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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force S. Sorce, Ed. 3 Internet-Draft Red Hat 4 Updates: 4120 (if approved) T. Yu, Ed. 5 Intended status: Standards Track T. Hardjono, Ed. 6 Expires: August 5, 2013 MIT Kerberos Consortium 7 Feb 2013 9 Kerberos Authorization Data Container Authenticated by Multiple MACs 10 draft-ietf-krb-wg-cammac-04 12 Abstract 14 Abstract: This document specifies a Kerberos Authorization Data 15 container that supersedes AD-KDC-ISSUED. It allows for multiple 16 Message Authentication Codes (MACs) or signatures to authenticate the 17 contained Authorization Data elements. 19 Status of this Memo 21 This Internet-Draft is submitted in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF). Note that other groups may also distribute 26 working documents as Internet-Drafts. The list of current Internet- 27 Drafts is at http://datatracker.ietf.org/drafts/current/. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 This Internet-Draft will expire on August 5, 2013. 36 Copyright Notice 38 Copyright (c) 2013 IETF Trust and the persons identified as the 39 document authors. All rights reserved. 41 This document is subject to BCP 78 and the IETF Trust's Legal 42 Provisions Relating to IETF Documents 43 (http://trustee.ietf.org/license-info) in effect on the date of 44 publication of this document. Please review these documents 45 carefully, as they describe your rights and restrictions with respect 46 to this document. Code Components extracted from this document must 47 include Simplified BSD License text as described in Section 4.e of 48 the Trust Legal Provisions and are provided without warranty as 49 described in the Simplified BSD License. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 54 2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3 55 3. Validation . . . . . . . . . . . . . . . . . . . . . . . . . . 3 56 4. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 57 4.1. AD-CAMMAC . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 5. Assigned numbers . . . . . . . . . . . . . . . . . . . . . . . 6 59 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 60 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 61 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 62 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 63 9.1. Normative References . . . . . . . . . . . . . . . . . . . 7 64 9.2. Informative References . . . . . . . . . . . . . . . . . . 8 65 Appendix A. Additional Stuff . . . . . . . . . . . . . . . . . . . 8 66 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8 68 1. Introduction 70 This document specifies a new Authorization Data container for 71 Kerberos, called AD-CAMMAC (Container Authenticated by Multiple 72 MACs), that supersedes AD-KDC-ISSUED. The container allows both the 73 receiving application service and the Key Distribution Center (KDC) 74 itself to verify the authenticity of the contained authorization 75 data. The AD-CAMMAC container can also include additional verifiers 76 that "trusted services" can use to verify the contained authorization 77 data. 79 2. Requirements Language 81 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 82 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 83 document are to be interpreted as described in RFC 2119 [RFC2119]. 85 3. Validation 87 Kerberos ticket authorization data are highly sensitive and must be 88 validated to insure that no tampering has occurred. Although 89 authorization data are in the encrypted part of a Kerberos ticket and 90 therefore have their integrity protected by the ticket encryption, 91 clients can request that KDCs insert potentially arbitrary 92 authorization data into tickets on their behalf. The Kerberos 93 protocol specifications allow this client behavior because the 94 originally envisioned usage of authorization data was to serve as 95 restrictions on the client's privileges. Services that need to 96 interpret specific authorization data as granting increased 97 privileges need some way to ensure that the KDC originated those 98 authorization data. 100 In order to validate any information, the receiving application 101 service needs to be able to cryptographically verify the data. This 102 is done by introducing a new AuthorizationData element called AD- 103 CAMMAC that contains enough information to bind the contents to a 104 principal in a way that a receiving application service can verify 105 autonomously without further contact with the KDC. 107 The following information is needed: 109 o The KDC MAC 111 o The Service MAC 112 o Optional Trusted Service MAC 114 The KDC MAC is required to allow the KDC to validate the data without 115 needing to recompute the contents at every Ticket Granting Service 116 (TGS) request. 118 The Service MAC is required so that the Service can verify that the 119 authorization data has been validated by the KDC. 121 The Trusted Service MAC is useful to verify the authenticity of the 122 contents on the same host, when the data is received by a less 123 trusted service and passed to a more trusted service on the same host 124 without the need for additional round trips to the KDC. 126 The ad-type for AD-CAMMAC is (TBD). 128 4. Encoding 130 The Kerberos protocol is defined in [RFC4120] using Abstract Syntax 131 Notation One (ASN.1) [X.680][X.690]. As such, this specification 132 also uses the ASN.1 syntax for specifying both the abstract layout of 133 the AD-CAMMAC attributes, as well as its encoding. 135 4.1. AD-CAMMAC 137 KerberosV5CAMMAC DEFINITIONS EXPLICIT TAGS ::= BEGIN 139 AD-CAMMAC ::= SEQUENCE { 140 elements [0] AuthorizationData, 141 kdc-verifier [1] Verifier-MAC, 142 svc-verifier [2] Verifier-MAC OPTIONAL, 143 other-verifiers [3] SEQUENCE OF Verifier 144 } 146 Verifier ::= CHOICE { 147 mac Verifier-MAC, 148 ... 149 } 151 Verifier-MAC ::= SEQUENCE { 152 identifier [0] PrincipalName OPTIONAL, 153 kvno [1] UInt32, 154 enctype [2] Int32, 155 mac [3] Checksum 156 } 158 AD-CAMMAC-BINDING ::= SEQUENCE { 159 cname [0] PrincipalName, 160 authtime [1] KerberosTime, 161 endtime [2] KerberosTime 162 } 164 END 166 elements: 167 A sequence of authorization data elements issued by the KDC. 168 These elements are the authorization data that the verifier fields 169 authenticate. 171 Verifier: 172 A CHOICE type that currently contains only one alternative: 173 Verifier-MAC. Future extensions might add support for public-key 174 signatures. 176 Verifier-MAC: 177 Contains a MAC computed over the encoding of the AuthorizationData 178 value in the elements field of the AD-CAMMAC. The identifier, 179 kvno, and enctype fields help the recipient locate the key 180 required for verifying the MAC. 182 AD-CAMMAC-BINDING: 183 An AuthorizationData element that binds the CAMMAC contents to the 184 enclosing ticket. This AuthorizationData element has ad-type 185 number TBD, and MUST be absent from the transmitted elements field 186 of the AD-CAMMAC. It MUST be included in the computation of the 187 Verifiers as if it were the first element. 189 kdc-verifier: 190 A Verifier-MAC where the key is the TGS key. The checksum type is 191 the mandatory checksum type for the TGS key. 193 svc-verifier: 194 A Verifier-MAC where the key is the long-term key of the service 195 for which the ticket is issued. The checksum type is the 196 mandatory checksum type for the long-term key of the service. 197 This field MUST be present if the service principal of the ticket 198 is not the local TGS, including when the ticket is a cross-realm 199 TGT. 201 other-verifiers: 202 A sequence of additional verifiers. In each additional Verifier- 203 MAC, the key is the long-term key of the principal name specified 204 in the identifier field. The PrincipalName MUST be present and be 205 a valid principal in the realm. KDCs MAY add one or more 'trusted 206 service' verifiers. Unless otherwise administratively configured, 207 the 'trusted service' SHOULD be found by replacing the service 208 identifier component of the principal name of the svc-verifier 209 with 'host'. The checksum type is the mandatory checksum type for 210 the long-term key (which one?) of the principal. The key usage is 211 TBD. 213 5. Assigned numbers 215 TBD 217 6. IANA Considerations 219 TBD. 221 7. Security Considerations 223 Although authorization data are generally conveyed within the 224 encrypted part of a ticket and are thereby protected by the existing 225 encryption methods on the ticket, some authorization data requires 226 the additional protection provided by the CAMMAC. 228 Extracting a CAMMAC from a ticket for use as a credential removes it 229 from the context of the ticket. In the general case, this could turn 230 it into a bearer token, with all of the associated security 231 implications. Also, the CAMMAC does not itself necessarily contain 232 sufficient information to identify the client principal (if the 233 encoding of AD-CAMMAC-BINDING is omitted from the transmitted 234 CAMMAC). Therefore, application protocols that rely on extracted 235 CAMMACs might need to duplicate a substantial portion of the ticket 236 contents and include that duplicated information in the authorization 237 data contained within the CAMMAC. 239 A KDC that needs to verify the contents of a CAMMAC in a non-TGS 240 service ticket MUST ensure that the CAMMAC in the ticket is the same 241 one that it inserted into the ticket. A malicious service could 242 substitute legitimate CAMMACs from other tickets that it has received 243 (but not fabricate completely new CAMMACs) into a service ticket. A 244 CAMMAC by itself does not contain sufficient information to 245 accomplish this. 247 8. Acknowledgements 249 TBD. 251 9. References 253 9.1. Normative References 255 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for 256 Kerberos 5", RFC 3961, February 2005. 258 [RFC3962] Raeburn, K., "Advanced Encryption Standard (AES) 259 Encryption for Kerberos 5", RFC 3962, February 2005. 261 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The 262 Kerberos Network Authentication Service (V5)", RFC 4120, 263 July 2005. 265 [X.680] ISO, "Information technology -- Abstract Syntax Notation 266 One (ASN.1): Specification of basic notation -- ITU-T 267 Recommendation X.680 (ISO/IEC International Standard 8824- 268 1:2008)", 2008. 270 [X.690] ISO, "Information technology -- ASN.1 encoding rules: 272 Specification of Basic Encoding Rules (BER), Canonical 273 Encoding Rules (CER) and Distinguished Encoding Rules 274 (DER) -- ITU-T Recommendation X.690 (ISO/IEC International 275 Standard 8825-1:2008)", 1997. 277 9.2. Informative References 279 [MIT-Athena] 280 Steiner, J., Neuman, B., and J. Schiller, "Kerberos: An 281 Authentication Service for Open Network Systems. In 282 Proceedings of the Winter 1988 Usenix Conference. 283 February.", 1988. 285 [RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network 286 Authentication Service (V5)", RFC 1510, September 1993. 288 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 289 Requirement Levels", BCP 14, RFC 2119, March 1997. 291 [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC 292 Text on Security Considerations", BCP 72, RFC 3552, 293 July 2003. 295 Appendix A. Additional Stuff 297 This becomes an Appendix. 299 Authors' Addresses 301 Simo Sorce (editor) 302 Red Hat 304 Email: ssorce@redhat.com 306 Tom Yu (editor) 307 MIT Kerberos Consortium 309 Email: tlyu@mit.edu 311 Thomas Hardjono (editor) 312 MIT Kerberos Consortium 314 Email: hardjono@mit.edu