Network Working Group N. Williams
Internet-Draft Cryptonector
Intended status: Standards Track August 13, 2013
Expires: February 14, 2014
Public Key-Based Kerberos Cross Realm Path Traversal Protocol Using
Kerberized Certification Authorities (kx509) and PKINIT
draft-williams-kitten-krb5-pkcross-02
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 security properties, including privacy protection
for the user relative to their home realm's infrastructure, as well a
support for leap-of-faith trust establishment, and automated cross-
realm keying. This protocol allows Kerberos to scale to large
numbers of realms.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . 3
2. The Protocol . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Exchange of Long-Term Cross-Realm Symmetric Keys . . . . . 4
3. Security Properties . . . . . . . . . . . . . . . . . . . 6
3.1. Automated Cross-Realm Keying . . . . . . . . . . . . . . . 6
3.2. Privacy Protection relative to home realm . . . . . . . . 6
3.3. Leap-of-Faith (LoF) / Trust-On-First-Use (TOFU) . . . . . 6
3.3.1. Requirements and Recommendations for LoF/TOFU
Authentication . . . . . . . . . . . . . . . . . . . . . . 6
4. Using DANE (DNSSEC) for Realm Certificate Validation . . . 8
5. Application Programming Interface Considerations . . . . . 9
5.1. API Considerations for LoF/TOFU Authentication . . . . . . 9
5.2. GSS-API Naming Considerations . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . 10
6.1. Loss of Cross-Realm Principal Trust Establishment
Information . . . . . . . . . . . . . . . . . . . . . . . 10
6.2. Security Considerations for LoF/TOFU . . . . . . . . . . . 10
6.3. On the Need for a Common Transit Path Policy Language . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . 14
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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. These keys are very difficult to
rollover, and when they are changed the result is often outages --
controlled outages where foreseen, but outages nonetheless. This
method of cross-realm keying does not scale, and has very poor
security properties. We seek to remediate this.
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.
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 Protocol
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. Generate a certificate signing request (CSR) [RFC2986], such that
the resulting certificate has an id-pkinit-san subject
alternative name (SAN) corresponding to the client's principal
name and realm;
3. Request a certificate from the kx509 [RFC6717] service run by the
source realm;
4. Request a TGT from the destination realm using PKINIT [RFC4556].
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 destination realm MUST NOT set the TRANSIT-POLICY-CHECKED flag on
the tickets they issue to clients whose foreign realm certificates
are not validated by the KDC. Destination realm administrators may
configure their realms to know specific foreign realm clients'
certificates.
The destination MUST include the trust path of the client's
certificate, if validated, in the 'transited' field of the issued
Ticket, using a mapping of the issuer names to the X.500 realm naming
style [XXX must specify this mapping; hopefully it can be the
identity function or close enough].
2.1. Exchange of Long-Term Cross-Realm Symmetric Keys
When the client principal is a TGS principal and its PKINIT AS-REQ
protocol data unit (PDU) has the USE-SESSION-KEY-AS-REALM-KEY
KDCOptions flag set then the client is requesting that the session
key of the ticket issued by the destination realm become the long-
term key for the corresponding krbtgt/DESTINATION@SOURCE principal.
The destination realm MUST validate the client principal's
certificate, building a trust path if need be, and validating it to a
trust anchor. The source and destination realm MAY have previously
exchange fingerprints of their respective key distribution service
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(KDC) public keys and/or certificates and/or the source realm's kx509
root or intermediate certification authority (CA), and such
previously exchanged material, if any, MUST be used for certificate
trust validation.
Realm administrators should use the procedure to setup symmetric
cross-realm keys as necessary to save clients from having to
frequently use kx509 and PKINIT as described in the preceding
section.
Where public key infrastructure (PKI) exists allowing this to happen
automatically, realms' KDCs MAY be configured to automatically key
cross-realm principals for any realms that their source realms'
clients request cross-realm TGTs for, but note that this presents a
denial of service (DoS) opportunity to the source realm's clients.
Source realm KDCs SHOULD only do this when a) they are configured to
do so, b) the requesting client principal is in the same realm, c)
the KDC has not spent too much effort recently providing this service
(i.e., KDCs should throttle attempts to establish symmetric cross-
realm keys in this manner), and d) up to some maximum number of
cross-realm principals.
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3. Security Properties
The proposed PKCROSS protocol has several useful properties described
below.
3.1. Automated Cross-Realm Keying
No more manual keying of cross-realm principals via exchanging
passwords on a telephone call (or similar).
3.2. Privacy Protection relative to home realm
This protocol protects the privacy of client principals vis-a-vis
their home realms: client principals' home realms need not know what
destination realms the clients are speaking to because client
principals need not ask their home realms.
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.
3.3. Leap-of-Faith (LoF) / Trust-On-First-Use (TOFU)
Clients need not validate the certificate trust path of destination
realms. When they do not, the services used through those
destination realms are as good as anonymous authentication. If the
client saves the root or intermediate or end entity certificates of
the destination realms that it cannot or does not validate, then the
client can check that on future occasions the destination realm's
certificate has not changed, and it may warn the user if it has.
This quite similar to how clients using the secure shell (SSH)
protocol [RFC4251] handle server authentication, and is commonly
known as "leap-of-faith" (LoF) or trust-on-first-use (TOFU). The
result is pseudonymous authentication.
Destination services too may apply apply LoF/TOFU: by not validating
the transit path of the client (e.g., if it's not in a white-list of
realms whose clients must have valid transit paths) and accepting
tickets without the TRANSITED-POLICY-CHECKED ticket flag set. The
destination service can save the client's certificate, if found in an
AD-CLIENT-CERTIFICATE authorization-data element in the client's
Ticket, and may use it later to ensure that it is talking to the same
client.
3.3.1. Requirements and Recommendations for LoF/TOFU Authentication
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o Implementations MUST NOT use LoF/TOFU to authenticate a target
service's realm without the approval of the user or without making
it clear that the realm is not fully authenticated (perhaps by
replacing the realm's name with a fingerprint of its public key /
certificate).
o Implementations MAY allow service administrators to establish
user-friendly aliases for client principal names that include
public key fingerprint material.
o Implementations MAY provide a way to automatically learn realm
name <-> public key / certificate bindings. Pinning [add
reference to HSTS] SHOULD be supported in that case. The user
MUST approve of each such mapping.
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4. Using DANE (DNSSEC) for Realm Certificate Validation
[Specify how to use DNS-Based Authentication of Named Entities (DANE)
[RFC6698] to authenticate the KDC certificates of realms with domain-
style names. Roughly: format the realm's name as a domainname, then
format the DANE TLSA resource record set's (RRset) domainname per-
DANE, using the KDC's port number. Note that the KDCs will usually
not speak TLS, though there is an extension for using TLS in the KDC
over TCP protocol. For example, the TLSA RRset for any KDC for the
DESTINATION.EXAMPLE realm might be named
_88._tcp.destination.example.]
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5. Application Programming Interface Considerations
For non-LoF/TOFU uses the main security consideration for
applications is that 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 little impact on
APIs here. However, an implementation may wish to provide
applications with interfaces for specifying policies, either named or
by value.
5.1. API Considerations for LoF/TOFU Authentication
For LoF/TOFU uses there is a critical requirement that APIs not
permit accidental aliasing of principal names as a result of LoF/TOFU
being used. The simplest way to do this is to use a fingerprint of
the peer principal's public key as their principal, and/or a
fingerprint of the peer principal's realm's public key as their
realm.
[[anchor1: For interoperability and compatibility we ought to specify
what fingerprint algorithm to use, perhaps one of the SSHv2
fingerprint algorithms, such as in RFC4255, but those use weaker
hashes...]]
5.2. GSS-API Naming Considerations
There are no GSS-API-specific considerations. The naming
considerations described in Section 5.1 and the naming attributes
defined in [I-D.williams-kitten-generic-naming-attributes] are
sufficient. Note however 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.
[[anchor2: Actually, we may need to specify some interfaces by which
to indicate that the user wishes to alias a pseudonymous name.
Perhaps we can do so by applying GSS_Set_name_attribute() to a peer
MN obtained from GSS_Inquire_context()?]]
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6. Security Considerations
[[anchor3: 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.
6.1. Loss of Cross-Realm Principal Trust Establishment Information
Note that once a cross-realm principal is symmetrically keyed no
information about how that keying operation took place will appear in
tickets issued by that TGS principal.
Note also that the Kebreros 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.
6.2. Security Considerations for LoF/TOFU
LoF/TOFU has additional security considerations. To start there is
the obvious susceptibility to peer impersonation / man-in-the-middle
(MITM) attacks on initial contact, which is mitigated by the
attacker's need to always remain in the middle in order to avoid
detection.
LoF/TOFU require the ability to remember peers' pseudonymous
identities -- their public keys (or certificates), otherwise one
remains vulnerable to peer impersonation / MITM attacks at all times.
This requires synchronization of peer pseudonym databases across
multiple devices (where users have multiple devices), which may not
always be possible or performed.
It is critical that existing applications not be broken by the
ability to use LoF/TOFU in new Kerberos implementations when those
applications are re-linked with newer Kerberos implementations. To
ensure this we require the use of public key fingerprints as
principal and/or realm names; local mappings of learned pseudonym
mappings onto semantically meaningful names are permitted where the
user can validate the mapping. But keep in mind that most users
never actually do much to verify peers' public keys in any
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application/protocol that provides LoF/TOFU [references for this
would be nice -Nico].
See Section 3.3.1 for additional requirements for LoF/TOFU
authentication.
6.3. 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.
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7. IANA Considerations
[[anchor4: Allocate the new KDCOptions flag (USE-SESSION-KEY-AS-
REALM-KEY) and authorization-data element (AD-CLIENT-CERTIFICATE).]]
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
November 2000.
[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.
[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.
[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-00
(work in progress), July 2013.
8.2. Informative References
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
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Author's Address
Nicolas Williams
Cryptonector, LLC
Email: nico@cryptonector.com
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