Network Working Group N. Williams
Internet-Draft Cryptonector
Intended status: Standards Track October 27, 2014
Expires: April 30, 2015
Public Key-Based Kerberos Cross Realm Path Traversal Protocol Using
Kerberized Certification Authorities (kx509) and PKINIT
draft-williams-kitten-krb5-pkcross-05
Abstract
This document specifies a protocol for obtaining cross-realm Kerberos
tickets using existing, related protocols: kerberized certification
authorities (kx509) and public key cryptography initial
authentication in Kerberos (PKINIT). The resulting protocol has a
number of desirable properties, primarily that it allows Kerberos to
scale to large numbers of realms.
Status of this Memo
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the Trust Legal Provisions and are provided without warranty as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . 3
2. The PKCROSS Protocol . . . . . . . . . . . . . . . . . . . 4
2.1. Client-Driven PKCROSS . . . . . . . . . . . . . . . . . . 4
2.2. TGS-Driven PKCROSS . . . . . . . . . . . . . . . . . . . . 4
2.2.1. Issuing cross-realm TGTs issued for PKCROSS-keyed
cross-realm TGS principals . . . . . . . . . . . . . . . . 5
2.2.2. Handling impatient clients . . . . . . . . . . . . . . . . 5
2.3. Stapled DANE . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Validation . . . . . . . . . . . . . . . . . . . . . . . . 6
2.5. Transit Path . . . . . . . . . . . . . . . . . . . . . . . 6
2.5.1. Transit path representation . . . . . . . . . . . . . . . 7
2.6. Exchange of Long-Term Cross-Realm Symmetric Keys . . . . . 7
3. Security Properties . . . . . . . . . . . . . . . . . . . 9
3.1. Automatic Cross-Realm Keying . . . . . . . . . . . . . . . 9
3.2. Scalability . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Simplified trust routing . . . . . . . . . . . . . . . . . 9
3.2.2. Simplified trust path validation . . . . . . . . . . . . . 9
3.3. Privacy Protection relative to home realm . . . . . . . . 10
4. Application Programming Interface Considerations . . . . . 11
4.1. GSS-API Considerations . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . 12
5.1. Loss of Cross-Realm Principal Trust Establishment
Information . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. On the Need for a Common Transit Path Policy Language . . 12
5.3. On the Need for Trust Routing . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . 14
7. TODO . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
Kerberos [RFC4120] supports meshes of many realms. The individual
relationships between realms must be manually keyed, usually with
keys derived from passwords. A full mesh wouldn't scale, therefore
the protocol calls for hierarchical trust universes. In practice
non-hierarchical but also non-fully-meshed relationships are used,
and these generally require distribution of trust routing information
to clients, services, and KDCs. With referrals it is possible to
reduce the need for client-side trust routing information, but KDCs
still need it, as do services (unless they accept KDC trust path
policy and the KDC applies it via the TRANSITED-POLICY-CHECKED ticket
flag).
These manually-exchanged keys are very difficult to rollover safely,
and when they are changed the result is often outages -- controlled
outages where foreseen, but outages nonetheless.
Manual cross-realm keying does not scale, and has very poor security
properties. We seek to remediate this using public key cryptography,
building on existing Kerberos specifications.
Distribution of trust routing (traditionally known as "capaths") and
trust path validation (also "capaths") information is difficult;
there is no standard protocol for it. Maintenance of it is a
thoroughly manual process.
Many years ago there was a proposal for exchanging cross-realm keys
using a public key infrastructure (PKI) [RFC5280]; that proposal went
by the name "PKCROSS". We appropriate that long-dead proposal's
name, but the protocol specified here is very different from the
original proposal.
PKCROSS can make Kerberos scale to large numbers of realms, will
remove the need for manual keying of cross-realm TGS principals, will
further reduce the need for maintenance and distribution of trust
routing information, and will tend to reduce the complexity of trust
path validation.
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. The PKCROSS Protocol
We provide two variants of the PKCROSS protocol: one that is client-
driven, and another that is driven by a Ticket Granting Service (TGS)
on behalf of its clients. The latter is based on the former, with
the TGS acting as a client. We begin with the client-driven case.
DNS-Based Authentication of Named Entities (DANE) [RFC6698] can and
should be used for realm CA certificate validation.
2.1. Client-Driven PKCROSS
A Kerberos client in with a ticket-granting ticket (TGT) for any one
source realm (usually but not necessarily the client's own realm)
wishing to acquire a TGT for a destination realm may use this
protocol instead of the traditional cross-realm ticket-granting
service (TGS) exchanges as follows:
1. Generate private key to a public key cryptosystem;
2. Request a certificate from the kx509 [RFC6717] service run by the
source realm;
3. Request a TGT from the destination realm using PKINIT [RFC4556]
and the client certificate obtained in step #2.
If the destination realm issues the requested Ticket then it SHOULD
include the client's certificate in an AD-CLIENT-CERTIFICATE
authorization-data element, and it MUST do so if it does not validate
the client's certificate to an acceptable trust anchor. The AD-
CLIENT-CERTIFICATE authorization-data MUST be in a KDC-signed
authorization-data container [XXX add reference to CAMMAC].
[[anchor1: QUESTION: Should the PKINIT request in step #3 be a TGS-
REQ with PKINIT pre-auth data?]]
[[anchor2: QUESTION: Should the PKINIT request in step #3 be required
to be used within a FAST tunnel?]]
2.2. TGS-Driven PKCROSS
A TGS can bootstrap ephemeral cross-realm trust principals on behalf
of its clients. This allows the cost of PKCROSS to be amortized over
many clients, and it allows participation by clients that do not
support client-driven PKCROSS (or whose PKCROSS requests are rejected
by the target).
In this mode the TGS uses the client-driven PKCROSS protocol,
modified as follows:
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o the TGS's client certificate MUST have an id-pkinit-san Subject
Alternative Name (SAN) identifying the source TGS as krbtgt/
SOURCE@SOURCE
o the TGS's client certificate MUST have an Extended Key Usage (EKU)
of id-pkcross-issuer (TBD)
The resulting TGT -which we shall term an "issuer TGT" (ITGT)- and
its session key can then be used by the source TGS to create cross-
realm TGTs for the source-to-target trust principal ("krbtgt/
TARGET@SOURCE").
This ITGT will be used to mint tickets as described below.
2.2.1. Issuing cross-realm TGTs issued for PKCROSS-keyed cross-realm
TGS principals
Cross-realm TGTs issued by a source TGS using an ITGT will not be
quite like normal Kerberos Tickets: their encrypted part contains an
AP-REQ using the ITGT acquired by the source TGS, and this AP-REQ is
"encrypted" with the null enctype, The AP-REQ's Authenitcator MUST
contain an authorization-data element that carries a) the name of the
client principal, b) the session key that the client should be using
with the cross-realm TGTs issued.
AD-PKCROSS-TGT-INFO ::= SEQUENCE {
cname [0] Principal, -- the client's realm is the
-- crealm from the ITGT's EncTicketPart
key [1] EncryptionKey
}
Figure 1: AD-PKCROSS-TGT-INFO
2.2.2. Handling impatient clients
Because the process of acquiring an ITGT might be slow, a TGS doing
so on behalf of a client could use a mechanism for instructing the
client to be patient. Existing clients would not handler a new error
code by waiting, therefore there is not much that can be done to keep
an impatient client from retrying at another KDC.
The existing KDC_ERR_SVC_UNAVAILABLE error code cannot be used as
often this causes the client to immediately retry the request at
another KDC. A new error code for indicating estimated time to
completion of request would be handy, but out of scope for this
document.
Note that there is a denial of service (DoS) attack by clients on
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willing source KDCs: the clients can ask the KDCs to acquire cross-
realm ITGTs for many target realms. Ideally the quality of service
for the Kerberos authentication service (AS) with PKINIT (and/or
other slow pre-authentication mechanisms) should be separate from
that of the Kerberos TGS co-located with it, and the PKCROSS-capable
TGS as well, so as to be able to throttle low-priority requests when
under load.
2.3. Stapled DANE
[[anchor3: TBD. We should use Google's serialization of DNS RRsets
needed for DANE validation. We will need a label for the TLSA RRs
for kx509 issuers.]]
2.4. Validation
KDCs processing PKINIT requests crossing realms MUST apply either or
both of:
o PKIX certificate validation
o DANE certificate validation
KDCs MUST reject PKINIT requests from clients of foreign realms whose
certificates cannot be validated, unless the client request the
anonymous principal name in the target's realm.
2.5. Transit Path
The combined Kerberos/PKIX/DNSSEC transit path MUST be represented in
any tickets issued using PKCROSS (see below). As usual, each realm's
KDCs in the mix can set the transit policy checked flag if a client's
transit path is acceptable per the realm's KDCs' local policy.
Two validation mechanisms are available: all PKIX [RFC5280]
validation methods, and DANE [RFC6698]. DANE validation records
SHOULD be stapled onto the client certificates by the issuing kx509
CA; alternatively, clients can staple <http://src.chromium.org/
viewvc/chrome/trunk/src/net/base/
dnssec_chain_verifier.cc?pathrev=167227> onto their PKINIT requests
using an authorization-data element, AD-PKINIT-CLIENT-DANE.
Additionally, when PKIX certificate validation is used, the trust
path should be encoded in an AD-INITIAL-VERIFIED-CAS authorization
data element, per-PKINIT.
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2.5.1. Transit path representation
The notional transit path for a ticket issued by a target realm's
KDCs includes:
o the source realm (never expressed in the 'transited' field of
Kerberos Tickets)
o all realms in the ITGT's transited field (in the TGS-driven
PKCROSS case)
o all issuers in the validation path for the kx509-issued
certificate, which are
* all issuers in the certificate's PKIX validation path when PKIX
validation is used
* all DNS zone domainnames transited from the source realm's
domainname to the root zone
o the target realm (also never expressed in the 'transited' field)
When using DANE for validation of the issuer's certificate the target
SHOULD represent the transit path as hierarchical from the source
realm's domain to the root domain, then direct from there to the
target's realm.
The notional transit path for a given client principal MUST be
encoded as usual, using the Kerberos X.500 and domain-style
representations of PKIX issuer names and DNS domainnames as
faithfully to the original as possible.
[[anchor4: QUESTION: Do we need a 100% faithful representation of the
transit path?]]
2.6. Exchange of Long-Term Cross-Realm Symmetric Keys
A KDC can acquire a TGT using PKCROSS whose session key then becomes
the long-lived, persistent symmetric key for a cross-realm principal
from the source realm to the target realm ("krbtgt/TARGET@SOURCE").
To do this the KDC MUST set the USE-SESSION-KEY-AS-REALM-KEY
KDCOptions flag (TBD) in its request for an ITGT from the target
realm. As usual, the target realm's KDC MUST validate the client
principal's certificate. The target realm's KDC MUST NOT return a
TGS-REP until the new principal is committed to its principal
database, and MUST set the endtime of the ITGT to the time at which
the source realm may begin using the new symmetrically-keyed
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principal.
The source realm's KDC MUST commit the new principal to its principal
database and MUST NOT begin using the new principal's long-term keys
until the new principal is available to all KDCs for the source realm
and the endtime of the ITGT passes.
Target KDCs SHOULD require manual pre-approval of such new cross-
realm principals. In small, isolated environments a KDC MAY be
configured to pre-approve all such new principals.
By default, source KDCs SHOULD NOT automatically request long-term
keying of cross-realm principals.
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3. Security Properties
The proposed PKCROSS protocol has several useful properties described
below.
3.1. Automatic Cross-Realm Keying
No more manual keying of cross-realm principals via exchanging
passwords in-person on a telephone call (or similar).
3.2. Scalability
Kerberos with commonplace symmetrically-keyed hierarchical cross-real
trusts can scale to a large universe of realms, but only if there are
top-level realms that are willing to pair-wise trust and "child"
realms. Such top-level realms do not exist in practice, leading to
an O(N^2) scaling problem for most two-label realms.
Leveraging a PKI, such as a PKIX PKI [RFC5280] or a DNSSEC PKI
[RFC4033] removes the need for either top-level realms (which are not
likely to ever be operated as commercial or even non-profit entities)
or O(N^2) pair-wise cross-realm symmetric keying.
The cost of this is having to add PKI trust paths to Kerberos trust
paths (though the resulting trust path length need not be much
different than before).
3.2.1. Simplified trust routing
For clients, relying on referrals (and TGS-driven PKCROSS) and/or
client-driven PKCROSS will greatly reduce the need for client-side
trust routing information.
Even KDCs won't need trust routing information.
3.2.2. Simplified trust path validation
For services that accept hierarchical trust paths, PKCROSS will
greatly reduce the complexity of trust path / transit validation.
Such services that also trust DANE/DNSSEC will need no trust path
validation information for any clients using PKCROSS to reach the
service. In many cases a very simple policy expressed in terms of
whitelists of top-level domains (TLDs) or near top-level domains
traversed, trust anchor sets, and trust of all zero-length transit
paths, will suffice.
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3.3. Privacy Protection relative to home realm
This protocol protects the privacy of client principals vis-a-vis
their home realms, when the clients use the client-driven PKCROSS
protocol.
This feature is generally and naturally available in PKI, and as this
protocol is based on a kerberized certification authority, this
protocol inherits this privacy feature from PKI.
The realms visited by the client may, of course, inform the client's
home realm, but in the event that they don't, the client does gain
this small measure of privacy. Of course, the privacy-conscious
client SHOULD attach an OCSP Response [RFC6960] to its PKINIT
request, per [RFC4557].
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4. Application Programming Interface Considerations
Improved scalability for Kerberos realm traversal implies larger
Kerberos universes, and the larger a universe of trust the more
important it is to have useful and expressive local policy for
evaluating the trustworthiness of any given transit path. Because in
most applications local policy should be a component external to the
application, there is mostly no impact on APIs here. However, an
implementation may wish to provide applications with interfaces for
specifying policies, either named or by value.
4.1. GSS-API Considerations
The naming attributes [RFC6680] defined in
[I-D.williams-kitten-generic-naming-attributes] provide access to
information about transit paths.
Note that information about how PKCROSS was used to establish
symmetrically-keyed cross-realm principals is lost and will not
appear in the transit path in tickets issued by KDCs reached via such
cross-realm principals.
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5. Security Considerations
[[anchor5: All the security considerations of Kerberos and PKI apply.
Security considerations are discussed throughout this document.]]
Scaling up the universe of realms reachable via any trust path
necessarily dilutes trust overall, but not for specific paths. On
the other hand, by shortening transit path lengths trust can be
improved, though some short transit paths will have been
symmetrically keyed using this PKCROSS protocol and therefore will be
longer than they appear to be. These are subjective notions of
trust, of course.
5.1. Loss of Cross-Realm Principal Trust Establishment Information
Once a cross-realm principal is symmetrically keyed the transit path
used to automatically key that principal will no longer appear in
subsequent cross-realm tickets issued by the target.
The Kerberos transit path encodes only realm names (including X.500-
style names, thus PKIX certificate subject and issuer names), and
lacks any public key information that might be useful for pinning.
However, the certificate validation path for each realm in a transit
path SHOULD be included in the transit path.
5.2. On the Need for a Common Transit Path Policy Language
There are no standard ways to express authorization policies for
trust transit paths for either Kerberos nor PKI. A standard language
for this would be extremely useful. Such a language should allow for
the expression of policies for both, clients and services. Such a
language should allow for the expression of complex realm/domain/
other naming, and should allow for HSTS-style pinning [add references
-Nico]. Such a language should allow for multiple paths where
desired, and should allow for more than path rejection: it should
also allow for reducing the entitlements assigned to a peer/realm for
authorization purposes.
The need for a standard transit path policy expression language is
not new, and such a language is broadly and generally needed.
Therefore such a language is outside this document's scope.
PKCROSS can greatly simplify the process of validating a ticket's
trust path, first by shortening the number of realms involved to two
(in the typical case), maybe three, second by making the actual trust
path (PKI or DANE) hierarchical, thus hopefully leaving much less
policy to express in a transit path policy language: whitelists of
domains and sub-domains, perhaps. But a common language would still
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be desirable.
5.3. On the Need for Trust Routing
A common language for trust routing is not necessary in a purely
hierarchical world, as in DANE. But since it's likely that there
will be some non-hierarchical, non-zero-length transit paths in many
deployments for a long time to come, a common language for trust
routing would be desirable as well. Routing protocols normally used
for network addresses could be used for discovery and distribution of
trust routing information as well. But note that there are subtle
differences between trust routing and trust path validation, even
though in traditional Kerberos deployments the same information is
used for both, with the trust validation policy effectively being
that the client must have taken the shortest, highest-priority path
specified in "capaths" configutation.
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6. IANA Considerations
[[anchor6: Allocate the new KDCOptions flag (USE-SESSION-KEY-AS-
REALM-KEY) and authorization-data element (AD-CLIENT-CERTIFICATE), as
well as the new EKU id-pkcross-issuer.]]
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7. TODO
o Provide a normative reference for DANE stapling.
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8. Acknowledgements
Although the author arrived at this "kx509 + PKINIT == PKCROSS" idea
independently, it is not an original idea. Henry Hotz and Jeffrey
Altman each conceived the same idea years earlier. It is a
relatively obvious idea when taking into account efforts to bridge
disparate security mechanisms and credentials infrastructures.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
[RFC4557] Zhu, L., Jaganathan, K., and N. Williams, "Online
Certificate Status Protocol (OCSP) Support for Public Key
Cryptography for Initial Authentication in Kerberos
(PKINIT)", RFC 4557, June 2006.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC6680] Williams, N., Johansson, L., Hartman, S., and S.
Josefsson, "Generic Security Service Application
Programming Interface (GSS-API) Naming Extensions",
RFC 6680, August 2012.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
[RFC6717] Hotz, H. and R. Allbery, "kx509 Kerberized Certificate
Issuance Protocol in Use in 2012", RFC 6717, August 2012.
[I-D.williams-kitten-generic-naming-attributes]
Williams, N., "Generic Naming Attributes for the Generic
Security Services Application Programming Interface (GSS-
API)", draft-williams-kitten-generic-naming-attributes-01
(work in progress), August 2013.
9.2. Informative References
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
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[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, June 2013.
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Author's Address
Nicolas Williams
Cryptonector, LLC
Email: nico@cryptonector.com
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