Kerberos Working Group A. Perez-Mendez
Internet-Draft R. Marin-Lopez
Intended status: Experimental F. Pereniguez-Garcia
Expires: March 5, 2013 G. Lopez-Millan
University of Murcia
Sep 2012
GSS-API pre-authentication for Kerberos
draft-perez-krb-wg-gss-preauth-02
Abstract
This document describes a pre-authentication mechanism for Kerberos
based on the Generic Security Service Application Program Interface
(GSS-API), which allows a Key Distribution Center (KDC) to
authenticate clients by using a GSS mechanism.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 5, 2013.
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document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definition of the Kerberos GSS padata . . . . . . . . . . . . 4
4. GSS Pre-authentication Operation . . . . . . . . . . . . . . . 5
4.1. Generation of GSS preauth requests . . . . . . . . . . . . 5
4.2. Processing of GSS preauth requests . . . . . . . . . . . . 6
4.3. Generation of GSS preauth responses . . . . . . . . . . . 6
4.4. Processing of GSS preauth responses . . . . . . . . . . . 7
5. Data in the KDC_ERR_PREAUTH_REQUIRED . . . . . . . . . . . . . 7
6. Derivation of the reply key from the GSS context . . . . . . . 7
7. KDC state management . . . . . . . . . . . . . . . . . . . . . 8
8. Support for federated users . . . . . . . . . . . . . . . . . 8
9. GSS channel bindings . . . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
11. Security Considerations . . . . . . . . . . . . . . . . . . . 9
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
13. Normative References . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
The GSS-API (Generic Security Service Application Programming
Interface) [RFC2743] provides a generic toolset of functions that
allow applications to establish security contexts in order to protect
their communications through security services such as
authentication, confidentiality and integrity protection. Thanks to
the GSS-API, applications remain independent from the specific
underlying mechanism used to establish the context and provide
security.
On the other hand, Kerberos [RFC4120] defines a process called pre-
authentication. This feature is intended to avoid the security risk
of providing tickets encrypted with the user's long-term key to
attackers, by requiring clients to proof their knowledge over these
credentials. The execution of a pre-authentication mechanism may
require the exchange of several KRB_AS_REQ/KRB_ERROR messages before
the KDC delivers the TGT requested by the client within a KRB_AS_REP.
These messages transport authentication information by means of pre-
authentication elements.
There exists a variety of pre-authentication mechanisms, like PKINIT
[RFC4556] and encrypted time-stamp [RFC4120]. Furthermore,
[I-D.ietf-krb-wg-preauth-framework] provides a generic framework for
Kerberos pre-authentication, which aims to describe the features that
a pre-authentication mechanism may provide (e.g. mutual
authentication, replace reply key, etc.). Additionally, in order to
simplify the definition of new pre-authentication mechanisms, it
defines a mechanism called FAST (Flexible Authentication Secure
Tunneling), which provides a generic and secure transport for pre-
authentication elements. More specifically, FAST establishes a
secure tunnel providing confidentiality and integrity protection
between the client and the KDC prior to the exchange of any specific
pre-authentication data. Within this tunnel, different pre-
authentication methods can be executed. This inner mechanism is
called a FAST factor. It is important to note that FAST factors
cannot usually be used outside the FAST pre-authentication method
since they assume the underlying security layer provided by FAST.
The aim of this draft is to define a new pre-authentication
mechanism, following the recommendations of
[I-D.ietf-krb-wg-preauth-framework], that relies on the GSS-API
security services to pre-authenticate clients. This pre-
authentication mechanism will allow the KDC to authenticate clients
making use of any current or future GSS mechanism, as long as they
satisfy the minimum security requirements described in this
specification. The Kerberos client will play the role of the GSS
initiator, while the Authentication Server (AS) in the KDC will play
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the role of the GSS acceptor.
1.1. Requirements Language
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 RFC 2119 [RFC2119].
2. Motivation
This work is mainly motivated by the necessity of a way to allow the
KDC to make use of the technologies defined in the ABFAB WG to
perform the access control of federated users. Specifically, the
ABFAB architecture requires relying parties to make use of the GSS-
EAP mechanism to perform authentication.
[I-D.perez-abfab-eap-gss-preauth] defines how GSS-EAP is transported
on top of the GSS pre-authentication mechanism defined in this
document.
3. Definition of the Kerberos GSS padata
To establish the security context, the GSS-API defines the exchange
of GSS tokens between the initiator and the acceptor. These tokens,
which contain mechanism-specific information, are completely opaque
to the application. However, how these tokens are transported
between the initiator and the responder depends on the specific
application. Since GSS-API is defined as independent of the
underlying communications service, its use does not require to
implement any specific security feature for the transport. For
instance, tokens could just be sent by means of plain UDP datagrams.
For this reason, security and ordered delivery of information must be
implemented by each specific GSS mechanism (if required).
Therefore, GSS tokens are the atomic piece of information from the
application point of view when using GSS-API, which require a proper
transport between the initiator (Kerberos client) and the acceptor
(AS). In particular, the proposed GSS-based pre-authentication
mechanism defines a new pre-authentication element (hereafter padata)
called PA-GSS, to transport a generic GSS token from the Kerberos
client to the AS and vice-versa. This padata also transport state
information required to maintain the KDC stateless (see section
Section 7.
PA-GSS To be defined (TBD)
A PA-GSS padata element contains the ASN.1 DER encoding of the PA-GSS
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structure:
PA-GSS ::= SEQUENCE {
sec-ctx-token [0] OCTET STRING,
state [1] EncryptedData OPTIONAL -- contains PA-GSS-STATE
}
PA-GSS-STATE ::= SEQUENCE {
timestamp [0] KerberosTime,
exported-sec-ctx-token [1] OCTET STRING,
...
}
The sec-ctx-token element of the PA-GSS structure contains the
output_token token returned by either, the GSS_Init_sec_context and
the GSS_Accept_sec_context calls. The state element of the PA-GSS
structure is optional, and will be absent in the first AS_REQ message
from the client to the KDC and in the last AS_REP message from the
KDC to the client. It contains a PA-GSS-STATE structure encrypted
with the first krbtgt key and a key usage in the 512-1023 range (to
be defined TBD). The state element is generated by the KDC, while
the client just copy it from the previously received PA-GSS structure
(if present).
The PA-GSS-STATE contains a timestamp element, meant to detect
possible replay situations, and a exported-sec-ctx-token element,
representing the whole GSS security context state corresponding to
the current authentication process. This value is generated by the
KDC by calling to the GSS_Export_sec_context, when the
GSS_Accept_sec_context returns GSS_S_CONTINUE_NEEDED.
4. GSS Pre-authentication Operation
4.1. Generation of GSS preauth requests
The Kerberos client (initiator) starts by calling to the
GSS_Init_sec_context function. In the first call to this function,
the client provides GSS_C_NO_CTX as the value of the context_handle
and NULL as the input_token, given that no context has been initiated
yet. When using multi round-trip GSS mechanisms, in subsequent calls
to this routine the client will use both, the context_handle value
obtained after the first call, and the input_token received from the
KDC. The mutual_req_flag, replay_det_req_flag and sequence_req_flag
MUST be set, as the GSS token is meant to be tranported over
cleartext channels.
The GSS_Init_sec_context returns a context_handle, an output_token
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and a status value. In this first call, the only acceptable status
value is GSS_S_CONTINUE_NEEDED, as the KDC is expected to provide a
token in order to continue with the context establishment process.
The Kerberos client creates a new PA-GSS padata, with the obtained
output_token as the sec-ctx-token element, and with the state element
absent. The PA-GSS padata is sent to the KDC through a KRB_AS_REQ
message.
4.2. Processing of GSS preauth requests
When the KDC (GSS acceptor) receives a KRB_AS_REQ message containing
a PA-GSS padata, but a state element (see Section 7) is not included,
the KDC assumes that this is the first message of a context
establishment, and thus GSS_C_NO_CTX is used as context_handle to
invoke the GSS_Accept_sec_context routine. Conversely, if a state
element is included, the KDC assumes that this message is part an
ongoing authentication and the value of the state element is
decrypted and used to recover the state of the authentication (see
Section 7). In both cases, after receiving the message, the KDC
calls to the GSS_Accept_sec_context function, using the adequate
context_handle value and using the received token in the PA-GSS
padata as input_token.
Once the execution of the GSS_Accept_sec_context function is
completed, the KDC obtains a context_handle, an output_token that
MUST be sent to the initiator in order to continue with the
authentication process, and a status value. If the obtained status
is GSS_S_COMPLETE, the client is considered authenticated. If the
status is GSS_S_CONTINUE_NEEDED, further information is required to
complete the process.
4.3. Generation of GSS preauth responses
Once the KDC has processed the input_token provided by the client (as
described in Section 4.2), two main different situations may occur
depending on the status value. If the client is successfully
authenticated (GSS_S_COMPLETE), the KDC will reply to the client with
a KRB_AS_REP message. This message will transport the final
output_token in a PA-GSS padata type. This PA-GSS padata will not
contain the state element. The reply key used to encrypt the enc-
part field of the KRB_AS_REP message is derived from the GSS security
context cryptographic material. Section 6 provides further details
regarding this derivation. At this moment, the KDC also verifies
that the cname provided in the AS_REQ matches the src_name obtained
through the final GSS_Accept_sec_ctx call (except when WELLKNOWN/
FEDERATED is used as cname Section 8).
On the contrary, if further data is required to complete the
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establishment process (GSS_S_CONTINUE_NEEDED), the KDC will reply to
the client with a KDC_ERR_MORE_PREAUTH_DATA_REQUIRED error message
[I-D.ietf-krb-wg-preauth-framework]. In the e-data field of the
message, the KDC will include the PA-GSS padata, containing both, the
GSS token (sec-ctx-token) and the exported GSS security context
(state) (see Section 7).
4.4. Processing of GSS preauth responses
When the client receives a KDC_ERR_MORE_PREAUTH_DATA_REQUIRED error,
it extracts the token from the PA-GSS element and invokes the
GSS_Init_sec_context function, as described in section Section 4.1.
If present, the state element of the PA-GSS padata is treated as an
opaque element, and it is simply copied and included into the
generated PA-GSS element without further processing.
On the other hand, when the client receives a KRB_AS_REP, it knows
the context establishment has finalized successfully. The client
invokes the GSS_Init_sec_context function using the transported GSS
token. Note that, to be consistent, this call MUST return
GSS_S_COMPLETE and not generate any output_token, since the KDC does
not expect further data from the client.
If the context establishment is completed correctly, the client MUST
use the same key derivation process followed by the KDC (Section 4.3)
to obtain the reply key to decrypt the enc-part of the KRB_AS_REP.
5. Data in the KDC_ERR_PREAUTH_REQUIRED
When the KDC sends a KDC_ERR_PREAUTH_REQUIRED error to the client, it
includes a sequence of padata, each corresponding to an acceptable
pre-authentication method. Optionally, these padata elements contain
data valuable for the client to configure the selected mechanism.
The data to be included in the padata for this message is described
in this section.
TBD. (For example, list of the OIDs of the GSS mechanisms supported
by the KDC)
6. Derivation of the reply key from the GSS context
The GSS pre-authentication mechanism proposed in this draft provides
the "Replacing-reply-key" facility
[I-D.ietf-krb-wg-preauth-framework].
After a successful authentication, client and KDC may decide to
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completely replace the reply key used to encrypt the KRB_AS_REP by a
new one that is cryptographically independent from the client's
password stored in client password on the Kerberos users database.
This additional keying material can be obtained by means of calls to
the GSS_Pseudo_random [RFC4401] function, using "KRB-GSS" as the
prf_in parameter.
7. KDC state management
The Kerberos standard [RFC4120] defines the KDC as a stateless
entity. This means that, if the GSS mechanism requires more than one
round-trip, the client MUST provide enough data to the KDC in the
following interactions to allow recovering the complete state of the
ongoing authentication. This is specially relevant when the client
switches from one KDC to different one (within the same realm) during
a pre-authentication process. This second KDC must be able to
continue with the process in a seamless way.
The GSS-API manages the so-called security contexts. They represent
the whole context of an authentication, including all the state and
relevant data of the ongoing security context. Thus, this
information MUST be serialized and sent to the client in order to
ensure that the KDC receiving it will be able to reconstruct the
associated state. In order to prevent attacks, this information must
be confidentiality and integrity protected using a key shared amongst
all the KDCs deployed in the realm, and must be sent along with a
timestamp to prevent replay attacks. How this information is encoded
is described in section Section 3.
To generate the serialized security context information, the
GSS_Export_sec_ctx() call is used. The main drawback of this
approach is that the current GSS-API specifications does not allow
the exportation of a security context which has not been completely
established. Nevertheless, some GSS mechanisms do allow the
exportation of partially established context (e.g.
[I-D.ietf-abfab-gss-eap]), and we expect that other GSS mechanisms
will do the same in the future.
8. Support for federated users
This draft supports the authentication of users belonging to a
different domain than the authenticating KDC. This is achieved by
letting the GSS-API to provide both, the client name and the reply
key to be used. That means that the requested username may not be
present in the KDC's database. To avoid the generation of an error
of type KDC_ERR_C_PRINCIPAL_UNKNOWN, when the Kerberos client knows
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it is operating in a federated environment, it MUST set the value of
the cname field of the KRB_AS_REQ message to a new wellknown value,
WELLKNOWN/FEDERATED, following the model proposed in [RFC6111]. In
this way, the KDC will be completely authenticated by the GSS-API
calls, and thus no local verification of credentials should be done.
9. GSS channel bindings
In order to link the GSS authentication with the actual Kerberos
exchange transporting GSS tokens, the DER-encoded KDC-REQ-BODY from
the AS-REQ is used as channel bindings.
10. Acknowledgements
This work is supported by the project MULTIGIGABIT EUROPEAN ACADEMIC
NETWORK (FP7-INFRASTRUCTURES-2009-1). It is also funded by a Seneca
Foundation grant from the Human Resources Researching Training
Program 2007. Authors finally thank the Funding Program for Research
Groups of Excellence with code 04552/GERM/06 granted by the Fundacion
Seneca.
11. Security Considerations
Protection of Request/Responses with FAST, restriction on GSS
mechanism, etc. TBD.
12. IANA Considerations
This document has no actions for IANA.
13. Normative References
[I-D.ietf-abfab-gss-eap]
Hartman, S. and J. Howlett, "A GSS-API Mechanism for the
Extensible Authentication Protocol",
draft-ietf-abfab-gss-eap-09 (work in progress),
August 2012.
[I-D.ietf-krb-wg-preauth-framework]
Hartman, S. and L. Zhu, "A Generalized Framework for
Kerberos Pre-Authentication",
draft-ietf-krb-wg-preauth-framework-17 (work in progress),
June 2010.
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[I-D.perez-abfab-eap-gss-preauth]
Perez-Mendez, A., Lopez, R., Pereniguez-Garcia, F., and G.
Lopez-Millan, "GSS-EAP pre-authentication for Kerberos",
draft-perez-abfab-eap-gss-preauth-01 (work in progress),
March 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API
Extension for the Generic Security Service Application
Program Interface (GSS-API)", RFC 4401, February 2006.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
[RFC6111] Zhu, L., "Additional Kerberos Naming Constraints",
RFC 6111, April 2011.
Authors' Addresses
Alejandro Perez-Mendez (Ed.)
University of Murcia
Campus de Espinardo S/N, Faculty of Computer Science
Murcia, 30100
Spain
Phone: +34 868 88 46 44
Email: alex@um.es
Rafa Marin-Lopez
University of Murcia
Campus de Espinardo S/N, Faculty of Computer Science
Murcia, 30100
Spain
Phone: +34 868 88 85 01
Email: rafa@um.es
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Fernando Pereniguez-Garcia
University of Murcia
Campus de Espinardo S/N, Faculty of Computer Science
Murcia, 30100
Spain
Phone: +34 868 88 78 82
Email: pereniguez@um.es
Gabriel Lopez-Millan
University of Murcia
Campus de Espinardo S/N, Faculty of Computer Science
Murcia, 30100
Spain
Phone: +34 868 88 85 04
Email: gabilm@um.es
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