Network Working Group G. Belingueres Internet-Draft Independent Consultant Expires: May 18, 2000 November 18, 1999 TCP Security Filter draft-belingueres-tcpsec-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 [1]. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. 1. Abstract This document explains how to install a security protocol in a virtual layer between TCP [2] and the application layer using TCP Filters. The method is incrementally deployable, as neither party will install the security layer without the other's consent. 2. Introduction Application layer protocols were originally used in the clear on the Internet. However, increased use of those protocols for sensitive applications has required security measures. The natural place to secure data is at the application level, where application-specific security requirements are perfectly known to be used to get the strongest security at the best performance. Unfortunately, this requires changing each and every application. Belingueres Expires May 18, 2000 [Page 1] Internet-Draft TCP Security Filter November 1999 Another alternative is to secure the data at the IP level, as is done in IPSec [3], so that the IP packet payload is secured. While this is application independent, it requires to change or extend the core internetworking stack, and maybe those changes or extensions are not so trivial. Also, a solution based on IPSec may be is not so convenient if it is intended to be used for TCP connections security only, as it is for Virtual Private Networks (VPN). The other alternative, widely used today, is to provide the security as a virtual transport layer, so that the application can secure connections with the parties involved in the communication. For example SSL, and its successor TLS [4] were designed to provide channel-oriented security, so that an application can authenticate the parties involved and send and receive data protected against eavesdropping and forging. I propose signaling security requirement above TCP by negotiation with a TCP option. One side sends an ordered list of which security protocol families it supports. The other side selects one from the list, which commits both sides to securing higher level protocol data accordingly. More precisely, during the exchange of TCP's SYN packets, one side initiates a filter negotiation by announcing what security filters (security protocol families) it is prepared to employ. The other responds in the next ACK packet by listing the security filter family that it is prepared to accept. Thence, each side applies the agreed upon security protocol family to secure the application layer data. It means that now the parties has to "instantiate" one security protocol that belongs to the chosen family, so that the application data can be securely transported. An example of where this could help is the transmission of HTTP [5] messages secured with SSL or TLS, combination commonly known as HTTPS [6]. This option is an example of a TCP filter option of the class described in [7]. 3. 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 RFC-2119 [8]. A protocol family is a set of security protocols that are either interoperable or provide backward compatibility with older versions of that protocol family. The term security protocol, security filter and filter is used interchangeably. Belingueres Expires May 1, 2000 [Page 2] Internet-Draft TCP Security Filter November 1999 4. Option Format We use one TCP option, of type TSEC (to be assigned by IANA), to signal the requirement for a security protocol. A type field indicates the operation. A security announcement MUST NOT appear except as specified by the [7] protocol. (All TCPs MUST ignore unknown options in SYN packets [9].) Security protocol related packets MUST NOT be sent unless both parties have agreed to the appropriate filter via the protocol [7]. Security protocol family IDs will be assigned by IANA. +--------+-------+-------------------------------+ | TSEC | len | Security protocol family list | +--------+-------+-------------------------------+ 1 1 ? The security filters, including any parameters, are fixed during the three-way handshake by the protocol [7]. +--------+-------+-------------------------------+------------+ | TSEC | len | Security protocol family list | param list | +--------+-------+-------------------------------+------------+ 1 1 ? ? 5. Behavior As per [7], by "initiator," we indicate the party that first includes security options in its SYN packet, and by "respondent," we indicate the other party. If the respondent (cf., [7] protocol) has indicated that it can accept a security protocol family, a sender MUST use it. Only the initial security protocol families and parameters that the parties support are determined by the security options in the initial 3-way handshake. Once the TCP connection is open, particular security protocols will include their own structures in the data stream, and some parts of those may be transmitted unsecured. Note that because of the stream nature of TCP, the unsecured portion may be sent in the same packet as secure data. Any necessary framing must be done by particular security protocols. Senders MUST honor the security protocol family specified by the respondent, as per [7]. Local dictates to the contrary require in- band communication to alter the security behavior (ie. the security protocol instantiated is the responsible to make the necessary changes); if the security protocol precludes such communication, then the session MUST be terminated and re-established with different (or absent) security options. Belingueres Expires May 1, 2000 [Page 3] Internet-Draft TCP Security Filter November 1999 6. Interactions 6.1. TCP Urgent Pointer If security protocols must note application requests to send urgent data, it is up to the security filter instantiated if it is important not to provide detectable "signals" to avoid traffic analysis attacks. In particular, TCP header's Urgent pointer travels in the clear. Beside that, Urgent Pointers are processed as described in [7]. 7. Security Considerations This document describes a technique to upgrade a TCP connection for use with security functionality, as required by the security protocol instantiated. This document is not about a security technique per se. Man-in-the-middle attacks can not be detected by the TCP Filter method, so an attacker could back down the list of the preferred security protocol families to the "less secure" of the family. It's up to the security protocol instantiated and the application that uses it to authenticate adequately the parties involved. If the security protocol instantiated provides such an adequate method of authentication, then the man-in-the-middle is detectable before application data is transmitted. Urgent Pointers have to be used carefully, provided they could be used to mount a traffic analysis attack. Therefore, it is up to the security filter to decide if and how they are used. 8. Open issues This document is probably to evolve as the [7] do, but there are some other issues related specifically to security. Some of those are: Filters could be allowed to modify the setting of a connection's Keep Alive timer? If they are allowed to do so, there could be problems with any upper layer protocol semantics? The semantics given in [7] to chose filters in the 3-way handshake is "relaxed", in the sense that it does not contemplates the case in witch the client's application does REQUIRE some filters to be allowed to open the TCP connection at all, witch it is commonly the case with security protocols. As defined now, is responsibility of the upper layer to check that their minimal set of filters are agreed. The intersection semantics (in the set theory sense) given in the simultaneous open case could have security implications, in the sense that the applications (or any other component) are not involved in deciding if it is right to accept the filters in the intersection of the filter lists. Belingueres Expires May 1, 2000 [Page 4] Internet-Draft TCP Security Filter November 1999 9. References 1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. 2 Postel, J., "Transport Control Protocol", STD 7, RFC 793, September 1981. 3 Kent, S., Atkinson, R., "Security Architecture for the Internet Protocol", RFC 2401, November 1998. 4 Allen, C., and Dierks, T., "Transport Layer Security Protocol v1.0", RFC 2246, January 1999. 5 Fielding, R. et. al., "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 6 Rescorla, E., "HTTP over TLS", Internet-Draft, Work in Progress. 7 Bellovin, S. et. al., "TCP Filters", Internet-Draft, Work in Progress. 8 Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 9 Braden, R., "Requirements for Internet Hosts -- Communication Layers", RFC1122, October 1989. 10. Acknowledgments 11. Appendix Initial security protocols family to be supported SHOULD include the following: "TLS FAMILY": SSL 2.0, SSL 3.0, TLS 1.0. (code 0x00). 12. Author's Addresses Gabriel Belingueres Independent Consultant Ines Indart CP 2747, Argentina Email: gaby@ieee.org Belingueres Expires May 1, 2000 [Page 5] Internet-Draft TCP Security Filter November 1999 Full Copyright Statement Copyright (C) The Internet Society (1999). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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