4 Network Working Group S. Josefsson
6 Intended status: Standards Track October 4, 2006
10 Using Kerberos V5 over the Transport Layer Security (TLS) protocol
11 draft-josefsson-kerberos5-starttls-01
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36 This Internet-Draft will expire on April 7, 2007.
40 Copyright (C) The Internet Society (2006).
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62 This document specify how the Kerberos V5 protocol can be transported
63 over the Transport Layer Security (TLS) protocol, to provide
64 additional security features.
69 1. Introduction and Background . . . . . . . . . . . . . . . . . 3
70 2. Kerberos V5 STARTTLS Extension . . . . . . . . . . . . . . . . 5
71 3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
72 4. STARTTLS aware KDC Discovery . . . . . . . . . . . . . . . . . 7
73 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
74 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
75 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
76 7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
77 7.2. Informative References . . . . . . . . . . . . . . . . . . 10
78 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
79 Intellectual Property and Copyright Statements . . . . . . . . . . 12
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116 1. Introduction and Background
118 This document describe how a Kerberos V5 [2] implementation may
119 upgrade communication between clients and Key Distribution Centers
120 (KDCs) to use the Transport Layer Security (TLS) [4] protocol.
122 The TLS protocol offer integrity and privacy protected exchanges that
123 can be authentication using X.509 certificates, OpenPGP keys [7], and
124 user name and passwords via SRP [6].
126 There are several reasons to use Kerberos V5 over TLS.
128 o Kerberos exchanges are privacy protected. Part of many Kerberos
129 packets are transfered without privacy protection (i.e.,
130 encryption). That part contains information, such as the client
131 principal name, the server principal name, the encryption types
132 supported by the client, the lifetime of tickets, etc. Revealing
133 such information is, in some threat models, considered a problem.
136 o Prevents downgrade attacks affecting encryption types. The
137 encryption type of the ticket in KDC-REQ are sent in the clear in
138 Kerberos 5. This allows an attacker to replace the encryption
139 type with a compromised mechanisms, e.g., 56-bit DES. Since
140 clients in general cannot know the encryption types other servers
141 support, it is difficult for the client to detect if there was a
142 man-in-the-middle or if the remote server simply did not support a
143 stronger mechanism. Clients could chose to refuse, e.g., 56-bit
144 DES altogether, but in some environments this leads to operational
148 o Additional authentication against the KDC. In some situations,
149 users are equipped with smart cards with a RSA authentication key.
150 In others, users have a OpenPGP client on their desktop, with a
151 public OpenPGP key known to the server. In some situations, the
152 policy may be that password authentication may only be done
156 o The TLS protocol has been studied by many parties. In some threat
157 models, the designer prefer to reduce the number of protocols that
158 can hurt the overall system security if they are compromised.
161 o Explicit server authentication of the KDC to the client. In
162 traditional Kerberos 5, authentication of the KDC is proved as a
163 side effect that the KDC knows your encryption key (i.e., your
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174 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
175 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
176 document are to be interpreted as described in RFC 2119 [1].
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228 2. Kerberos V5 STARTTLS Extension
230 The STARTTLS extension uses the Kerberos V5 TCP extension mechanism
231 [3]. The extension uses bit #TBD in the extension bitmask.
233 The protocol is as follows. After the server has sent the 4-octet
234 value 0x00000000 to indicate support of this extension, the stream
235 will be controlled by the TLS protocol and its framing. The TLS
236 protocol is initiated by the client.
238 Typically, the client initiate the TLS handshake protocol by sending
239 a client hello, and the server responds, and the handshake continues
240 until it either succeed or fails.
242 If for any reason the handshake fails, the STARTTLS protocol will
243 also fail, and the TLS error is used as the error indication.
245 If the handshake succeeds, the Kerberos V5 authentication protocol is
246 performed within the protected TLS channel, like a normal TCP
247 Kerberos V5 exchange. In particular, this means that every Kerberos
248 V5 packet will be prefixed by a 4-octet length field, that indicate
249 the length of the Kerberos V5 packet.
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286 A complete packet flow for a successful AS-REQ/REP exchange protected
287 by this mechanism will be as follows. The "STARTTLS-bit" is a
288 4-octet value with only the bit allocated for this extension set.
292 [ Kerberos V5 TCP extension mechanism negotiation starts ]
294 [0x70000000 & STARTTLS-bit] -------->
298 [ TLS negotiation starts ]
301 ClientHello -------->
306 <-------- ServerHelloDone
315 [ Kerberos V5 negotiation starts ]
317 Kerberos V5 AS-REQ -------->
321 * Indicates optional or situation-dependent messages that are not
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340 4. STARTTLS aware KDC Discovery
342 Section 7.2.3 of Kerberos V5 [2] describe how Domain Name System
343 (DNS) SRV records [5] can be used to find the address of an KDC.
344 Using the terminology of Section 7.2.3 of RFC 4120, we define a new
345 Proto of "tls" to indicate that the particular KDC is intended to
346 support this STARTTLS extension. The Service, Realm, TTL, Class,
347 SRV, Priority, Weight, Port and Target have the same meaning as in
352 _kerberos._tls.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com.
353 _kerberos._tls.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com.
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396 5. IANA Considerations
398 The IANA is requested to allocate a bit in the "Kerberos TCP
399 Extensions" registry for the extension described in this document, as
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452 6. Security Considerations
454 The security considerations in Kerberos V5, TLS, and the extension
455 mechanism framework are inherited.
457 To protect against the inherent downgrade attack in the extension
458 framework, it is suggested that implementations offer a policy to
459 require that this extension is successfully negotiated. For
460 interoperability with implementations that do not support this
461 extension, it is suggested that the policy is disabled by default.
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510 7.1. Normative References
512 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
513 Levels", BCP 14, RFC 2119, March 1997.
515 [2] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos
516 Network Authentication Service (V5)", RFC 4120, July 2005.
518 [3] Josefsson, S., "Extended Kerberos Version 5 Key Distribution
519 Center (KDC) Exchanges Over TCP",
520 draft-ietf-krb-wg-tcp-expansion-01 (work in progress),
523 [4] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
524 Protocol Version 1.1", RFC 4346, April 2006.
526 [5] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
527 specifying the location of services (DNS SRV)", RFC 2782,
530 7.2. Informative References
532 [6] Taylor, D., "Using SRP for TLS Authentication",
533 draft-ietf-tls-srp-12 (work in progress), June 2006.
535 [7] Mavroyanopoulos, N., "Using OpenPGP keys for TLS
536 authentication", draft-ietf-tls-openpgp-keys-10 (work in
537 progress), June 2006.
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569 Email: simon@josefsson.org
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620 Full Copyright Statement
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