Network Working Group S. Hartman Internet-Draft Painless Security Intended status: Informational D. Zhang Expires: January 6, 2011 Huawei July 5, 2010 Operations Model for Router Keying draft-hartman-karp-ops-model-00.txt Abstract Developing an operational and management model for routing protocol security that works across protocols will be critical to the success of routing protocol security efforts. This document discusses issues and begins to consider development of these models. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and 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 January 6, 2011. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of Hartman & Zhang Expires January 6, 2011 [Page 1] Internet-Draft Operations Model for Router Keying July 2010 publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4 3. Breakdown of KARP configuration . . . . . . . . . . . . . . . 5 4. Credentials and Authorization . . . . . . . . . . . . . . . . 6 4.1. Preshared Keys . . . . . . . . . . . . . . . . . . . . . . 6 4.2. Public Keys . . . . . . . . . . . . . . . . . . . . . . . 8 4.3. Public Key Infrastructure . . . . . . . . . . . . . . . . 8 4.4. The role of Central Servers . . . . . . . . . . . . . . . 9 5. Grouping Peers Together . . . . . . . . . . . . . . . . . . . 10 6. Administrator Involvement . . . . . . . . . . . . . . . . . . 11 7. Upgrade Considerations . . . . . . . . . . . . . . . . . . . . 12 8. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 13 9. Security Considerations . . . . . . . . . . . . . . . . . . . 14 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 11.1. Normative References . . . . . . . . . . . . . . . . . . . 16 11.2. Informative References . . . . . . . . . . . . . . . . . . 16 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17 Hartman & Zhang Expires January 6, 2011 [Page 2] Internet-Draft Operations Model for Router Keying July 2010 1. Introduction The KARP working group is designing improvements to the cryptographic authentication of IETF routing protocols. These improvements include improvements to how integrity functions are handled within each protocol as well as designing an automated key management solution. This document discusses issues to consider when thinking about the operational and management model for KARP. Each implementation will take its own approach to management; this is one area for vendor differentiation. However, it is desirable to have a common baseline for the management objects allowing administrators, security architects and protocol designers to understand what management capabilities they can depend on in heterogeneous environments. Similarly, designing and deploying the protocol will be easier with thought paid to a common operational model. This will also help with the design of NetConf schemas or MIBs later. Hartman & Zhang Expires January 6, 2011 [Page 3] Internet-Draft Operations Model for Router Keying July 2010 2. Requirements notation 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]. Hartman & Zhang Expires January 6, 2011 [Page 4] Internet-Draft Operations Model for Router Keying July 2010 3. Breakdown of KARP configuration There are multiple ways of structuring configuration information. For example consider OSPF [RFC2328]. Each OSPF link needs to use the same authentication configuration, including the set of keys used for reception and the set of keys used for transmission. One approach would be to configure each key as a property of the link. Another approach would be to notice that it is common to use the same authentication information across an area and configure the keys as a property of the area. Doing so makes configuration easier in a common case at the expense of generality. If keys are configured per-area then the system may not support configuring different keys for links in the same area. Another option would be to have some sort of inheritance where default configurations are made per-area unless overridden per-interface. An implementation could directly store the information about a given key in its configuration scope. That is, if the key was configured on an interface, then the parameters of that key would be stored with the interface. Alternatively, some abstract key object could be associated with multiple interfaces. This approach might be more complicated but would make it easier to update a key if the same key were used for multiple protocols or in multiple configuration scopes. As discussed in Section 4.1, key separation is an important concern when the same key is used in multiple contexts. Which of these approaches seems best may depend on the types of authentication being used. The following section discusses this. Hartman & Zhang Expires January 6, 2011 [Page 5] Internet-Draft Operations Model for Router Keying July 2010 4. Credentials and Authorization Several methods for authentication have been proposed for KARP. The simplest is preshared keys used directly as traffic keys. In this mode, the traffic integrity keys are directly configured. This is the mode supported by today's routing protocols. Preshared keys could also be used by an automated key management protocol. In this mode, preshared keys would be used for authentication. However traffic keys would be generated by some key agreement mechanism or transported in a key encryption key derived from the preshared key. This mode may provide better replay protection. Also, in the absence of active attackers, key agreement strategies such as Diffie-Hellman can be used to produce high-quality traffic keys even from relatively weak preshared keys. Public keys can be used for authentication. The design guide [I-D.ietf-karp-design-guide] describes a mode in which routers have the hashes of peer routers' public keys. In this mode, a traditional public-key infrastructure is not required. The advantage of this mode is that a router only contains its own keying material, limiting the scope of a compromise. The disadvantage is that when a router is added or deleted from the set of authorized routers, all routers that peer need to be updated. Note that self-signed certificates are a common way of communicating public-keys in this style of authentication. Certificates signed by a certification authority or some other PKI could be used. The advantage of this approach is that routers may not need to be directly updated when peers are added or removed. The disadvantage is that more complexity is required. Each of these approaches has a different set of management and operational requirements. Key differences include how authorization is handled and how identity works. This section discusses these differences. 4.1. Preshared Keys In the protocol, manual preshared keys are either unnamed or named by a small integer key ID. Implementations that support multiple keys for protocols that have no names for keys need to try all possible keys before deciding a packet cannot be validated. Typically key IDs are names valid only in the scope of one interface or peer. Manual preshared keys are often known by a group of peers not just one peer. This is an interesting security property: it is impossible to identify the peer sending a message cryptographically; it is only Hartman & Zhang Expires January 6, 2011 [Page 6] Internet-Draft Operations Model for Router Keying July 2010 possible to identify a group of peers using cryptographic means. As a consequence, authorization is typically based on knowing the preshared key rather than on being a particular peer. Note that once an authorization decision is made, the peer can assert its identity; this identity is trusted just as the routing information from the peer is trusted. However, for the process of authorization, it would be more complicated to identify peers this way and would not gain a security benefit in most deployments. Preshared keys that are used via automatic key management have not been specified. Their naming and authorization may differ. In particular, such keys may end up being known only by two peers. Alternatively they may also be known by a group of peers. Authorization could potentially be based on peer identity, although it is likely that knowing the right key will be sufficient. There does not appear to be a compelling reason to decouple the authorization of a key for some purpose from authorization of peers holding that key to perform the authorized function. Care needs to be taken when symmetric keys are used for multiple purposes. Consider the implications of using the same preshared key for two interfaces: it becomes impossible to distinguish a router on one interface from a router on another interface. So, a router that is trusted to participate in a routing protocol on one interface becomes implicitly trusted for the other interfaces that share the key. For many cases, such as OSPF routers in the same area, there is no significant advantage that an attacker could gain from this trust within the KARP threat model. However, other protocols, such as RIP, permit routes from a particular interface to be filtered. For these protocols, participation in one interface might be more advantageous than another. More subtle problems with key separation can appear in protocol design. Two protocols that use the same traffic keys may work together in unintended ways permitting one protocol to be used to attack the other. Consider two hypothetical protocols. Protocol A starts its messages with a set of extensions that are ignored if not understood. Protocol B has a fixed header at the beginning of its messages but ends messages with extension information. It may be that the same message is valid both as part of protocol A and protocol B. An attacker may be able to gain an advantage by getting a router to generate this message with one protocol under situations where the other protocol would not generate the message. This hypothetical example is overly simplistic; real-world attacks exploiting key separation weaknesses tend to be complicated and involve specific properties of the cryptographic functions involved. The key point is that whenever the same key is used in multiple protocols, attacks may be possible. All the involved protocols need Hartman & Zhang Expires January 6, 2011 [Page 7] Internet-Draft Operations Model for Router Keying July 2010 to be analyzed to understand the scope of potential attacks. Key separation attacks interact with the KARP operational model in a number of ways. Administrators need to be aware of situations where using the same manual traffic key with two different protocols (or the same protocol in different contexts) creates attack opportunities. Design teams should consider how their protocol might interact with other routing protocols and describe any attacks discovered so that administrators can understand the operational implications. When designing automated key management or new cryptographic authentication within routing protocols, we need to be aware that administrators expect to be able to use the same preshared keys in multiple contexts. As a result, we should use appropriate key derivation functions so that different cryptographic keys are used even when the same initial input key is used. 4.2. Public Keys Outside of a PKI, public keys are expected to be known by the hash of a key or (potentially self-signed) certificate. The Session Description Protocol provides a standardized mechanism for naming keys (in that case certificates) based on hashes. KARP SHOULD adopt this approach or another approach already standardized within the IETF rather than inventing a new mechanism for naming public keys. A public key is typically expected to belong to one peer. As a peer generates new keys and retires old keys, its public key may change. For this reason, from a management standpoint, peers should be thought of as associated with multiple public keys rather than as containing a single public key hash as an attribute of the peer object. Authorization of public keys could be done either by key hash or by peer identity. Performing authorizations by peer identity should make it easier to update the key of a peer without risk of losing authorizations for that peer. However management interfaces need to be carefully designed to avoid making this extra level of indirection complicated for operators. 4.3. Public Key Infrastructure When a PKI is used, certificates are used. The certificate binds a key to a name of a peer. The key management protocol is responsible for exchanging certificates and validating them to a trust anchor. Authorization needs to be done in terms of peer identities not in terms of keys. One reason for this is that when a peer changes its key, the new certificate needs to be sufficient for authentication to Hartman & Zhang Expires January 6, 2011 [Page 8] Internet-Draft Operations Model for Router Keying July 2010 continue functioning even though the key has never been seen before. Potentially authorization could be performed in terms of groups of peers rather than single peers. An advantage of this is that it may be possible to add a new router with no authentication related configuration of the peers of that router. Assuming that potentially self-signed certificates are used by routers that wish to use public keys but that do not need a PKI, then PKI and the infrastructureless mode of public-key operation described in the previous section can work well together. One router could identify its peers based on names and use certificate validation. Another router could use hashes of certificates. This could be very useful for border routers between two organizations. Smaller organizations could use public keys and larger organizations could use PKI. 4.4. The role of Central Servers An area to explore is the role of central servers like RADIUS or directories. As discussed in the design-guide, a system where keys are pushed by a central management system is undesirable as an end result for KARP. However central servers may play a role in authorization and key rollover. For example a node could send a hash of a public key to a RADIUS server. If central servers do play a role it will be critical to make sure that they are not required during routine operation or a cold-start of a network. They are more likely to play a role in enrollment of new peers or key migration/compromise. Another area where central servers may play a role is for group key agreement. As an example, [I-D.liu-ospfv3-automated-keying-req] discusses the potential need for key agreement servers in OSPF. Other routing protocols that use multicast or broadcast such as IS-IS are likely to need a similar approach. Hartman & Zhang Expires January 6, 2011 [Page 9] Internet-Draft Operations Model for Router Keying July 2010 5. Grouping Peers Together Discuss abstractions to manage interfaces, ASes, etc that have similar authorization sets. Hartman & Zhang Expires January 6, 2011 [Page 10] Internet-Draft Operations Model for Router Keying July 2010 6. Administrator Involvement One key operational question is what areas will administrator involvement be required. Likely areas where involvement may be useful includes enrollment of new peers. Hartman & Zhang Expires January 6, 2011 [Page 11] Internet-Draft Operations Model for Router Keying July 2010 7. Upgrade Considerations It needs to be possible to deploy automated key management in an organization without either having to disable existing security or disrupting routing. As a result, it needs to be possible to perform a phased upgrade from manual keying to automated key management. Hartman & Zhang Expires January 6, 2011 [Page 12] Internet-Draft Operations Model for Router Keying July 2010 8. Related Work Discuss draft-housley-saag-*, draft-polk-saag-*, the discussions in the KARP framework, etc. Hartman & Zhang Expires January 6, 2011 [Page 13] Internet-Draft Operations Model for Router Keying July 2010 9. Security Considerations This document does not define a protocol. It does discuss the operational and management implications of several security technologies. Hartman & Zhang Expires January 6, 2011 [Page 14] Internet-Draft Operations Model for Router Keying July 2010 10. Acknowledgments Funding for Sam Hartman's work on this memo is provided by Huawei. Hartman & Zhang Expires January 6, 2011 [Page 15] Internet-Draft Operations Model for Router Keying July 2010 11. References 11.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 11.2. Informative References [I-D.ietf-karp-design-guide] Lebovitz, G. and M. Bhatia, "Keying and Authentication for Routing Protocols (KARP) Design Guidelines", draft-ietf-karp-design-guide-00 (work in progress), February 2010. [I-D.liu-ospfv3-automated-keying-req] Liu, Y., "OSPFv3 Automated Group Keying Requirements", draft-liu-ospfv3-automated-keying-req-01 (work in progress), July 2007. [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. Hartman & Zhang Expires January 6, 2011 [Page 16] Internet-Draft Operations Model for Router Keying July 2010 Authors' Addresses Sam Hartman Painless Security Email: hartmans-ietf@mit.edu Dacheng Zhang Huawei Email: zhangdacheng@huawei.com Hartman & Zhang Expires January 6, 2011 [Page 17]