Network Working Group R. Van Rein
Internet-Draft ARPA2.net
Intended status: Experimental February 28, 2019
Expires: September 1, 2019
Diameter Messages in Kerberos5 AuthorizationData
draft-vanrein-krb5-authzdata-diameter-01
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. Use with Realm Crossover . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Normative References . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Kerberos5 [RFC4120] is a single-signon system that provides the users
of its realms with authentication tickets to individual services.
Such tickets tend to be valid for the remaining user session,
typically up to a day. Service tickets are provided by a central
realm service known as the Key Distribution Center or KDC.
The KDC may not be able to supply tickets for all services. When it
knows that a service belongs to another realm, and when it can locate
a crossover key to the service's KDC, it will forward the user to a
remote realm through a realm-crossing ticket. The user follows such
tickets under current Kerberos semantics [RFC6806].
Though designed for authentication and not authorisation, the realm-
centric position of a KDC makes it a suitable place to control both
aspects of access control. The message formats can indeed 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 by the Kerberos client.
All this data is protected in transit; requests to the KDC are
encrypted with the TGT secret or a sub-key; tickets carry it to a
service using a key shared between the server and KDC. The KDC can
prove to a ticket-receiving service that it originated certain parts
of AuthorizationData by wrapping it in CAMMAC [RFC7751] structures.
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 an extensible
network access application that lends itself for authorisation
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applications. These Diameter frames are herein proposed as an
AuthorizationData format in Kerberos.
The purpose of using Diameter in Kerberos is to employ the existing
authentication infrastructure of Kerberos to also pass authorisation
settings between hosts. The standardised format of Diameter
simplifies access to services in foreign realms; this is useful to
the InternetWide.org purpose to Bring Your Own IDentity (BYOID),
which requires passing 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. Formally, we
define
AD-DIAMETER ::= -- A concatenation of byte strings:
-- 1. one Diameter Header, as defined
-- in Section 3 of RFC 6733
-- 2. zero or more AVP Headers, with
-- applicable AVP Data, as defined
-- in Section 4 of RFC 6733
-- The whole adhering to the Command
-- Code Format Specification defined
-- in Section 3.2 of RFC 6733 and its
-- extensions.
and shall use it under this specification when the preceding ad-type
field has value TBD assigned for this purpose. Note that AD-DIAMETER
does not follow an ASN.1 encoding.
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 by the Kerberos framework when AuthorizationData is used to carry
the Diameter frames.
Diameter distinguishes between sessions and connections. Connections
represent coherent message carriers such as TCP or SCTP connections,
whereas a session specifies a conceptual relationship that may span
multiple connections. Each connection may carry multiple sessions.
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Requests over one connection may even be responded to over another.
For the purposes of Diameter, passing messages via 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.
The average Diameter exchange is a request/response interaction
between a resource requesting access and an oracle granting or
withholding it. This model does not fit well on the Kerberos system,
when the KDC is an intermediate. A modified form does hold; the
client may request certain kinds of access in a Diameter frame, and
the KDC may filter, modify and pass it on. It is also possible for
the KDC to use just the client identity to state privileges that are
granted to it.
An AuthorizationData field with a Diameter message reaching the
service from an known-aware KDC can be treated as a pro-active hint,
to prepare an answer that might come up when interpreting a service
request. This is possible because the KDC is always aware of the
service being targeted by a requested ticket. 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 establishment of realm crossover. This
loosely addresses the requirement for Capabilities Exchange.
This 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. By default, a KDC will pass AuthorizationData
that the client supplies as-is, though it reserves the right to make
modifications. This means that clients under a KDC unaware of
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Diameter-formatted AuthorizationData might send any claims they like
to a foreign realm, and that the KDC cannot be considered a Diameter
end point unless it adds a proof of origination.
3. Use with Realm Crossover
When crossing over between realms under independent administrative
control, matters of trust and security arise. This has implications
for the interpretation of the AuthorizationData fields, including the
form described herein.
To overcome this problem, the client KDC must use CAMMAC [RFC7751] to
protectively wrap the AuthorizationData for AD-DIAMETER. Trusting
services should validate the origin to be the intended KDC. This
usually means that the KDC of the service realm validates the AD-
DIAMETER data as having originated from the client realm.
With the possible exception for manual overrides, it is not safe to
rely on any other delivery form of AD-DIAMETER data from another
realm.
4. Security Considerations
This specification suggests some leniency in terms of attempting to
access Diameter services that may extend beyond available
capabilities. To mitigate any risk, unrecognised messages could be
silently ignored. In other uses of Diameter, this would instead
cause an explicit error message. As long as no actions are taken on
unrecognised content, this should not impact security.
Diameter messaging parties must take responsibility for what they
send. Kerberos KDCs may pass AuthorizationData without looking, so
any such fields with AD-DIAMETER data must be sent under CAMMAC
protection to prove the agreement of the originating KDC.
During realm crossing, privileges may be passed in from a KDC of
another realm. It is important for any service realm to be mindful
of this. Whether this concern is implemented in individual services
or generally dealt with in the realm's KDC is a local operational
choice of the service realm; the KDC for the service realm always
participates in realm crossover, because it needs to supply the
service ticket to the client that crosses over.
During realm crossing, the client's KDC releases a crossover ticket
to reach a remote realm. The information contained in this ticket's
AuthorizationData may be visible to all services in the remote realm,
and is therefore a privacy concern. It may be necessary to either
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supply only generic or selected information (such as descriptive
attributes) or use only one or a few foreign services per realm.
5. IANA Considerations
When IANA takes on the registration of AuthorizationData tags, it
will take the following allocation into account:
TODO
6. 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>.
[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>.
[RFC7751] Sorce, S. and T. Yu, "Kerberos Authorization Data
Container Authenticated by Multiple Message Authentication
Codes (MACs)", RFC 7751, DOI 10.17487/RFC7751, March 2016,
<https://www.rfc-editor.org/info/rfc7751>.
Appendix A. Acknowledgements
Thanks to the Kerberos list at MIT and especially Greg Hudson, for a
lot of information about Kerberos and GSSAPI.
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.
We thank NLnet foundation and SURFnet for funding (parts of) this
work.
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Author's Address
Rick van Rein
ARPA2.net
Haarlebrink 5
Enschede, Overijssel 7544 WP
The Netherlands
Email: rick@openfortress.nl
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