Internet DRAFT - draft-belingueres-tcpsec
draft-belingueres-tcpsec
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].
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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.
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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.
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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.
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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.
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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.
<draft-ietf-tls-https-04.txt>
7 Bellovin, S. et. al., "TCP Filters", Internet-Draft, Work in
Progress. <draft-bellovin-tcpfilt-00.txt>
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
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