Diameter Messages in Kerberos5 AuthorizationData
draft-vanrein-krb5-authzdata-diameter-00

Network Working Group                                        R. Van Rein
Internet-Draft                                                 ARPA2.net
Intended status: Experimental                           October 20, 2018
Expires: April 23, 2019

            Diameter Messages in Kerberos5 AuthorizationData
                draft-vanrein-krb5-authzdata-diameter-00

Abstract

   The Kerberos5 infrastructure is concerned with authentication, but it
   can also carry AuthorizationData in a variety of formats.  Diameter
   is an extensible standard for the expression of authorisation
   information.  This specification defines an embedding of Diameter
   data in the AuthorizationData fields of Kerberos5.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Embedding Diameter in Kerberos5 . . . . . . . . . . . . . . .   3
   3.  InternetWide Usage Pattern  . . . . . . . . . . . . . . . . .   4
   4.  Example Application . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Kerberos5 [RFC4120] is a single-signon system that automatically
   supplies users with authentication tickets to individual services.
   Such tickets tend to last as long as a maximum user session, usually
   up to a day.  They are provided by a central realm service known as
   the Key Distribution Center or KDC.

   Though designed for authention rather than authorisation, KDC
   exchanges lend themselves well to facilitate both.  The message
   formats can carry AuthorizationData consisting of a numerical tag and
   data to be interpreted according to that tag.  This specification
   adds a tag value to describe AuthorizationData holding Diameter
   protocol messages.

   AuthorizationData can be included in requests to the KDC, and the KDC
   can include AuthorizationData in a ticket.  When requesting a service
   ticket from the KDC, a single-signon ticket with possible
   AuthorizationData is attached to that request.  All this data is
   protected in transit; requests to the KDC are encrypted with the TGT
   secret or a sub-secret; tickets pass it to a service using a key
   shared between the server and KDC.

   Diameter frames [RFC6733] hold a list of Attribute-Value Pairs (AVP),
   with optional nesting into group AVPs.  The format is easily parsed
   and applications exist (or can be defined) to capture a class of
   usage scenarios.  Among the existing applications is a network access
   application that lends itself for most authorisation applications.

   The purpose of using Diameter in this place is to employ the existing
   communication infrastructure of Kerberos to pass authorisation
   settings between hosts.  The standardised format simplifies access to
   services in foreign realms; this is useful to the InternetWide.org
   purpose to Bring Your Own IDentity (BYOID), which requires passing

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   standardised authentication and authorisation information between
   collaborating but otherwise independent parties.

2.  Embedding Diameter in Kerberos5

   Diameter messages consist of a header choosing an application,
   followed by the AVPs that are meaningful within that application.
   The header guides the interpretation of the AVPs and is therefore a
   meaningful part in proper understanding of the exchange.  This is why
   inclusion of Diameter as Kerberos AuthorizationData must not be
   limited to a set of AVPs but instead include a header.

   Diameter messages are always carried over a protected transport such
   as SCTP with DTLS or TCP with TLS, where the purpose is to mutually
   authenticate Diameter Peers in protection of in-transit data against
   rogue alterations and to conceal sensitive data from similarly
   undesirable parties.  These security requirements are alternatively
   met when AuthorizationData is used to carry the Diameter frames.

   Diameter makes a distinction between sessions and connections.
   Connections represent coherent message carriers such as TCP or SCTP
   connections, whereas a session is more conceptual relationship that
   may span multiple connections and connections may carry multiple
   sessions.  Requests over one connection may even be responded to over
   another.  For the purposes of Diameter, passing messages through
   Kerberos can be viewed as an alternative form of connection; the
   collaboration between KDC and services can be considered a
   connection, consisting of a multitude of individual packages but
   together forming a coherent message carrier.

   A normal exchange under Diameter is a request leading to a response.
   This can be established by inclusion of a request in a message going
   to a KDC, and a response coming out of it.  We can also imagine
   responses to be supplied as a pro-active hint, to prepare an answer
   that might come up when interpreting a service request.  This is
   possible because the client KDC is asked for a service ticket,
   allowing it to preview upcoming inquiries.  The hint provided would
   stand for the entire duration of the service ticket, effectively
   turning such tickets into client-held caches of their authorisations.

   Diameter connections must start interactions with a Capabilities
   Exchange.  This specification answers that with a default setup with
   the Network Access Application [RFC7155] and possible overrides to
   take place during administrative setup, such as during the creation
   of service keys or the esstablishment of realm crossover.  This
   looseley addresses the requirement for Capabilities Exchange.

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   The looseness is warranted because the customary need for a
   Capabilities Exchange is not fully applicable to the use of Diameter
   messages in AuthorizationData, not even when crossing realms.  First,
   this can be used as a pro-active mechanism and it is generally safe
   to ignore any misunderstood Diameter messages.  Second, tickets tend
   to be cached for a day, which makes their generation less resource-
   demanding.  Third, the purpose can help avoid traditional Diameter
   traffic, thus limiting the danger of a lot of spurious network
   traffic.

   Peers that process Kerberos messages should not be considered
   Diameter processing nodes, as they may be just passing traffic,
   except for the end points that produce and consume Diameter messages
   in AuthorizationData.  Intermediate nodes may choose to process data
   though; for instance, during realm crossover the KDC of the service
   domain may remove authorisations that it does not find acceptable;
   the service may assume that it's realm is managed thusly.  In general
   however, Kerberos defaults to passing AuthorizationData without
   change, meaning that Kerberos nodes cannot be held accountable for
   Diameter message content that it does not interpret.

3.  InternetWide Usage Pattern

   This section is not normative; it serves to present context.

   The following patterns are proposed for the InternetWide.org project,
   which aims to support secure and private interactions across realm
   boundaries for standard Internet protocols, including but certainly
   not limited to HTTP.

   A general principle in the InternetWide Architecture is that users
   can Bring Your Own IDentity (BYOID) which is implemented with names
   with a local part and a domain part, basically like an email address
   or Netword Access Identifier.  When crossing over to a remote realm,
   the intention is to allow a change to an alias that may be specific
   to that remote realm, thus allowing better control of online
   presence.

   The InternetWide Architecture treats user services such as email and
   web as what Diameter calls Network Access Servers; they grant access
   to a facility, and need authorisation from the client's home realm.
   Diameter was designed to allow this across realms, and the inclusion
   of these forms in Kerberos tickets can greatly improve the
   scalability of such interactions.

   The Kerberos mechanism is very efficient for use across realms, as is
   required under BYOID.  Other current mechanisms tend to be less
   efficient due to:

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   o  Everything is connected through HTTP over TLS;

   o  The employed cryptography is public-key based;

   o  There is only limited opportunity for caching;

   o  Authorisation is performed on-demand;

   o  Authorisation signatures must be separately verified;

   o  The result is not protected against quantum computing.

   In contrast, the proposed AuthorizationData used in Kerberos with
   realm crossover permits:

   o  All protocols can be used, with authentication in only a few
      exchanges;

   o  The employed cryptography is symmetric;

   o  AuthorizationData is cached for about a day with the client; realm
      crossover is cached in the KDC;

   o  Authorisation is done pro-actively during authentication;

   o  AuthorizationData is protected along with the authentication of a
      service ticket;

   o  Kerberos can withstand quantum computing.

   At the start of a session, a user logs in to the KDC for his realm.
   During the day, service tickets are requested.  When the KDC needs
   to, it will forward the user to a remote realm through realm-crossing
   tickets, that the user follows using current Kerberos semantics
   [RFC6806].

   For services in serviced realms, the KDC can insert dedicated
   authorisation information into requested service tickets.

   The KDC cannot be configured to grant the supply of information to
   remote realms during the dissimination of realm-crossing tickets.
   Such information should not be specialised for a service name found
   in the request that caused it to release a realm-crossing ticket,
   because clients tend to cache these tickets and thus be deprived of
   this refinement for future use in the same realm.  It is however
   possible to supply more kinds of information, or generally usable
   information, in such realm-crossing tickets.

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   The KDC of a service realm may prune information that does not apply,
   or is considered improper, for a service for which it received a
   request.  This can be included into a specific ticket for a specific
   service.  Some authorisation information during realm crossover is
   the prerogative of the client's KDC, other information is the
   prerogative of the service's KDC.  Identity is a good example of
   client-sided control, whereas rights to write to a resource tend to
   be decided about on the service side.  The InternetWide Architecture
   envisions standardised contracts between client and service realms
   that may enhance the information that could pass.

   A targeted service can retrieve authorisation information from a
   service ticket, and be sure that it came from the KDC that
   constructed this ticket.  In case of a user from a realm that is not
   local to the service's realm, the service may derive trust from any
   filtering that it knows is being done in the KDC for the service
   realm.

4.  Example Application

   This section is not normative; it serves as an example.

   Shell services are often made available through OpenSSH [RFC4251] and
   it is common to provide services (such as file transfer) as well as
   tunneling (such as to a server port behind a firewall).  Kerberos
   access to these services take the form of rather general "host"
   tickets that is also used for several other purposes on the same
   host.

   The multitude of access opportunities with one ticket calls for some
   restriction, but a change of the ticket naming convention is not
   practical.  Additional content describing authorised purposes is.
   Diameter messages can be useful to accommodate this.  It may also
   help to provide access to shared environments, where each client is
   given a different kind of access profile.  Even access to clients
   without a local account may be possible under such conditions.

   Everyday setups of OpenSSH are isolated; it may use a plugin module
   to add Kerberos5 authentication and decide on the access and session
   properties.  These setups tend to work in isolation without
   connectivity to a Diameter infrastructure, but having such data can
   nonetheless be beneficial.

5.  Security Considerations

   This specification suggests some leniency in terms of attempting to
   access Diameter services that may not be as formally negotiated.
   This does mean that unrecognised messages should be silently ignored.

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   In typical uses of Diameter, this would rather cause an explicit
   error message.  As long as no actions are taken on unrecognised
   content, this should not impact security, however.

   Diameter messaging parties must take responsibility for what they
   send.  Kerberos peers pass AuthorisationData without looking, so
   these peers cannot be held responsible and should be viewed as
   communication channels, not Diameter peers.

   During realm crossing, the right to setup certain authorisation
   fields may vary.  It is important for any trusting realm to be
   mindful of this.  Whether this concern is implemented in individual
   services or generally dealt with in the realm's KDC is an operational
   choice that can be made locally to the realm; the realm's KDC is
   involved in realm crossover at the time that a service ticket is
   requested by the foreign client.

   During realm crossing, the client's KDC releases a general ticket for
   a remote realm.  The information contained in this ticket's
   AuthorizationData may be visible to all services in the remote realm,
   and so is subject to privacy concerns.  It may be necessary to either
   supply only generic information (such as descriptive attributes) or
   use only one or a few foreign services per realm.

6.  IANA Considerations

   When IANA takes on the registration of AuthorizationData tags, it
   will take the following allocation into account:

   TODO

7.  References

7.1.  Normative References

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,
              <https://www.rfc-editor.org/info/rfc4120>.

   [RFC6733]  Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
              Ed., "Diameter Base Protocol", RFC 6733,
              DOI 10.17487/RFC6733, October 2012,
              <https://www.rfc-editor.org/info/rfc6733>.

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   [RFC6806]  Hartman, S., Ed., Raeburn, K., and L. Zhu, "Kerberos
              Principal Name Canonicalization and Cross-Realm
              Referrals", RFC 6806, DOI 10.17487/RFC6806, November 2012,
              <https://www.rfc-editor.org/info/rfc6806>.

   [RFC7155]  Zorn, G., Ed., "Diameter Network Access Server
              Application", RFC 7155, DOI 10.17487/RFC7155, April 2014,
              <https://www.rfc-editor.org/info/rfc7155>.

7.2.  Informative References

   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
              January 2006, <https://www.rfc-editor.org/info/rfc4251>.

Appendix A.  Acknowledgements

   This work was conducted as part of the InternetWide.org project, and
   aims to support authorisation data that crosses over between
   platforms and realms.  Implementation projects under this
   architecture are named ARPA2 projects.

Author's Address

   Rick van Rein
   ARPA2.net
   Haarlebrink 5
   Enschede, Overijssel  7544 WP
   The Netherlands

   Email: rick@openfortress.nl

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