Network Working Group S. Josefsson
Internet-Draft November 2002
Expires: May 2, 2003
A Kerberos 5 SASL Mechanism
draft-josefsson-sasl-kerberos5-00
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Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document specifies a Simple Authentication and Security Layer
(SASL) [3] mechanism for Kerberos 5 Client/Server Authentication [1],
with optional initial Authentication Service (AS) and/or
Ticket-Granting-Service (TGS) exchanges.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Kerberos version 5 mechanism . . . . . . . . . . . . . . . . . 3
3. Theory of operation . . . . . . . . . . . . . . . . . . . . . 5
3.1 Separate SASL and Kerberos 5 server . . . . . . . . . . . . . 5
3.2 Combined SASL and Kerberos 5 server . . . . . . . . . . . . . 6
4. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
Normative References . . . . . . . . . . . . . . . . . . . . . 9
Informative References . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 10
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1. Introduction
Kerberos 5 provides client and optional server authentication,
usually employing symmetric encryption and a trusted (symmetric) key
distribution center. Specifying Kerberos 5 authentication for each
network protocol where there is a need to use Kerberos 5 is a tedious
task. However, as many protocols already specify support for the
SASL framework, by specifying a Kerberos 5 SASL mechanism, support
for Kerberos 5 in many protocols is accomplished. Even for protocols
that do not support SASL, specifying SASL support (and thereby
implicitely Kerberos 5) is often advantageous over specifying
Kerberos 5 support directly. The advantages include better
flexibility if or when new Kerberos versions are released, and
perhaps more commonly, when circumstances demand that other
authentication methods (supported by SASL) should be used.
It should be mentioned that Kerberos 5 authentication via SASL is
already possible, by using the Generic Security Service Application
Program Interface [6] framework. However, GSSAPI adds some amount of
overhead, both in terms of code complexity, code size and additional
network round trips. More importantly, GSSAPI do not support the
authentication steps (AS and TGS). These are some of the motivation
behind this "slimmer" Kerberos 5 SASL mechanism.
2. Kerberos version 5 mechanism
The mechanism name associated with Kerberos version 5 is
"KERBEROS_V5". The exchange consists of one initial server packet
(containing some parameters and a challenge, described below), and
then an unfixed number of messages containing Kerberos 5 packets,
with the last exchange being an AP-REQ, and optional AP-REP, for the
desired SASL service on a format described below.
The normal packet exchange, after the required initial server packet,
are one optional AS-REQ and AS-REP exchange, one optional TGS-REQ and
TGS-REP exchange and then the AP-REQ packet and optional AP-REP
reply. The only steps that are required by this SASL mechanism is
the initial server packet and the final AP-REQ and optional AP-REP
exchange. The AP-REP is sent when and only when mutual
authentication is required. The AP-REQ is for the SASL service that
is requested. The AP-REQ must contain authenticated application data
on a format described below. The AS and TGS exchanges is usually
used by clients to aquire the proper tickets required for the AP
phase. It is not expected that any other Kerberos 5 packets will be
exchanged, but this mechanism do not disallow such packets in order
to make it possible to use this SASL mechanism with future Kerberos
extensions.
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As discussed above, the client is allowed to send any valid Kerberos
5 message and the server should handle any message, subject to local
policy restrictions. If the server do not understand the meaning of
a packet or do not wish to respond to it (e.g., it cannot proxy a
TGS-REQ), it SHOULD respond with a empty response and await further
packets. Reasons for aborting the authentication phase instead of
sending an empty response includes if the number of received packets
exceeds a pre-defined limit. AS-REQ and TGS-REQ packets destined for
non-local Kerberos Key Distribution Centers (KDCs) is proxied to the
correct server by the SASL server. No provisions are made in the
protocol to allow the client to specify the addresses of the KDCs,
instead the SASL server is required to discover this information
(usually by static configuration or by using DNS). It is legitimate
for the SASL server to abort the authentication phase with an error
saying that the indicated realm was not found or is restricted by
policy (i.e., a policy that only permits local KDC requests is
permitted).
Since it is expected that clients may not yet have IP addresses when
they invoke this SASL mechanism (invoking this mechanism may be one
step in aquiring an IP address), clients commonly leave the address
fields in the AS-REQ empty.
The initial server packet should contain one octet containing a bit
mask of supported security layers, four octets indicating the maximum
cipher-text buffer size the server is able to receive (or 0 if no
security layers are supported) in network byte order, and then 16
octets containing random data (see [4] on how random data might be
generated).
The last exchange must be an AP-REQ for the desired SASL service,
optionaly followed by an AP-REP. The SASL service is translated into
a Kerberos principal and realm as follows: The first element of the
principal is the service name specified in the protocol that uses
SASL. The second element is the address of the SASL server, usually
expressed as a hostname. The realm should be the Kerberos realm of
the SASL server. The checksum field in the Authenticator of the
AP-REQ must be on a string where the first octet indicate the desired
security layer requested by the client (a bitmask with one bit set,
which must also be set in the server offered security layer bitmask),
the next four octets indicate the maximum cipher-text buffer size the
client is able to receive in network byte order (or 0 if a security
layer is not indicated by the first octet), followed by the entire
initial server packet, in turn followed by the desired authorization
identity (which can be empty to indicate that the authorization
identity should be the same as the authentication identity provided
by the Kerberos ticket in the AP-REQ).
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Upon decrypting and verifying the ticket and authenticator (i.e.,
standard AP-REQ processing), the server verifies that the contained
security layer bit mask, server maximum cipher-text buffer size, and
the random data equals the data provided in the original server
challenge. The server also verify that the client selected security
level matches one of the offered security levels. Should the
verification be successful, the server must verify that the principal
identified in the Kerberos ticket is authorized to connect to the
service as the authorization identity specified by the client (or, if
absent, the Kerberos ticket principal). Unless the client requested
mutual authentication, the authentication process is complete.
If the client requested mutual authentication, the server constructs
an AP-REP according to Kerberos 5. The checksum field in the
Authenticator of the AP-REP must contain one octet with the desired
security level (must be equal to what the client requested) followed
by the application the client provided in the AP-REQ Authenticator
checksum.
The security layers and their corresponding bit-masks are as follows:
Bit 0 No security layer
Bit 1 Integrity (KRB-SAFE) protection
Bit 2 Privacy (KRB-PRIV) protection
Bit 3 Mutual authentication is required (AP option MUTUAL-
REQUIRED must also be present).
Other bit-masks may be defined in the future; bits which are not
understood must be negotiated off.
3. Theory of operation
This section describes, for illustrion only, two scenarios where this
mechanism can be used.
3.1 Separate SASL and Kerberos 5 server
Normally a SASL server is an application server such as a mail
system. The server is configured to belong to at least one Kerberos
5 realm, and the server shares a symmetric key with the Kerberos 5
Key Distribution Center in that realm. The server cannot perform the
initial Kerberos AS and TGS operation itself but rather acts as a
proxy, and once the user acquired credentials the server it is able
to carry out the AP-REQ/AP-REP phase using its symmetric key. The
steps are as follows:
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0) Server sends initial token.
1) Client constructs AS-REQ using username and realm supplied by user,
in order to acquire a ticket granting ticket.
2) Server finds address of KDC and forwards the AS-REQ to and waits
for the AS-REP response which it forwards back to the client.
3) Client parses AS-REP and constructs a TGS-REQ using the ticket
granting ticket encryption key, in order to acquire a ticket for
the server.
4) Server finds address of KDC and forwards the TGS-REQ to and waits
for the TGS-REP response which it forwards back to the client.
5) Client parses TGS-REP and generates the AP-REQ using the session
encryption key.
4) Server parses AP-REQ and generates the AP-REP if requested.
5) Client optionally parses AP-REP.
3.2 Combined SASL and Kerberos 5 server
Kerberos 5 is usually a distributed security system, but we wish to
point out that this Kerberos 5 SASL mechanism may be used in a
standalone SASL server to provide the security advantages that the
Kerberos 5 Authentication Service (AS) provides over other methods.
Specifically, the SASL server may use a legacy password database such
as a CRAM-MD5 clear text password file to generate Kerberos 5
principals "on the fly". Authentication may thus proceed as follows:
0) Server sends initial token.
1) Client constructs AS-REQ using username supplied by user, in order to
acquire a ticket for the server directly.
2) Server parses AS-REQ and generates AS-REP based on password in database.
3) Client parses AS-REP and construct a ticket and the AP-REQ using
the session encryption key.
4) Server parses AP-REQ and generates the AP-REP if requested.
5) Client optionally parses AP-REP.
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This may be extended further, i.e. by using the password and
Kerberos 5 pre-authentication in step 1.
4. Example
The following is one Kerberos version 5 login scenarios to the IMAP4
protocol (note that the line breaks in the sample authenticators are
for editorial clarity and are not in real authenticators).
S: * OK IMAP4rev1 server
C: . AUTHENTICATE KERBEROS_V5
S: + AAAAAAB2ZXJ5IHJhbmRvbSBkYXRh
C: aoGNMIGKoQMCAQWiAwIBCqMCMACkejB4oAcDBQAAAAAAoRAwDqADAgEBoQcwBRsD
amFzog8bDUpPU0VGU1NPTi5PUkejIjAgoAMCAQGhGTAXGwZrcmJ0Z3QbDUpPU0VG
U1NPTi5PUkelERgPMjAwMzAxMDgyMzAwMDBapwYCBEbYfGaoCzAJAgESAgEQAgED
S: + a4ICITCCAh2gAwIBBaEDAgELow8bDUpPU0VGU1NPTi5PUkekEDAOoAMCAQGhBzAF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C: boIBpTCCAaGgAwIBBaEDAgEOogcDBQAAAAAAo4IBFGGCARAwggEMoAMCAQWhDxsN
Sk9TRUZTU09OLk9SR6IiMCCgAwIBAaEZMBcbBGltYXAbD3l4YS5leHR1bmRvLmNv
baOBzzCBzKADAgEQoQMCAQGigb8Egbxn6oQtjVF1OlzSSm2TimVUM4xEcaHUeZ0Q
TxqpVmmej0OhVXwtHKnrNY2v/+edOzNS8yojmmKaKN5cnUHnvLwgS/W0Xx55bhFW
zo0I+x0kAaOef7HHf5XsfinYybp5qVaLihjJXK0IrbYkeZy/B6tpAfsIuKfIRVXL
jI9DYt9a+sHVLpE+yHMiCxM//JWOnCJsnD2T5IeuHgjBf6D9DXVwzJTMJZRs3zVm
Vo4NhAoqIDsFtdZ3ca+bcaBA2aR0MHKgAwIBEKEDAgEBomYEZC0aevz6IesKrIx+
m8/qfYjYx/cho274VGzMrREKEg9lK8s1ajkeq4yrNQWxblwA3zx6Q2HzX0DHomhj
6Lm1TnB8VKJziZmFocyG3nicOuORf1oqO+qLmr9S3yNQUseKBFOEfX8=
S: . OK KERBEROS_V5 successful
(XXX this is not a real example)
5. Security Considerations
The authentication phase is belived to be no less secure than the
Client/Server Authentication exchange described in the Kerberos 5
protocol.
If no security layer is negotiated, the connection is subject to
active man-in-the-middle attackers that hijack the connection after
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authentication has been completed.
When security layers are used, it is believed that the communication
channel negotiated by this specification is no less secure than the
KRB_SAFE and KRB_PRIV primitives. In other words, it is believed
that if an attack that breaches integrity or privacy of this
mechanism, the same attack also applies to the Kerberos 5
specification, and vice versa.
Server implementations should be aware that this proxying function
can be abused, and MAY implement precaution against this if it is
considered a threat. Useful precautions include limiting the size of
packets forwarded, and the number of packets, to abort the SASL
exchange when the limit is reached.
Server implementations should make sure the method to look up KDC for
the client indicated realm does not cause security problems. In
particular, trusting unprotected DNS lookups to find the KDC of a
realm may be considered as dangerous by a server.
The forward-compatibility behaviour of returning empty responses to
unsupported commands may be abused as a covert channel.
The reason for the client to send, in the Authenticator checksum
field, not only the server random number but the entire initial
server packet with the security layer bitmask and maximum cipher-text
buffer size accepted by server, is to prevent an attacker from
downgrading the security layer ultimately selected. The random
number ties the client and server to the same network session,
prevent man-in-the-middle attacks assuming a Kerberos 5 security
layer is chosen and that the Kerberos 5 security layer is secure.
The security considerations of Kerberos 5 and SASL are inherited.
Some immediates consequences of this follows (this is an inconclusive
summary):
Note that some of the Kerberos 5 encryption types are considered
weak, implementations must decide which algorithms are trusted.
Note that Kerberos 5 do not authorize users, it only authenticate
users. Applications using this mechanism must thus perform checks,
not described in detail in this document, to make sure the
authenticated user is authorized to the service she is requesting.
Note that the SASL framework is subject to "downgrade" attacks where
an attacker forces a weaker SASL mechanism to be used. The use of,
e.g., TLS [5] can be used to mitigate this.
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Normative References
[1] Kohl, J. and B. Neuman, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Myers, J., "Simple Authentication and Security Layer (SASL)",
RFC 2222, October 1997.
Informative References
[4] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[5] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and
P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January
1999.
[6] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
Author's Address
Simon Josefsson
Drottningholmsv. 70
Stockholm 112 42
Sweden
EMail: simon@josefsson.org
Acknowledgements
Text and ideas was borrowed from the Kerberos version 4 SASL
mechanism in RFC 2222.
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