INTERNET-DRAFT Tom Yu draft-yu-krb-wg-kerberos-extensions-00.txt MIT Expires: 09 August 2004 09 February 2004 The Kerberos Network Authentication Service (Version 5) Status of This Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document describes version 5 of the Kerberos network authentication protocol. It describes changes to the protocol which allow for extensions to be made to the protocol without creating interoperability problems. [ This document is a VERY rough draft. Many sections are not yet fully filled out. The main purpose is to illustrate the beginnings of a new document structure as a starting point. ] Key Words for Requirements The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", and "MAY" in this document are to be interpreted as described in RFC 2119. Yu Expires: Aug 2004 [Page 1] Internet-Draft yu-krb-extensions-00 09 Feb 2004 Table of Contents Status of This Memo ....................................... 1 Copyright Notice .......................................... 1 Abstract .................................................. 1 Key Words for Requirements ................................ 1 Table of Contents ......................................... 2 1. Introduction .......................................... 4 1.1. Kerberos Protocol Overview .......................... 4 1.2. Overview of Document ................................ 5 2. Extensibility ......................................... 5 3. Criticality ........................................... 6 4. Use of ASN.1 .......................................... 6 4.1. Module Header ....................................... 6 4.2. Top-Level Type ...................................... 7 4.3. Parameterized Types ................................. 7 4.4. Constraints ......................................... 8 4.5. New Types ........................................... 8 5. Basic Types ........................................... 8 5.1. Constrained Integer Types ........................... 8 5.2. KerberosTime ........................................ 9 5.3. KerberosString ...................................... 9 6. Principals ............................................ 10 6.1. Name Types .......................................... 10 6.2. Principal Name Reuse ................................ 11 7. Types Relating to Encryption .......................... 11 7.1. EncryptedData ....................................... 11 7.2. EncryptionKey ....................................... 13 7.3. Checksums ........................................... 13 7.3.1. ChecksumOf ........................................ 14 7.3.2. Signed ............................................ 15 8. Tickets ............................................... 15 8.1. Timestamps .......................................... 16 8.2. Ticket Flags ........................................ 16 8.2.1. Flags Relating to Initial Ticket Acquisition ...... 17 8.2.2. Invalid Tickets ................................... 17 8.2.3. OK as Delegate .................................... 18 8.3. Renewable Tickets ................................... 18 8.4. Postdated Tickets ................................... 19 8.5. Proxiable and Proxy Tickets ......................... 20 8.6. Forwardable Tickets ................................. 21 8.7. Transited Realms .................................... 21 8.8. Authorization Data .................................. 21 8.9. Encrypted Part of Ticket ............................ 21 8.10. Cleartext Part of Ticket ........................... 22 9. Credential Acquisition ................................ 23 9.1. KDC-REQ ............................................. 24 9.2. PA-DATA ............................................. 26 9.3. KDC-REQ Processing .................................. 26 9.3.1. Handling Replays .................................. 26 9.3.2. Request Validation ................................ 26 Yu Expires: Aug 2004 [Page 2] Internet-Draft yu-krb-extensions-00 09 Feb 2004 9.3.2.1. AS-REQ Authentication ........................... 27 9.3.2.2. TGS-REQ Authentication .......................... 27 9.3.2.3. Principal Validation ............................ 27 9.3.3. Timestamp Handling ................................ 27 9.3.3.1. AS-REQ Timestamp Processing ..................... 28 9.3.3.2. TGS-REQ Timestamp Processing .................... 29 9.3.4. Key Selection ..................................... 29 9.3.5. Checking For Revoked Tickets ...................... 30 9.4. Reply Validation .................................... 30 10. Application Authentication ........................... 30 11. Session Key Use ...................................... 30 11.1. KRB-SAFE ........................................... 30 11.2. KRB-PRIV ........................................... 30 11.3. KRB-CRED ........................................... 30 12. Security Considerations .............................. 30 12.1. Time Synchronization ............................... 30 12.2. Replays ............................................ 30 12.3. Principal Name Reuse ............................... 30 12.4. Password Guessing .................................. 30 12.5. Forward Secrecy .................................... 30 12.6. Authorization ...................................... 31 12.7. Login Authentication ............................... 31 Appendices ................................................ 31 A. ASN.1 Module (Normative) .............................. 31 B. Kerberos and Character Encodings (Informative) ........ 60 C. Kerberos History (Informative) ........................ 62 Normative References ...................................... 62 Informative References .................................... 63 Acknowledgments ........................................... 63 Author's Address .......................................... 63 Full Copyright Statement .................................. 63 Yu Expires: Aug 2004 [Page 3] Internet-Draft yu-krb-extensions-00 09 Feb 2004 1. Introduction The Kerberos network authentication protocol is a trusted third-party protocol utilizing symmetric-key cryptography. It assumes that all communications between parties in the protocol may be arbitrarily tampered with or monitored, and that the security of the overall system depends only on the effectiveness of the cryptographic techniques and the secrecy of the keys used. The protocol authenticates an application client's identity to an application server, and likewise authenticates the application server's identity to the application client. These assurances are made possible by the client and the server sharing secrets with the trusted third party: the Kerberos server, also known as the Key Distribution Center (KDC). In addition, the protocol establishes an ephemeral shared secret (the session key) between the client and the server, allowing the protection of further communications between them. 1.1. Kerberos Protocol Overview Kerberos comprises three main sub-protocols: credentials acquisition, application authentication, and session key usage. A client acquires credentials by asking the for KDC a credential for a service; the KDC issues the credential, consisting of a ticket and a session key. The ticket, containing the client's identity, timestamps, expiration time, and a session key, is a encrypted in a key known to the application server. The KDC encrypts the credential using a key known to the client, and transmits the credential to the client. There are two means of requesting credentials: the Authentication Service (AS) exchange, and the Ticket-Granting Service (TGS) exchange. The AS exchange typically involves a client using a password-derived key to decrypt the response. The TGS exchange involves the KDC behaving as an application, which the client authenticates to using a Ticket-Granting Ticket (TGT). The client usually obtains the TGT by using the AS exchange. Application authentication consists of the client establishing the session key with the application server by transmitting the ticket to the application server, along with an authenticator. The authenticator contains a timestamp and additional data encrypted using the ticket's session key. The application server decrypts the ticket, extracting the session key. The application server then uses the session key to decrypt the authenticator. Upon successful decryption of the authenticator, the application server knows that the data in the authenticator were sent by the client named in the associated ticket. Additionally, since authenticators expire more quickly than tickets, the application server has some assurance that the transaction is not a replay. The application server may send an encrypted acknowledgment to the client, verifying its identity to the client. Yu Expires: Aug 2004 [Page 4] Internet-Draft yu-krb-extensions-00 09 Feb 2004 Once application authentication has occurred, the client and server may use the established session key to protect further traffic. This protection may consist of protection of integrity only, or of protection of confidentiality and integrity. Additional measures exist for a client to forward credentials to a server. The entire scheme depends on loosely synchronized clocks. Synchronization of the clock on the KDC with the application server clock allows the application server to accurately determine whether a credential is expired. Likewise, synchronization of the clock on the client with the application server clock prevents replay attacks utilizing the same credential. Careful design of the application protocol may allow replay prevention without requiring client-server clock synchronization. Following the establishment of a session key between the application client and the application server, the Kerberos protocol provides messages that use the session key to protect the integrity or confidentiality of communications between the client and the server. Additionally, the client may forward credentials to the application server. The credentials acquisition protocol takes place over specific, defined transports (UDP and TCP). Application protocols define which transport to use for the session key establishment protocol and for messages using the session key; the application may choose to perform its own encapsulation of the Kerberos messages, for example. 1.2. Overview of Document The remainder of this document begins by describing the general frameworks for protocol extensibility, including whether to interpret unknown extensions as critical. It then defines the protocol messages and exchanges. The definition of the Kerberos protocol uses Abstract Syntax Notation One (ASN.1) [X680], which specifies notation for describing the abstract content of protocol messages. This document defines a number of base types using ASN.1; these base types subsequently appear in multiple types which define actual protocol messages. Definitions of principal names and of tickets, which are central to the protocol, also appear preceding the protocol message definitions. 2. Extensibility As originally defined in [RFC1510], the Kerberos protocol does not readily allow for backwards-compatible extensions to the protocol. Various proposals to extend the Kerberos protocol have appeared since RFC 1510, many of them creating problems for backwards compatibility. This document adopts the technique of creating new extensible types which encode to messages which are very similar to RFC 1510 messages Yu Expires: Aug 2004 [Page 5] Internet-Draft yu-krb-extensions-00 09 Feb 2004 on the wire. This similarity allows implementors to use shared code paths for encoding and decoding both new and old messages. The protocol defined in RFC 1510 already contains some elements allowing for limited backwards-compatible extensions to the protocol. Most of these elements consist of "typed holes"; these are octet strings whose contents have types defined by an assigned number. This document adds a number of typed holes to types which have previously lacked typed holes. This document also describes procedures for the IETF to use the extensibility model of ASN.1 make further backwards-compatible extensions of the Kerberos protocol, if typed holes prove inadequate for some desired extension. 3. Criticality In general, implementations SHOULD treat unknown extension, flags, etc. as non-critical; i.e., they should ignore them when they do not understand them. Exceptions are clearly marked. An implementation SHOULD NOT reject a request merely because it does not understand some element of the request. As a related consequence, implementations SHOULD handle talking to other implementations which do not implement some requested options. This may require designers of extensions or options to provide means detect whether extensions or options are rejected, or whether such extensions or options are merely not understood, or (perhaps maliciously) deleted in transit. 4. Use of ASN.1 Kerberos uses the ASN.1 Distinguished Encoding Rules (DER) [X690]. Even though ASN.1 theoretically allows the description of protocol messages to be independent of the encoding rules used to encode the messages, Kerberos messages MUST be encoded with DER. Subtleties in the semantics of the notation, such as whether tags carry any semantic content to the application, may cause the use of other ASN.1 encoding rules to be problematic. Implementors not using existing ASN.1 compilers or support libraries are cautioned to thoroughly read and understand the actual ASN.1 specification to ensure correct implementation behavior. There is more complexity in the notation than is immediately obvious, and some tutorials and guides to ASN.1 are misleading or erroneous. 4.1. Module Header The type definitions in this section assume an ASN.1 module definition of the following form: Yu Expires: Aug 2004 [Page 6] Internet-Draft yu-krb-extensions-00 09 Feb 2004 KerberosV5Spec3 { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) modules(4) krb5spec3(4) } DEFINITIONS EXPLICIT TAGS ::= BEGIN -- Rest of definitions here END This specifies that the tagging context for the module will be explicit and that automatic tagging is not done. Some other publications [RFC1510] [RFC1964] erroneously specify an object identifier (OID) having an incorrect value of "5" for the "dod" component of the OID. In the case of RFC 1964, use of the "correct" OID value would result in a change in the wire protocol; therefore, the RFC 1964 OID remains unchanged for now. 4.2. Top-Level Type The ASN.1 type "KRB-PDU" is a CHOICE over all the types (Protocol Data Units or PDUs) which an application may directly reference. Applications SHOULD NOT transmit any types other than those which are alternatives of the KRB-PDU CHOICE. -- top-level type -- -- Applications should not directly reference any types -- other than KRB-PDU and its component types. -- KRB-PDU ::= CHOICE { ticket Ticket, as-req AS-REQ, as-rep AS-REP, tgs-req TGS-REQ, tgs-rep TGS-REP, ap-req AP-REQ, ap-rep AP-REP, krb-safe KRB-SAFE, krb-priv KRB-PRIV, krb-cred KRB-CRED, tgt-req TGT-REQ, krb-error KRB-ERROR, ... } 4.3. Parameterized Types This document uses ASN.1 parameterized types [X683] to make definitions of types more readable. For some types, some or all of Yu Expires: Aug 2004 [Page 7] Internet-Draft yu-krb-extensions-00 09 Feb 2004 the parameters are advisory, i.e., they are not encoded in any form for transmission in a protocol message. These advisory parameters can describe implementation behavior associated with the type. 4.4. Constraints This document uses ASN.1 constraints, including the "UserDefinedConstraint" syntax [X682]. Some constraints can be handled automatically by tools that can parse them. Uses of the "UserDefinedConstraint" syntax (the "CONSTRAINED BY" syntax) will typically need to have behavior manually coded; these uses provide a formalized way of conveying intended implementation behavior. 4.5. New Types This document defines a number of new ASN.1 types. The names of these types will typically have a suffix like "Ext", indicating that the types are intended to support extensibility. Types original to RFC 1510 have been renamed to have a suffix like "1510". The "Ext" and "1510" types often contain a number of common elements; these common elements have a suffix like "Common". The "1510" types have various ASN.1 constraints applied to them; the constraints limit the possible values of the "1510" types to those permitted by RFC 1510 or by [KCLAR]. The "Ext" types may have different constraints, typically to force string values to be sent as UTF-8. 5. Basic Types Certain ASN.1 types in Kerberos appear in numerous other types. 5.1. Constrained Integer Types In [RFC1510], many types contained references to the unconstrained INTEGER type. Since an unconstrained INTEGER may contain any possible abstract integer value, most of the unconstrained references to INTEGER in [RFC1510] have been constrained to 32 bits or smaller. -- signed values representable in 32 bits -- -- These are often used as assigned numbers for various things. Int32 ::= INTEGER (-2147483648..2147483647) -- unsigned 32 bit values UInt32 ::= INTEGER (0..4294967295) The "Int32" type often contains an assigned number identifying the type of a protocol element. Unless otherwise stated, non-negative values are registered, and negative values are available for local use. Yu Expires: Aug 2004 [Page 8] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- microseconds Microseconds ::= INTEGER (0..999999) -- sequence numbers -- -- We may want to increase this to 2**64 and define a UInt64 -- type. SeqNum ::= UInt32 -- nonces -- -- Likewise, we may want to make this UInt64. Nonce ::= UInt32 While these types have different abstract types from their equivalents in [RFC1510], their DER encodings remain identical. 5.2. KerberosTime -- must not have fractional seconds KerberosTime ::= GeneralizedTime The timestamps used in Kerberos are encoded as GeneralizedTimes. A KerberosTime value MUST NOT include any fractional portions of the seconds. As required by the DER, it further MUST NOT include any separators, and it specifies the UTC time zone (Z). Example: The only valid format for UTC time 6 minutes, 27 seconds after 9 pm on 6 November 1985 is "19851106210627Z". 5.3. KerberosString -- used for names and for error messages KerberosString ::= CHOICE { ia5 GeneralString (IA5String), utf8 UTF8String, ... -- no extension may be sent -- to an rfc1510 implementation -- } The KerberosString type is used for strings in various places in the Kerberos protocol. For compatibility with RFC 1510, GeneralString values constrained to IA5String (US-ASCII) are permitted in messages exchanged with RFC 1510 implementations. The new protocol messages contain strings encoded as UTF-8. KerberosString values are present in principal names and in error messages. Control characters SHOULD NOT be used in principal names, and used with caution in error messages. Yu Expires: Aug 2004 [Page 9] Internet-Draft yu-krb-extensions-00 09 Feb 2004 For detailed background regarding the history of character string use in Kerberos, as well as discussion of some compatibility issues, see Appendix B. 6. Principals Principals are participants in the Kerberos protocol. A "realm" consists of principals in one administrative domain, served by one KDC (or one replicated set of KDCs). Each principal name has an arbitrary number of components, though typical principal names will only have one or two components. A principal name is meant to be readable by and meaningful to humans, especially in a realm lacking a centrally adminstered authorization infrastructure. Realm ::= KerberosString PrincipalName ::= SEQUENCE { name-type [0] NameType, -- May have zero elements, or individual elements may be -- zero-length, but this is not recommended. name-string [1] SEQUENCE OF KerberosString } -- assigned numbers for name types (used in principal names) NameType ::= Int32 Kerberos realm names are KerberosStrings. Realms MUST NOT contain a character with the code 0 (the US-ASCII NUL). Most realms will usually consist of several components separated by periods (.), in the style of Internet Domain Names, or separated by slashes (/) in the style of X.500 names. name-type Specifies the type of name that follows. The name-type SHOULD be treated as a hint. Ignoring the name type, no two names can be the same (i.e., at least one of the components, or the realm, must be different). name-string Encodes a sequence of components that form a name, each component encoded as a KerberosString. Taken together, a PrincipalName and a Realm form a principal identifier. Most PrincipalNames will have only a few components (typically one or two). 6.1. Name Types Yu Expires: Aug 2004 [Page 10] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- Name type not known nt-unknown NameType ::= 0 -- Just the name of the principal as in DCE, or for users nt-principal NameType ::= 1 -- Service and other unique instance (krbtgt) nt-srv-inst NameType ::= 2 -- Service with host name as instance (telnet, rcommands) nt-srv-hst NameType ::= 3 -- Service with host as remaining components nt-srv-xhst NameType ::= 4 -- Unique ID nt-uid NameType ::= 5 -- Encoded X.509 Distingished name [RFC 2253] nt-x500-principal NameType ::= 6 -- Name in form of SMTP email name (e.g. user@foo.com) nt-smtp-name NameType ::= 7 -- Enterprise name - may be mapped to principal name nt-enterprise NameType ::= 10 6.2. Principal Name Reuse Realm administrators SHOULD use extreme caution when considering reusing a principal name. A service administrator might explicitly enter principal names into a local access control list (ACL) for the service. If such local ACLs exist independently of a centrally administered authorization infrastructure, realm administrators SHOULD NOT reuse principal names until confirming that all extant ACL entries referencing that principal name have been updated. Failure to perform this check can result in a security vulnerability, as a new principal can inadvertently inherit unauthorized privileges upon receiving a reused principal name. An organization whose Kerberos- authenticated services all use a centrally-adminstered authorization infrastructure may not need to take these precautions regarding principal name reuse. 7. Types Relating to Encryption Many Kerberos protocol messages contain encryptions of various data types. Kerberos protocol messages also contain checksums (signatures) of various types. 7.1. EncryptedData The "EncryptedData" type contains the encryption of another data type. The recipient uses fields within EncryptedData to determine which key to use for decryption. Yu Expires: Aug 2004 [Page 11] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- "Type" specifies which ASN.1 type is encrypted to the -- ciphertext in the EncryptedData. "Keys" specifies a set of -- keys of which one key may be used to encrypt the data. -- "KeyUsages" specifies a set of key usages, one of which may -- be used to encrypt. -- -- None of the parameters is transmitted over the wire. EncryptedData { Type, KeyToUse:Keys, KeyUsage:KeyUsages } ::= SEQUENCE { etype [0] EType, kvno [1] UInt32 OPTIONAL, cipher [2] OCTET STRING (CONSTRAINED BY { -- must be encryption of -- OCTET STRING (CONTAINING Type), -- with one of the keys -- KeyToUse:Keys, -- with key usage being one of -- KeyUsage:KeyUsages }), ... } -- Assigned numbers denoting encryption mechanisms. EType ::= Int32 -- Assigned numbers denoting key usages. KeyUsage ::= UInt32 EType Integer type for assigned numbers for encryption algorithms. Defined in [KCRYPTO] KeyUsage Integer type for assigned numbers for key usages. Key usage values are inputs to the encryption and decryption functions described in [KCRYPTO]. Type Advisory parameter indicating the ASN.1 type whose DER encoding is the plaintext encrypted into the EncryptedData. Keys Advisory parameter indicating which key to use to perform the encryption. If "Keys" indicate multiple "KeyToUse" values, the detailed description of the containing message will indicate which key to use under which conditions. Yu Expires: Aug 2004 [Page 12] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- KeyToUse identifies which key is to be used to encrypt or -- sign a given value. -- -- Values of KeyToUse are never actually transmitted over the -- wire, or even used directly by the implementation in any -- way, as key usages are; it exists primarily to identify -- which key gets used for what purpose. Thus, the specific -- numeric values associated with this type are irrelevant. KeyToUse ::= ENUMERATED { -- unspecified key-unspecified, -- server long term key key-server, -- client long term key key-client, -- key selected by KDC for encryption of a KDC-REP key-kdc-rep, -- session key from ticket key-session, -- subsession key negotiated via AP-REQ/AP-REP key-subsession, ... } KeyUsages Advisory parameter indicating which "KeyUsage" value is used to encrypt. If "KeyUsages" indicates multiple "KeyUsage" values, the detailed description of the containing message will indicate which key usage to use under which conditions. 7.2. EncryptionKey The "EncryptionKey" type holds an encryption key. EncryptionKey ::= SEQUENCE { keytype [0] EType, keyvalue [1] OCTET STRING } keytype This "EType" identifies the encryption algorithm, described in [KCRYPTO]. keyvalue Contains the actual key. 7.3. Checksums Yu Expires: Aug 2004 [Page 13] Internet-Draft yu-krb-extensions-00 09 Feb 2004 Several types contain checksums (actually signatures) of data. CksumType ::= Int32 -- The parameters specify which key to use to produce the -- signature, as well as which key usage to use. The -- parameters are not actually sent over the wire. Checksum { KeyToUse:Keys, KeyUsage:KeyUsages } ::= SEQUENCE { cksumtype [0] CksumType, checksum [1] OCTET STRING (CONSTRAINED BY { -- signed using one of the keys -- KeyToUse:Keys, -- with key usage being one of -- KeyUsage:KeyUsages }) } CksumType Integer type for assigned numbers for signature algorithms. Defined in [KCRYPTO] Keys As in EncryptedData KeyUsages As in EncryptedData cksumtype Signature algorithm used to produce the signature. checksum The actual checksum. 7.3.1. ChecksumOf ChecksumOf is like "Checksum", but specifies which type is signed. -- a Checksum that must contain the checksum of a particular type ChecksumOf { Type, KeyToUse:Keys, KeyUsage:KeyUsages } ::= Checksum { Keys, KeyUsages } (WITH COMPONENTS { ..., checksum (CONSTRAINED BY { -- must be checksum of -- OCTET STRING (CONTAINING Type) }) }) Yu Expires: Aug 2004 [Page 14] Internet-Draft yu-krb-extensions-00 09 Feb 2004 Type Indicates the ASN.1 type whose DER encoding is signed. 7.3.2. Signed Signed is like "ChecksumOf", but contains an actual instance of the type being signed in addition to the signature. -- parameterized type for wrapping authenticated plaintext Signed { InnerType, KeyToUse:Keys, KeyUsage:KeyUsages } ::= SEQUENCE { cksum [0] ChecksumOf { InnerType, Keys, KeyUsages } OPTIONAL, inner [1] InnerType, ... } 8. Tickets The important fields of a ticket are in the encrypted part. The components in common between the RFC 1510 and the extensible versions are EncTicketPartCommon ::= SEQUENCE { flags [0] TicketFlags, key [1] EncryptionKey, crealm [2] Realm, cname [3] PrincipalName, transited [4] TransitedEncoding, authtime [5] KerberosTime, starttime [6] KerberosTime OPTIONAL, endtime [7] KerberosTime, renew-till [8] KerberosTime OPTIONAL, caddr [9] HostAddresses OPTIONAL, authorization-data [10] AuthorizationData OPTIONAL, ... } crealm This field contains the client's realm. cname This field contains the client's name. caddr This field lists the network addresses (if absent, all addresses are permitted) from which the ticket is valid. Yu Expires: Aug 2004 [Page 15] Internet-Draft yu-krb-extensions-00 09 Feb 2004 Descriptions of the other fields appear in the following sections. 8.1. Timestamps Three of the ticket timestamps may be requested from the KDC. The timestamps may differ from those requested, depending on site policy. authtime The time at which the client authenticated, as recorded by the KDC. starttime The earliest time when the ticket is valid. If not present, the ticket is valid starting at the authtime. This is requested as the "from" field of KDC-REQ-BODY-COMMON. endtime This time is requested in the "till" field of KDC-REQ-BODY- COMMON. Contains the time after which the ticket will not be honored (its expiration time). Note that individual services MAY place their own limits on the life of a ticket and MAY reject tickets which have not yet expired. As such, this is really an upper bound on the expiration time for the ticket. renew-till This time is requested in the "rtime" field of KDC-REQ-BODY- COMMON. It is only present in tickets that have the "renewable" flag set in the flags field. It indicates the maximum endtime that may be included in a renewal. It can be thought of as the absolute expiration time for the ticket, including all renewals. 8.2. Ticket Flags A number of flags may be set in the ticket, further defining some of its capabilities. Some of these flags map to flags in a KDC request. Yu Expires: Aug 2004 [Page 16] Internet-Draft yu-krb-extensions-00 09 Feb 2004 TicketFlags ::= KerberosFlags { TicketFlagsBits } TicketFlagsBits ::= BIT STRING { reserved (0), forwardable (1), forwarded (2), proxiable (3), proxy (4), may-postdate (5), postdated (6), invalid (7), renewable (8), initial (9), pre-authent (10), hw-authent (11), transited-policy-checked (12), ok-as-delegate (13), anonymous (14), cksummed-ticket (15) } 8.2.1. Flags Relating to Initial Ticket Acquisition [ adapted KCLAR 2.1. ] Several flags indicate the details of how the initial ticket was acquired. initial The "initial" flag indicates that a ticket was issued using the AS protocol, rather than issued based on a ticket-granting ticket. Application servers (e.g., a password-changing program) requiring a client's definite knowledge of its secret keycan insist that this flag be set in any tickets they accept, and thus be assured that the client's key was recently presented to the application client. pre-authent The "pre-authent" flag indicates that some sort of pre- authentication was used during the AS exchange. hw-authent The "hw-authent" flag indicates that some sort of hardware-based pre-authentication occurred during the AS exchange. Both the "pre-authent" and the "hw-authent" flags may be present with or without the "initial" flag; such tickets with the "initial" flag clear are ones which are derived from initial tickets with the "pre- authent" or "hw-authent" flags set. Yu Expires: Aug 2004 [Page 17] Internet-Draft yu-krb-extensions-00 09 Feb 2004 8.2.2. Invalid Tickets [ KCLAR 2.2. ] The "invalid" flag indicates that a ticket is invalid. Application servers MUST reject tickets which have this flag set. A postdated ticket will be issued in this form. Invalid tickets MUST be validated by the KDC before use, by presenting them to the KDC in a TGS request with the "validate" option specified. The KDC will only validate tickets after their starttime has passed. The validation is required so that postdated tickets which have been stolen before their starttime can be rendered permanently invalid (through a hot- list mechanism -- see Section 9.3.5). 8.2.3. OK as Delegate [ KCLAR 2.8. ] For some applications a client may need to delegate authority to a server to act on its behalf in contacting other services. This requires that the client forward credentials to an intermediate server. The ability for a client to obtain a service ticket to a server conveys no information to the client about whether the server should be trusted to accept delegated credentials. The "ok-as- delegate" flag provides a way for a KDC to communicate local realm policy to a client regarding whether an intermediate server is trusted to accept such credentials. The copy of the ticket flags visible to the client may have the "ok- as-delegate" flag set to indicate to the client that the server specified in the ticket has been determined by policy of the realm to be a suitable recipient of delegation. A client can use the presence of this flag to help it make a decision whether to delegate credentials (either grant a proxy or a forwarded ticket-granting ticket) to this server. It is acceptable to ignore the value of this flag. When setting this flag, an administrator should consider the security and placement of the server on which the service will run, as well as whether the service requires the use of delegated credentials. 8.3. Renewable Tickets [ adapted KCLAR 2.3. ] Renewable tickets can be used to mitigate the consequences of ticket theft. Applications may desire to hold tickets which can be valid for long periods of time. However, this can expose their credentials to potential theft for equally long periods, and those stolen credentials would be valid until the expiration time of the ticket(s). Simply using short-lived tickets and obtaining new ones periodically would require the client to have long-term access to its Yu Expires: Aug 2004 [Page 18] Internet-Draft yu-krb-extensions-00 09 Feb 2004 secret key, an even greater risk. Renewable tickets have two "expiration times": the first is when the current instance of the ticket expires, and the second is the latest permissible value for an individual expiration time. An application client must periodically (i.e., before it expires) present a renewable ticket to the KDC, with the "renew" option set in the KDC request. The KDC will issue a new ticket with a new session key and a later expiration time. All other fields of the ticket are left unmodified by the renewal process. When the latest permissible expiration time arrives, the ticket expires permanently. At each renewal, the KDC MAY consult a hot-list to determine if the ticket had been reported stolen since its last renewal; it will refuse to renew such stolen tickets, and thus the usable lifetime of stolen tickets is reduced. The "renewable" flag in a ticket is normally only interpreted by the ticket-granting service. It can usually be ignored by application servers. However, some particularly careful application servers MAY disallow renewable tickets. If a renewable ticket is not renewed by its expiration time, the KDC will not renew the ticket. The "renewable" flag is clear by default, but a client can request it be set by setting the "renewable" option in the AS-REQ message. If it is set, then the "renew-till" field in the ticket contains the time after which the ticket may not be renewed. 8.4. Postdated Tickets [ KCLAR 2.4. ] Applications may occasionally need to obtain tickets for use much later, e.g., a batch submission system would need tickets to be valid at the time the batch job is serviced. However, it is dangerous to hold valid tickets in a batch queue, since they will be on-line longer and more prone to theft. Postdated tickets provide a way to obtain these tickets from the KDC at job submission time, but to leave them "dormant" until they are activated and validated by a further request of the KDC. If a ticket theft were reported in the interim, the KDC would refuse to validate the ticket, and the thief would be foiled. The "may-postdate" flag in a ticket is normally only interpreted by the TGS. It can be ignored by application servers. This flag MUST be set in a ticket-granting ticket in order for the KDC to issue a postdated ticket based on the presented ticket. It is reset by default; it MAY be requested by a client by setting the "allow- postdate" option in the AS-REQ [?also TGS-REQ?] message. This flag does not allow a client to obtain a postdated ticket-granting ticket; postdated ticket-granting tickets can only by obtained by requesting Yu Expires: Aug 2004 [Page 19] Internet-Draft yu-krb-extensions-00 09 Feb 2004 the postdating in the AS-REQ message. The life (endtime-starttime) of a postdated ticket will be the remaining life of the ticket- granting ticket at the time of the request, unless the "renewable" option is also set, in which case it can be the full life (endtime- starttime) of the ticket-granting ticket. The KDC MAY limit how far in the future a ticket may be postdated. The "postdated" flag indicates that a ticket has been postdated. The application server can check the authtime field in the ticket to see when the original authentication occurred. Some services MAY choose to reject postdated tickets, or they may only accept them within a certain period after the original authentication. When the KDC issues a "postdated" ticket, it will also be marked as "invalid", so that the application client MUST present the ticket to the KDC for validation before use. 8.5. Proxiable and Proxy Tickets [ KCLAR 2.5. ] At times it may be necessary for a principal to allow a service to perform an operation on its behalf. The service must be able to take on the identity of the client, but only for a particular purpose. A principal can allow a service to take on the principal's identity for a particular purpose by granting it a proxy. The process of granting a proxy using the "proxy" and "proxiable" flags is used to provide credentials for use with specific services. Though conceptually also a proxy, users wishing to delegate their identity in a form usable for all purposes MUST use the ticket forwarding mechanism described in the next section to forward a ticket-granting ticket. The "proxiable" flag in a ticket is normally only interpreted by the ticket-granting service. It can be ignored by application servers. When set, this flag tells the ticket-granting server that it is OK to issue a new ticket (but not a ticket-granting ticket) with a different network address based on this ticket. This flag is set if requested by the client on initial authentication. By default, the client will request that it be set when requesting a ticket-granting ticket, and reset when requesting any other ticket. This flag allows a client to pass a proxy to a server to perform a remote request on its behalf (e.g. a print service client can give the print server a proxy to access the client's files on a particular file server in order to satisfy a print request). In order to complicate the use of stolen credentials, Kerberos tickets may contain a set of network addresses from which they are valid. When granting a proxy, the client MUST specify the new network address from which the proxy is to be used, or indicate that the Yu Expires: Aug 2004 [Page 20] Internet-Draft yu-krb-extensions-00 09 Feb 2004 proxy is to be issued for use from any address. The "proxy" flag is set in a ticket by the TGS when it issues a proxy ticket. Application servers MAY check this flag and at their option they MAY require additional authentication from the agent presenting the proxy in order to provide an audit trail. 8.6. Forwardable Tickets [ KCLAR 2.6. ] 8.7. Transited Realms [ KCLAR 2.7., plus new stuff ] 8.8. Authorization Data 8.9. Encrypted Part of Ticket The complete definition of the encrypted part is -- Encrypted part of ticket EncTicketPart ::= CHOICE { rfc1510 [APPLICATION 3] EncTicketPart1510, ext [APPLICATION 5] EncTicketPartExt } EncTicketPart1510 ::= SEQUENCE { -- effectively drops the extension marker COMPONENTS OF EncTicketPartCommon } (WITH COMPONENTS { ..., -- explicitly force IA5 in strings crealm (WITH COMPONENTS { ia5 PRESENT }), cname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 PRESENT })) }) }) Yu Expires: Aug 2004 [Page 21] Internet-Draft yu-krb-extensions-00 09 Feb 2004 EncTicketPartExt ::= EncTicketPartCommon (WITH COMPONENTS { ..., -- explicitly force UTF-8 in strings crealm (WITH COMPONENTS { ia5 ABSENT }), cname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 ABSENT })) }), -- explicitly constrain caddr to be non-empty if present caddr (SIZE (1..MAX)), -- explicitly constrain authorization-data to be non-empty -- if present authorization-data (SIZE (1..MAX)) }) 8.10. Cleartext Part of Ticket Ticket ::= CHOICE { rfc1510 [APPLICATION 1] Ticket1510, ext [APPLICATION 4] Signed { TicketExt, { key-server }, { ku-Ticket-cksum } } } -- takes a parameter specifying which type gets encrypted TicketCommon { EncPart } ::= SEQUENCE { tkt-vno [0] INTEGER (5), realm [1] Realm, sname [2] PrincipalName, enc-part [3] EncryptedData { EncPart, { key-server }, { ku-Ticket } }, extensions [4] TicketExtensions OPTIONAL, ... } Yu Expires: Aug 2004 [Page 22] Internet-Draft yu-krb-extensions-00 09 Feb 2004 Ticket1510 ::= SEQUENCE { -- "COMPONENTS OF" drops the extension marker from -- TicketCommon COMPONENTS OF TicketCommon { EncTicketPart1510 } } (WITH COMPONENTS { ..., -- explicitly force IA5 in strings realm (WITH COMPONENTS { ia5 PRESENT }), sname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 PRESENT })) }), extensions ABSENT }) -- APPLICATION tag goes inside Signed{} as well as outside, -- to prevent possible substitution attacks. TicketExt ::= [APPLICATION 4] TicketCommon (WITH COMPONENTS { ..., -- explicitly force UTF-8 in strings realm (WITH COMPONENTS { ia5 ABSENT }), sname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 ABSENT })) }) }) TEType ::= Int32 TICKETEXTENSION ::= TYPEDHOLE { TEType } -- ticket extensions: for TicketExt only TicketExtensions ::= SEQUENCE (SIZE (1..MAX)) OF SEQUENCE { te-type [0] TICKETEXTENSION.&id ({TicketExtension-Set}), te-data [1] OCTET STRING (TICKETEXTENSION.&Type) ({TicketExtension-Set}{@te-type}) } -- no mandatory ticket extensions currently TicketExtensionSet TICKETEXTENSION ::= { ... } 9. Credential Acquisition Yu Expires: Aug 2004 [Page 23] Internet-Draft yu-krb-extensions-00 09 Feb 2004 There are two exchanges that can be used for acquiring credentials: the AS exchange and the TGS exchange. These exchanges have many similarities, and this document describes them in parallel, noting which behaviors are specific to one type of exchange. The AS request (AS-REQ) and TGS request (TGS-REQ) are both forms of KDC requests (KDC-REQ). Likewise, the AS reply (AS-REP) and TGS reply (TGS-REP) are forms of KDC replies (KDC-REP). 9.1. KDC-REQ The KDC-REQ has a large number of fields in common between the RFC 1510 and the extensible versions. KDC-REQ-COMMON ::= SEQUENCE { -- NOTE: first tag is [1], not [0] pvno [1] INTEGER (5), msg-type [2] INTEGER (10 -- AS-REQ.rfc1510 -- | 12 -- TGS-REQ.rfc1510 -- | 6 -- AS-REQ.ext -- | 8 -- TGS-REQ.ext -- ), padata [3] SEQUENCE OF PA-DATA OPTIONAL -- NOTE: not empty } Yu Expires: Aug 2004 [Page 24] Internet-Draft yu-krb-extensions-00 09 Feb 2004 KDC-REQ-BODY-COMMON ::= SEQUENCE { kdc-options [0] KDCOptions, cname [1] PrincipalName OPTIONAL -- Used only in AS-REQ --, realm [2] Realm -- Server's realm; also client's in AS-REQ --, sname [3] PrincipalName OPTIONAL, from [4] KerberosTime OPTIONAL, till [5] KerberosTime OPTIONAL -- was required in rfc1510; -- still required for compat versions -- of messages --, rtime [6] KerberosTime OPTIONAL, nonce [7] Nonce, etype [8] SEQUENCE OF EType -- in preference order --, addresses [9] HostAddresses OPTIONAL, enc-authorization-data [10] EncryptedData { AuthorizationData, { key-session | key-subsession }, { ku-TGSReqAuthData-subkey | ku-TGSReqAuthData-sesskey } } OPTIONAL, additional-tickets [11] SEQUENCE OF Ticket OPTIONAL -- NOTE: not empty --, ... } Many fields of KDC-REQ-BODY-COMMON correspond directly to fields of an EncTicketPartCommon. The KDC copies most of them unchanged, provided that their values meet site policy. kdc-options These flags do not correspond directly to "flags" in EncTicketPartCommon. [ insert mapping table here ] cname This field is copied to the "cname" field in EncTicketPartCommon. The "cname" field is required in an AS- REQ; the client places its own name here. In a TGS-REQ, the "cname" in the ticket in the AP-REQ takes precedence. realm This field is the server's realm, and also holds the client's realm in an AS-REQ. Yu Expires: Aug 2004 [Page 25] Internet-Draft yu-krb-extensions-00 09 Feb 2004 The "from", "till", and "rtime" fields correspond to the "starttime", "endtime", and "renew-till" fields of EncTicketPartCommon. addresses This field corresponds to the "caddr" field of EncTicketPartCommon. enc-authorization-data For TGS-REQ, this field contains authorization data encrypted using either the TGT session key or the AP-REQ subsession key; these may be copied into the "authorization-data" field of EncTicketPartCommon if policy permits. 9.2. PA-DATA PA-DATA have multiple uses in the Kerberos protocol. They may pre- authenticate an AS-REQ; they may also modify several of the encryption keys used in a KDC-REP. PA-DATA may also provide "hints" to the client about which long-term key (usually password-derived) to use. PA-DATA may also include "hints" about which pre-authentication mechanisms to use, or include data for input into a pre- authentication mechanism. 9.3. KDC-REQ Processing Processing of a KDC-REQ proceeds through several steps. An implementation need not perform these steps exactly as described, as long as the resulting behavior is as if the steps were performed as described. The KDC performs replay handling on receipt of the request; it then validates the request, adjusts timestamps, and selects the keys used in the reply. It copies data from the request into the issued ticket, adjusting for policy. The KDC then transmits the reply to the client. 9.3.1. Handling Replays Because Kerberos can run over unreliable transports such as UDP, the KDC MUST be prepared to retransmit responses in case they are lost. If a KDC receives a request identical to one it has recently successfully processed, the KDC MUST respond with an KDC-REP message rather than a replay error. In order to reduce the amount of ciphertext given to a potential attacker, KDCs MAY send the same response generated when the request was first handled. KDCs MUST obey this replay behavior even if the actual transport in use is reliable. If the AP-REQ which authenticates a TGS-REQ is a replay, and the entire request is not identical to a recently successfully processed request, the KDC SHOULD return "krb-ap-err-repeat", as is appropriate for AP-REQ processing. Yu Expires: Aug 2004 [Page 26] Internet-Draft yu-krb-extensions-00 09 Feb 2004 9.3.2. Request Validation 9.3.2.1. AS-REQ Authentication Site policy determines whether a given client principal is required to provide some pre-authentication prior to receiving an AS-REP. Since the default reply key is typically the client's long-term password-based key, an attacker may easily request known plaintext (in the form of an AS-REP) upon which to mount a dictionary attack. Pre-authentication can limit the possibility of such an attack. If site policy requires pre-authentication for a client principal, and no pre-authentication is provided, the KDC returns the error "kdc-err-preauth-required". Accompanying this error are "e-data" which include hints telling the client which pre-authentication mechanisms to use, or data for input to pre-authentication mechanisms (e.g., input to challenge-response systems). If pre-authentication fails, the KDC returns the error "kdc-err-preauth-failed". [ may need additional changes based on Sam's preauth draft ] 9.3.2.2. TGS-REQ Authentication A TGS-REQ has an accompanying AP-REQ, which is included in the "pa- tgs-req". The KDC MUST validate the checksum in the Authenticator of the AP-REQ, which is computed over the KDC-REQ-BODY-1510 or KDC-REQ- BODY-EXT (for TGS-REQ-1510 or TGS-REQ-EXT, respectively) of the request. [ padata not signed by authenticator! ] Any error from the AP-REQ validation process SHOULD be returned in a KRB-ERROR message. The service principal in the ticket of the AP-REQ may be a ticket- granting service principal, or a normal application service principal. An AP-REQ ticket which is not a ticket-granting ticket MUST NOT be used to issue a ticket for any service other than the one named in the ticket. In this case, the "renew", "validate", or "proxy" [?also forwarded?] option must be set in the request. 9.3.2.3. Principal Validation If the client principal in an AS-REQ is unknown, the KDC returns the error "kdc-err-c-principal-unknown". If the server principal is unknown, the KDC returns the error "kdc-err-c-principal-unknown". 9.3.3. Timestamp Handling [ some aspects of timestamp handling, especially regarding postdating and renewal, are difficult to read in KCLAR... needs closer examination here ] For the AS exchange, the "authtime" of a ticket is set to the local time at the KDC. For the TGS exchange, the KDC sets the "authtime" to that of the ticket in the AP-REQ authenticating the TGS-REQ. Yu Expires: Aug 2004 [Page 27] Internet-Draft yu-krb-extensions-00 09 Feb 2004 [?application server can spoof the authtime. security issues for hot-list?] [ MIT implementation may change authtime of renewed tickets; needs check... ] Processing of "starttime" (requested in the "from" field) differs depending on whether the "postdated" option is set in the request. If the "postdated" option is not set, and the requested "starttime" is in the future beyond the window of acceptable clock skew, the KDC returns the error "kdc-err-cannot-postdate". If the "postdated" option is not set, and the requested "starttime" is absent or does not indicate a time in the future beyond the acceptable clock skew, the KDC sets the "starttime" to the KDC's current time. The "postdated" option MUST NOT be honored if the ticket is being requested by TGS-REQ and the "may-postdate" is not set in the TGT. Otherwise, if the "postdated" option is set, and site policy permits, the KDC sets the "starttime" as requested, and sets the "invalid" flag in the new ticket. 9.3.3.1. AS-REQ Timestamp Processing The "endtime" of the ticket will be set to the earlier of the requested "till" time and a time determined by local policy, possibly determined using factors specific to the realm or principal. For example, the expiration time MAY be set to the earliest of the following: * The expiration time (till) requested. * The ticket's start time plus the maximum allowable lifetime associated with the client principal from the authentication server's database. * The ticket's start time plus the maximum allowable lifetime associated with the server principal. * The ticket's start time plus the maximum lifetime set by the policy of the local realm. If the requested expiration time minus the start time (as determined above) is less than a site-determined minimum lifetime, an error message with code "kdc-err-never-valid" is returned. If the requested expiration time for the ticket exceeds what was determined as above, and if the "renewable-ok" option was requested, then the "renewable" flag is set in the new ticket, and the "renew-till" value is set as if the "renewable" option were requested. If the "renewable" option has been requested or if the "renewable-ok" option has been set and a renewable ticket is to be issued, then the renew-till field MAY be set to the earliest of: Yu Expires: Aug 2004 [Page 28] Internet-Draft yu-krb-extensions-00 09 Feb 2004 * Its requested value. * The start time of the ticket plus the minimum of the two maximum renewable lifetimes associated with the principals' database entries. * The start time of the ticket plus the maximum renewable lifetime set by the policy of the local realm. 9.3.3.2. TGS-REQ Timestamp Processing If the TGS-REQ has the TGT in its AP-REQ, and the TGS-REQ requests an "endtime" (in the "till" field), then the "endtime" of the new ticket is set to the minimum of (a) the requested "endtime" value, (b) the "endtime" in the TGT, and (c) an "endtime" determined by site policy on ticket lifetimes. If the new ticket is a renewal, the "endtime" of the new ticket is bounded by (a) the requested "endtime" value, (b) the value of the "renew-till" value of the old, and (c) the "starttime" of the new ticket plus the life (endtime - starttime) of the old ticket. [ the previous sentence is a bit confusing; adapted from KCLAR 3.3.3. ] When handling a TGS-REQ, a KDC MUST NOT issue a postdated ticket with a "starttime", "endtime", or "renew-till" time later than the "renew- till" time of the TGT. 9.3.4. Key Selection Three keys are involved in creating a KDC-REP. The reply key is used to encrypt the encrypted part of the KDC-REP. The session key is stored in the encrypted part of the ticket, and is also present in the part of the reply which is visible to the client. The ticket key is used to encrypt the ticket. These keys all have initial values for a given exchange; pre-authentication and other extension mechanisms may change the value used for any of these keys. [ again, may need changes based on Sam's preauth draft ] The set of encryption types the client will understand appears in the "etype" field of KDC-REQ-BODY-COMMON. The KDC limits the set of possible reply keys and the set of session key encryption types based on the "etype" field. For the AS exchange, the reply key is initially a long-term key of the client, limited to those encryption types specified by the "etype" field. The KDC SHOULD use the first valid strong "etype" for which an encryption key is available. For the TGS exchange, the reply key is initially the subsession key of the Authenticator, or, if that is not present, the session key of the ticket used to authenticate the TGS-REQ. Yu Expires: Aug 2004 [Page 29] Internet-Draft yu-krb-extensions-00 09 Feb 2004 The ticket key is initially the long-term key of the service. User- to-user authentication sets the ticket key to be the session key of the additional ticket in the request. The session key is initially randomly generated, and has an encryption type which both the client and the server will understand. Typically, the KDC has prior knowledge of which encryption types the server will understand. It selects the first valid strong "etype" listed the request which the server also will understand. 9.3.5. Checking For Revoked Tickets 9.4. Reply Validation 10. Application Authentication 11. Session Key Use 11.1. KRB-SAFE 11.2. KRB-PRIV 11.3. KRB-CRED 12. Security Considerations 12.1. Time Synchronization Time synchronization between the KDC and application servers is necessary to prevent acceptance of expired tickets. Time synchronization is needed between application servers and clients to prevent replay attacks if a replay cache is being used. If negotiated subsession keys are used to encrypt application data, replay caches may not be necessary. 12.2. Replays 12.3. Principal Name Reuse See Section 6.2. 12.4. Password Guessing 12.5. Forward Secrecy [from KCLAR 10.; needs some rewriting] The Kerberos protocol in its basic form does not provide perfect forward secrecy for communications. If traffic has been recorded by an eavesdropper, then messages encrypted using the KRB-PRIV message, Yu Expires: Aug 2004 [Page 30] Internet-Draft yu-krb-extensions-00 09 Feb 2004 or messages encrypted using application-specific encryption under keys exchanged using Kerberos can be decrypted if any of the user's, application server's, or KDC's key is subsequently discovered. This is because the session key used to encrypt such messages is transmitted over the network encrypted in the key of the application server, and also encrypted under the session key from the user's ticket-granting ticket when returned to the user in the TGS-REP message. The session key from the ticket-granting ticket was sent to the user in the AS-REP message encrypted in the user's secret key, and embedded in the ticket-granting ticket, which was encrypted in the key of the KDC. Application requiring perfect forward secrecy must exchange keys through mechanisms that provide such assurance, but may use Kerberos for authentication of the encrypted channel established through such other means. 12.6. Authorization As an authentication service, Kerberos provides a means of verifying the identity of principals on a network. Kerberos does not, by itself, provide authorization. Applications SHOULD NOT accept the mere issuance of a service ticket by the Kerberos server as granting authority to use the service. 12.7. Login Authentication Some applications, particularly those which provide login access when provided with a password, SHOULD NOT treat successful acquisition of credentials as sufficient proof of the user's identity. An attacker posing as a user could generate an illegitimate KDC-REP message which decrypts properly. To authenticate a user logging on to a local system, the credentials obtained SHOULD be used in a TGS exchange to obtain credentials for a local service. Successful use of those credentials to authenticate to the local service assures that the initially obtained credentials are from a valid KDC. Appendices A. ASN.1 Module (Normative) KerberosV5Spec3 { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) modules(4) krb5spec3(4) } DEFINITIONS EXPLICIT TAGS ::= BEGIN Yu Expires: Aug 2004 [Page 31] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- OID arc for KerberosV5 -- -- This OID may be used to identify Kerberos protocol messages -- encapsulated in other protocols. -- -- This OID also designates the OID arc for KerberosV5-related -- OIDs. -- -- NOTE: RFC 1510 had an incorrect value (5) for "dod" in its -- OID. id-krb5 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) } -- top-level type -- -- Applications should not directly reference any types -- other than KRB-PDU and its component types. -- KRB-PDU ::= CHOICE { ticket Ticket, as-req AS-REQ, as-rep AS-REP, tgs-req TGS-REQ, tgs-rep TGS-REP, ap-req AP-REQ, ap-rep AP-REP, krb-safe KRB-SAFE, krb-priv KRB-PRIV, krb-cred KRB-CRED, tgt-req TGT-REQ, krb-error KRB-ERROR, ... } -- -- *** basic types -- -- signed values representable in 32 bits -- -- These are often used as assigned numbers for various things. Int32 ::= INTEGER (-2147483648..2147483647) -- unsigned 32 bit values UInt32 ::= INTEGER (0..4294967295) Yu Expires: Aug 2004 [Page 32] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- microseconds Microseconds ::= INTEGER (0..999999) -- sequence numbers -- -- We may want to increase this to 2**64 and define a UInt64 -- type. SeqNum ::= UInt32 -- nonces -- -- Likewise, we may want to make this UInt64. Nonce ::= UInt32 -- must not have fractional seconds KerberosTime ::= GeneralizedTime -- used for names and for error messages KerberosString ::= CHOICE { ia5 GeneralString (IA5String), utf8 UTF8String, ... -- no extension may be sent -- to an rfc1510 implementation -- } -- used for language tags LangTag ::= PrintableString (FROM ("A".."Z" | "a".."z" | "0".."9" | "-")) Realm ::= KerberosString PrincipalName ::= SEQUENCE { name-type [0] NameType, -- May have zero elements, or individual elements may be -- zero-length, but this is not recommended. name-string [1] SEQUENCE OF KerberosString } -- assigned numbers for name types (used in principal names) NameType ::= Int32 Yu Expires: Aug 2004 [Page 33] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- Name type not known nt-unknown NameType ::= 0 -- Just the name of the principal as in DCE, or for users nt-principal NameType ::= 1 -- Service and other unique instance (krbtgt) nt-srv-inst NameType ::= 2 -- Service with host name as instance (telnet, rcommands) nt-srv-hst NameType ::= 3 -- Service with host as remaining components nt-srv-xhst NameType ::= 4 -- Unique ID nt-uid NameType ::= 5 -- Encoded X.509 Distingished name [RFC 2253] nt-x500-principal NameType ::= 6 -- Name in form of SMTP email name (e.g. user@foo.com) nt-smtp-name NameType ::= 7 -- Enterprise name - may be mapped to principal name nt-enterprise NameType ::= 10 -- Yet another refinement to kludge around historical -- implementation bugs... we still send at least 32 bits, but -- this parameterized type allows us to actually use named bit -- string syntax to define flags, sort of. KerberosFlags { NamedBits } ::= BIT STRING (SIZE (32..MAX)) (CONSTRAINED BY { -- must be a valid value of -- NamedBits -- but if the value to be sent would otherwise be shorter -- than 32 bits, it must be padded with trailing zero bits -- to 32 bits. Otherwise, no trailing zero bits may be -- present. }) AddrType ::= Int32 HostAddress ::= SEQUENCE { addr-type [0] AddrType, address [1] OCTET STRING } -- NOTE: HostAddresses is always used as an OPTIONAL field and -- should not be a zero-length SEQUENCE OF. -- -- The extensible messages explicitly constrain this to be -- non-empty. HostAddresses ::= SEQUENCE OF HostAddress Yu Expires: Aug 2004 [Page 34] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- -- *** typed hole support -- -- Object class for generic typed holes, e.g., padata, -- authorizationdata. -- -- Its parameter specifies the name of integer type used as a -- unique identifier; usually this type is an aliased Int32. -- -- Usually, the &Type field will be an OctetstringHole, but if -- there is a need to use a non-ASN.1 encoded type, it may be -- simply an OCTET STRING, possibly with some comments -- describing its contents. TYPEDHOLE { IntType } ::= CLASS { &id-int IntType UNIQUE, &id-oid RELATIVE-OID UNIQUE OPTIONAL, &Type, &desc ObjectDescriptor OPTIONAL } WITH SYNTAX { SYNTAX &Type IDENTIFIED BY &id-int [ OID &id-oid ] [ DESCRIPTION &desc ] } -- An octet string wrapping another ASN.1 type. OctetstringHole { Type } ::= OCTET STRING (CONTAINING Type) -- -- *** crypto-related types and assignments -- Yu Expires: Aug 2004 [Page 35] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- -- Actual identifier names are provisional and subject to -- change. -- ku-pa-enc-ts KeyUsage ::= 1 ku-Ticket KeyUsage ::= 2 ku-EncASRepPart KeyUsage ::= 3 ku-TGSReqAuthData-sesskey KeyUsage ::= 4 ku-TGSReqAuthData-subkey KeyUsage ::= 5 ku-pa-TGSReq-cksum KeyUsage ::= 6 ku-pa-TGSReq-authenticator KeyUsage ::= 7 ku-EncTGSRepPart-sesskey KeyUsage ::= 8 ku-EncTGSRepPart-subkey KeyUsage ::= 9 ku-Authenticator-cksum KeyUsage ::= 10 ku-APReq-authenticator KeyUsage ::= 11 ku-EncAPRepPart KeyUsage ::= 12 ku-EncKrbPrivPart KeyUsage ::= 13 ku-EncKrbCredPart KeyUsage ::= 14 ku-KrbSafe-cksum KeyUsage ::= 15 ku-ad-KDCIssued-cksum KeyUsage ::= 19 -- The following numbers are provisional... conflicts may exist elsewhere. ku-Ticket-cksum KeyUsage ::= 25 ku-ASReq-cksum KeyUsage ::= 26 ku-TGSReq-cksum KeyUsage ::= 27 ku-ASRep-cksum KeyUsage ::= 28 ku-TGSRep-cksum KeyUsage ::= 29 ku-APReq-cksum KeyUsage ::= 30 ku-APRep-cksum KeyUsage ::= 31 ku-KrbCred-cksum KeyUsage ::= 32 ku-KrbError-cksum KeyUsage ::= 33 ku-KDCRep-cksum KeyUsage ::= 34 Yu Expires: Aug 2004 [Page 36] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- assigned numbers for encryption schemes et-des-cbc-crc EType ::= 1 et-des-cbc-md4 EType ::= 2 et-des-cbc-md5 EType ::= 3 -- [reserved] 4 et-des3-cbc-md5 EType ::= 5 -- [reserved] 6 et-des3-cbc-sha1 EType ::= 7 et-dsaWithSHA1-CmsOID EType ::= 9 et-md5WithRSAEncryption-CmsOID EType ::= 10 et-sha1WithRSAEncryption-CmsOID EType ::= 11 et-rc2CBC-EnvOID EType ::= 12 et-rsaEncryption-EnvOID EType ::= 13 et-rsaES-OAEP-ENV-OID EType ::= 14 et-des-ede3-cbc-Env-OID EType ::= 15 et-des3-cbc-sha1-kd EType ::= 16 et-aes128-cts-hmac-sha1-96 EType ::= 17 -- AES et-aes256-cts-hmac-sha1-96 EType ::= 18 -- AES et-rc4-hmac EType ::= 23 -- Microsoft et-rc4-hmac-exp EType ::= 24 -- Microsoft et-subkey-keymaterial EType ::= 65 -- opaque; PacketCable -- "Type" specifies which ASN.1 type is encrypted to the -- ciphertext in the EncryptedData. "Keys" specifies a set of -- keys of which one key may be used to encrypt the data. -- "KeyUsages" specifies a set of key usages, one of which may -- be used to encrypt. -- -- None of the parameters is transmitted over the wire. EncryptedData { Type, KeyToUse:Keys, KeyUsage:KeyUsages } ::= SEQUENCE { etype [0] EType, kvno [1] UInt32 OPTIONAL, cipher [2] OCTET STRING (CONSTRAINED BY { -- must be encryption of -- OCTET STRING (CONTAINING Type), -- with one of the keys -- KeyToUse:Keys, -- with key usage being one of -- KeyUsage:KeyUsages }), ... } -- Assigned numbers denoting encryption mechanisms. EType ::= Int32 -- Assigned numbers denoting key usages. KeyUsage ::= UInt32 Yu Expires: Aug 2004 [Page 37] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- KeyToUse identifies which key is to be used to encrypt or -- sign a given value. -- -- Values of KeyToUse are never actually transmitted over the -- wire, or even used directly by the implementation in any -- way, as key usages are; it exists primarily to identify -- which key gets used for what purpose. Thus, the specific -- numeric values associated with this type are irrelevant. KeyToUse ::= ENUMERATED { -- unspecified key-unspecified, -- server long term key key-server, -- client long term key key-client, -- key selected by KDC for encryption of a KDC-REP key-kdc-rep, -- session key from ticket key-session, -- subsession key negotiated via AP-REQ/AP-REP key-subsession, ... } EncryptionKey ::= SEQUENCE { keytype [0] EType, keyvalue [1] OCTET STRING } CksumType ::= Int32 -- The parameters specify which key to use to produce the -- signature, as well as which key usage to use. The -- parameters are not actually sent over the wire. Checksum { KeyToUse:Keys, KeyUsage:KeyUsages } ::= SEQUENCE { cksumtype [0] CksumType, checksum [1] OCTET STRING (CONSTRAINED BY { -- signed using one of the keys -- KeyToUse:Keys, -- with key usage being one of -- KeyUsage:KeyUsages }) } Yu Expires: Aug 2004 [Page 38] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- a Checksum that must contain the checksum of a particular type ChecksumOf { Type, KeyToUse:Keys, KeyUsage:KeyUsages } ::= Checksum { Keys, KeyUsages } (WITH COMPONENTS { ..., checksum (CONSTRAINED BY { -- must be checksum of -- OCTET STRING (CONTAINING Type) }) }) -- parameterized type for wrapping authenticated plaintext Signed { InnerType, KeyToUse:Keys, KeyUsage:KeyUsages } ::= SEQUENCE { cksum [0] ChecksumOf { InnerType, Keys, KeyUsages } OPTIONAL, inner [1] InnerType, ... } -- -- *** Tickets -- Ticket ::= CHOICE { rfc1510 [APPLICATION 1] Ticket1510, ext [APPLICATION 4] Signed { TicketExt, { key-server }, { ku-Ticket-cksum } } } -- takes a parameter specifying which type gets encrypted TicketCommon { EncPart } ::= SEQUENCE { tkt-vno [0] INTEGER (5), realm [1] Realm, sname [2] PrincipalName, enc-part [3] EncryptedData { EncPart, { key-server }, { ku-Ticket } }, extensions [4] TicketExtensions OPTIONAL, ... } Yu Expires: Aug 2004 [Page 39] Internet-Draft yu-krb-extensions-00 09 Feb 2004 Ticket1510 ::= SEQUENCE { -- "COMPONENTS OF" drops the extension marker from -- TicketCommon COMPONENTS OF TicketCommon { EncTicketPart1510 } } (WITH COMPONENTS { ..., -- explicitly force IA5 in strings realm (WITH COMPONENTS { ia5 PRESENT }), sname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 PRESENT })) }), extensions ABSENT }) -- APPLICATION tag goes inside Signed{} as well as outside, -- to prevent possible substitution attacks. TicketExt ::= [APPLICATION 4] TicketCommon (WITH COMPONENTS { ..., -- explicitly force UTF-8 in strings realm (WITH COMPONENTS { ia5 ABSENT }), sname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 ABSENT })) }) }) -- Encrypted part of ticket EncTicketPart ::= CHOICE { rfc1510 [APPLICATION 3] EncTicketPart1510, ext [APPLICATION 5] EncTicketPartExt } Yu Expires: Aug 2004 [Page 40] Internet-Draft yu-krb-extensions-00 09 Feb 2004 EncTicketPartCommon ::= SEQUENCE { flags [0] TicketFlags, key [1] EncryptionKey, crealm [2] Realm, cname [3] PrincipalName, transited [4] TransitedEncoding, authtime [5] KerberosTime, starttime [6] KerberosTime OPTIONAL, endtime [7] KerberosTime, renew-till [8] KerberosTime OPTIONAL, caddr [9] HostAddresses OPTIONAL, authorization-data [10] AuthorizationData OPTIONAL, ... } EncTicketPart1510 ::= SEQUENCE { -- effectively drops the extension marker COMPONENTS OF EncTicketPartCommon } (WITH COMPONENTS { ..., -- explicitly force IA5 in strings crealm (WITH COMPONENTS { ia5 PRESENT }), cname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 PRESENT })) }) }) EncTicketPartExt ::= EncTicketPartCommon (WITH COMPONENTS { ..., -- explicitly force UTF-8 in strings crealm (WITH COMPONENTS { ia5 ABSENT }), cname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 ABSENT })) }), -- explicitly constrain caddr to be non-empty if present caddr (SIZE (1..MAX)), -- explicitly constrain authorization-data to be non-empty -- if present authorization-data (SIZE (1..MAX)) }) Yu Expires: Aug 2004 [Page 41] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- -- *** Authorization Data -- ADType ::= Int32 AUTHDATA ::= TYPEDHOLE { ADType } -- NOTE: AuthorizationData is always used as an OPTIONAL field and -- should not be a zero-length SEQUENCE OF. -- -- The extensible messages explicitly constrain this to be non-empty. AuthorizationData ::= SEQUENCE OF SEQUENCE { ad-type [0] AUTHDATA.&id-int ({Authdata-Set}), ad-data [1] OCTET STRING (AUTHDATA.&Type) ({Authdata-Set}{@ad-type}) } -- Mandatory AuthorizationData Authdata-Set AUTHDATA ::= { ad-if-relevant | ad-kdcissued | ad-and-or | ad-mandatory-for-kdc , ... } ad-if-relevant AUTHDATA ::= { SYNTAX OctetstringHole { AuthorizationData } IDENTIFIED BY 1 DESCRIPTION "Encapsulates another AuthorizationData. Intended for application servers; receiving application servers MAY ignore." } ad-kdcissued AUTHDATA ::= { SYNTAX OctetstringHole { AD-KDCIssued } IDENTIFIED BY 4 DESCRIPTION "KDC-issued privilege attributes" } Yu Expires: Aug 2004 [Page 42] Internet-Draft yu-krb-extensions-00 09 Feb 2004 AD-KDCIssued ::= SEQUENCE { ad-checksum [0] ChecksumOf { AuthorizationData, { key-session }, { ku-ad-KDCIssued-cksum }}, i-realm [1] Realm OPTIONAL, i-sname [2] PrincipalName OPTIONAL, elements [3] AuthorizationData } AD-AND-OR ::= SEQUENCE { condition-count [0] INTEGER, elements [1] AuthorizationData } AD-MANDATORY-FOR-KDC ::= AuthorizationData ad-and-or AUTHDATA ::= { SYNTAX OctetstringHole { AD-AND-OR } IDENTIFIED BY 5 DESCRIPTION "And/Or" } AD-AND-OR ::= SEQUENCE { condition-count [0] INTEGER, elements [1] AuthorizationData } ad-mandatory-for-kdc AUTHDATA ::= { SYNTAX OctetstringHole { AuthorizationData } IDENTIFIED BY 8 DESCRIPTION "KDCs MUST interpret any AuthorizationData wrapped in this." } TrType ::= Int32 -- must be registered -- encoded Transited field TransitedEncoding ::= SEQUENCE { tr-type [0] TrType, contents [1] OCTET STRING } Yu Expires: Aug 2004 [Page 43] Internet-Draft yu-krb-extensions-00 09 Feb 2004 TEType ::= Int32 TICKETEXTENSION ::= TYPEDHOLE { TEType } -- ticket extensions: for TicketExt only TicketExtensions ::= SEQUENCE (SIZE (1..MAX)) OF SEQUENCE { te-type [0] TICKETEXTENSION.&id ({TicketExtension-Set}), te-data [1] OCTET STRING (TICKETEXTENSION.&Type) ({TicketExtension-Set}{@te-type}) } -- no mandatory ticket extensions currently TicketExtensionSet TICKETEXTENSION ::= { ... } TicketFlags ::= KerberosFlags { TicketFlagsBits } TicketFlagsBits ::= BIT STRING { reserved (0), forwardable (1), forwarded (2), proxiable (3), proxy (4), may-postdate (5), postdated (6), invalid (7), renewable (8), initial (9), pre-authent (10), hw-authent (11), transited-policy-checked (12), ok-as-delegate (13), anonymous (14), cksummed-ticket (15) } -- -- *** KDC protocol -- Yu Expires: Aug 2004 [Page 44] Internet-Draft yu-krb-extensions-00 09 Feb 2004 AS-REQ ::= CHOICE { rfc1510 [APPLICATION 10] KDC-REQ-1510 (WITH COMPONENTS { ..., msg-type (10), -- AS-REQ must include client name req-body (WITH COMPONENTS { ..., cname PRESENT }) }), ext [APPLICATION 6] Signed { -- APPLICATION tag goes inside Signed{} as well as outside, -- to prevent possible substitution attacks. [APPLICATION 6] KDC-REQ-EXT, -- not sure this is correct key to use; do we even want -- to sign AS-REQ? { key-client }, { ku-ASReq-cksum } } (WITH COMPONENTS { ..., msg-type (6), -- AS-REQ must include client name req-body (WITH COMPONENTS { ..., cname PRESENT }) }) } TGS-REQ ::= CHOICE { rfc1510 [APPLICATION 12] KDC-REQ-1510 (WITH COMPONENTS { ..., msg-type (12), -- client name optional in TGS-REQ req-body (WITH COMPONENTS { ..., cname ABSENT }) }), ext [APPLICATION 8] Signed { -- APPLICATION tag goes inside Signed{} as well as outside, -- to prevent possible substitution attacks. [APPLICATION 8] KDC-REQ-EXT, { key-session }, { ku-TGSReq-cksum } } (WITH COMPONENTS { ..., msg-type (8), -- client name optional in TGS-REQ req-body (WITH COMPONENTS { ..., cname ABSENT }) }) } Yu Expires: Aug 2004 [Page 45] Internet-Draft yu-krb-extensions-00 09 Feb 2004 KDC-REQ-COMMON ::= SEQUENCE { -- NOTE: first tag is [1], not [0] pvno [1] INTEGER (5), msg-type [2] INTEGER (10 -- AS-REQ.rfc1510 -- | 12 -- TGS-REQ.rfc1510 -- | 6 -- AS-REQ.ext -- | 8 -- TGS-REQ.ext -- ), padata [3] SEQUENCE OF PA-DATA OPTIONAL -- NOTE: not empty } KDC-REQ-1510 ::= SEQUENCE { COMPONENTS OF KDC-REQ-COMMON, req-body [4] KDC-REQ-BODY-1510 } (WITH COMPONENTS { ..., msg-type (10 | 12) }) -- APPLICATION tag goes inside Signed{} as well as outside, -- to prevent possible substitution attacks. KDC-REQ-EXT ::= SEQUENCE { COMPONENTS OF KDC-REQ-COMMON, req-body [4] KDC-REQ-BODY-EXT, lang-tags [5] SEQUENCE (SIZE (1..MAX)) OF LangTag OPTIONAL, ... } (WITH COMPONENTS { ..., msg-type (6 | 8), padata (SIZE (1..MAX)) }) Yu Expires: Aug 2004 [Page 46] Internet-Draft yu-krb-extensions-00 09 Feb 2004 KDC-REQ-BODY-COMMON ::= SEQUENCE { kdc-options [0] KDCOptions, cname [1] PrincipalName OPTIONAL -- Used only in AS-REQ --, realm [2] Realm -- Server's realm; also client's in AS-REQ --, sname [3] PrincipalName OPTIONAL, from [4] KerberosTime OPTIONAL, till [5] KerberosTime OPTIONAL -- was required in rfc1510; -- still required for compat versions -- of messages --, rtime [6] KerberosTime OPTIONAL, nonce [7] Nonce, etype [8] SEQUENCE OF EType -- in preference order --, addresses [9] HostAddresses OPTIONAL, enc-authorization-data [10] EncryptedData { AuthorizationData, { key-session | key-subsession }, { ku-TGSReqAuthData-subkey | ku-TGSReqAuthData-sesskey } } OPTIONAL, additional-tickets [11] SEQUENCE OF Ticket OPTIONAL -- NOTE: not empty --, ... } KDC-REQ-BODY-1510 ::= SEQUENCE { -- effectively drops the extension marker COMPONENTS OF KDC-REQ-BODY-COMMON } (WITH COMPONENTS { ..., cname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 PRESENT })) }), realm (WITH COMPONENTS { ia5 PRESENT }), sname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 PRESENT })) }), till PRESENT }) Yu Expires: Aug 2004 [Page 47] Internet-Draft yu-krb-extensions-00 09 Feb 2004 KDC-REQ-BODY-EXT ::= KDC-REQ-BODY-COMMON (WITH COMPONENTS { ..., cname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 ABSENT })) }), realm (WITH COMPONENTS { ia5 ABSENT }), sname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 ABSENT })) }), addresses (SIZE (1..MAX)), enc-authorization-data (EncryptedData { AuthorizationData (SIZE (1..MAX)), { key-session | key-subsession }, { ku-TGSReqAuthData-subkey | ku-TGSReqAuthData-sesskey } }), additional-tickets (SIZE (1..MAX)) }) KDCOptions ::= KerberosFlags { KDCOptionsBits } KDCOptionsBits ::= BIT STRING { reserved (0), forwardable (1), forwarded (2), proxiable (3), proxy (4), allow-postdate (5), postdated (6), unused7 (7), renewable (8), unused9 (9), unused10 (10), unused11 (11), unused12 (12), unused13 (13), requestanonymous (14), canonicalize (15), disable-transited-check (26), renewable-ok (27), enc-tkt-in-skey (28), renew (30), validate (31) -- XXX need "need ticket1" flag? } Yu Expires: Aug 2004 [Page 48] Internet-Draft yu-krb-extensions-00 09 Feb 2004 AS-REP ::= CHOICE { rfc1510 [APPLICATION 11] KDC-REP-1510 { EncASRepPart } (WITH COMPONENTS { ..., msg-type (11) }), ext [APPLICATION 7] Signed { [APPLICATION 7] KDC-REP-EXT { EncASRepPart }, { key-reply }, { ku-ASRep-cksum } } (WITH COMPONENTS { ..., msg-type (7) }) } TGS-REP ::= CHOICE { rfc1510 [APPLICATION 13] KDC-REP-1510 { EncTGSRepPart } (WITH COMPONENTS { ..., msg-type (13) }), ext [APPLICATION 9] Signed { [APPLICATION 9] KDC-REP-EXT { EncTGSRepPart }, { key-reply }, { ku-TGSRep-cksum } } (WITH COMPONENTS { ..., msg-type (9) }) } KDC-REP-COMMON { EncPart } ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (11 -- AS-REP.rfc1510 -- | 13 -- TGS.rfc1510 -- | 7 -- AS-REP.ext -- | 9 -- TGS-REP.ext -- ), padata [2] SEQUENCE OF PA-DATA OPTIONAL, crealm [3] Realm, cname [4] PrincipalName, ticket [5] Ticket, enc-part [6] EncryptedData { EncPart, { key-reply }, -- maybe reach into EncryptedData in AS-REP/TGS-REP definitions -- to apply constraints on key usages? { ku-EncASRepPart -- if AS-REP -- | ku-EncTGSRepPart-subkey -- if TGS-REP and using -- -- Authenticator session key -- | ku-EncTGSRepPart-sesskey -- if TGS-REP and using -- -- subsession key -- } }, -- In extensible version, KDC signs original request -- to avoid replay attacks agaginst client. req-cksum [7] ChecksumOf { CHOICE { as-req AS-REQ, tgs-req TGS-REQ }, { key-reply }, { ku-KDCRep-cksum }} OPTIONAL, lang-tag [8] LangTag OPTIONAL, ... } Yu Expires: Aug 2004 [Page 49] Internet-Draft yu-krb-extensions-00 09 Feb 2004 KDC-REP-1510 { EncPart } ::= SEQUENCE { -- effectively drops the extension marker COMPONENTS OF KDC-REP-COMMON { EncPart } } (WITH COMPONENTS { ..., msg-type (11 | 13), crealm (WITH COMPONENTS { ia5 PRESENT}), cname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 PRESENT })) }), req-cksum ABSENT, lang-tag ABSENT }) KDC-REP-EXT { EncPart } ::= KDC-REP-COMMON { EncPart } (WITH COMPONENTS { ..., msg-type (7 | 9), crealm (WITH COMPONENTS { ia5 ABSENT }), cname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 ABSENT })) }) }) EncASRepPart ::= [APPLICATION 25] EncKDCRepPart EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart EncKDCRepPart ::= SEQUENCE { key [0] EncryptionKey, last-req [1] LastReq, nonce [2] Nonce, key-expiration [3] KerberosTime OPTIONAL, flags [4] TicketFlags, authtime [5] KerberosTime, starttime [6] KerberosTime OPTIONAL, endtime [7] KerberosTime, renew-till [8] KerberosTime OPTIONAL, srealm [9] Realm, sname [10] PrincipalName, caddr [11] HostAddresses OPTIONAL, ... -- ASN.1 extensions must be excluded -- when sending to rfc1510 implementation } Yu Expires: Aug 2004 [Page 50] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- convert to use object class? LRType ::= Int32 LastReq ::= SEQUENCE OF SEQUENCE { lr-type [0] LRType, lr-value [1] KerberosTime } -- -- *** preauth -- PaDataType ::= Int32 -- TYPEDHOLE class that uses PaDataType as its unique ID type. PADATA-OBJ ::= TYPEDHOLE { PaDataType } PA-DATA ::= SEQUENCE { -- NOTE: first tag is [1], not [0] padata-type [1] CHOICE { -- example of possible use of RELATIVE-OIDs int PADATA-OBJ.&id-int ({PaDataSet}), oid PADATA-OBJ.&id-oid ({PaDataSet}{@int}) }, padata-value [2] OCTET STRING (PADATA-OBJ.&Type) ({PaDataSet}{@padata-type.int}) } PaDataSet PADATA-OBJ ::= { pa-tgs-req | pa-enc-timestamp | pa-etype-info | pa-etype-info2 | pa-pw-salt | pa-as-req , ... } pa-tgs-req PADATA-OBJ ::= { SYNTAX OctetstringHole { AP-REQ } IDENTIFIED BY 1 DESCRIPTION "AP-REQ authenticating a TGS-REQ" } Yu Expires: Aug 2004 [Page 51] Internet-Draft yu-krb-extensions-00 09 Feb 2004 pa-enc-timestamp PADATA-OBJ ::= { SYNTAX OctetstringHole { PA-ENC-TIMESTAMP } IDENTIFIED BY 2 DESCRIPTION "Encrypted timestamp preauth; Encryption key used is client's long-term key." } PA-ENC-TIMESTAMP ::= EncryptedData { PA-ENC-TS-ENC, { key-client }, { ku-pa-enc-ts } } PA-ENC-TS-ENC ::= SEQUENCE { patimestamp [0] KerberosTime -- client's time --, pausec [1] Microseconds OPTIONAL } pa-etype-info PADATA-OBJ ::= { SYNTAX OctetstringHole { ETYPE-INFO } IDENTIFIED BY 11 DESCRIPTION "Hints returned in AS-REP or KRB-ERROR to help client choose a password-derived key, either for preauthentication or for decryption of the reply." } ETYPE-INFO ::= SEQUENCE OF ETYPE-INFO-ENTRY ETYPE-INFO-ENTRY ::= SEQUENCE { etype [0] EType, salt [1] OCTET STRING OPTIONAL } pa-etype-info2 PADATA-OBJ ::= { SYNTAX OctetstringHole { ETYPE-INFO2 } IDENTIFIED BY 19 DESCRIPTION "Similar to etype-info, but with parameters provided for the string-to-key function." } ETYPE-INFO2 ::= SEQUENCE (SIZE (1..MAX)) OF ETYPE-INFO-ENTRY ETYPE-INFO2-ENTRY ::= SEQUENCE { etype [0] EType, salt [1] KerberosString OPTIONAL, s2kparams [2] OCTET STRING OPTIONAL } Yu Expires: Aug 2004 [Page 52] Internet-Draft yu-krb-extensions-00 09 Feb 2004 pa-pw-salt PADATA-OBJ ::= { SYNTAX OCTET STRING (CONSTRAINED BY { -- Must consist of the salt string to be used by the -- client, in an unspecified character encoding. -- }) IDENTIFIED BY 3 DESCRIPTION "Obsolescent. Salt for client's long-term key. Its character encoding is unspecified." } pa-as-req PADATA-OBJ ::= { SYNTAX OctetstringHole { AS-REQ (WITH COMPONENTS { ext }) } IDENTIFIED BY 42 -- provisional DESCRIPTION "An extensible AS-REQ may be sent as a padata in a non-extensible AS-REQ to allow for backwards compatibility." } -- -- *** Application session setup -- AP-REQ ::= CHOICE { rfc1510 [APPLICATION 14] AP-REQ-1510, ext [APPLICATION 18] Signed { AP-REQ-EXT, { key-session }, { ku-APReq-cksum } } } AP-REQ-COMMON ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (14 | 18), ap-options [2] APOptions, ticket [3] Ticket, authenticator [4] EncryptedData { Authenticator, { key-session }, { ku-APReq-authenticator | ku-pa-TGSReq-authenticator } }, extensions [5] ApReqExtensions OPTIONAL, ... } Yu Expires: Aug 2004 [Page 53] Internet-Draft yu-krb-extensions-00 09 Feb 2004 AP-REQ-1510 ::= SEQUENCE { -- effectively drops the extension marker COMPONENTS OF AP-REQ-COMMON } (WITH COMPONENTS { ..., msg-type (14), extensions ABSENT }) AP-REQ-EXT ::= AP-REQ-COMMON (WITH COMPONENTS { ..., msg-type (18), -- The following constraints on Authenticator assume that -- we want to restrict the use of AP-REQ-EXT with TicketExt only, -- since that is the only way we can enforce UTF-8. authenticator (EncryptedData { Authenticator (WITH COMPONENTS { ..., crealm (WITH COMPONENTS { ia5 ABSENT }), cname (WITH COMPONENTS { ..., name-string (WITH COMPONENT (WITH COMPONENTS { ia5 ABSENT })) }), authorization-data (SIZE (1..MAX)) }), { key-session }, { ku-APReq-authenticator }}) }) ApReqExtType ::= Int32 ApReqExtensions ::= SEQUENCE (SIZE (1..MAX)) OF SEQUENCE { apReqExt-Type [0] ApReqExtType, apReqExt-Data [1] OCTET STRING } APOptions ::= KerberosFlags { APOptionsBits } APOptionsBits ::= BIT STRING { reserved (0), use-session-key (1), mutual-required (2) } Yu Expires: Aug 2004 [Page 54] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- plaintext of authenticator Authenticator ::= [APPLICATION 2] SEQUENCE { authenticator-vno [0] INTEGER (5), crealm [1] Realm, cname [2] PrincipalName, cksum [3] Checksum {{ key-session }, { ku-Authenticator-cksum | ku-pa-TGSReq-cksum }} OPTIONAL, cusec [4] Microseconds, ctime [5] KerberosTime, subkey [6] EncryptionKey OPTIONAL, seq-number [7] SeqNum OPTIONAL, authorization-data [8] AuthorizationData OPTIONAL } AP-REP ::= CHOICE { rfc1510 [APPLICATION 15] AP-REP-1510, ext [APPLICATION 19] Signed { AP-REP-EXT, { key-session | key-subsession }, { ku-APRep-cksum }} } AP-REP-COMMON { EncPart } ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (15 | 19), enc-part [2] EncryptedData { EncPart, { key-session | key-subsession }, { ku-EncAPRepPart }}, extensions [3] ApRepExtensions OPTIONAL, ... } AP-REP-1510 ::= SEQUENCE { -- effectively drops the extension marker COMPONENTS OF AP-REP-COMMON { EncAPRepPart1510 } } (WITH COMPONENTS { ..., msg-type (15), extensions ABSENT }) AP-REP-EXT ::= [APPLICATION 19] AP-REP-COMMON { EncAPRepPartExt } (WITH COMPONENTS { ..., msg-type (19) }) Yu Expires: Aug 2004 [Page 55] Internet-Draft yu-krb-extensions-00 09 Feb 2004 ApRepExtType ::= Int32 -- convert to use object class? ApRepExtensions ::= SEQUENCE (SIZE (1..MAX)) OF SEQUENCE { apRepExt-Type [0] ApRepExtType, apRepExt-Data [1] OCTET STRING } EncAPRepPart ::= CHOICE { rfc1510 [APPLICATION 27] EncAPRepPart1510, ext [APPLICATION 31] EncAPRepPartExt } EncAPRepPart1510 ::= SEQUENCE { COMPONENTS OF ENCAPRepPartCom } (WITH COMPONENTS { ..., authorization-data ABSENT }) EncAPRepPartExt ::= EncAPRepPartCom EncAPRepPartCom ::= SEQUENCE { ctime [0] KerberosTime, cusec [1] Microseconds, subkey [2] EncryptionKey OPTIONAL, seq-number [3] SeqNum OPTIONAL, authorization-data [4] AuthorizationData OPTIONAL, ... } -- -- *** Application messages -- Yu Expires: Aug 2004 [Page 56] Internet-Draft yu-krb-extensions-00 09 Feb 2004 -- Do we chew up another tag for KRB-SAFE-EXT? That would allow us to -- make safe-body optional, allowing for a GSS-MIC sort of message. KRB-SAFE ::= [APPLICATION 20] SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (20), safe-body [2] KRB-SAFE-BODY, cksum [3] ChecksumOf { KRB-SAFE-BODY, { key-session | key-subsession }, { ku-KrbSafe-cksum }}, ... -- ASN.1 extensions must be excluded -- when sending to rfc1510 implementations } KRB-SAFE-BODY ::= SEQUENCE { user-data [0] OCTET STRING, timestamp [1] KerberosTime OPTIONAL, usec [2] Microseconds OPTIONAL, seq-number [3] SeqNum OPTIONAL, s-address [4] HostAddress, r-address [5] HostAddress OPTIONAL, ... -- ASN.1 extensions must be excluded -- when sending to rfc1510 implementations } KRB-PRIV ::= [APPLICATION 21] SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (21), enc-part [3] EncryptedData { EncKrbPrivPart, { key-session | key-subsession }, { ku-EncKrbPrivPart }}, ... } EncKrbPrivPart ::= [APPLICATION 28] SEQUENCE { user-data [0] OCTET STRING, timestamp [1] KerberosTime OPTIONAL, usec [2] Microseconds OPTIONAL, seq-number [3] SeqNum OPTIONAL, s-address [4] HostAddress -- sender's addr --, r-address [5] HostAddress OPTIONAL -- recip's addr --, ... -- ASN.1 extensions must be excluded -- when sending to rfc1510 implementations } Yu Expires: Aug 2004 [Page 57] Internet-Draft yu-krb-extensions-00 09 Feb 2004 KRB-CRED ::= CHOICE { rfc1510 [APPLICATION 22] KRB-CRED-1510, ext [APPLICATION 24] Signed { KRB-CRED-EXT, { key-session | key-subsession }, { ku-KrbCred-cksum }} } KRB-CRED-COMMON ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (22 | 24), tickets [2] SEQUENCE OF Ticket, enc-part [3] EncryptedData { EncKrbCredPart, { key-session | key-subsession }, { ku-EncKrbCredPart }}, ... } KRB-CRED-1510 ::= SEQUENCE { -- effectively drops the extension marker COMPONENTS OF KRB-CRED-COMMON } (WITH COMPONENTS { ..., msg-type (22) }) KRB-CRED-EXT ::= [APPLICATION 24] KRB-CRED-COMMON (WITH COMPONENTS { ..., msg-type (24) }) EncKrbCredPart ::= [APPLICATION 29] SEQUENCE { ticket-info [0] SEQUENCE OF KrbCredInfo, nonce [1] Nonce OPTIONAL, timestamp [2] KerberosTime OPTIONAL, usec [3] Microseconds OPTIONAL, s-address [4] HostAddress OPTIONAL, r-address [5] HostAddress OPTIONAL } Yu Expires: Aug 2004 [Page 58] Internet-Draft yu-krb-extensions-00 09 Feb 2004 KrbCredInfo ::= SEQUENCE { key [0] EncryptionKey, prealm [1] Realm OPTIONAL, pname [2] PrincipalName OPTIONAL, flags [3] TicketFlags OPTIONAL, authtime [4] KerberosTime OPTIONAL, starttime [5] KerberosTime OPTIONAL, endtime [6] KerberosTime OPTIONAL, renew-till [7] KerberosTime OPTIONAL, srealm [8] Realm OPTIONAL, sname [9] PrincipalName OPTIONAL, caddr [10] HostAddresses OPTIONAL } TGT-REQ ::= [APPLICATION 16] SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (16), sname [2] PrincipalName OPTIONAL, srealm [3] Realm OPTIONAL, ... } -- -- *** Error messages -- ErrCode ::= Int32 KRB-ERROR ::= CHOICE { rfc1510 [APPLICATION 30] KRB-ERROR-1510, ext [APPLICATION 23] Signed { KRB-ERROR-EXT, { ku-KrbError-cksum } } } Yu Expires: Aug 2004 [Page 59] Internet-Draft yu-krb-extensions-00 09 Feb 2004 KRB-ERROR-COMMON ::= SEQUENCE { pvno [0] INTEGER (5), msg-type [1] INTEGER (30 | 23), ctime [2] KerberosTime OPTIONAL, cusec [3] Microseconds OPTIONAL, stime [4] KerberosTime, susec [5] Microseconds, error-code [6] ErrCode, crealm [7] Realm OPTIONAL, cname [8] PrincipalName OPTIONAL, realm [9] Realm -- Correct realm --, sname [10] PrincipalName -- Correct name --, e-text [11] KerberosString OPTIONAL, e-data [12] OCTET STRING OPTIONAL, typed-data [13] TYPED-DATA OPTIONAL, nonce [14] Nonce OPTIONAL, lang-tag [15] LangTag OPTIONAL, ... } KRB-ERROR-1510 ::= SEQUENCE { -- effectively drops the extension marker COMPONENTS OF KRB-ERROR-COMMON } (WITH COMPONENTS { ..., msg-type (30), typed-data ABSENT, nonce ABSENT, lang-tag ABSENT }) KRB-ERROR-EXT ::= [APPLICATION 23] KRB-ERROR-COMMON (WITH COMPONENTS { ..., msg-type (23) }) TDType ::= Int32 -- convert to information object class later TYPED-DATA ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE { data-type [0] TDType, data-value [1] OCTET STRING OPTIONAL } END Yu Expires: Aug 2004 [Page 60] Internet-Draft yu-krb-extensions-00 09 Feb 2004 B. Kerberos and Character Encodings (Informative) [adapted from KCLAR 5.2.1] The original specification of the Kerberos protocol in RFC 1510 uses GeneralString in numerous places for human-readable string data. Historical implementations of Kerberos cannot utilize the full power of GeneralString. This ASN.1 type requires the use of designation and invocation escape sequences as specified in ISO-2022/ECMA-35 [ISO-2022/ECMA-35] to switch character sets, and the default character set that is designated as G0 is the ISO-646/ECMA-6 [ISO-646,ECMA-6] International Reference Version (IRV) (aka U.S. ASCII), which mostly works. ISO-2022/ECMA-35 defines four character-set code elements (G0..G3) and two Control-function code elements (C0..C1). DER previously prohibited the designation of character sets as any but the G0 and C0 sets. This had the side effect of prohibiting the use of ISO-8859 (ISO Latin) [ISO-8859] character-sets or any other character-sets that utilize a 96-character set, since it is prohibited by ISO-2022/ECMA-35 to designate them as the G0 code element. Recent revisions to the ASN.1 standards resolve this contradiction. In practice, many implementations treat RFC 1510 GeneralStrings as if they were 8-bit strings of whichever character set the implementation defaults to, without regard for correct usage of character-set designation escape sequences. The default character set is often determined by the current user's operating system dependent locale. At least one major implementation places unescaped UTF-8 encoded Unicode characters in the GeneralString. This failure to conform to the GeneralString specifications results in interoperability issues when conflicting character encodings are utilized by the Kerberos clients, services, and KDC. This unfortunate situation is the result of improper documentation of the restrictions of the ASN.1 GeneralString type in prior Kerberos specifications. [the following should probably be rewritten and moved into the principal name section] For compatibility, implementations MAY choose to accept GeneralString values that contain characters other than those permitted by IA5String, but they should be aware that character set designation codes will likely be absent, and that the encoding should probably be treated as locale-specific in almost every way. Implementations MAY also choose to emit GeneralString values that are beyond those permitted by IA5String, but should be aware that doing so is extraordinarily risky from an interoperability perspective. Yu Expires: Aug 2004 [Page 61] Internet-Draft yu-krb-extensions-00 09 Feb 2004 Some existing implementations use GeneralString to encode unescaped locale-specific characters. This is a violation of the ASN.1 standard. Most of these implementations encode US-ASCII in the left- hand half, so as long the implementation transmits only US-ASCII, the ASN.1 standard is not violated in this regard. As soon as such an implementation encodes unescaped locale-specific characters with the high bit set, it violates the ASN.1 standard. Other implementations have been known to use GeneralString to contain a UTF-8 encoding. This also violates the ASN.1 standard, since UTF-8 is a different encoding, not a 94 or 96 character "G" set as defined by ISO 2022. It is believed that these implementations do not even use the ISO 2022 escape sequence to change the character encoding. Even if implementations were to announce the change of encoding by using that escape sequence, the ASN.1 standard prohibits the use of any escape sequences other than those used to designate/invoke "G" or "C" sets allowed by GeneralString. C. Kerberos History (Informative) [Adapted from KCLAR "BACKGROUND"] The Kerberos model is based in part on Needham and Schroeder's trusted third-party authentication protocol [NS78] and on modifications suggested by Denning and Sacco [DS81]. The original design and implementation of Kerberos Versions 1 through 4 was the work of two former Project Athena staff members, Steve Miller of Digital Equipment Corporation and Clifford Neuman (now at the Information Sciences Institute of the University of Southern California), along with Jerome Saltzer, Technical Director of Project Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many other members of Project Athena have also contributed to the work on Kerberos. Version 5 of the Kerberos protocol (described in this document) has evolved from Version 4 based on new requirements and desires for features not available in Version 4. The design of Version 5 of the Kerberos protocol was led by Clifford Neuman and John Kohl with much input from the community. The development of the MIT reference implementation was led at MIT by John Kohl and Theodore Ts'o, with help and contributed code from many others. Since RFC1510 was issued, extensions and revisions to the protocol have been proposed by many individuals. Some of these proposals are reflected in this document. Where such changes involved significant effort, the document cites the contribution of the proposer. Reference implementations of both version 4 and version 5 of Kerberos are publicly available and commercial implementations have been developed and are widely used. Details on the differences between Kerberos Versions 4 and 5 can be found in [KNT94]. Yu Expires: Aug 2004 [Page 62] Internet-Draft yu-krb-extensions-00 09 Feb 2004 Normative References [KCRYPTO] [RFC2119] [X680] [X681] [X682] [X683] [X690] Informative References [DS81] [KCLAR] [KNT94] [NS78] [RFC1510] [RFC1964] [ISO8859] Acknowledgments Some stuff lifted from draft-ietf-krb-wg-kerberos-clarifications-04. Author's Address Tom Yu 77 Massachusetts Ave Cambridge, MA 02139 USA tlyu@mit.edu Full Copyright Statement Copyright (C) The Internet Society (2004). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published Yu Expires: Aug 2004 [Page 63] Internet-Draft yu-krb-extensions-00 09 Feb 2004 and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Yu Expires: Aug 2004 [Page 64]