Network Working Group Steven M. Bellovin
Internet Draft AT&T Labs Research
Expiration Date: April 2003 October 2002
Guidelines for Mandating the Use of IPsec
draft-bellovin-useipsec-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Abstract
The Security Considerations sections of many Internet Drafts say, in
effect, "just use IPsec". While this is sometimes correct, more
often it will leave users without real, interoperable security
mechanisms. This memo offers some guidance on when IPsec should and
should not be specified.
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1. Introduction
The Security Considerations sections of many Internet Drafts say, in
effect, "just use IPsec". While this is sometimes correct, more
often it will leave users without real, interoperable security
mechanisms. IPsec is often unavailable in the likely endpoints.
Even if it is available, it may not provide the proper granularity of
protection. Finally, if it is available and appropriate, the
document mandating it needs to specify just how it is to be used.
Recall that the goal is realistic, interoperable security.
Specifying, as the only security mechanism, a configuration which is
unavailable to -- and hence unusable by -- a majority of the user
community is tantamount to saying "turn off security".
For further guidance on security considerations (including discussion
of IPsec), see [SECCONS].
NOTE: Many of the arguments below relate to the capabilities of
current implementations of IPsec. These may change over time; this
advice based on the knowledge available to the IETF at publication
time.
2. WARNING
The design of security protocols is a subtle and difficult art. The
cautions here about specifying use of IPsec should NOT be taken to
mean that that you should invent your own new security protocol for
each new application. If IPsec is a bad choice, use another
standardized, well-understood security protocol. Don't roll your
own.
3. The Pieces of IPsec
IPsec is composed of a number of different pieces. These can be used
to provide confidentiality, integrity, and replay protection; though
some these can be configured manually, in general a key management
component is used. Additionally, the decision about whether and how
to use IPsec is controlled by a policy database of some sort.
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3.1. AH and ESP
The Authentication Header (AH) [RFC2402] and the Encapsulating
Security Protocol (ESP) [RFC2406] are the over-the-wire security
protocols. Both provide (optional) replay protection. ESP typically
is used to provide confidentiality (encryption), integrity and
authentication for traffic. ESP also can provide integrity and
authentication without confidentiality, which makes it a good
alternative to AH in most cases where confidentiality is not a
required or desired service. Finally, ESP can be used to provide
confidentiality alone, although this is not recommended [Bell96].
The difference in integrity protection offered by AH is that AH
protects portions of the preceding IP header, including the source
and destination address. However, when ESP is used in tunnel mode
(see section 4.2), and if integrity/authentication is enabled, the IP
header seen by the source and destination hosts is completely
protected anyway.
AH can also protect those IP options that need to be seen by
intermediate routers, but must be intact and authentic when delivered
to the receiving system. At this time, use (and existence) of such
IP options is extremely rare.
If an application requires such protection, and if the information to
be protected cannot be inferred from the key management process, AH
must be used. (ESP is generally regarded as easier to implement;
however, virtually all IPsec packages support both.) If
confidentiality is required, ESP must be used. It is possible to use
AH in conjunction with ESP, but this combination is rarely required.
3.2. Transport and Tunnel Mode
AH and ESP can both be used in either transport mode or tunnel mode.
In tunnel mode, the IPsec header is followed by an inner IP header;
this is the normal usage for Virtual Private Networks (VPN), and it
is generally required whenever either end of the IPsec-protected path
is not the ultimate IP destination, e.g., when IPsec is implemented
in a firewall, router, etc. Transport mode is preferred for point-
to-point communication, though tunnel mode can be used for this
purpose.
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3.3. Key Management
Any cryptographic system requires key management. IPsec provides for
both manual and automatic key management schemes. Manual key
management is easy; however, it doesn't scale very well. Also,
IPsec's replay protection mechanisms are not available if manual key
management is used.
One automated key exchange mechanism is available, Internet Key
Exchange (IKE) [RFC2409]. A new, simpler version of IKE is currently
being designed. A second mechanism, Kerberized Internet Key Exchange
(KINK) [KINK], is being defined. It, of course, uses Kerberos, and
is suitable if and only if a Kerberos infrastructure is available.
IKE can use public key certificates or shared secrets. Shared secret
authentication is simpler, but doesn't scale as well, since each
endpoint must share a unique secret with every peer with which it can
communicate, in order to uniquely authenticate these peers.
IKE also has public key variants. In most real-world situations,
these use locally-issued certificates. That is, the administrator of
the system or network concerned will issue certificates to all
authorized users; these certificates are useful only for IPsec.
It is sometimes possible to use certificates from an existing PKI
with IKE. In practice, this is rare. Furthermore, there not only is
no global PKI for the Internet, there probably never will be one.
Designing a structure which assumes such a PKI is a mistake. In
particular, assuming that an arbitrary node will have an "authentic"
certificate, issued by a mutually trusted third party and vouching
for that node's identity, is wrong. Again, such a PKI does not and
probably will not exist. Public key IKE is generally a good idea,
but almost always with locally-issued certificates.
Note that public key schemes require a substantial amount of
computation. Protocol designers should consider whether or not such
computations are feasible on devices of interest to their clientele.
Using certificates roughly doubles the number of large
exponentiations that must be performed, compared with shared secret
versions of IKE.
Today, even low-powered devices can generally perform enough
computation to set up a limited number of security associations;
concentration points, such as firewalls or VoIP servers, may require
hardware assists, especially if many peers are expected to create
security associations at about the same time.
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3.4. Applications Program Interface (API)
It is, in some sense, a misnomer to speak of the API as a part of
IPsec, since that piece is missing on many systems. To the extent
that it does exist, it isn't standardized. The problem is simple:
it is difficult or impossible to request IPsec protection, or to tell
if was used for given inbound packets or connections.
Router- or firewall-based IPsec implementations pose even greater
problems, since there is no standardized over-the-wire protocol for
communicating this information from outboard encryptors to hosts.
4. Availability of IPsec in Target Devices
Although IPsec is now widely implemented, and is available for
current releases of most host operating systems, it is less available
for embedded systems. Few hubs, network address translators, etc.,
implement it, especially at the low end. It is generally
inappropriate to rely on IPsec when many of the endpoints are in this
category.
Even for host-to-host use, IPsec availability (and experience, and
ease of use) has generally been for VPNs. Hosts that support IPsec
for VPN use do not always support it on a point-to-point basis,
especially via a stable, well-defined API ur user interface.
Finally, few implementations support multiple layers of IPsec. If a
telecommuter is using IPsec in VPN mode to access an organizational
network, he or she may not be able to employ a second level of IPsec
to protect an application connection to a host within the
organization. (We note that such support is, in fact, mandated by
Case 4 of Section 4.5 of [RFC2041]. Nevertheless, it is not widely
available.) The likelihood of such deployment scenarios should be
taken into account when deciding whether or not to mandate IPsec.
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5. Endpoints
[RFC2401] describes many different forms of endpoint identifier.
These include source addresses (both IPv4 and IPv6), host names
(possibly as embedded in X.500 certificates), and user IDs (again,
possibly as embedded in a certificate). Not all forms of identifier
are available on all implementations; in particular, user-granularity
identification is not common. This is especially a concern for
multi-users systems, where it may not be possible to use different
certificates to distinguish between traffic from two different users.
Again, we note that the ability to provide fine-grained protection,
such as keying each connection separately, and with per-user
credentials, was one of the original design goals of IPsec.
Nevertheless, only a few platforms support it. Indeed, some
implementations do not even support using port numbers when deciding
whether or not to apply IPsec protection.
6. Selectors and the SPD
Section 4.4 of [RFC2401] describes the Security Policy Database (SPD)
and "selectors" used to decide what traffic should be protected by
IPsec. Choices include source and destination addresses (or address
ranges), protocol numbers (i.e., 6 for TCP and 17 for UDP), and port
numbers for TCP and UDP. Protocols whose protection requirements
cannot be described in such terms are poorer candidates for IPsecp in
particular, it becomes impossible to apply protection at any finer
grain than "destination host". Thus, traffic embedded in an L2TP
[RFC2661] session cannot be protected selectively by IPsec above the
L2TP layer, because IPsec has no selectors defined that let it peer
into the L2TP packet to find the TCP port numbers. Similarly, SCTP
[RFC2960] did not exist when [RFC2401] was written; thus, protecting
individual SCTP applications on the basis of port number could not be
done until a new document is written that defines new selectors for
IPsec.
Implementation issues are also a concern here. As before, too many
vendors have not implemented the full specifications; too many IPsec
implementations are not capable of using port numbers in their
selectors. Protection of traffic between two hosts is thus on an all
or nothing basis when these non-compliant implementations are
employed.
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7. Broadcast and Multicast
Although the designers of IPsec tried to leave room for protection of
multicast traffic, the design has not yet been completed. There is,
as yet, no key management for the general case. Worse yet, an
important component of over-the-wire IPsec -- replay protection -- is
very difficult to implement in a multi-sender situation. IPsec is
thus inappropriate for such protocols unless and until suitable key
management and replay protection mechanisms are defined and available
in the target domain. (Single-sender multicast can be supported, if
suitable key management is available; the MSEC working group is
developing such a protocol.)
8. Mandating IPsec
Despite all of the caveats given above, it may still be appropriate
to use IPsec in particular situations. The range of choices make it
mandatory to define precisely how IPsec is to be used. Authors of
RFCs that rely on IPsec must specify the following:
(a) What selectors the initiator of the conversation (the client,
in client-server architectures) should use? In particular,
what addresses, port numbers, etc., are to be used?
(b) What IPsec protocol is to be used: AH or ESP? What mode is
to be employed: transport mode or tunnel mode?
(c) What form of key management is appropriate?
(d) What security policy database entry types should be used by
the responder (i.e., the server) when deciding whether or not
to accept the IPsec connection request.
(e) What form of identification and authentication should be used.
(f) Which of the many variants of IKE must be supported.
(g) Is suitable IPsec support available in likely configurations
of the products that would have to employ IPsec?
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9. Example
Suppose that the designers of the Border Gateway Protocl (BGP)
[RFC1771] wished to use IPsec for security, rather than the mechanism
described in [2385]. Does it meet these criteria? (Note that the
deeper security issues raised by BGP are not addressed by IPsec or
any other transmission security mechanism. See [Kent00a] and
[Kent00b] for more details.)
Selectors
The issue of selectors is easy. BGP already runs between
manully-configured pairs of hosts on TCP port 179. The
appropriate selector would be the pair of BGP speakers, for
that port only. Note that the router's "loopback address" is
almost certainly the address to use.
Mode
Clearly, transport mode is the proper choice. The information
being communicated is generally not confidential, so
encryption need not be used. Either AH or ESP can be used; if
ESP is used, the sender's IP address would need to be checked
against the IP address asserted in the key management
exchange. (This check is mandated by [RFC2401].)
Key Management
To permit replay detection, an automated key management system
should be used, most likely IKE.
Security Policy
Connections should be accepted only from the designated peer.
Authentication
Given the number of BGP-speaking routers used internally by
large ISPs, it is likely that shared key mechanisms are
inadequate. Consequently, certificate-based IKE must be
supported. However, shared secret mode is reasonable on
peering links, or (perhaps) on links between ISPs and
customers. Whatever scheme is used, it must tie back to a
source IP address in some fashion, since other BGP policies
are expressed in terms of the peer's IP address.
Availability
For this scenario, availability is the crucial question. Do
likely BGP speakers -- both backbone routers and access
routers -- support the profile of IPsec described above? Will
use of IPsec, with its attendant expensive cryptographic
operations, raise the issue of new denial of service attacks?
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The working group and the IESG must make these determinations
before deciding to use IPsec to protect BGP.
10. Security Considerations
IPsec provides transmission security and simple access control only.
There are many other dimensions to protocol security that are beyond
the scope of this memo. Within its scope, the security of any
resulting protocol depends heavily on the accuracy of the analysis
that resulted in a decision to use IPsec.
11. Acknowledgments
Ran Atkinson, Barbara Fraser, Paul Hoffman, and Stephen Kent made
many useful suggestions.
12. References
[Bell96] "Problem Areas for the IP Security Protocols", S.M.
Bellovin, Proc. Sixth Usenix Security Symposium, 1996, pp.
205-214.
[Kent00a] "Secure Border Gateway Protocol (Secure-BGP)", S. Kent, C.
Lynn, and K. Seo, IEEE Journal on Selected Areas in
Communications 18:4, April 2000, pp. 582-592.
[Kent00b] "Secure Border Gateway Protocol (S-BGP) -- Real World
Performance and Deployment Issues", S. Kent, C. Lynn, J.
Mikkelson, and K. Seo, Proc. Network and Distributed System
Security Symposium, February 2000.
[KINK] "Kerberized Internet Negotiation of Keys (KINK)", M. Thomas
and J. Vilhuber, work in progress, 2002.
[RFC1771] "A Border Gateway Protocol 4 (BGP-4)", Y. Rekhter and T.
Li. RFC 1771, March 1995.
[RFC2401] "Security Architecture for the Internet Protocol", S. Kent
and R. Atkinson, RFC 2401, November 1998.
[RFC2402] "IP Authentication Header", S. Kent and R. Atkinson, RFC
2401, November 1998.
[RFC2406] "IP Encapsulating Security Payload (ESP)", S. Kent and R.
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Atkinson, RFC 2401, November 1998.
[RFC2409] "The Internet Key Exchange (IKE)", D. Harkins and D.
Carrel. RFC 2409, November 1998.
[RFC2661] "Layer Two Tunneling Protocol L2TP", W. Townsley, A.
Valencia, A. Rubens, G. Pall, G. Zorn, B. Palter. RFC 2661,
August 1999.
[RFC2960] "Stream Control Transmission Protocol", R. Stewart, Q. Xie,
K. Morneault, C. Sharp, H. Schwarzbauer, T. Taylor, I. Rytina,
M. Kalla, L. Zhang, and V. Paxson. RFC 2960, October 2000.
[SECCONS] "Guidelines for Writing RFC Text on Security
Considerations", E. Rescorla and B. Korver, work in progress,
2002.
13. Author Information
Steven M. Bellovin
AT&T Labs Research
Shannon Laboratory
180 Park Avenue
Florham Park, NJ 07932
Phone: +1 973-360-8656
email: bellovin@acm.org
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