NETWORK WORKING GROUP N. Williams Internet-Draft Sun Expires: September 1, 2007 M. Richardson SSW February 28, 2007 Better-Than-Nothing-Security: An Unauthenticated Mode of IPsec draft-ietf-btns-core-02.txt 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 September 1, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Williams & Richardson Expires September 1, 2007 [Page 1] Internet-Draft BTNS IPsec February 2007 Abstract This document specifies how to use the Internet Key Exchange (IKE) protocols, such as IKEv1 and IKEv2, to setup "unauthenticated" security associations (SAs) for use with the IPsec Encapsulating Security Payload (ESP) and the IPsec Authentication Header (AH). No IKE extensions are needed, but Peer Authorization Database (PAD) and Security Policy Database (SPD) extensions are specified. Unauthenticated IPsec is herein referred to by its popular acronym, "BTNS" (Better Than Nothing Security). Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Conventions used in this document . . . . . . . . . . . . . 3 2. BTNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Example #1: sgA . . . . . . . . . . . . . . . . . . . . . . 6 3.2. Example #2: Q . . . . . . . . . . . . . . . . . . . . . . . 8 3.3. Example #3: C . . . . . . . . . . . . . . . . . . . . . . . 9 3.4. Miscaellaneous examples . . . . . . . . . . . . . . . . . . 9 4. Security Considerations . . . . . . . . . . . . . . . . . . 10 4.1. Connection-Latching and Channel Binding . . . . . . . . . . 10 4.2. Leap-of-Faith (LoF) for BTNS . . . . . . . . . . . . . . . . 10 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.1. Normative References . . . . . . . . . . . . . . . . . . . . 12 5.2. Informative References . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14 Intellectual Property and Copyright Statements . . . . . . . 15 Williams & Richardson Expires September 1, 2007 [Page 2] Internet-Draft BTNS IPsec February 2007 1. Introduction Here we describe how to establish unauthenticated IPsec SAs using IKEv1 [RFC2408] [RFC2409] or IKEv2 [RFC4306] and unauthenticated public keys. No new on-the-wire protocol elements are added to IKE or IKEv2. The [RFC4301] processing model is assumed. This document does not define an opportunistic BTNS mode of IPsec whereby nodes may fallback on unprotected IP when their peers do not support IKE or IKEv2, nor does it describe "leap-of-faith" modes, or "connection latching." See [I-D.ietf-btns-prob-and-applic] for the applicability and uses of BTNS. 1.1. 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]. Williams & Richardson Expires September 1, 2007 [Page 3] Internet-Draft BTNS IPsec February 2007 2. BTNS The IPsec processing model, IKE and IKEv2 are hereby modified as follows: o A new ID type is added, 'PUBLICKEY'; IDs of this type have public keys as values. This ID type is not used on the wire. o A BTNS-specific PAD entry. This entry is intended to be the last entry in the PAD when BTNS is enabled. A peer that matches no other PAD entries is to be "authenticated" by verifying that the signature in its AUTH (or SIG) payload in the IKEv2 (or v1) exchange with the public key from the peer's CERT payload. The peer's ID MUST then be coerced to be of 'PUBLICKEY' type with the peer's public key as its value. o A new flag for SPD entries: 'BTNS_OK'. Traffic to/from peers that match the BTNS PAD entry will only match SPD entries that have the BTNS_OK flag set. The SPD may be searched by address or by ID (of type PUBLICKEY, of course, for BTNS peers), as per the IPsec processing model [RFC4301]; searching by ID in this case requires creation of SPD entries that are bound to public key values (this could be used to build "leap-of-faith" behaviour, for example). Nodes MUST reject IKE_SA proposals from peers that match non-BTNS PAD entries but fail to authenticate properly. Nodes wishing to be treated as BTNS nodes by their peers SHOULD use bare RSA key CERT payloads and MAY use certificates known not to have been pre-shared with their peers or outside their trust anchors (e.g., self-signed certificates). RSA keys for use in BTNS may be generated at any time, but "connection latching" [I-D.ietf-btns-connection-latching] requires that they remain constant between IKE exchanges setting up SAs for latched connections. To preserve standard IPsec access control semantics BTNS responders MUST NOT allow BTNS peers to assert addresses which could be asserted by non-BTNS peers. This can be achieved by processing the PAD in order both, when peers authenticate and when BTNS peers negotiate child SAs -- in the first case the PAD is searched for a matching PAD entry as usual, and in the second it is searched to make sure that BTNS peers' asserted child SA traffic selectors do not conflict with non-BTNS PAD entries. Note that in general, if there are multiple PAD entries with wildcard matching on peer ID then all but the last one should constrain the traffic selectors for matching peers. Note that nodes may unwittingly match peers' BTNS PAD entries and be Williams & Richardson Expires September 1, 2007 [Page 4] Internet-Draft BTNS IPsec February 2007 authenticated as BTNS nodes. This may be used in specifying the "latching" of traffic flows to peer IDs [I-D.ietf-btns-connection-latching]. Williams & Richardson Expires September 1, 2007 [Page 5] Internet-Draft BTNS IPsec February 2007 3. Usage Scenarios In order to explain the above rules a number of scenarios will be postulated. The goal here is to demonstrate that the above rules are both sufficient and required. To explain the scenarios a reference diagram describing a canonical network will be used. It is as follows: [Q] [R] AS1 . . AS2 [A]----+----[SG-A].......+....+.......[SG-B]-------[B] ...... \ ..PI.. ----[btns-B] ...... [btns-C].....+....+.......[btns-D] Figure 1: Reference Network Diagram In this diagram, there are six end-nodes: A, B, C and D. Two of the systems are security gateways: SG-A, SG-B, protecting networks on which [A] and [B] reside. There is a node [Q] which is IPsec and BTNS capable, and node [R] is a simple node, with no IPsec or BTNS capability. Nodes [C] and [D] are BTNS capable. We will examine interactions between the BTNS enabled nodes, and the IPsec enabled nodes. Nodes C and Q have a fixed addresses. Node D non-fixed addresses. PI is the Public Internet ("The Wild"). 3.1. Example #1: sgA The machine that we will care about will be [SG-A], a firewall device of some kind which we wish to configure to respond to BTNS connections from [C] SG-A has the following "VPN" PAD and SPD entries: Child SA Rule Remote ID IDs allowed SPD Search by ---- --------- ----------- ------------- 1 by-IP 2 by-IP 3 PUBLICKEY:any ANY by-IP The last entry is the BTNS entry. Williams & Richardson Expires September 1, 2007 [Page 6] Internet-Draft BTNS IPsec February 2007 Figure 2: SG-A PAD table Here is any address that is not part of A's network, and is not claimed by any other entry. Note that sgA's PAD entry has one and only one wildcard PAD entry: the BTNS catch-all PAD entry, so, as described in Section 2. and are from [RFC4301] section 4.4.3 Rule Local Remote Next Layer BTNS Action addr addr Protocol ok ---- ----- ------ ---------- ---- ----------------------- 1 A R ANY N/A BYPASS 2 A Q ANY no PROTECT(ESP,tunnel,AES, SHA256) 3 A B-net ANY no PROTECT(ESP,tunnel,AES, SHA256) 4 A ANY ANY yes PROTECT(ESP,transport, integr+conf) Figure 3: SG-A SPD table The processing by sgA of various peers then is as follows: o Q does not match PAD entry #1, but does match PAD entry #2; PAD processing stops, then the SPD is searched by Q's ID to find entry #2; CHILD SAs are then allowed that have sgA's and Q's addresses as the end-point addresses. o sgB matches PAD entry #1; PAD processing stops, then the SPD is searched by sgB's ID to find SPD entry #3; CHILD SAs are then allowed that have sgA's address and any addresses from B's network as the end-point addresses. o R does not initiate any IKE_SAs; its traffic to A is bypassed by SPD entry #1. o C does not match PAD entries #1 or #2, but does match entry #3, the BTNS wildcard PAD entry; the SPD is searched by C's address and SPD entry #4 is matched. CHILD SAs are then allowed that have sgA's address and C's address as the end-point addresses provided that C's address is neither Q's nor any of B's (see Section 2). o Rogue BTNS nodes attempting to assert Q's or B's addresses will either match the PAD entries for Q or B and fail to authenticate as Q or B, in which case they are rejected, or they will match PAD Williams & Richardson Expires September 1, 2007 [Page 7] Internet-Draft BTNS IPsec February 2007 entry #3 but will not be allowed to create CHILD SAs with Q's or B's addresses as traffic selectors. o Rogue BTNS nodes attempting to assert C's address are allowed. Protection for C requires additional bindings of C's specific BTNS ID (that is, its public key) to its traffic flows through connection-latching and channel binding, or leap-of-faith, none of which are described here. 3.2. Example #2: Q Q is either a BITS or native IPsec implementation; if it is a native implementation it may have IPsec-aware applications, specifically NFSv4 (TCP port 2049). In any case, Q wants to communicate with A generally, and with BTNS peers for NFSv4 only. It's PAD and SPD are configured as follows: Child SA Rule Remote ID IDs allowed SPD Search by ---- --------- ----------- ------------- 1 by-IP 2 PUBLICKEY:any ANY by-IP The last entry is the BTNS entry. Figure 4: Q PAD table Rule Local Remote Next Layer BTNS Action addr addr Protocol ok ---- ----- ------ ---------- ---- ----------------------- 1 Q A ANY no PROTECT(ESP,tunnel,AES, SHA256) 2 Q ANY ANY yes PROTECT(ESP,transport, and integr+conf) port 2049 Figure 5: SG-A SPD table The same analysis shown above in Section 3.1 applies here with respect to sgA, C and rogue peers, except that C is allowed only access to the NFSv4 service on Q. Additionally sgB is treated as a BTNS peer as it is not known to Q, and therefore any host behind sgB can access the NFSv4 service on Q (and, because Q has no formal relationship with sgB, rogues can impersonate B). Williams & Richardson Expires September 1, 2007 [Page 8] Internet-Draft BTNS IPsec February 2007 3.3. Example #3: C C only supports BTNS and wants to use BTNS to protect NFSv4 traffic. It's PAD and SPD are configured as follows: Child SA Rule Remote ID IDs allowed SPD Search by ---- --------- ----------- ------------- 1 PUBLICKEY:any ANY by-IP The last entry is the BTNS entry. Figure 6: Q PAD table Rule Local Remote Next Layer BTNS Action addr addr Protocol ok ---- ----- ------ ---------- ---- ----------------------- 1 C ANY ANY yes PROTECT(ESP,transport, and integr+conf) port 2049 2 C ANY ANY N/A BYPASS Figure 7: SG-A SPD table 3.4. Miscaellaneous examples If sgA were not BTNS-capable then it would not have PAD and SPD entries #3 and #4, respectively. Then C would be rejected as usual under the standard IPsec model [RFC4301]. Similarly, if Q were not BTNS-capable then it would not have PAD and SPD entries #2. Then C would be rejected as usual under the standard IPsec model [RFC4301]. Williams & Richardson Expires September 1, 2007 [Page 9] Internet-Draft BTNS IPsec February 2007 4. Security Considerations Unauthenticated security association negotiation is subject to MITM attacks and should be used with care. Where security infrastructures are lacking this may indeed be better than nothing. Use with applications that bind authentication at higher network layers to secure channels at lower layers may provide one secure way to use unauthenticated IPsec, but this is not specified herein. Use of multiple wildcard PAD entries can be problematic. Where it is important that addresses and node identities be tightly bound it is important that such PAD entries limit the addresses that matching peers can assert for their CHILD SAs to non-overlapping address spaces. In practice this may be difficult to configure; if it is not feasible to configure systems in this way then either BTNS should not be used or BTNS PAD entries should constrain matching peers only to using services for which authentication is not normally necessary or where IPsec-aware/connection-latching applications are used. 4.1. Connection-Latching and Channel Binding BTNS is subject to MITM attacks. One way to protect against MITM attacks subsequent to initial communications is to use "connection latching" [I-D.ietf-btns-connection-latching], whereby ULPs cooperate with IPsec in native IPsec implementations to bind individual packet flows to sequences of SAs whose end-point IDs (public keys, in the case of BTNS end-points) and other characteristics (e.g., quality of protection) must all be the same. MITMs can be detected by using application-layer authentication frameworks and/or mechanisms, such as the GSS-API [RFC2743], with channel binding [I-D.ietf-nfsv4-channel-bindings] [I-D.ietf-kitten-gssapi-channel-bindings], where the channels to be bound to application-layer authentication are latched connections and where the channel bindings data strongly identify the end-points of the latched connection (e.g., the public keys of the end-points). 4.2. Leap-of-Faith (LoF) for BTNS "Leap of faith" is the term generally used for the habit of accepting that a given key identifies a peer that one wanted to talk to without strong evidence for that proposition. Specifically this is a common mode of operation for Secure Shell [RFC4251] clients where, when a server is encountered for the first time the client may ask the user whether to accept the server's public key as its identity and, if so, records the server's name (as given by the user) and public key in a database. Williams & Richardson Expires September 1, 2007 [Page 10] Internet-Draft BTNS IPsec February 2007 Leap of Faith can work in a similar way for BTNS nodes, but it is currently still being designed and specified by the IETF BTNS WG. Williams & Richardson Expires September 1, 2007 [Page 11] Internet-Draft BTNS IPsec February 2007 5. References 5.1. Normative References [I-D.ietf-btns-connection-latching] Williams, N., "IPsec Channels: Connection Latching", draft-ietf-btns-connection-latching-00 (work in progress), February 2006. [I-D.ietf-btns-prob-and-applic] Touch, J., "Problem and Applicability Statement for Better Than Nothing Security (BTNS)", draft-ietf-btns-prob-and-applic-05 (work in progress), February 2007. [I-D.ietf-kitten-gssapi-channel-bindings] Williams, N., "Clarifications and Extensions to the GSS- API for the Use of Channel Bindings", draft-ietf-kitten-gssapi-channel-bindings-02 (work in progress), June 2006. [I-D.ietf-nfsv4-channel-bindings] Williams, N., "On the Use of Channel Bindings to Secure Channels", draft-ietf-nfsv4-channel-bindings-04 (work in progress), June 2006. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2408] Maughan, D., Schneider, M., and M. Schertler, "Internet Security Association and Key Management Protocol (ISAKMP)", RFC 2408, November 1998. [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. 5.2. Informative References [RFC2743] Linn, J., "Generic Security Service Application Program Interface Version 2, Update 1", RFC 2743, January 2000. [RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Williams & Richardson Expires September 1, 2007 [Page 12] Internet-Draft BTNS IPsec February 2007 Protocol Architecture", RFC 4251, January 2006. Williams & Richardson Expires September 1, 2007 [Page 13] Internet-Draft BTNS IPsec February 2007 Authors' Addresses Nicolas Williams Sun Microsystems 5300 Riata Trace Ct Austin, TX 78727 US Email: Nicolas.Williams@sun.com Michael C. Richardson Sandelman Software Works 470 Dawson Avenue Ottawa, ON K1Z 5V7 CA Email: mcr@sandelman.ottawa.on.ca URI: http://www.sandelman.ottawa.on.ca/ Williams & Richardson Expires September 1, 2007 [Page 14] Internet-Draft BTNS IPsec February 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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, THE IETF TRUST 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. 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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). Williams & Richardson Expires September 1, 2007 [Page 15]