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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 KERBEROS WORKING GROUP Johansson 3 Internet-Draft SUNET 4 Intended status: Standards Track July 29, 2012 5 Expires: January 30, 2013 7 An information model for Kerberos version 5 8 draft-ietf-krb-wg-kdc-model-13 10 Abstract 12 This document describes an information model for Kerberos version 5 13 from the point of view of an administrative service. There is no 14 standard for administrating a kerberos 5 KDC. This document 15 describes the services exposed by an administrative interface to a 16 KDC. 18 Status of this Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on January 30, 2013. 35 Copyright Notice 37 Copyright (c) 2012 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 This document may contain material from IETF Documents or IETF 51 Contributions published or made publicly available before November 52 10, 2008. The person(s) controlling the copyright in some of this 53 material may not have granted the IETF Trust the right to allow 54 modifications of such material outside the IETF Standards Process. 55 Without obtaining an adequate license from the person(s) controlling 56 the copyright in such materials, this document may not be modified 57 outside the IETF Standards Process, and derivative works of it may 58 not be created outside the IETF Standards Process, except to format 59 it for publication as an RFC or to translate it into languages other 60 than English. 62 Table of Contents 64 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 65 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4 66 3. Information model demarcation . . . . . . . . . . . . . . . . 5 67 4. Information model specification . . . . . . . . . . . . . . . 6 68 4.1. Principal . . . . . . . . . . . . . . . . . . . . . . . . 6 69 4.1.1. Principal: Attributes . . . . . . . . . . . . . . . . 6 70 4.1.2. Principal: Associations . . . . . . . . . . . . . . . 7 71 4.2. KeySet . . . . . . . . . . . . . . . . . . . . . . . . . . 8 72 4.2.1. KeySet: Attributes . . . . . . . . . . . . . . . . . . 8 73 4.2.2. KeySet: Associations . . . . . . . . . . . . . . . . . 8 74 4.3. Key . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 75 4.3.1. Key: Attributes . . . . . . . . . . . . . . . . . . . 9 76 4.3.2. Key: Associations . . . . . . . . . . . . . . . . . . 9 77 4.3.3. Key: Remarks . . . . . . . . . . . . . . . . . . . . . 10 78 4.4. Policy . . . . . . . . . . . . . . . . . . . . . . . . . . 10 79 4.4.1. Policy: Attributes . . . . . . . . . . . . . . . . . . 10 80 4.4.2. Mandatory-to-implement Policy . . . . . . . . . . . . 11 81 5. Implementation Scenarios . . . . . . . . . . . . . . . . . . . 12 82 5.1. LDAP backend to KDC . . . . . . . . . . . . . . . . . . . 12 83 5.2. LDAP frontend to KDC . . . . . . . . . . . . . . . . . . . 12 84 5.3. SOAP . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 85 5.4. Netconf . . . . . . . . . . . . . . . . . . . . . . . . . 12 86 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 87 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 88 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 89 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 90 9.1. Normative References . . . . . . . . . . . . . . . . . . . 16 91 9.2. Informative References . . . . . . . . . . . . . . . . . . 16 92 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17 94 1. Introduction 96 The Kerberos version 5 authentication service described in [RFC4120] 97 describes how a Key Distribution Center (KDC) provides authentication 98 to clients. The standard does not stipulate how a KDC is managed and 99 several "kadmin" servers have evolved. This document describes the 100 services required to administer a KDC and the underlying information 101 model assumed by a kadmin-type service. 103 The information model is written in terms of "attributes" and 104 "services" or "interfaces" but the use of these particular words must 105 not be taken to imply any particular modeling paradigm. Neither an 106 object oriented model nor an LDAP [RFC4510] schema is intended. The 107 author has attempted to describe in natural language the intended 108 semantics and syntax of the components of the model. An LDAP schema 109 (for instance) based on this model will be more precise in the 110 expression of the syntax while preserving the semantics of this 111 model. 113 Implementations of this document MAY decide to change the names used 114 (e.g. principalName). If so an implementation MUST provide a name to 115 name mapping to this document. In particular schema languages may 116 have different conventions for caseing, eg camelCase vs use of '_' 117 and '-' to separate 'words' in a name. Implementations MUST call out 118 such conventions explicitly. 120 Implementations of this document MUST be able to support default 121 values for attributes as well as the ability to specify syntax for 122 attribute values. 124 2. Requirements notation 126 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 127 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 128 document are to be interpreted as described in [RFC2119]. 130 This document describes an information model for kerberos 5 but does 131 not directly describe any mapping onto a particular schema- or 132 modelling language. Hence an implementation of this model consists 133 of a mapping to such a language - e.g. an LDAP or SQL schema. The 134 precise interpretation of terms from [RFC2119] therefore require some 135 extra explanation. 137 The terms MUST or REQUIRED means that schema implementing this model 138 must have a way to represent the feature (i.e that it is mandatory to 139 implement) but that unless otherwise specified the feature may 140 represent an optional element in the chosen schema definition 141 language. 143 However MUST also means that a KDC or administrative interface 144 implementing this information model MUST provide the feature and 145 associated behavior consistent with schema. 147 For instance, principalLastFailedAuthentication (cf below) represents 148 the last time an authentication failed for a principal. In an LDAP 149 schema (for instance) this may be represented as an optional 150 attribute even though all KDCs implementing this specification must 151 support this attribute. 153 The terms MAY or OPTIONAL means that the feature is optional to 154 implement by a KDC or administrative interface implementing this 155 information model. MAY also means that the feature is optional to 156 implement in schema. 158 Implementors of schema should be aware that unless there is a way to 159 represent critical but optional elements in the schema definition 160 language confusion may arise when optional elements are used but not 161 understood by all implementations in a particular deployment. 163 3. Information model demarcation 165 The information model specified in the next chapter describes 166 objects, properties of those objects and relations between those 167 objects. These elements comprise an abstract view of the data 168 represented in a KDC. It is important to understand that the 169 information model is not a schema. In particular the way objects are 170 compared for equality beyond that which is implied by the 171 specification of a syntax is not part of this specification. Nor is 172 ordering specified between elements of a particular syntax. 174 Further work on Kerberos will undoubtedly prompt updates to this 175 information model to reflect changes in the functions performed by 176 the KDC. Such extensions to the information model should always use 177 a normative reference to the relevant RFCs detailing the change in 178 KDC function. 180 This model describes a number of elements related to password policy 181 management. Not all of the elements in this model are unique to 182 Kerberos; an LDAP implementation of this model should incorporate 183 existing LDAP schema where functional overlap exists, rather than 184 defining additional Kerberos-specific elements. 186 4. Information model specification 188 4.1. Principal 190 The fundamental entity stored in a KDC is the principal. The 191 Principal is associated to keys and generalizes the "user" concept. 192 The Principal MUST be implemented in full and MUST NOT be OPTIONAL in 193 an implementation 195 4.1.1. Principal: Attributes 197 4.1.1.1. principalName 199 The principalName MUST uniquely identify the Principal within the 200 administrative context of the KDC. The principalName MUST be 201 equivalent to the string representation of the Principal name 202 (section 2.1.1 of [RFC1964]) including, if applicable for the name 203 type, the realm. 205 The attribute MAY be multi-valued if the implementation supports 206 aliases and/or enterprise names. In that case exactly one of the 207 principalName values MAY be designated the canonical principalName 208 and if the implementation supports enctypes which require salt then 209 exactly one of the values of principalName MAY be designated as the 210 canonical salting principalName. 212 Implementations (i.e. schema) that support enterprise names and/or 213 aliases SHOULD provide for efficient lookup of Principal objects 214 based on alias/enterprise name. 216 4.1.1.2. principalNotUsedBefore 218 The Principal MUST not be used before this date. The syntax of the 219 attribute MUST be Internet Date/Time Format from [RFC3339]. The 220 attribute MUST be single-valued. 222 4.1.1.3. principalNotUsedAfter 224 The Principal MUST not be used after this date. The syntax of the 225 attribute MUST be Internet Date/Time Format from [RFC3339]. The 226 attribute MUST be single-valued. 228 4.1.1.4. principalIsDisabled 230 A boolean attribute used to disable a Principal. The attribute 231 SHOULD default to boolean FALSE. 233 4.1.1.5. principalLastCredentialChangeTime 235 This single-valued attribute contains the time of the last successful 236 change of credential (e.g. password or private key) associated with 237 this Principal. The syntax of the attribute MUST be Internet Date/ 238 Time Format from [RFC3339]. 240 4.1.1.6. principalCreateTime 242 This single-valued attribute contains the time and date when this 243 Principal was created. The syntax of the attribute MUST be Internet 244 Date/Time Format from [RFC3339]. 246 4.1.1.7. principalModifyTime 248 This single-valued attribute contains the time and date when this 249 Principal was last modified excluding credentials change. The syntax 250 of the attribute MUST be Internet Date/Time Format from [RFC3339]. 252 4.1.1.8. principalMaximumTicketLifetime 254 This single-valued attribute contains the time in seconds 255 representing the maximum lifetime for tickets issued for this 256 Principal. 258 4.1.1.9. principalMaximumRenewableTicketLifetime 260 This single-valued attribute contains the delta time in seconds 261 representing the maximum amount of time a ticket may be renewed for 262 this Principal. 264 4.1.1.10. principalAllowedEnctype 266 This OPTIONAL multi-valued attribute lists the enctypes allowed for 267 this principal. If empty or absent any enctype supported by the 268 implementation is allowed for this Principal. 270 This attribute is intended as a policy attribute and restricts all 271 uses of enctypes including server, client, and session keys. Data 272 models MAY choose to use policy objects in order to represent more 273 complex decision mechanisms. 275 4.1.2. Principal: Associations 277 Each Principal MAY be associated with 0 or more KeySet and MAY be 278 associated with 0 or more Policies. The KeySet is represented as an 279 object in this model since it has attributes associated with it (the 280 key version number). In typical situations the Principal is 281 associated with exactly 1 KeySet but implementations MUST NOT assume 282 this case, i.e. an implementation of this standard MUST be able to 283 handle the general case of multiple KeySet associated with each 284 principal. Multiple KeySets may for instance be useful when 285 performing a key rollover for a principal. 287 4.2. KeySet 289 In Kerberos principals are associated with zero or more symmetric 290 secret keys, and each key has a key version number (kvno) and 291 enctype. In this model we group keys by kvno into KeySet objects. A 292 Principal can have zero or more KeySet objects associated with it, 293 each of which MUST have one or more keys. Each KeySet is associated 294 with exactly one principal. Schemas derived from this model MAY lack 295 a direct analogue of KeySet as described in this document. 297 It is expected that most Kerberos implementations will use a special- 298 purpose interface for setting and changing Principal passwords and 299 keys. 301 If a server supports an enctype for a Principal that enctype must be 302 present in at least one key for the Principal in question. For any 303 given enctype a KeySet MUST NOT contain more than one Key with that 304 enctype. 306 The security of Kerberos 5 depends absolutely on the confidentiality 307 and integrity of the Key objects stored in the KDC. Implementations 308 of this standard MUST facilitate, to the extent possible, an 309 administrator's ability to place more restrictive access controls on 310 KeySets than on other Principal data, and to arrange for more secure 311 backup for KeySets. 313 4.2.1. KeySet: Attributes 315 4.2.1.1. kvno 317 Also knowns as the key version number. This is a single-valued 318 attribute containing a non-negative integer. This number is 319 incremembed by one each time a key in the KeySet is changed. 321 4.2.2. KeySet: Associations 323 To each KeySet MUST be associated a set of 1 or more Keys. 325 4.3. Key 327 Implementations of this model MUST NOT REQUIRE keys to be 328 represented. 330 4.3.1. Key: Attributes 332 4.3.1.1. keyEncryptionType 334 The enctype SHOULD be represented as an enumeration of the enctypes 335 supported by the KDC using the string name ("encryption type") of the 336 enctype from the IANA registry of Kerberos Encryption Type Numbers. 337 One example is 'aes128-cts-hmac-sha1-96'. 339 4.3.1.2. keyValue 341 The binary representation of the key data. This MUST be a single- 342 valued octet string. 344 4.3.1.3. keySaltValue 346 The binary representation of the key salt. This MUST be a single- 347 valued octet string. 349 4.3.1.4. keyStringToKeyParameter 351 This MUST be a single-valued octet string representing an opaque 352 parameter associated with the enctype. This parameter is specified 353 in the "string-to-key" method in section 3 of [RFC3961]. 355 4.3.1.5. keyNotUsedBefore 357 This key MUST NOT be used before this date. The syntax of the 358 attribute MUST be semantically equivalent with the standard ISO date 359 format. This MUST be a single-valued attribute. 361 4.3.1.6. keyNotUsedAfter 363 This key MUST NOT be used after this date. The syntax of the 364 attribute MUST be semantically equivalent with the standard ISO date 365 format. This MUST be a single-valued attribute. 367 4.3.1.7. keyIsDisabled 369 This is a boolean attribute which SHOULD be set to false by default. 370 If this attribute is true the key MUST NOT be used. This is used to 371 temporarily disable a key. 373 4.3.2. Key: Associations 375 None 377 4.3.3. Key: Remarks 379 The security of the keys is an absolute requirement for the operation 380 of Kerberos 5. If keys are implemented adequate protection from 381 unauthorized modification and disclosure MUST be available and 382 REQUIRED by the implementation. 384 4.4. Policy 386 Implementations SHOULD implement policy but MAY allow them to be 387 OPTIONAL. The Policy should be thought of as a 'typed hole'. i.e. an 388 opaque binary value paired with an identifier of type of data 389 contained in the binary value. Both attributes (type and value) must 390 be present. 392 4.4.1. Policy: Attributes 394 4.4.1.1. policyIdentifier 396 The policyIdentifier MUST be globally unique. Possible types of 397 identifiers include: 399 An Object Identifier (OID) [RFC1778] 401 A URI [RFC3986] 403 A UUID [RFC4122] 405 The use of OIDs is RECOMMENDED for this purpose. 407 4.4.1.2. policyIsCritical 409 This boolean attribute indicates that the KDC MUST be able to 410 correctly interpret and apply this policy for the Principal to be 411 used. 413 4.4.1.3. policyContent 415 This is an optional single opaque binary value used to store a 416 representation of the policy. In general a policy cannot be fully 417 expressed using attribute-value pairs. The policyContent is OPTIONAL 418 in the sense that an implementation MAY use it to store an opaque 419 value for those policy-types which are not directly representable in 420 that implementation. 422 4.4.1.4. policyUse 424 This is an optional single enumerated string value used to describe 425 the use of the policy. Implementations SHOULD provide this attribute 426 and MUST (if the attribute is implemented) describe the enumerated 427 set of possible values. The intent is that this attribute be useful 428 in providing an initial context-based filtering. 430 4.4.2. Mandatory-to-implement Policy 432 All implementations that represent Policy objects MUST be able to 433 represent the policies listed in this section. Implementations are 434 not required to use the same underlying data-representation for the 435 policyContent binary value but SHOULD use the same OIDs as the 436 policyIdentifier. In general the expression of policy may require a 437 Turing-complete language. This specification does not attempt to 438 model policy expression language. 440 4.4.2.1. Password Quality Policy 442 Password quality policy controls the requirements placed by the KDC 443 on new passwords. This policy SHOULD be identified by the OID 444 .1. 446 4.4.2.2. Password Management Policy 448 Password management policy controls how passwords are changed. This 449 policy SHOULD be identified by the OID .2. 451 4.4.2.3. Keying Policy 453 A keying policy specifies the association of enctypes with new 454 principals, e.g. when a Principal is created one of the applicable 455 keying policies is used to determine the set of keys to associate 456 with the principal. This policy SHOULD be identified by the OID 457 .3. 459 4.4.2.4. Ticket Flag Policy 461 A ticket flag policy specifies the ticket flags allowed for tickets 462 issued for a principal. This policy SHOULD be identified by the OID 463 .4. 465 5. Implementation Scenarios 467 There are several ways to implement an administrative service for 468 Kerberos 5 based on this information model. In this section we list 469 a few of them. 471 5.1. LDAP backend to KDC 473 Given an LDAP schema implementation of this information model it 474 would be possible to build an administrative service by back-ending 475 the KDC to a directory server where principals and keys are stored. 476 Using the security mechanisms available on the directory server keys 477 are protected from access by anyone apart from the KDC. 478 Administration of the principals, policy, and other non-key data is 479 done through the directory server while the keys are modified using 480 the set/change password protocol 481 [I-D.ietf-krb-wg-kerberos-set-passwd]. 483 5.2. LDAP frontend to KDC 485 An alternative way to provide a directory interface to the KDC is to 486 implement an LDAP-frontend to the KDC which exposes all non-key 487 objects as entries and attributes. As in the example above all keys 488 are modified using the set/change password protocol 489 [I-D.ietf-krb-wg-kerberos-set-passwd]. In this scenario the 490 implementation would typically not use a traditional LDAP 491 implementation but treat LDAP as an access protocol to data in the 492 native KDC database. 494 5.3. SOAP 496 Given an XML schema implementation of this information model it would 497 be possible to build a SOAP interface to the KDC. This demonstrates 498 the value of creating an abstract information model which is mappable 499 to multiple schema representations. 501 5.4. Netconf 503 Given a YAML implementation of this information model it would be 504 possible to create a Netconf-based interface to the KDC, enabling 505 management of the KDC from standard network management applications. 507 6. Security Considerations 509 This document describes an abstract information model for Kerberos 5. 510 The Kerberos 5 protocol depends on the security of the keys stored in 511 the KDC. The model described here assumes that keys MUST NOT be 512 transported in the clear over the network and furthermore that keys 513 are treated as write-only attributes that SHALL only be modified 514 (using the administrative interface) by the change-password protocol 515 [I-D.ietf-krb-wg-kerberos-set-passwd]. 517 Exposing the object model of a KDC typically implies that objects can 518 be modified and/or deleted. In a KDC not all principals are created 519 equal, so that for instance deleting krbtgt/EXAMPLE.COM@EXAMPLE.COM 520 effectively disables the EXAMPLE.COM realm. Hence access control is 521 paramount to the security of any implementation. This document does 522 not mandate access control. This only implies that access control is 523 beyond the scope of the standard information model, i.e. that access 524 control may not be accessible via any protocol based on this model. 525 If access control objects are exposed via an extension to this model 526 the presence of access control may in itself provide points of attack 527 by giving away information about principals with elevated rights etc. 529 7. IANA Considerations 531 This document requires the allocation of several OIDs marked in 532 section 4.4.2 above. IANA should allocate a new arc under 533 1.3.6.1.5.2.5 (iso.org.dod.internet.security.kerberosV5.policies) 534 named "kdcPolicy" and assign each of the policy OIDs a new number 535 under this arc. 537 8. Acknowledgments 539 The author wishes to extend his thanks to Love Hoernquist-Aestrand 540 and Sam Hartman for their important 541 contributions to this document. 543 9. References 545 9.1. Normative References 547 [RFC1778] Howes, T., Kille, S., Yeong, W., and C. Robbins, "The 548 String Representation of Standard Attribute Syntaxes", 549 RFC 1778, March 1995. 551 [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", 552 RFC 1964, June 1996. 554 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 555 Requirement Levels", BCP 14, RFC 2119, March 1997. 557 [RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the 558 Internet: Timestamps", RFC 3339, July 2002. 560 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for 561 Kerberos 5", RFC 3961, February 2005. 563 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 564 Resource Identifier (URI): Generic Syntax", STD 66, 565 RFC 3986, January 2005. 567 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The 568 Kerberos Network Authentication Service (V5)", RFC 4120, 569 July 2005. 571 [RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally 572 Unique IDentifier (UUID) URN Namespace", RFC 4122, 573 July 2005. 575 9.2. Informative References 577 [I-D.ietf-krb-wg-kerberos-set-passwd] 578 Williams, N., "Kerberos Set/Change Key/Password Protocol 579 Version 2", draft-ietf-krb-wg-kerberos-set-passwd-08 (work 580 in progress), November 2008. 582 [RFC4510] Zeilenga, K., "Lightweight Directory Access Protocol 583 (LDAP): Technical Specification Road Map", RFC 4510, 584 June 2006. 586 Author's Address 588 Leif Johansson 589 Swedish University Network 590 Thulegatan 11 591 Stockholm 593 Email: leifj@sunet.se 594 URI: http://www.sunet.se