Kerberos Working Group L. Zhu Internet-Draft Microsoft Corporation Updates: 4120 (if approved) S. Hartman Intended status: Standards Track MIT Expires: April 26, 2007 October 23, 2006 A Generalized Framework for Kerberos Pre-Authentication draft-ietf-krb-wg-preauth-framework-03 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. This Internet-Draft will expire on April 26, 2007. Copyright Notice Copyright (C) The Internet Society (2006). Abstract Kerberos is a protocol for verifying the identity of principals (e.g., a workstation user or a network server) on an open network. The Kerberos protocol provides a mechanism called pre-authentication for proving the identity of a principal and for better protecting the long-term secret of the principal. This document describes a model for Kerberos pre-authentication Zhu & Hartman Expires April 26, 2007 [Page 1]
Internet-Draft Kerberos Preauth Framework October 2006 mechanisms. The model describes what state in the Kerberos request a pre-authentication mechanism is likely to change. It also describes how multiple pre-authentication mechanisms used in the same request will interact. This document also provides common tools needed by multiple pre- authentication mechanisms. One of such tools is a secure channel between the client and the KDC with a reply key delivery mechanism, this secure channel can be used to protect the authentication exchange thus eliminate offline dictionary attacks. With these tools, it is straightforward to chain multiple authentication factors or add a plugin to, for example, utilize a different key management system, or support a new key agreement algorithm. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 3. Model for Pre-Authentication . . . . . . . . . . . . . . . . . 4 3.1. Information Managed by the Pre-authentication Model . . . 5 3.2. Initial Pre-authentication Required Error . . . . . . . . 7 3.3. Client to KDC . . . . . . . . . . . . . . . . . . . . . . 8 3.4. KDC to Client . . . . . . . . . . . . . . . . . . . . . . 9 4. Pre-Authentication Facilities . . . . . . . . . . . . . . . . 10 4.1. Client-authentication Facility . . . . . . . . . . . . . . 11 4.2. Strengthening-reply-key Facility . . . . . . . . . . . . . 11 4.3. Replacing-reply-key Facility . . . . . . . . . . . . . . . 12 4.4. KDC-authentication Facility . . . . . . . . . . . . . . . 13 5. Requirements for Pre-Authentication Mechanisms . . . . . . . . 13 6. Tools for Use in Pre-Authentication Mechanisms . . . . . . . . 14 6.1. Combining Keys . . . . . . . . . . . . . . . . . . . . . . 14 6.2. Protecting Requests/Responses . . . . . . . . . . . . . . 15 6.3. Managing States for the KDC . . . . . . . . . . . . . . . 15 6.4. Pre-authentication Set . . . . . . . . . . . . . . . . . . 17 6.5. Definition of Kerberos FAST Padata . . . . . . . . . . . . 18 6.5.1. FAST Request . . . . . . . . . . . . . . . . . . . . . 18 6.5.2. FAST Response . . . . . . . . . . . . . . . . . . . . 22 6.6. Authentication Strength Indication . . . . . . . . . . . . 25 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 8. Security Considerations . . . . . . . . . . . . . . . . . . . 25 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26 10.1. Normative References . . . . . . . . . . . . . . . . . . . 26 10.2. Informative References . . . . . . . . . . . . . . . . . . 26 Appendix A. ASN.1 module . . . . . . . . . . . . . . . . . . . . 26 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29 Intellectual Property and Copyright Statements . . . . . . . . . . 30 Zhu & Hartman Expires April 26, 2007 [Page 2]
Internet-Draft Kerberos Preauth Framework October 2006 1. Introduction The core Kerberos specification [RFC4120] treats pre-authentication data as an opaque typed hole in the messages to the KDC that may influence the reply key used to encrypt the KDC reply. This generality has been useful: pre-authentication data is used for a variety of extensions to the protocol, many outside the expectations of the initial designers. However, this generality makes designing more common types of pre-authentication mechanisms difficult. Each mechanism needs to specify how it interacts with other mechanisms. Also, problems like combining a key with the long-term secret or proving the identity of the user are common to multiple mechanisms. Where there are generally well-accepted solutions to these problems, it is desirable to standardize one of these solutions so mechanisms can avoid duplication of work. In other cases, a modular approach to these problems is appropriated. The modular approach will allow new and better solutions to common pre-authentication problems to be used by existing mechanisms as they are developed. This document specifies a framework for Kerberos pre-authentication mechanisms. It defines the common set of functions pre- authentication mechanisms perform as well as how these functions affect the state of the request and reply. In addition several common tools needed by pre-authentication mechanisms are provided. Unlike [RFC3961], this framework is not complete--it does not describe all the inputs and outputs for the pre-authentication mechanisms. Pre-Authentication mechanism designers should try to be consistent with this framework because doing so will make their mechanisms easier to implement. Kerberos implementations are likely to have plugin architectures for pre-authentication; such architectures are likely to support mechanisms that follow this framework plus commonly used extensions. One of these common tools is the flexible authentication secure tunneling (FAST) padata. FAST provides a protected channel between the client and the KDC, and it also delivers a reply key within the protected channel. Based on FAST, pre-authentication mechanisms can extend Kerberos with ease, to support, for example, password authenticated key exchange (PAKE) protocols with zero knowledge password proof (ZKPP) [EKE] [IEEE1363.2]. Any pre-authentication mechanism can be encapsulated in the padata field Section 6.5 of FAST. A pre-authentication type thus carried within FAST is called a FAST factor. A FAST factor MUST NOT be used outside of FAST unless its specification explicitly allows so. Note that FAST without a FAST factor for authentication does NOT by itself authenticate the client or the KDC. New pre-authentication mechanisms SHOULD design FAST factors, instead Zhu & Hartman Expires April 26, 2007 [Page 3]
Internet-Draft Kerberos Preauth Framework October 2006 of full-blown pre-authentication mechanisms. A conversation consists of all messages that are necessary to complete the mutual authentication between the client and the KDC. A conversation is the smallest logic unit for messages exchanged between the client and the KDC. The KDC need to manage mulitple authentication sets frequently need to keep track of KDC states during a convesation, standard solutions are provided for these common problems. This document should be read only after reading the documents describing the Kerberos cryptography framework [RFC3961] and the core Kerberos protocol [RFC4120]. This document freely uses terminology and notation from these documents without reference or further explanation. 2. Conventions Used in This Document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. The word padata is used as the shorthand of pre-authentication data. A conversation is used to refer to all authentication messages exchanged between the client and the KDC. 3. Model for Pre-Authentication When a Kerberos client wishes to obtain a ticket using the authentication server, it sends an initial Authentication Service (AS) request. If pre-authentication is required but not being used, then the KDC will respond with a KDC_ERR_PREAUTH_REQUIRED error. Alternatively, if the client knows what pre-authentication to use, it MAY optimize away a round-trip and send an initial request with padata included in the initial request. If the client includes the wrong padata, the KDC MAY return KDC_ERR_PREAUTH_FAILED with no indication of what padata should have been included. In that case, the client MUST retry with no padata and examine the error data of the KDC_ERR_PREAUTH_REQUIRED error. If the KDC includes pre- authentication information in the accompanying error data of KDC_ERR_PREAUTH_FAILED, the client SHOULD process the error data as that of the KDC_ERR_PREAUTH_REQUIRED error, and then retry. The conventional KDC maintains no state between two requests; subsequent requests may even be processed by a different KDC. On the other hand, the client treats a series of exchanges with KDCs as a Zhu & Hartman Expires April 26, 2007 [Page 4]
Internet-Draft Kerberos Preauth Framework October 2006 single authentication session. Each exchange accumulates state and hopefully brings the client closer to a successful authentication. These models for state management are in apparent conflict. For many of the simpler pre-authentication scenarios, the client uses one round trip to find out what mechanisms the KDC supports. Then the next request contains sufficient pre-authentication for the KDC to be able to return a successful reply. For these simple scenarios, the client only sends one request with pre-authentication data and so the authentication session is trivial. For more complex authentication sessions, the KDC needs to provide the client with a cookie to include in future requests to capture the current state of the authentication session. Handling of multiple round-trip mechanisms is discussed in Section 6.3. This framework specifies the behavior of Kerberos pre-authentication mechanisms used to identify users or to modify the reply key used to encrypt the KDC reply. The PA-DATA typed hole may be used to carry extensions to Kerberos that have nothing to do with proving the identity of the user or establishing a reply key. Such extensions are outside the scope of this framework. However mechanisms that do accomplish these goals should follow this framework. This framework specifies the minimum state that a Kerberos implementation needs to maintain while handling a request in order to process pre-authentication. It also specifies how Kerberos implementations process the padata at each step of the AS request process. 3.1. Information Managed by the Pre-authentication Model The following information is maintained by the client and KDC as each request is being processed: o The reply key used to encrypt the KDC reply o How strongly the identity of the client has been authenticated o Whether the reply key has been used in this authentication session o Whether the reply key has been replaced in this authentication session o Whether the contents of the KDC reply can be verified by the client principal Zhu & Hartman Expires April 26, 2007 [Page 5]
Internet-Draft Kerberos Preauth Framework October 2006 o Whether the contents of the KDC reply can be verified by the client machine Conceptually, the reply key is initially the long-term key of the principal. However, principals can have multiple long-term keys because of support for multiple encryption types, salts and string2key parameters. As described in section 5.2.7.5 of the Kerberos protocol [RFC4120], the KDC sends PA-ETYPE-INFO2 to notify the client what types of keys are available. Thus in full generality, the reply key in the pre-authentication model is actually a set of keys. At the beginning of a request, it is initialized to the set of long-term keys advertised in the PA-ETYPE-INFO2 element on the KDC. If multiple reply keys are available, the client chooses which one to use. Thus the client does not need to treat the reply key as a set. At the beginning of a handling a request, the client picks a reply key to use. KDC implementations MAY choose to offer only one key in the PA-ETYPE- INFO2 element. Since the KDC already knows the client's list of supported enctypes from the request, no interoperability problems are created by choosing a single possible reply key. This way, the KDC implementation avoids the complexity of treating the reply key as a set. When the padata in the request is verified by the KDC, then the client is known to have that key, therefore the KDC SHOULD pick the same key as the reply key. At the beginning of handling a message on both the client and the KDC, the client's identity is not authenticated. A mechanism may indicate that it has successfully authenticated the client's identity. This information is useful to keep track of on the client in order to know what pre-authentication mechanisms should be used. The KDC needs to keep track of whether the client is authenticated because the primary purpose of pre-authentication is to authenticate the client identity before issuing a ticket. The handling of authentication strength using various authentication mechanisms is discussed in Section 6.6. Initially the reply key has not been used. A pre-authentication mechanism that uses the reply key either directly to encrypt or checksum some data or indirectly in the generation of new keys MUST indicate that the reply key is used. This state is maintained by the client and the KDC to enforce the security requirement stated in Section 4.3 that the reply key cannot be used after it is replaced. Initially the reply key has not been replaced. If a mechanism implements the Replace Reply Key facility discussed in Section 4.3, Zhu & Hartman Expires April 26, 2007 [Page 6]
Internet-Draft Kerberos Preauth Framework October 2006 then the state MUST be updated to indicate that the reply key has been replaced. Once the reply key has been replaced, knowledge of the reply key is insufficient to authenticate the client. The reply key is marked replaced in exactly the same situations as the KDC reply is marked as not being verified to the client principal. However, while mechanisms can verify the KDC reply to the client, once the reply key is replaced, then the reply key remains replaced for the remainder of the authentication session. Without pre-authentication, the client knows that the KDC reply is authentic and has not been modified because it is encrypted in a long-term key of the client. Only the KDC and the client know that key. So at the start of handling any message the KDC reply is presumed to be verified using the client principal's long-term key. Any pre-authentication mechanism that sets a new reply key not based on the principal's long-term secret MUST either verify the KDC reply some other way or indicate that the reply is not verified. If a mechanism indicates that the reply is not verified then the client implementation MUST return an error unless a subsequent mechanism verifies the reply. The KDC needs to track this state so it can avoid generating a reply that is not verified. The typical Kerberos request does not provide a way for the client machine to know that it is talking to the correct KDC. Someone who can inject packets into the network between the client machine and the KDC and who knows the password that the user will give to the client machine can generate a KDC reply that will decrypt properly. So, if the client machine needs to authenticate that the user is in fact the named principal, then the client machine needs to do a TGS request for itself as a service. Some pre-authentication mechanisms may provide a way for the client to authenticate the KDC. Examples of this include signing the reply with a well-known public key or providing a ticket for the client machine as a service in addition to the requested ticket. 3.2. Initial Pre-authentication Required Error Typically a client starts an authentication session by sending an initial request with no pre-authentication. If the KDC requires pre- authentication, then it returns a KDC_ERR_PREAUTH_REQUIRED message. After the first reply with the KDC_ERR_PREAUTH_REQUIRED error code, the KDC returns the error code KDC_ERR_MORE_PREAUTH_DATA_NEEDED for pre-authentication configurations that use multi-round-trip mechanisms; see Section 3.4 for details of that case. The KDC needs to choose which mechanisms to offer the client. The client needs to be able to choose what mechanisms to use from the first message. For example consider the KDC that will accept Zhu & Hartman Expires April 26, 2007 [Page 7]
Internet-Draft Kerberos Preauth Framework October 2006 mechanism A followed by mechanism B or alternatively the single mechanism C. A client that supports A and C needs to know that it should not bother trying A. Mechanisms can either be sufficient on their own or can be part of an authentication set--a group of mechanisms that all need to successfully complete in order to authenticate a client. Some mechanisms may only be useful in authentication sets; others may be useful alone or in authentication sets. For the second group of mechanisms, KDC policy dictates whether the mechanism will be part of an authentication set or offered alone. For each mechanism that is offered alone, the KDC includes the pre-authentication type ID of the mechanism in the padata sequence returned in the KDC_ERR_PREAUTH_REQUIRED error. The KDC SHOULD NOT send data that is encrypted in the long-term password-based key of the principal. Doing so has the same security exposures as the Kerberos protocol without pre-authentication. There are few situations where pre-authentication is desirable and where the KDC needs to expose cipher text encrypted in a weak key before the client has proven knowledge of that key. 3.3. Client to KDC This description assumes a client has already received a KDC_ERR_PREAUTH_REQUIRED from the KDC. If the client performs optimistic pre-authentication then the client needs to optimistically choose the information it would normally receive from that error response. The client starts by initializing the pre-authentication state as specified. It then processes the padata in the KDC_ERR_PREAUTH_REQUIRED. When processing the response to the KDC_ERR_PREAUTH_REQUIRED, the client MAY ignore any padata it chooses unless doing so violates a specification to which the client conforms. Clients MUST NOT ignore the padata defined in Section 6.3. Clients SHOULD process padata unrelated to this framework or other means of authenticating the user. Clients SHOULD choose one authentication set or mechanism that could lead to authenticating the user and ignore the rest. Since the list of mechanisms offered by the KDC is in the decreasing preference order, clients typically choose the first mechanism that the client can usefully perform. If a client chooses to ignore a padata it MUST NOT process the padata, allow the padata to affect the pre- authentication state, nor respond to the padata. For each padata the client chooses to process, the client processes Zhu & Hartman Expires April 26, 2007 [Page 8]
Internet-Draft Kerberos Preauth Framework October 2006 the padata and modifies the pre-authentication state as required by that mechanism. Padata are processed in the order received from the KDC. After processing the padata in the KDC error, the client generates a new request. It processes the pre-authentication mechanisms in the order in which they will appear in the next request, updating the state as appropriate. The request is sent when it is complete. 3.4. KDC to Client When a KDC receives an AS request from a client, it needs to determine whether it will respond with an error or a AS reply. There are many causes for an error to be generated that have nothing to do with pre-authentication; they are discussed in the core Kerberos specification. From the standpoint of evaluating the pre-authentication, the KDC first starts by initializing the pre-authentication state. It then processes the padata in the request. As mentioned in Section 3.3, the KDC MAY ignore padata that is inappropriate for the configuration and MUST ignore padata of an unknown type. At this point the KDC decides whether it will issue a pre- authentication required error or a reply. Typically a KDC will issue a reply if the client's identity has been authenticated to a sufficient degree. In the case of a KDC_ERR_PREAUTH_REQUIRED error, the KDC first starts by initializing the pre-authentication state. Then it processes any padata in the client's request in the order provided by the client. Mechanisms that are not understood by the KDC are ignored. Mechanisms that are inappropriate for the client principal or the request SHOULD also be ignored. Next, it generates padata for the error response, modifying the pre-authentication state appropriately as each mechanism is processed. The KDC chooses the order in which it will generate padata (and thus the order of padata in the response), but it needs to modify the pre-authentication state consistently with the choice of order. For example, if some mechanism establishes an authenticated client identity, then the subsequent mechanisms in the generated response receive this state as input. After the padata is generated, the error response is sent. Typically the errors with the code KDC_ERR_MORE_PREAUTH_DATA_NEEDED in a converstation will include KDC state as discussed in Section 6.3. To generate a final reply, the KDC generates the padata modifying the pre-authentication state as necessary. Then it generates the final Zhu & Hartman Expires April 26, 2007 [Page 9]
Internet-Draft Kerberos Preauth Framework October 2006 response, encrypting it in the current pre-authentication reply key. 4. Pre-Authentication Facilities Pre-Authentication mechanisms can be thought of as providing various conceptual facilities. This serves two useful purposes. First, mechanism authors can choose only to solve one specific small problem. It is often useful for a mechanism designed to offer key management not to directly provide client authentication but instead to allow one or more other mechanisms to handle this need. Secondly, thinking about the abstract services that a mechanism provides yields a minimum set of security requirements that all mechanisms providing that facility must meet. These security requirements are not complete; mechanisms will have additional security requirements based on the specific protocol they employ. A mechanism is not constrained to only offering one of these facilities. While such mechanisms can be designed and are sometimes useful, many pre-authentication mechanisms implement several facilities. By combining multiple facilities in a single mechanism, it is often easier to construct a secure, simple solution than by solving the problem in full generality. Even when mechanisms provide multiple facilities, they need to meet the security requirements for all the facilities they provide. According to Kerberos extensibility rules (Section 1.5 of the Kerberos specification [RFC4120]), an extension MUST NOT change the semantics of a message unless a recipient is known to understand that extension. Because a client does not know that the KDC supports a particular pre-authentication mechanism when it sends an initial request, a pre-authentication mechanism MUST NOT change the semantics of the request in a way that will break a KDC that does not understand that mechanism. Similarly, KDCs MUST not send messages to clients that affect the core semantics unless the client has indicated support for the message. The only state in this model that would break the interpretation of a message is changing the expected reply key. If one mechanism changed the reply key and a later mechanism used that reply key, then a KDC that interpreted the second mechanism but not the first would fail to interpret the request correctly. In order to avoid this problem, extensions that change core semantics are typically divided into two parts. The first part proposes a change to the core semantic--for example proposes a new reply key. The second part acknowledges that the extension is understood and that the change takes effect. Section 4.2 discusses how to design mechanisms that modify the reply key to be split into a proposal and acceptance without requiring Zhu & Hartman Expires April 26, 2007 [Page 10]
Internet-Draft Kerberos Preauth Framework October 2006 additional round trips to use the new reply key in subsequent pre- authentication. Other changes in the state described in Section 3.1 can safely be ignored by a KDC that does not understand a mechanism. Mechanisms that modify the behavior of the request outside the scope of this framework need to carefully consider the Kerberos extensibility rules to avoid similar problems. 4.1. Client-authentication Facility The client authentication facility proves the identity of a user to the KDC before a ticket is issued. Examples of mechanisms implementing this facility include the encrypted timestamp facility defined in Section 5.2.7.2 of the Kerberos specification [RFC4120]. Mechanisms that provide this facility are expected to mark the client as authenticated. Mechanisms implementing this facility SHOULD require the client to prove knowledge of the reply key before transmitting a successful KDC reply. Otherwise, an attacker can intercept the pre-authentication exchange and get a reply to attack. One way of proving the client knows the reply key is to implement the Replace Reply Key facility along with this facility. The PKINIT mechanism [RFC4556] implements Client Authentication alongside Replace Reply Key. If the reply key has been replaced, then mechanisms such as encrypted-timestamp that rely on knowledge of the reply key to authenticate the client MUST NOT be used. 4.2. Strengthening-reply-key Facility Particularly, when dealing with keys based on passwords, it is desirable to increase the strength of the key by adding additional secrets to it. Examples of sources of additional secrets include the results of a Diffie-Hellman key exchange or key bits from the output of a smart card [RFC4556]. Typically these additional secrets can be first combined with the existing reply key and then converted to a protocol key using tools defined in Section 6.1. If a mechanism implementing this facility wishes to modify the reply key before knowing that the other party in the exchange supports the mechanism, it proposes modifying the reply key. The other party then includes a message indicating that the proposal is accepted if it is understood and meets policy. In many cases it is desirable to use the new reply key for client authentication and for other facilities. Waiting for the other party to accept the proposal and actually modify the reply key state would add an additional round trip to the exchange. Instead, mechanism designers are encouraged to include a typed hole for additional padata in the message that proposes the Zhu & Hartman Expires April 26, 2007 [Page 11]
Internet-Draft Kerberos Preauth Framework October 2006 reply key change. The padata included in the typed hole are generated assuming the new reply key. If the other party accepts the proposal, then these padata are interpreted as if they were included immediately following the proposal. The party generating the proposal can determine whether the padata were processed based on whether the proposal for the reply key is accepted. The specific formats of the proposal message, including where padata are are included is a matter for the mechanism specification. Similarly, the format of the message accepting the proposal is mechanism-specific. Mechanisms implementing this facility and including a typed hole for additional padata MUST checksum that padata using a keyed checksum or encrypt the padata. Typically the reply key is used to protect the padata. If you are only minimally increasing the strength of the reply key, this may give the attacker access to something too close to the original reply key. However, binding the padata to the new reply key seems potentially important from a security standpoint. There may also be objections to this from a double encryption standpoint because we also recommend client authentication facilities be tied to the reply key. 4.3. Replacing-reply-key Facility The Replace Reply Key facility replaces the key in which a successful AS reply will be encrypted. This facility can only be used in cases where knowledge of the reply key is not used to authenticate the client. The new reply key MUST be communicated to the client and the KDC in a secure manner. Mechanisms implementing this facility MUST mark the reply key as replaced in the pre-authentication state. Mechanisms implementing this facility MUST either provide a mechanism to verify the KDC reply to the client or mark the reply as unverified in the pre-authentication state. Mechanisms implementing this facility SHOULD NOT be used if a previous mechanism has used the reply key. As with the strengthening-reply-key facility, Kerberos extensibility rules require that the reply key not be changed unless both sides of the exchange understand the extension. In the case of this facility it will likely be more common for both sides to know that the facility is available by the time that the new key is available to be used. However, mechanism designers can use a container for padata in a proposal message as discussed in Section 4.2 if appropriate. Zhu & Hartman Expires April 26, 2007 [Page 12]
Internet-Draft Kerberos Preauth Framework October 2006 4.4. KDC-authentication Facility This facility verifies that the reply comes from the expected KDC. In traditional Kerberos, the KDC and the client share a key, so if the KDC reply can be decrypted then the client knows that a trusted KDC responded. Note that the client machine cannot trust the client unless the machine is presented with a service ticket for it (typically the machine can retrieve this ticket by itself). However, if the reply key is replaced, some mechanism is required to verify the KDC. Pre-authentication mechanisms providing this facility allow a client to determine that the expected KDC has responded even after the reply key is replaced. They mark the pre-authentication state as having been verified. 5. Requirements for Pre-Authentication Mechanisms This section lists requirements for specifications of pre- authentication mechanisms. For each message in the pre-authentication mechanism, the specification describes the pa-type value to be used and the contents of the message. The processing of the message by the sender and recipient is also specified. This specification needs to include all modifications to the pre-authentication state. Generally mechanisms have a message that can be sent in the error data of the KDC_ERR_PREAUTH_REQUIRED error message or in an authentication set. If the client need information such as, for example, trusted certificate authorities in order to determine if it can use the mechanism, then this information should be in that message. In addition, such mechanisms should also define a pa-hint to be included in authentication sets. Often, the same information included in the padata-value is appropriate to include in the pa- hint. In order to ease security analysis the mechanism specification should describe what facilities from this document are offered by the mechanism. For each facility, the security consideration section of the mechanism specification should show that the security requirements of that facility are met. This requirement is applicable to any FAST factor that is used in FAST to provide authentication information. Significant problems have resulted in the specification of Kerberos protocols because much of the KDC exchange is not protected against authentication. The security considerations section should discuss unauthenticated plaintext attacks. It should either show that Zhu & Hartman Expires April 26, 2007 [Page 13]
Internet-Draft Kerberos Preauth Framework October 2006 plaintext is protected or discuss what harm an attacker could do by modifying the plaintext. It is generally acceptable for an attacker to be able to cause the protocol negotiation to fail by modifying plaintext. More significant attacks should be evaluated carefully. 6. Tools for Use in Pre-Authentication Mechanisms This section describes common tools needed by multiple pre- authentication mechanisms. By using these tools mechanism designers can use a modular approach to specify mechanism details and ease security analysis. 6.1. Combining Keys Frequently a weaker key need to be combined with a strong key before use. For example, passwords are typically limited in size and insufficiently random, therefore it is desirable to increase the strength of the keys based on passwords by adding additional secrets to it. Additional source of secrecy can come from a hardware token. This section provides a standard way to combine two keys into one. The function KRB-FX-CF1() produces a new key based on two existing keys of the same enctype and it is base on the primitives encrypt(), random-to-key() and K-truncate() described in [RFC3961]. KRB-FX-CF1(protocol key, protocol key, octet string) -> (resulting key) The KRB-FX-CF1() function takes two protocol keys and an octet string as input, and output a new key of the same enctype. encrypt(B, initial-cipher-state, pepper) -> (state-1, cipher-text-1) encrypt(A, initial-cipher-state, cipher-text-1) -> (state-2, cipher-text-2) K-truncate(cipher-text-2) -> bitstring-3 random-to-key(bitstring-3) -> final-key KRB-FX-CF1(A, B, pepper) -> final-key Where initial-cipher-state is defined in [RFC3961] and the key- generation seed length K is specified by the enctype profile [RFC3961]. The length of the parameter pepper MUST be chosen such that cipher-text-2 has at least K bits. If the input parameter pepper is too short for encrypt(), it MUST first be padded with all Zhu & Hartman Expires April 26, 2007 [Page 14]
Internet-Draft Kerberos Preauth Framework October 2006 zeroes to the next shortest length that encryt() can operate on. KRB-FX-CF1() has the following properties: o The knowledge of the final-key does not reveal either key A or key B. o Without the knowledge of key A, it is infeasible to find the value of the final-key within the lifetime of key A. o Without the knowledge of key B, it is infeasible to find the value of final key within the lifetime of key B. o Typically Key A is stronger than Key B. The lifetime of final-key is no worse than that of Key A. Any mechanism that uses KRB-FX-CF1() MUST show the security requirements are met base on these properties. 6.2. Protecting Requests/Responses Mechanism designers SHOULD provide integrity protection of the messages in a conversation whenever feasible Sensitive data MUST be encrypted when sent over the wire. Non- sensitive data that have privacy implications are encouraged to be encrypted as well. If there are more than one roundtrip for an authentication exchange, mechanism designers SHOULD allow either the client or the KDC provide a checksum of all the messages exchanged on the wire, that is then verified by the receiver. Primitives defined in [RFC3961] are RECOMMENDED for integrity protection and confidentiality. Mechanisms based on these primitives have the benefit of crypto-agility provided by [RFC3961]. The advantage afforded by crypto-agility is the ability to avoid a multi- year standardization and deployment cycle to fix a problem specific to a particular algorithm, when real attacks do arise against that algorithm. New mechanisms MUST NOT be hard-wired to use a specific algorithm. 6.3. Managing States for the KDC For any conversation that consists of more than two messages, the KDC likely need to keep track of KDC states for incomplete authentication exchanges and destroy the states of a conversation when the Zhu & Hartman Expires April 26, 2007 [Page 15]
Internet-Draft Kerberos Preauth Framework October 2006 authentication completes successful or fails, or the KDC times out. When the KDC times out, the KDC returns an error message with the code KDC_ERR_PREAUTH_TIMED_OUT. KDC_ERR_PREAUTH_TIMED_OUT TBA Upnon receipt of this error, the client MUST abort the existing conversation, and restart a new one. An example, where more than one message from the client is needed, is when the client is authenticated based on a challenge-response scheme. In that case, the KDC need to keep track of the challenge issued for a client authentication request. The PA-FX-COOKIE pdata type is defined in this section to facilitate state management. PA_FX_COOKIE TBA The corresponding padata-value field [RFC4120] contains the Distinguished Encoding Rules (DER) [X60] [X690] encoding of the following Abstract Syntax Notation One (ASN.1) type PA-FX-COOKIE: PA-FX-COOKIE ::= SEQUENCE { Cookie [1] OCTET STRING, -- Opaque data, for use to associate all the messages in a -- single conversation between the client and the KDC. -- This can be generated by either the client or the KDC. -- The receiver MUST copy the exact Cookie encapsulated in -- a PA_FX_COOKIE data element into the next message of the -- same conversation. ... } The PA-FX-COOKIE structure contains a opaque cookie that is a logic identifier of all the messages in a conversation. The PA_FX_COOKIE can be initially sent by the client or the KDC, the receiver MUST copy the Cookie into a PA_FX_COOKIE padata and include it in the next message, if any, in the same conversation. The content of the PA_FX_COOKIE padata is a local matter of the sender. Implementations MUST NOT include any sensitive or private data in the PA-FX-COOKIE structure. If at least one more message for a mechanism or a mechanism set is expected by the KDC, the KDC returns a KDC_ERR_MORE_PREAUTH_DATA_NEEDED error with a PA_FX_COOKIE to Zhu & Hartman Expires April 26, 2007 [Page 16]
Internet-Draft Kerberos Preauth Framework October 2006 identify the conversation with the client. KDC_ERR_MORE_PREAUTH_DATA_NEEDED TBA If a PA_FX_COOKIE is included in the client request, the KDC then MUST copy the exact cookie into the response. 6.4. Pre-authentication Set If all mechanisms in a group need to successfully complete in order to authenticate a client, the client and the KDC SHOULD use the PA_AUTHENTICATION_SET padata element. A PA_AUTHENTICATION_SET padata element contains the ASN.1 DER encoding of the PA-AUTHENTICATION-SET structure: PA-AUTHENTICATION-SET ::= SEQUENCE OF PA-AUTHENTICATION-SET-ELEM PA-AUTHENTICATION-SET-ELEM ::= SEQUENCE { pa-type [1] Int32, -- same as padata-type. pa-hint [2] OCTET STRING, -- hint data. ... } The pa-type field of the PA-AUTHENTICATION-SET-ELEM structure contains the corresponding value of padata-type in PA-DATA [RFC4120]. Associated with the pa-type is a pa-hint, which is an octet-string specified by the pre-authentication mechanism. This hint may provide information for the client which helps it determine whether the mechanism can be used. For example a public-key mechanism might include the certificate authorities it trusts in the hint info. Most mechanisms today do not specify hint info; if a mechanism does not specify hint info the KDC MUST NOT send a hint for that mechanism. To allow future revisions of mechanism specifications to add hint info, clients MUST ignore hint info received for mechanisms that the client believes do not support hint info. When indicating which sets of padata are supported, the KDC includes a PA-AUTHENTICATION-SET padata element for each authentication set. The client element sends the padata-value for the first mechanism in the authentication set, when the first mechanism completes, the client and the KDC will proceed with the second mechanism, and so on. The PA_FX_COOKIE as defined in Section 6.3 MUST be sent along with the first message that contains a PA-AUTHENTICATION-SET, in order to keep track of KDC states. Zhu & Hartman Expires April 26, 2007 [Page 17]
Internet-Draft Kerberos Preauth Framework October 2006 6.5. Definition of Kerberos FAST Padata The cipher text exposure of encrypted timestamp pre-authentication data is a security concern for Kerberos. Attackers can lauch offline dictionary attack using the cipher text. The FAST pre-authentication padata is a tool to mitigate this threat. FAST also provides solutions to common problems for pre-authentication mechanisms such as binding of the request and the reply, freshness guarantee of the authentication. FAST itself, however, does not authenticate the client or the KDC, instead, it provides a typed hole to allow pre- authentication data be carried with the FAST messages. A pre- authentication data element used within FAST is called a FAST factor. A FAST factor represents the minimal work required for extending Kerberos to support a new authentication scheme. A FAST factor MUST NOT be used outside of FAST unless its specification explicitly allows so. The FAST typed hole can also be used as a generic one not intended to prove the client's identity, or establish the reply key. New pre-authentication mechanisms SHOULD design as FAST factors, instead of full-blown pre-authentication mechanisms. A FAST mechanism factor when used within FAST to authenticate the client or the KDC is a pre-authentication mechanism, as such the specification of such a FAST factor SHOULD specify which facilities it provides per Section 5. Implementations of the pre-authentication framework SHOULD use encrypted timestamp pre-authentication, if that is the mechanism to authenticate the client, as a FAST factor to avoid security exposure. The encrypted timestamp FAST factor MUST fill out the encrypted rep- key-package field as described in this section. This pre- authentication mechanism provides the following facilities: client- authentication, replacing-reply-key, KDC-authentication. It does not provide the strengthening-reply-key facility. The security considerations section of this document provides an explaination why the security requirements are met. FAST employs an armoring scheme. The armor can be a host TGT, or an anonymous TGT obtained based on anonymous PKINIT [KRB-ANON], or a pre-shared long term key such as a host key. The rest of this section describes the messages used by FAST. 6.5.1. FAST Request A padata type PA_FX_FAST is defined for the FAST Kerberos pre- authentication padata. The corresponding padata-value field [RFC4120] contains the DER encoding of the ASN.1 type PA-FX-FAST- Zhu & Hartman Expires April 26, 2007 [Page 18]
Internet-Draft Kerberos Preauth Framework October 2006 REQUEST. PA-FX-FAST-REQUEST ::= CHOICE { armored-data [1] KrbFastAmoredReq, ... } KrbFastAmoredReq ::= SEQUENCE { armor [1] KrbFastArmor OPTIONAL, -- Contains the armor that determines the armor key. -- MUST be present in the initial AS-REQ in a converstation, -- MUST be absent in any subsequent AS-REQ. -- MUST be absent in TGS-REQ. req-checksum [2] Checksum, -- Checksum performed over the type KDC-REQ-BODY. -- The checksum key is the armor key, and the checksum -- type is the required checksum type for the enctype of -- the armor key. enc-fast-req [3] EncryptedData, -- KrbFastReq -- -- The encryption key is the armor key, and the key usage -- number is TBA. ... } The PA-FX-FAST-REQUEST contains a KrbFastAmoredReq structure. The KrbFastAmoredReq encapsulates the encrypted padata. The key used to encrypt the KrbFastReq structure in the KrbFastAmoredReq is called the armor key, and the key usage number for that encryption is TBA. When a KrbFastAmoredReq is included in an AS request, the KrbFastArmor field MUST be present in the initial AS-REQ in a converstation, specifying the armor key being used. The armor field MUST be absent in any subsequent AS-REQ of the same converstation. Thus the armor key is specified explicitly in the initial AS-REQ in a converstation, and implicitly thereafter. When a KrbFastAmoredReq is included in a TGS request, the KrbFastArmor field MUST be absent. In which case, the subkey in the AP-REQ authenticator in the PA-TGS-REQ MUST be present, and the armor key is implicitly that subkey. 6.5.1.1. FAST Armor The ArmorData structure is used to identify the armor key. It contains two fields: The armor-type identifies the type of armor data, and the armor-value as an OCTET STRING contains the data. Zhu & Hartman Expires April 26, 2007 [Page 19]
Internet-Draft Kerberos Preauth Framework October 2006 KrbFastArmor ::= SEQUENCE { armor-type [1] Int32, -- Type of the armor. armor-value [2] OCTET STRING, -- Value of the armor. ... } The value of the armor key is a matter of the armor type specification. The following types of armors are defined: FX_FAST_ARMOR_AP_REQUEST 1 FX_FAST_ARMOR_KEY_ID 2 6.5.1.1.1. Ticket Based Armors The FX_FAST_ARMOR_AP_REQUEST armor type is based on a Kerberos ticket. The content of a FX_FAST_ARMOR_AP_REQUEST is an AP-REQ encoded in DER. The subkey field in the AP-REQ MUST be present. And the armor key is the subkey in the AP-REQ authenticator. If the client has a TGT for the expected KDC, it can use that ticket to construct the AP-REQ. If not, the client can use anonymous PKINIT as described in [KRB-ANON] to obtain a TGT anonymously and use that to construct a FX_FAST_ARMOR_AP_REQUEST armor. 6.5.1.1.2. Key Based Armors The FX_FAST_ARMOR_KEY_ID armor type contains an identifier of a key shared between the client host and the KDC. The content and the encoding of the armor-data is a local matter of the client and the KDC. The FX_FAST_ARMOR_KEY_ID value is an identifier of the armor key. The FX_FAST_ARMOR_KEY_ID armor is useful when the client host and the KDC does have a shared key and it is beneficial to minimize the number of messages exchanged between the client and the KDC, namely eliminating the messages to obtain a host ticket based on the host key. Conforming implementations MUST implement the FX_FAST_ARMOR_AP_REQUEST armor. The req-checksum field contains a checksum that is performed over the type KDC-REQ-BODY. The checksum key is the armor key, and the checksum type is the required checksum type for the enctype of the armor key. The enc-fast-req field contains an encrypted KrbFastReq structure. Zhu & Hartman Expires April 26, 2007 [Page 20]
Internet-Draft Kerberos Preauth Framework October 2006 The KrbFastReq structure contains the following information: KrbFastReq ::= SEQUENCE { fast-options [0] FastOptions, -- Additional options. padata [1] SEQUENCE OF PA-DATA, -- padata typed holes. timestamp [2] KerberosTime, usec [3] Microseconds, -- timestamp and usec represent the time of the client -- host. req-nonce [4] OCTET STRING, -- At least 128 octets in length, randomly filled using -- a PRNG by the client for each message request. ... } The fast-options field indicates various options to modify the behavior of the KDC. The meanings of the options are as follows: FastOptions ::= KerberosFlags -- reserved(0), -- anonymous(1), -- kdc-referrals(16) Bits Name Description ----------------------------------------------------------------- 0 RESERVED Reserved for future expansion of this field. 1 anonymous Requesting the KDC to hide client names in the KDC response, as described next in this section. 16 kdc-referrals Requesting the KDC to follow referrals, as described next in this section. Bits 1 through 15 (with bit 2 and bit 15 included) are critical options. If the KDC does not understand the critical option, it MUST fail the request. Bit 16 and onward (with bit 16 included) are non- critical options. The KDC ignores an unknown non-critical option. The anonymous Option The Kerberos response defined in [RFC4120] contains the client identity in clear text, This makes traffic analysis straightforward. The anonymous option is designed to complicate traffic analysis against the client-KDC exchange. If the anonymous option is set, the KDC implementing PA_FX_FAST MUST identify out the client as the anonymous principal in the KDC Zhu & Hartman Expires April 26, 2007 [Page 21]
Internet-Draft Kerberos Preauth Framework October 2006 reply and the error response. Thus this option is set by the client to hide the client identity in the KDC response. The kdc-referrals Option The Kerberos client described in [RFC4120] has to request referral TGTs along the authentication path in order to get a service ticket for the target service. The Kerberos client described in the [REFERRALS] need to contain the AS specified in the error response in order to complete client referrals. In many cases, it is desirable to keep the client's involvement minimal. For example, the client may contact the KDC via a satellite link that has high latency, or the client has limited computational capabilities. The kdc-referrals option is designed to minimize the number of KDC response messages that the client need to process. If the kdc-referrals option is set, the KDC that honors this option acts as the client to follow AS referrals and TGS referrals [REFERRALS], and return the ticket thus-obtained using the reply key expected by the client. The kdc-referrals option can be implemented when the KDC knows the reply key. KDC can igore kdc-referrals option when it does not understand it or it does not allow it based on local policy. The client MUST be able to process the KDC responses when this option is not honored by the KDC. The padata field contains a list of PA-DATA structures as described in Section 5.2.7 in [RFC4120]. These PA-DATA structures can contain FAST factors. They can also be used as generic typed-holes to contain data not intended for proving the client's identity or establishing a reply, but for protocol extensibility. The timestamp and usec fields represent the time of the client host, these fields have the same semantics as the corresponding- identically-named fields in Section 5.6.1 of [RFC4120]. The req-nonce field is randomly filled using a PRNG by the client for each message request. It MUST have at least 128 octets in length. 6.5.2. FAST Response The KDC that supports the PA_FX_FAST padata MUST include a PA_FX_FAST padata element in the KDC reply and/or the error response. The KDC can include a PA_FX_FAST padata element in the error response when the client and the KDC agreed upon the armor key. The corresponding padata-value field [RFC4120] in the KDC response is the DER encoding of the ASN.1 type PA-FX-FAST-REPLY. Zhu & Hartman Expires April 26, 2007 [Page 22]
Internet-Draft Kerberos Preauth Framework October 2006 PA-FX-FAST-REPLY ::= CHOICE { armored-data [1] KrbFastArmoredRep, ... } KrbFastArmoredRep ::= SEQUENCE { enc-fast-rep [1] EncryptedData, -- KrbFastResponse -- -- The encryption key is the armor key in the request, and -- the key usage number is TBA. ... } The PA-FX-FAST-REPLY structure contains a KrbFastArmoredRep structure. The KrbFastArmoredRep structure encapsulates the KDC reply in the encrypted form. The KrbFastResponse is encrypted with the armor key used in the corresponding request, and the key usage number is TBA. The Kerberos client who does not receive a PA-FX-FAST-REPLY in the KDC response to a PA-FX-FAST-REQUEST MUST reject the reply based on a local policy. The Kerberos client MAY process an error message without a PA-FX-FAST-REPLY, if that is only intended to return better error information to the application, typically for trouble-shooing purposes. The KrbFastResponse structure contains the following information: KrbFastResponse ::= SEQUENCE { padata [1] SEQUENCE OF PA-DATA, -- padata typed holes. finish [2] KrbFastFinish OPTIONAL, -- MUST be present if the client is authenticated, -- absent otherwise. -- Typically this is present if and only if the containing -- message is the last one in a conversation. rep-nonce [3] OCTET STRING, -- At least 128 octets in length, randomly filled using -- a PRNG by the KDC for each KDC response. ... } The padata field in the KrbFastResponse structure contains a list of PA-DATA structures as described in Section 5.2.7 of [RFC4120]. These PA-DATA structures are used to carry data completing the exchange for the FAST factors. They can also be used as generic typed-holes for protocol extensibility. The finish field contains a KrbFastFinish structure. It is filled by Zhu & Hartman Expires April 26, 2007 [Page 23]
Internet-Draft Kerberos Preauth Framework October 2006 the KDC to indicate the client has been authenticated, it MUST be absent otherwise. This field can only be present in an AS-REP or a TGS-REP when a ticket is returned, and typically the containing message is the last one in a conversation. The KrbFastFinish structure contains the following information: KrbFastFinish ::= SEQUENCE { authtime [1] KerberosTime, usec [2] Microseconds, -- timestamp and usec represent the time on the KDC when -- the reply was generated. rep-key-package [3] EncryptedData OPTIONAL, -- EncryptionKey -- -- This, if present, replaces the reply key for AS and TGS. -- The encryption key is the client key, unless otherwise -- specified. The key usage number is TBA. crealm [4] Realm, cname [5] PrincipalName, -- Contains the client realm and the client name. checksum [6] Checksum, -- Checksum performed over all the messages in the -- conversation, except the containing message. -- The checksum key is the ticket session key of the reply -- ticket, and the checksum type is the required checksum -- type of that key. ... } The timestamp and usec fields represent the time on the KDC when the reply was generated, these fields have the same semantics as the corresponding-identically-named fields in Section 5.6.1 of [RFC4120]. The client MUST use the KDC's time in these fields thereafter when using the returned ticket. This KDC time in AS-REP may not match the authtime in the reply ticket if the kdc-referrals option is requested and honored by the KDC. The rep-key-package field, if present, contains the reply key encrypted using the client key unless otherwise specified. The key usage number is TBA. When the encrypted timestamp FAST factor is used in the request, the rep-key-package field MUST be present. If a KrbFastArmoredRep is included in the reply, the reply key MUST NOT be the client key. The client key can be used to encrypt the reply key enclosed in the KrbFastArmoredRep. The cname and crealm fields identifies the authenticated client. Zhu & Hartman Expires April 26, 2007 [Page 24]
Internet-Draft Kerberos Preauth Framework October 2006 The checksum field contains a checksum of all the prior messages in the conversation excluding the containing message. The checksum key is the ticket session key of the reply ticket, and the checksum type is the required checksum type of that key. The rep-nonce field is randomly filled using a PRNG by the KDC, for each KDC response, and it MUST have at least 128 octets in length. The client MUST include a PA_FX_COOKIE as defined in Section 6.3, if it includes a PA_FX_FAST in the request. 6.6. Authentication Strength Indication Implementations that have pre-authentication mechanisms offering significantly different strengths of client authentication MAY choose to keep track of the strength of the authentication used as an input into policy decisions. For example, some principals might require strong pre-authentication, while less sensitive principals can use relatively weak forms of pre-authentication like encrypted timestamp. An AuthorizationData data type AD-Authentication-Strength is defined. AD-Authentication-Strength TBA The corresponding ad-data field contains the DER encoding of the pre- authentication data set as defined in Section 6.4. This set contains all the pre-authentication mechanisms that were used to authenticate the client. If only one pre-authentication mechanism was used to authenticate the client, the pre-authentication set contains one element. The AD-Authentication-Strength element MUST be included in the AD-IF- RELEVANT, thus it can be ignored if it is unknown to the receiver. 7. IANA Considerations This document defines FAST factors, these are mini- and light- weighted- pre-authentication mechanisms. A new IANA registry should be setup for registering FAST factor IDs. 8. Security Considerations The kdc-referrals option in the Kerberos FAST padata requests the KDC to act as the client to follow referrals. This can overload the KDC. To limit the damages of denied of service using this option, KDCs MAY restrict the number of simultaneous active requests with this option Zhu & Hartman Expires April 26, 2007 [Page 25]
Internet-Draft Kerberos Preauth Framework October 2006 for any given client principal. Because the client secrets are known only to the client and the KDC, the verification of the encrypted timestamp proves the client's identity, the verification of the encrypted rep-key-package in the KDC reply proves that the expected KDC responded. The encrypted reply key is contained in the rep-key-package in the PA-FX-FAST- REPLY. Therefore, the encrypted timestamp FAST factor as a pre- authentication mechanism offers the following facilities: client- authentication, replacing-reply-key, KDC-authentication. There is no un-authenticated cleartext introduced by the encrypted timestamp FAST factor. 9. Acknowledgements Serveral suggestions from Jeffery Hutzman based on early revisions of this documents led significant improvements of this document. 10. References 10.1. Normative References [KRB-ANON] Zhu, L., Leach, P. and Jaganathan, K., "Kerberos Anonymity Support", draft-ietf-krb-wg-anon, work in progress. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for Kerberos 5", RFC 3961, February 2005. [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005. [REFERALS] Raeburn, K. et al, "Generating KDC Referrals to Locate Kerberos Realms", draft-ietf-krb-wg-kerberos-referrals, work in progress. [X680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002, Information technology - Abstract Syntax Notation One (ASN.1): Specification of basic notation. Zhu & Hartman Expires April 26, 2007 [Page 26]
Internet-Draft Kerberos Preauth Framework October 2006 [X690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002, Information technology - ASN.1 encoding Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER). 10.2. Informative References [EKE] Bellovin, S. M. and M. Merritt. "Augmented Encrypted Key Exchange: A Password-Based Protocol Secure Against Dictionary Attacks and Password File Compromise". Proceedings of the 1st ACM Conference on Computer and Communications Security, ACM Press, November 1993. [IEEE1363.2] IEEE P1363.2: Password-Based Public-Key Cryptography [RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)", RFC 4556, June 2006. Appendix A. ASN.1 module KerberosPreauthFramework { iso(1) identified-organization(3) dod(6) internet(1) security(5) kerberosV5(2) modules(4) preauth-framework(3) } DEFINITIONS EXPLICIT TAGS ::= BEGIN PA-FX-COOKIE ::= SEQUENCE { Cookie [1] OCTET STRING, -- Opaque data, for use to associate all the messages in a -- single conversation between the client and the KDC. -- This can be generated by either the client or the KDC. -- The receiver MUST copy the exact Cookie encapsulated in -- a PA_FX_COOKIE data element into the next message of the -- same conversation. ... } PA-AUTHENTICATION-SET ::= SEQUENCE OF PA-AUTHENTICATION-SET-ELEM PA-AUTHENTICATION-SET-ELEM ::= SEQUENCE { pa-type [1] Int32, -- same as padata-type. pa-hint [2] OCTET STRING, -- hint data. ... ... } PA-FX-FAST-REQUEST ::= CHOICE { armored-data [1] KrbFastAmoredReq, ... } KrbFastAmoredReq ::= SEQUENCE { armor [1] KrbFastArmor OPTIONAL, -- Contains the armor that determines the armor key. -- MUST be present in AS-REQ. -- MUST be absent in TGS-REQ. req-checksum [2] Checksum, -- Checksum performed over the type KDC-REQ-BODY. -- The checksum key is the armor key, and the checksum -- type is the required checksum type for the enctype of -- the armor key. enc-fast-req [3] EncryptedData, -- KrbFastReq -- -- The encryption key is the armor key, and the key usage -- number is TBA. ... } Zhu & Hartman Expires April 26, 2007 [Page 27]
Internet-Draft Kerberos Preauth Framework October 2006 KrbFastArmor ::= SEQUENCE { armor-type [1] Int32, -- Type of the armor. armor-value [2] OCTET STRING, -- Value of the armor. ... } KrbFastReq ::= SEQUENCE { fast-options [0] FastOptions, -- Additional options. padata [1] SEQUENCE OF PA-DATA, -- padata typed holes. timestamp [2] KerberosTime, usec [3] Microseconds, -- timestamp and usec represent the time of the client -- host. req-nonce [4] OCTET STRING, -- At least 128 octets in length, randomly filled using -- a PRNG by the client for each message request. ... } FastOptions ::= KerberosFlags -- reserved(0), -- anonymous(1), -- kdc-referrals(16) PA-FX-FAST-REPLY ::= SEQUENCE { enc-fast-rep [1] EncryptedData, -- KrbFastResponse -- -- The encryption key is the armor key in the request, and -- the key usage number is TBA. ... } KrbFastResponse ::= SEQUENCE { padata [1] SEQUENCE OF PA-DATA, -- padata typed holes. finish [2] KrbFastFinish OPTIONAL, -- MUST be present if the client is authenticated, -- absent otherwise. -- Typically this is present if and only if the containing -- message is the last one in a conversation. rep-nonce [3] OCTET STRING, -- At least 128 octets in length, randomly filled using -- a PRNG by the KDC for each KDC response. ... } Zhu & Hartman Expires April 26, 2007 [Page 28]
Internet-Draft Kerberos Preauth Framework October 2006 KrbFastFinish ::= SEQUENCE { timestamp [1] KerberosTime, usec [2] Microseconds, -- timestamp and usec represent the time on the KDC when -- the reply was generated. rep-key-package [3] EncryptedData OPTIONAL, -- EncryptionKey -- -- This, if present, replaces the reply key for AS and TGS. -- The encryption key is the client key, unless otherwise -- specified. The key usage number is TBA. crealm [4] Realm, cname [5] PrincipalName, -- Contains the client realm and the client name. checksum [6] Checksum, -- Checksum performed over all the messages in the -- conversation, except the containing message. -- The checksum key is the ticket session key of the reply -- ticket, and the checksum type is the required checksum -- type of that key. ... } END Authors' Addresses Larry Zhu Microsoft Corporation One Microsoft Way Redmond, WA 98052 US Email: lzhu@microsoft.com Sam hartman MIT Email: hartmans@mit.edu Zhu & Hartman Expires April 26, 2007 [Page 29]
Internet-Draft Kerberos Preauth Framework October 2006 Full Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Zhu & Hartman Expires April 26, 2007 [Page 30]
