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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group S. Josefsson 3 Internet-Draft SJD AB 4 Intended status: Informational August 14, 2010 5 Expires: February 15, 2011 7 Using Kerberos V5 over the Transport Layer Security (TLS) protocol 8 draft-josefsson-kerberos5-starttls-09 10 Abstract 12 This document specify how the Kerberos V5 protocol can be transported 13 over the Transport Layer Security (TLS) protocol, to provide 14 additional security features. 16 Status of this Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at http://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on February 15, 2011. 33 Copyright Notice 35 Copyright (c) 2010 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (http://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 This document may contain material from IETF Documents or IETF 49 Contributions published or made publicly available before November 50 10, 2008. The person(s) controlling the copyright in some of this 51 material may not have granted the IETF Trust the right to allow 52 modifications of such material outside the IETF Standards Process. 53 Without obtaining an adequate license from the person(s) controlling 54 the copyright in such materials, this document may not be modified 55 outside the IETF Standards Process, and derivative works of it may 56 not be created outside the IETF Standards Process, except to format 57 it for publication as an RFC or to translate it into languages other 58 than English. 60 Table of Contents 62 1. Introduction and Background . . . . . . . . . . . . . . . . . 3 63 2. Kerberos V5 STARTTLS Extension . . . . . . . . . . . . . . . . 4 64 3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 65 4. STARTTLS aware KDC Discovery . . . . . . . . . . . . . . . . . 6 66 5. Server Certificates . . . . . . . . . . . . . . . . . . . . . 7 67 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 68 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 69 8. Security Considerations . . . . . . . . . . . . . . . . . . . 11 70 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 71 9.1. Normative References . . . . . . . . . . . . . . . . . . . 12 72 9.2. Informative References . . . . . . . . . . . . . . . . . . 12 73 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13 75 1. Introduction and Background 77 This document describe how a Kerberos V5 [RFC4120] implementation may 78 upgrade communication between clients and Key Distribution Centers 79 (KDCs) to use the Transport Layer Security (TLS) [RFC5246] protocol. 81 The TLS protocol offer integrity and privacy protected exchanges that 82 can be authentication using X.509 certificates, OpenPGP keys 83 [RFC5081], and user name and passwords via Secure Remote Password 84 (SRP) [RFC5054]. 86 There are several reasons to use Kerberos V5 over TLS. 88 o Prevents downgrade attacks affecting, e.g., encryption types and 89 pre-auth data negotiation. The encryption type field in KDC-REQ, 90 and the METHOD-DATA field with the requested pre-auth types from 91 the server in KDC_ERR_PREAUTH_REQUIRED errors in KDC-REP, are sent 92 without integrity or privacy protection in Kerberos 5. This 93 allows an active attacker to replace the encryption type with a 94 compromised encryption type, e.g., 56-bit DES, or request that 95 clients should use a broken pre-auth type. Since clients in 96 general cannot know the encryption types other servers support, or 97 the pre-auth types servers prefer or require, it is difficult for 98 the client to detect if there was a man-in-the-middle or if the 99 remote server simply did not support a stronger encryption type or 100 preferred another pre-auth type. 102 o Kerberos exchanges are privacy protected. Part of many Kerberos 103 packets are transferred without privacy protection (i.e., 104 encryption). That part contains information, such as the client 105 principal name, the server principal name, the encryption types 106 supported by the client, the lifetime of tickets, etc. Revealing 107 such information is, in some threat models, considered a problem. 109 o Additional authentication against the KDC. In some situations, 110 users are equipped with smart cards with a RSA authentication key. 111 In others, users have a OpenPGP client on their desktop, with a 112 public OpenPGP key known to the server. 114 o Explicit server authentication of the KDC to the client. In 115 traditional Kerberos 5, authentication of the KDC is proved as a 116 side effect that the KDC knows your encryption key (i.e., your 117 password). 119 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 120 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 121 document are to be interpreted as described in RFC 2119 [RFC2119]. 123 2. Kerberos V5 STARTTLS Extension 125 The STARTTLS extension uses the Kerberos V5 TCP extension mechanism 126 [RFC5021]. The extension uses bit #TBD in the extension bitmask. 128 The protocol is as follows. The client requests the extension by 129 setting the STARTTLS bit in the TCP extension mechanism bitmask. 130 (How to deal with extension negotiation failures at this point is 131 described in [RFC5021].) After the server has sent the 4-octet value 132 0x00000000 to indicate support of this extension, the stream will be 133 controlled by the TLS protocol and its framing. The TLS protocol is 134 initiated by the client. 136 Typically, the client initiate the TLS handshake protocol by sending 137 a client hello, and the server responds, and the handshake continues 138 until it either succeed or fails. 140 If for any reason the handshake fails, the STARTTLS protocol will 141 also fail, and the TLS error is used as the error indication. In 142 this case, no further messages can be exchanged over the same TCP 143 session. 145 If the handshake succeeds, the Kerberos V5 authentication protocol is 146 performed within the protected TLS channel, like a normal TCP 147 Kerberos V5 exchange. In particular, this means that every Kerberos 148 V5 packet will be prefixed by a 4-octet length field, that indicate 149 the length of the Kerberos V5 packet. 151 When no further Kerberos V5 messages needs to be transferred in the 152 TLS session, the TLS session MUST be shut down properly using the 153 close_notify alert. When the TLS session is shut down, the TCP 154 connection cannot be re-used to send any further data and MUST be 155 closed. 157 3. Examples 159 A complete packet flow for a successful AS-REQ/REP exchange protected 160 by this mechanism will be as follows. The "STARTTLS-bit" is a 161 4-octet value with only the bit allocated for this extension set, and 162 | is the binary OR operation. 164 Client Server 166 [ Kerberos V5 TCP extension mechanism negotiation starts ] 168 0x80000000 | STARTTLS-bit --------> 169 0x00000000 170 <-------- 172 [ TLS negotiation starts ] 174 ClientHello --------> 175 ServerHello 176 Certificate* 177 ServerKeyExchange* 178 CertificateRequest* 179 <-------- ServerHelloDone 180 Certificate* 181 ClientKeyExchange 182 CertificateVerify* 183 [ChangeCipherSpec] 184 Finished --------> 185 [ChangeCipherSpec] 186 <-------- Finished 188 [ Kerberos V5 negotiation starts ] 190 4 octet length field 191 Kerberos V5 AS-REQ --------> 192 4 octet length field 193 Kerberos V5 AS-REP 194 <-------- 196 * Indicates optional or situation-dependent messages that are not 197 always sent. 199 4. STARTTLS aware KDC Discovery 201 Section 7.2.3 of Kerberos V5 [RFC4120] describe how Domain Name 202 System (DNS) SRV records [RFC2782] can be used to find the address of 203 an KDC. We define a new Service of "kerberos-tls" to indicate that 204 the particular KDC is intended to support this STARTTLS extension. 205 The Proto (tcp), Realm, TTL, Class, SRV, Priority, Weight, Port and 206 Target have the same meaning as in RFC 4120. 208 For example: 210 _kerberos-tls._tcp.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com. 211 _kerberos-tls._tcp.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com. 213 5. Server Certificates 215 The TLS protocol may be used in a mode that provides server 216 authentication using, for example, X.509 and OpenPGP. 218 A goal for the protocol described in this memo is that it should be 219 as easy to implement and deploy on clients as support for UDP/TCP. 220 Since many client environments do not have access to long-term 221 storage, or to long-term storage that is sufficiently secure to 222 enable validation of server certificates, the Kerberos V5 STARTTLS 223 protocol does not require clients to verify server certificates. If 224 server certification had been required, then environments with 225 constrained clients such as those mentioned would be forced to 226 disable TLS; this would arguably be worse than TLS without server 227 certificate validation as use of TLS, even without server certificate 228 validation, protects against some attacks that Kerberos V5 over UDP/ 229 TCP do not. For example, even without server certificate validation, 230 TLS does protect against passive network sniffing aimed at tracking 231 Kerberos service usage by a given client. 233 Note however that use of TLS without server certificate verification 234 opens up for a range of active attacks such as man-in-the-middle. 236 When clients have the ability, they MUST validate the server 237 certificate. For this reason, if a KDC presents a X.509 server 238 certificate over TLS, it MUST contain an otherName Subject 239 Alternative Name (SAN) identified using a type-id of id-krb5starttls- 240 san. The intention is to bind the server certificate to the Kerberos 241 realm for the purpose of using Kerberos V5 STARTTLS. The value field 242 of the otherName should contain the realm as the "Realm" ASN.1 type. 244 id-krb5starttls-san OBJECT IDENTIFIER ::= 245 { iso(1) identified-organization(3) dod(6) internet(1) 246 private(4) enterprise(1) gnu(11591) 247 shishi(6) krb5starttls-san(1) } 249 To validate a server certificate, the client MAY use local 250 configuration (e.g., a list that maps the Kerberos realm to a copy of 251 the server's certificate) and compare that with the authentication 252 information provided from the server via TLS. For illustration, the 253 server certificate could be a X.509 certificate or an OpenPGP key. 254 In this mode, the client need no processing related to id- 255 krb5starttls-san. 257 When the server presents a X.509 server certificate, clients MAY use 258 "Certification Path Validation" as described in [RFC5280] to validate 259 the KDC server certificate. In addition, unless the client can 260 otherwise verify that the server certificate is bound to the KDC of 261 the target realm, the client MUST verify that the server certificate 262 contains the id-krb5starttls-san SAN and that the value is identical 263 to the intended Kerberos realm. 265 6. IANA Considerations 267 The IANA is requested to allocate a bit in the "Kerberos TCP 268 Extensions" registry for the extension described in this document, as 269 per [RFC5021]. 271 7. Acknowledgements 273 Miguel A. Garcia, Jeffrey Hutzelman, Sam Hartman, Magnus Nystroem, 274 and Peter Saint-Andre (in alphabetical order) provided comments that 275 improved the protocol and document. 277 8. Security Considerations 279 The security considerations in Kerberos V5, TLS, and the Kerberos V5 280 TCP extension mechanism are inherited. 282 Note that TLS does not protect against Man-In-The-Middle (MITM) 283 attacks unless clients verify the KDC's credentials (X.509 284 certificate, OpenPGP key, etc) correctly. Although certificate 285 validation adds an extra layer of protection, that is not considered 286 strictly necessary to improve the security profile of Kerberos V5 as 287 outlined in this document. 289 If server authentication is used, some information about the server 290 (such as its name) is visible to passive attackers. 292 To protect against the inherent downgrade attack in the extension 293 framework, implementations SHOULD offer a policy mode that requires 294 this extension to always be successfully negotiated, for a particular 295 realm, or generally. For interoperability with implementations that 296 do not support this extension, the policy mode SHOULD be disabled by 297 default. 299 9. References 301 9.1. Normative References 303 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 304 Requirement Levels", BCP 14, RFC 2119, March 1997. 306 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 307 specifying the location of services (DNS SRV)", RFC 2782, 308 February 2000. 310 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The 311 Kerberos Network Authentication Service (V5)", RFC 4120, 312 July 2005. 314 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 315 (TLS) Protocol Version 1.2", RFC 5246, August 2008. 317 [RFC5021] Josefsson, S., "Extended Kerberos Version 5 Key 318 Distribution Center (KDC) Exchanges over TCP", RFC 5021, 319 August 2007. 321 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 322 Housley, R., and W. Polk, "Internet X.509 Public Key 323 Infrastructure Certificate and Certificate Revocation List 324 (CRL) Profile", RFC 5280, May 2008. 326 9.2. Informative References 328 [RFC5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin, 329 "Using the Secure Remote Password (SRP) Protocol for TLS 330 Authentication", RFC 5054, November 2007. 332 [RFC5081] Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport 333 Layer Security (TLS) Authentication", RFC 5081, 334 November 2007. 336 Author's Address 338 Simon Josefsson 339 Simon Josefsson Datakonsult AB 340 Hagagatan 24 341 Stockholm 113 47 342 Sweden 344 Email: simon@josefsson.org 345 URI: http://josefsson.org/