client indicates the use of public-key authentication by including a special preauthenticator in the initial request. This preauthenticator contains the client's public-key data and a signature.
  2. The KDC tests the client's request against its authentication policy and trusted Certification Authorities (CAs).
  3. If the request passes the verification tests, the KDC replies as usual, but the reply is encrypted using either:
    1. a key generated through a Diffie-Hellman (DH) key exchange with the client, signed using the KDC's signature key; or
    2. a symmetric encryption key, signed using the KDC's signature key and encrypted using the client's public key.
      Any keying material required by the client to obtain the encryption key for decrypting the KDC reply is returned in a pre-authentication field accompanying the usual reply.
  4. The client validates the KDC's signature, obtains the encryption key, decrypts the reply, and then proceeds as usual.

Given that actual work to handle PKINIT is already implemented in MIT Kerberos, high level scope for FreeIPA integration includes the following changes:

  • Changes to the FreeIPA server and replica installers to request PKINIT-compatible certificates for use in KDC configuration
  • Addition of a special profile with Kerberos KDC specific extensions
  • Changes to certificate management plugin (`cert.py`) to allow generating PKINIT-compatible certificate for KDC using the profile defined above
  • Creation of a special principal to assist in Anonymous PKINIT initial authentication


Since the External Authentication workflow now isolates FreeIPA framework from all Kerberos keys/ccaches care must be taken to ensure that the password logins to the master (e.g. via WebUI) are able to leverage some FAST armoring mechanisms to enable login using OTP. To allow this, local anonymous PKINIT may be performed using a self-signed KDC keypair. Note that such PKINIT setup is not usable on enrolled clients and should be employed only in cases when proper external PKINIT certificates signed by IPA CA are not available, e.g. in CA-less case or when PKINIT certificate issuance was suppressed by installer option. In order to differentiate between client-consumable PKINIT and the local variant, the former capability will be advertised in LDAP and an API will be provided to display its' status on individual masters.

Implementation

Addition of a special profile with Kerberos KDC specific extensions

A profile named KDCs_PKINIT_Certs is added to Dogtag and is loaded into the LDAP store by means of an upgrade script.

Changes to certificate management plugin

Certificate management plugin (`cert.py`) is changed to allow generating PKINIT-compatible certificate for KDC using the KDCs_PKINIT_Certs profile. The difference with normal certificate issuing workflow is that the public certificate is not stored in the LDAP entry for the coresponding principal, `krbtgt/REALM@REALM`. Since KDC pre-authentication pkinit module is configured through `kdc.conf`, there is no need to store the public certificate in the LDAP at all.

As a result, each KDC will have own certificate issued for `krbtgt/REALM@REALM` using KDCs_PKINIT_Certs profile.

Creation of a special principal for Anonymous PKINIT

A special principal, `WELLKNOWN/ANONYMOUS@REALM`, is created during KDC configuration stage. A pre-authentication is set to be required for the principal because PKINIT use requires pre-authentication mechanism.

FreeIPA server and replica installer changes

MIT Kerberos KDC can be built with either NSS and OpenSSL libraries for PKINIT support. Depending on the compile time default, when KDC is configured to enable PKINIT, KDC and CA certificates are configured in a different way. Both Red Hat Enterprise Linux and Fedora distributions build MIT KDC with OpenSSL. Thus, KDC configuration must specify separately a private key and a public certificate of the KDC certificates.

Certmonger is capable to generate PKI certificates pair in separate files but FreeIPA installer framework does not support this option. This means certmonger shim in FreeIPA installer framework is extended to support issuing certificates in ceparate private and public key files.

Backup and Restore

When backing the server up, KDC certificate pair needs to be backed up. When restoring, KDC certificate pair needs to be properly restored, including certmonger tracking of it.

Feature Management

Since anonymous PKINIT will now always be required given the changes during Privilege separation, ipa pkinit-anonymous command will be deprecated and do nothing but issue a warning. This is a safeguard to prevent the locking of anonymous principal and hence breaking password-based authentication to IPA framework (e.g. WebUI logins).

ipa config-show will be enhanced to display active KDCs supporting client PKINIT. To improve the usability of PKINIT in mixed topologies, a separate command (ipa pkinit-status) will be provided to query PKINIT status on selected master or on the whole topology.

To allow PKINIT authentication for users, their PKI certificates should include SAN extensions that map them directly to their primary Kerberos principal name. More details can be found in the PKINIT section of the krb5.conf manual page.

Configuration

For deployment with integrated CA PKINIT is always enabled. For deployments with external CA it is possible to supply externally signed KDC certificate pair.

The exact PKINIT configuration when deploying new replica depends on the domain level, FreeIPA version and CA status of the remote master as is shown in the following table:

Remote master configuration PKINIT feature status*
<4.5, no CA local PKINIT
<4.5 with CA try out full PKINIT, fall back to local configuration if it fails
4.5 no CA, no external PKINIT cert local PKINIT
4.5 no CA, external PKINIT cert external PKINIT
4.5 with CA full PKINIT
  • status: local PKINIT: locally configured KDC with self-signed certificate, external PKINIT: proper PKINIT configured externally provided KDC certificate and CA cert, full PKINIT: proper PKINIT configured using certificate signed by IPA CA

In addition, ipa-server-certinstall command should be extended to allow for installing a PKINIT certificate from PKCS#12 file or request a new PKINIT keypair signed by integrated CA. This should allow users to configure proper PKINIT support for clients post-hoc after installing/upgrading masters with no client PKINIT support.

NOTE: The design does not consider completely disabled anonymous PKINIT as a configuration option. In order to emulate disabled PKINIT the administrator can provide a PKCS#12 file via ipa-server-certinstall and just not distribute the trusted CA certificate as a pkinit anchor to the clients. In this way the local PKINIT will work but clients would not be able to issue anonymous tickets to clients. Since this state will be recorded as a 'external PKINIT', the upgrade code will not attempt to configure full PKINIT.

Upgrade

On upgrade PKINIT is not automatically enabled. To enable PKINIT, run ipa-pkinit-manage command.

The exact upgrade path depends on the state of the feature on the master. We shall distinguish the following configurations:

  • PKINIT is not configured at all (pre 4.5 masters)
  • only local PKINIT w/ self-signed KDC keypair is configured (DL0 masters, CA-less masters without supplied PKINT certificates)
  • PKINIT is configured with supplied 3rd party certificate
  • full PKINIT is configured with certificates signed by IPA CA
Server configuration absent PKINIT local PKINIT external PKINIT full PKINIT
no CA configure local PKINIT no action no action N/A
with CA on the master configure full PKINIT configure full PKINIT no action no action
with CA not on the master try to configure full PKINIT, fallback to local PKINIT try to configure full PKINIT, fallback to local PKINIT no action no action

As can be inferred from the table, there may be failures arising from PKINIT cert request being routed to a CA master which does not hold the KDCs_PKINIT_Certs profile or does not yet possess upgraded cert.py plugin. In this case the upgrader will issue a warning and fall back to issuing self-signed PKINIT certificates.

Update Sep 1 2017

Following information should be incorporated in the design page in order to be up to date with what was implemented in FreeIPA 4.5.x

When you install 4.5 with --no-pkinit, the installer will generate self-signed certificate for PKINIT. This certificate is only used and trusted by IPA Web UI running on the same server to obtain an anonymous ticket.

How to upgrade that certificate to a full-featured PKINIT KDC certificate depends on what is your CA.

A new tool, ipa-pkinit-manage, exists in FreeIPA 4.5.

ipa-pkinit-manage status

will tell you current status. "Enabled" means you have properly working PKINIT infrastructure on this system. "Disabled" means you only have self-signed PKINIT.

ipa-pkinit-manage enable

will try to request PKINIT KDC certificate from IdM CA. You should make sure that all IdM CAs are at the same level (FreeIPA 4.5+) before running this command.

If you are not using IdM CA (external CA is in use), the tool will tell you to use ipa-server-certinstall to install an externally procured KDC certificate. That certificate should have following properties:

  • it be issued with CN=fqdn,$SUBJECT_BASE
  • it should have Kerberos principal krbtgt/REALM@REALM
  • it should have id-pkinit-KPkdc OID (1.3.6.1.5.2.3.5)

You can see /usr/share/ipa/profiles/KDCs_PKINIT_Certs.cfg which is the KDC profile we use to issue this certificate with integrated Dogtag CA. Compare it to /usr/share/ipa/profiles/caIPAserviceCert.cfg to see the difference to a 'normal' IPA service certificate.

How to Use

FreeIPA clients are always configured to trust FreeIPA CA in their /etc/krb5.conf. Thus, if PKINIT KDC certificate is issued by FreeIPA CA, no additional configuration on the client side is needed.

To use Anonymous PKINIT, make sure ipa pkinit-anonymous enable is run (default if installed with PKINIT enabled).

Then any client can use

   kinit -n

to request an anonymous Kerberos ticket.

The ticket granting ticket (TGT) obtained as result of the kinit -n request can only be used to create a FAST channel for second factor authentication:

   kinit -n
   klist
   ARMOR_CCACHE=$(klist|grep cache:|cut -d' ' -f3-)
   kinit -T $ARMOR_CCACHE principal@REALM 

-T option of kinit allows to specify existing credentials cache with a valid TGT to create a FAST channel between the Kerberos client and the KDC when requesting the initial ticket for the specified principal@REALM. This allows to safely pass details of the multi-factor pre-authentication exchange to the KDC.

To query whether the master supports client PKINIT via config-show:

  $ ipa config-show
  Maximum username length: 32
  Home directory base: /home
  Default shell: /bin/sh
  Default users group: ipausers
  ...
  IPA masters supporting PKINIT: master1.ipa.test
  ...

To obtain the same information via ipa pkinit-status:

  $ ipa pkinit-status
  Server name: master1.ipa.test
  PKINIT status: enabled
  ...
  Server name: replica1.ipa.test
  PKINIT status: disabled
  ...