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-<A HREF="toc.html">Contents</A>
-<A HREF="politics.html">Previous</A>
-<A HREF="mail.html">Next</A>
-<HR>
-<H1><A name="ipsec.detail">The IPsec protocols</A></H1>
-<P>This section provides information on the IPsec protocols which
- FreeS/WAN implements. For more detail, see the<A href="rfc.html"> RFCs</A>
-.</P>
-<P>The basic idea of IPsec is to provide security functions,<A href="glossary.html#authentication">
- authentication</A> and<A href="glossary.html#encryption"> encryption</A>
-, at the IP (Internet Protocol) level. This requires a higher-level
- protocol (IKE) to set things up for the IP-level services (ESP and AH).</P>
-<H2><A NAME="27_1">Protocols and phases</A></H2>
-<P>Three protocols are used in an IPsec implementation:</P>
-<DL>
-<DT>ESP, Encapsulating Security Payload</DT>
-<DD>Encrypts and/or authenticates data</DD>
-<DT>AH, Authentication Header</DT>
-<DD>Provides a packet authentication service</DD>
-<DT>IKE, Internet Key Exchange</DT>
-<DD>Negotiates connection parameters, including keys, for the other two</DD>
-</DL>
-<P>The term "IPsec" (also written as IPSEC) is slightly ambiguous. In
- some contexts, it includes all three of the above but in other contexts
- it refers only to AH and ESP.</P>
-<P>There is more detail below, but a quick summary of how the whole
- thing works is:</P>
-<DL>
-<DT>Phase one IKE (main mode exchange)</DT>
-<DD>sets up a keying channel (ISAKMP SA) between the two gateways</DD>
-<DT>Phase two IKE (quick mode exchange)</DT>
-<DD>sets up data channels (IPsec SAs)</DD>
-<DT>IPsec proper</DT>
-<DD>exchanges data using AH or ESP</DD>
-</DL>
-<P>Both phases of IKE are repeated periodically to automate re-keying.</P>
-<H2><A name="others">Applying IPsec</A></H2>
-<P>Authentication and encryption functions for network data can, of
- course, be provided at other levels. Many security protocols work at
- levels above IP.</P>
-<UL>
-<LI><A href="glossary.html#PGP">PGP</A> encrypts and authenticates mail
- messages</LI>
-<LI><A href="glossary.html#SSH">SSH</A> authenticates remote logins and
- then encrypts the session</LI>
-<LI><A href="glossary.html#SSL">SSL</A> or<A href="glossary.html#TLS">
- TLS</A> provides security at the sockets layer, e.g. for secure web
- browsing</LI>
-</UL>
-<P>and so on. Other techniques work at levels below IP. For example,
- data on a communications circuit or an entire network can be encrypted
- by specialised hardware. This is common practice in high-security
- applications.</P>
-<H3><A name="advantages">Advantages of IPsec</A></H3>
-<P>There are, however, advantages to doing it at the IP level instead
- of, or as well as, at other levels.</P>
-<P>IPsec is the<STRONG> most general way to provide these services for
- the Internet</STRONG>.</P>
-<UL>
-<LI>Higher-level services protect a<EM> single protocol</EM>; for
- example PGP protects mail.</LI>
-<LI>Lower level services protect a<EM> single medium</EM>; for example a
- pair of encryption boxes on the ends of a line make wiretaps on that
- line useless unless the attacker is capable of breaking the encryption.</LI>
-</UL>
-<P>IPsec, however, can protect<EM> any protocol</EM> running above IP
- and<EM> any medium</EM> which IP runs over. More to the point, it can
- protect a mixture of application protocols running over a complex
- combination of media. This is the normal situation for Internet
- communication; IPsec is the only general solution.</P>
-<P>IPsec can also provide some security services "in the background",
- with<STRONG> no visible impact on users</STRONG>. To use<A href="glossary.html#PGP">
- PGP</A> encryption and signatures on mail, for example, the user must
- at least:</P>
-<UL>
-<LI>remember his or her passphrase,</LI>
-<LI>keep it secure</LI>
-<LI>follow procedures to validate correspondents' keys</LI>
-</UL>
-<P>These systems can be designed so that the burden on users is not
- onerous, but any system will place some requirements on users. No such
- system can hope to be secure if users are sloppy about meeting those
- requirements. The author has seen username and password stuck on
- terminals with post-it notes in an allegedly secure environment, for
- example.</P>
-<H3><A name="limitations">Limitations of IPsec</A></H3>
-<P>IPsec is designed to secure IP links between machines. It does that
- well, but it is important to remember that there are many things it
- does not do. Some of the important limitations are:</P>
-<DL>
-<DT><A name="depends">IPsec cannot be secure if your system isn't</A></DT>
-<DD>System security on IPsec gateway machines is an essential
- requirement if IPsec is to function as designed. No system can be
- trusted if the underlying machine has been subverted. See books on Unix
- security such as<A href="biblio.html#practical"> Garfinkel and Spafford</A>
- or our web references for<A href="web.html#linsec"> Linux security</A>
- or more general<A href="web.html#compsec"> computer security</A>.
-<P>Of course, there is another side to this. IPsec can be a powerful
- tool for improving system and network security. For example, requiring
- packet authentication makes various spoofing attacks harder and IPsec
- tunnels can be extremely useful for secure remote administration of
- various things.</P>
-</DD>
-<DT><A name="not-end-to-end">IPsec is not end-to-end</A></DT>
-<DD>IPsec cannot provide the same end-to-end security as systems working
- at higher levels. IPsec encrypts an IP connection between two machines,
- which is quite a different thing than encrypting messages between users
- or between applications.
-<P>For example, if you need mail encrypted from the sender's desktop to
- the recipient's desktop and decryptable only by the recipient, use<A href="glossary.html#PGP">
- PGP</A> or another such system. IPsec can encrypt any or all of the
- links involved -- between the two mail servers, or between either
- server and its clients. It could even be used to secure a direct IP
- link from the sender's desktop machine to the recipient's, cutting out
- any sort of network snoop. What it cannot ensure is end-to-end
- user-to-user security. If only IPsec is used to secure mail, then
- anyone with appropriate privileges on any machine where that mail is
- stored (at either end or on any store-and-forward servers in the path)
- can read it.</P>
-<P>In another common setup, IPsec encrypts packets at a security gateway
- machine as they leave the sender's site and decrypts them on arrival at
- the gateway to the recipient's site. This does provide a useful
- security service -- only encrypted data is passed over the Internet --
- but it does not even come close to providing an end-to-end service. In
- particular, anyone with appropriate privileges on either site's LAN can
- intercept the message in unencrypted form.</P>
-</DD>
-<DT><A name="notpanacea">IPsec cannot do everything</A></DT>
-<DD>IPsec also cannot provide all the functions of systems working at
- higher levels of the protocol stack. If you need a document
- electronically signed by a particular person, then you need his or her<A
-href="glossary.html#signature"> digital signature</A> and a<A href="glossary.html#public">
- public key cryptosystem</A> to verify it with.
-<P>Note, however, that IPsec authentication of the underlying
- communication can make various attacks on higher-level protocols more
- difficult. In particular, authentication prevents<A href="glossary.html#middle">
- man-in-the-middle attacks</A>.</P>
-</DD>
-<DT><A name="no_user">IPsec authenticates machines, not users</A></DT>
-<DD>IPsec uses strong authentication mechanisms to control which
- messages go to which machines, but it does not have the concept of user
- ID, which is vital to many other security mechansims and policies. This
- means some care must be taken in fitting the various security
- mechansims on a network together. For example, if you need to control
- which users access your database server, you need some non-IPsec
- mechansim for that. IPsec can control which machines connect to the
- server, and can ensure that data transfer to those machines is done
- securely, but that is all. Either the machines themselves must control
- user access or there must be some form of user authentication to the
- database, independent of IPsec.</DD>
-<DT><A name="DoS">IPsec does not stop denial of service attacks</A></DT>
-<DD><A href="glossary.html#DOS">Denial of service</A> attacks aim at
- causing a system to crash, overload, or become confused so that
- legitimate users cannot get whatever services the system is supposed to
- provide. These are quite different from attacks in which the attacker
- seeks either to use the service himself or to subvert the service into
- delivering incorrect results.
-<P>IPsec shifts the ground for DoS attacks; the attacks possible against
- systems using IPsec are different than those that might be used against
- other systems. It does not, however, eliminate the possibility of such
- attacks.</P>
-</DD>
-<DT><A name="traffic">IPsec does not stop traffic analysis</A></DT>
-<DD><A href="glossary.html#traffic">Traffic analysis</A> is the attempt
- to derive intelligence from messages without regard for their contents.
- In the case of IPsec, it would mean analysis based on things visible in
- the unencrypted headers of encrypted packets -- source and destination
- gateway addresses, packet size, et cetera. Given the resources to
- acquire such data and some skill in analysing it (both of which any
- national intelligence agency should have), this can be a very powerful
- technique.
-<P>IPsec is not designed to defend against this. Partial defenses are
- certainly possible, and some are<A href="#traffic.resist"> described
- below</A>, but it is not clear that any complete defense can be
- provided.</P>
-</DD>
-</DL>
-<H3><A name="uses">IPsec is a general mechanism for securing IP</A></H3>
-<P>While IPsec does not provide all functions of a mail encryption
- package, it can encrypt your mail. In particular, it can ensure that
- all mail passing between a pair or a group of sites is encrypted. An
- attacker looking only at external traffic, without access to anything
- on or behind the IPsec gateway, cannot read your mail. He or she is
- stymied by IPsec just as he or she would be by<A href="glossary.html#PGP">
- PGP</A>.</P>
-<P>The advantage is that IPsec can provide the same protection for<STRONG>
- anything transmitted over IP</STRONG>. In a corporate network example,
- PGP lets the branch offices exchange secure mail with head office. SSL
- and SSH allow them to securely view web pages, connect as terminals to
- machines, and so on. IPsec can support all those applications, plus
- database queries, file sharing (NFS or Windows), other protocols
- encapsulated in IP (Netware, Appletalk, ...), phone-over-IP,
- video-over-IP, ... anything-over-IP. The only limitation is that IP
- Multicast is not yet supported, though there are Internet Draft
- documents for that.</P>
-<P>IPsec creates<STRONG> secure tunnels through untrusted networks</STRONG>
-. Sites connected by these tunnels form VPNs,<A href="glossary.html#VPN">
- Virtual Private Networks</A>.</P>
-<P>IPsec gateways can be installed wherever they are required.</P>
-<UL>
-<LI>One organisation might choose to install IPsec only on firewalls
- between their LANs and the Internet. This would allow them to create a
- VPN linking several offices. It would provide protection against anyone
- outside their sites.</LI>
-<LI>Another might install IPsec on departmental servers so everything on
- the corporate backbone net was encrypted. This would protect messages
- on that net from everyone except the sending and receiving department.</LI>
-<LI>Another might be less concerned with information secrecy and more
- with controlling access to certain resources. They might use IPsec
- packet authentication as part of an access control mechanism, with or
- without also using the IPsec encryption service.</LI>
-<LI>It is even possible (assuming adequate processing power and an IPsec
- implementation in each node) to make every machine its own IPsec
- gateway so that everything on a LAN is encrypted. This protects
- information from everyone outside the sending and receiving machine.</LI>
-<LI>These techniques can be combined in various ways. One might, for
- example, require authentication everywhere on a network while using
- encryption only for a few links.</LI>
-</UL>
-<P>Which of these, or of the many other possible variants, to use is up
- to you.<STRONG> IPsec provides mechanisms; you provide the policy</STRONG>
-.</P>
-<P><STRONG>No end user action is required</STRONG> for IPsec security to
- be used; they don't even have to know about it. The site
- administrators, of course, do have to know about it and to put some
- effort into making it work. Poor administration can compromise IPsec as
- badly as the post-it notes mentioned above. It seems reasonable,
- though, for organisations to hope their system administrators are
- generally both more security-conscious than end users and more able to
- follow computer security procedures. If not, at least there are fewer
- of them to educate or replace.</P>
-<P>IPsec can be, and often should be, used with along with security
- protocols at other levels. If two sites communicate with each other via
- the Internet, then IPsec is the obvious way to protect that
- communication. If two others have a direct link between them, either
- link encryption or IPsec would make sense. Choose one or use both.
- Whatever you use at and below the IP level, use other things as
- required above that level. Whatever you use above the IP level,
- consider what can be done with IPsec to make attacks on the higher
- levels harder. For example,<A href="glossary.html#middle">
- man-in-the-middle attacks</A> on various protocols become difficult if
- authentication at packet level is in use on the potential victims'
- communication channel.</P>
-<H3><A name="authonly">Using authentication without encryption</A></H3>
-<P>Where appropriate, IPsec can provide authentication without
- encryption. One might do this, for example:</P>
-<UL>
-<LI>where the data is public but one wants to be sure of getting the
- right data, for example on some web sites</LI>
-<LI>where encryption is judged unnecessary, for example on some company
- or department LANs</LI>
-<LI>where strong encryption is provided at link level, below IP</LI>
-<LI>where strong encryption is provided in other protocols, above IP
-<BR> Note that IPsec authentication may make some attacks on those
- protocols harder.</LI>
-</UL>
-<P>Authentication has lower overheads than encryption.</P>
-<P>The protocols provide four ways to build such connections, using
- either an AH-only connection or ESP using null encryption, and in
- either manually or automatically keyed mode. FreeS/WAN supports only
- one of these, manually keyed AH-only connections, and<STRONG> we do not
- recommend using that</STRONG>. Our reasons are discussed under<A href="#traffic.resist">
- Resisting traffic analysis</A> a few sections further along.</P>
-<H3><A name="encnoauth">Encryption without authentication is dangerous</A>
-</H3>
-<P>Originally, the IPsec encryption protocol<A href="glossary.html#ESP">
- ESP</A> didn't do integrity checking. It only did encryption. Steve
- Bellovin found many ways to attack ESP used without authentication. See
- his paper<A href="http://www.research.att.com/~smb/papers/badesp.ps">
- Problem areas for the IP Security Protocols</A>. To make a secure
- connection, you had to add an<A href="glossary.html#AH"> AH</A>
- Authentication Header as well as ESP. Rather than incur the overhead of
- several layers (and rather than provide an ESP layer that didn't
- actually protect the traffic), the IPsec working group built integrity
- and replay checking directly into ESP.</P>
-<P>Today, typical usage is one of:</P>
-<UL>
-<LI>ESP for encryption and authentication</LI>
-<LI>AH for authentication alone</LI>
-</UL>
-<P>Other variants are allowed by the standard, but not much used:</P>
-<DL>
-<DT>ESP encryption without authentication</DT>
-<DD><STRONG>Bellovin has demonstrated fatal flaws in this. Do not use.</STRONG>
-</DD>
-<DT>ESP encryption with AH authentication</DT>
-<DD>This has higher overheads than using the authentication in ESP, and
- no obvious benefit in most cases. The exception might be a network
- where AH authentication was widely or universally used. If you're going
- to do AH to conform with network policy, why authenticate again in the
- ESP layer?</DD>
-<DT>Authenticate twice, with AH and with ESP</DT>
-<DD>Why? Of course, some folk consider "belt and suspenders" the
- sensible approach to security. If you're among them, you might use both
- protocols here. You might also use both to satisfy different parts of a
- security policy. For example, an organisation might require AH
- authentication everywhere but two users within the organisation might
- use ESP as well.</DD>
-<DT>ESP authentication without encryption</DT>
-<DD>The standard allows this, calling it "null encryption". FreeS/WAN
- does not support it. We recommend that you use AH instead if
- authentication is all you require. AH authenticates parts of the IP
- header, which ESP-null does not do.</DD>
-</DL>
-<P>Some of these variants cannot be used with FreeS/WAN because we do
- not support ESP-null and do not support automatic keying of AH-only
- connections.</P>
-<P>There are fairly frequent suggestions that AH be dropped entirely
- from the IPsec specifications since ESP and null encryption can handle
- that situation. It is not clear whether this will occur. My guess is
- that it is unlikely.</P>
-<H3><A name="multilayer">Multiple layers of IPsec processing are
- possible</A></H3>
-<P>The above describes combinations possible on a single IPsec
- connection. In a complex network you may have several layers of IPsec
- in play, with any of the above combinations at each layer.</P>
-<P>For example, a connection from a desktop machine to a database server
- might require AH authentication. Working with other host, network and
- database security measures, AH might be just the thing for access
- control. You might decide not to use ESP encryption on such packets,
- since it uses resources and might complicate network debugging. Within
- the site where the server is, then, only AH would be used on those
- packets.</P>
-<P>Users at another office, however, might have their whole connection
- (AH headers and all) passing over an IPsec tunnel connecting their
- office to the one with the database server. Such a tunnel should use
- ESP encryption and authentication. You need authentication in this
- layer because without authentication the encryption is vulnerable and
- the gateway cannot verify the AH authentication. The AH is between
- client and database server; the gateways aren't party to it.</P>
-<P>In this situation, some packets would get multiple layers of IPsec
- applied to them, AH on an end-to-end client-to-server basis and ESP
- from one office's security gateway to the other.</P>
-<H3><A name="traffic.resist">Resisting traffic analysis</A></H3>
-<P><A href="glossary.html#traffic">Traffic analysis</A> is the attempt
- to derive useful intelligence from encrypted traffic without breaking
- the encryption.</P>
-<P>Is your CEO exchanging email with a venture capital firm? With
- bankruptcy trustees? With an executive recruiting agency? With the
- holder of some important patents? If an eavesdropper learns about any
- of those, then he has interesting intelligence on your company, whether
- or not he can read the messages themselves.</P>
-<P>Even just knowing that there is network traffic between two sites may
- tell an analyst something useful, especially when combined with
- whatever other information he or she may have. For example, if you know
- Company A is having cashflow problems and Company B is looking for
- aquisitions, then knowing that packets are passing between the two is
- interesting. It is more interesting if you can tell it is email, and
- perhaps yet more if you know the sender and recipient.</P>
-<P>Except in the simplest cases, traffic analysis is hard to do well. It
- requires both considerable resources and considerable analytic skill.
- However, intelligence agencies of various nations have been doing it
- for centuries and many of them are likely quite good at it by now.
- Various commercial organisations, especially those working on "targeted
- marketing" may also be quite good at analysing certain types of
- traffic.</P>
-<P>In general, defending against traffic analysis is also difficult.
- Inventing a really good defense could get you a PhD and some
- interesting job offers.</P>
-<P>IPsec is not designed to stop traffic analysis and we know of no
- plausible method of extending it to do so. That said, there are ways to
- make traffic analysis harder. This section describes them.</P>
-<H4><A name="extra">Using "unnecessary" encryption</A></H4>
-<P>One might choose to use encryption even where it appears unnecessary
- in order to make analysis more difficult. Consider two offices which
- pass a small volume of business data between them using IPsec and also
- transfer large volumes of Usenet news. At first glance, it would seem
- silly to encrypt the newsfeed, except possibly for any newsgroups that
- are internal to the company. Why encrypt data that is all publicly
- available from many sites?</P>
-<P>However, if we encrypt a lot of news and send it down the same
- connection as our business data, we make<A href="glossary.html#traffic">
- traffic analysis</A> much harder. A snoop cannot now make inferences
- based on patterns in the volume, direction, sizes, sender, destination,
- or timing of our business messages. Those messages are hidden in a mass
- of news messages encapsulated in the same way.</P>
-<P>If we're going to do this we need to ensure that keys change often
- enough to remain secure even with high volumes and with the adversary
- able to get plaintext of much of the data. We also need to look at
- other attacks this might open up. For example, can the adversary use a
- chosen plaintext attack, deliberately posting news articles which, when
- we receive and encrypt them, will help break our encryption? Or can he
- block our business data transmission by flooding us with silly news
- articles? Or ...</P>
-<P>Also, note that this does not provide complete protection against
- traffic analysis. A clever adversary might still deduce useful
- intelligence from statistical analysis (perhaps comparing the input
- newsfeed to encrypted output, or comparing the streams we send to
- different branch offices), or by looking for small packets which might
- indicate establishment of TCP connections, or ...</P>
-<P>As a general rule, though, to improve resistance to traffic analysis,
- you should<STRONG> encrypt as much traffic as possible, not just as
- much as seems necessary.</STRONG></P>
-<H4><A name="multi-encrypt">Using multiple encryption</A></H4>
-<P>This also applies to using multiple layers of encryption. If you have
- an IPsec tunnel between two branch offices, it might appear silly to
- send<A href="glossary.html#PGP"> PGP</A>-encrypted email through that
- tunnel. However, if you suspect someone is snooping your traffic, then
- it does make sense:</P>
-<UL>
-<LI>it protects the mail headers; they cannot even see who is mailing
- who</LI>
-<LI>it protects against user bungles or software malfunctions that
- accidentally send messages in the clear</LI>
-<LI>it makes any attack on the mail encryption much harder; they have to
- break IPsec or break into your network before they can start on the
- mail encryption</LI>
-</UL>
-<P>Similar arguments apply for<A href="glossary.html#SSL"> SSL</A>
--encrypted web traffic or<A href="glossary.html#SSH"> SSH</A>-encrypted
- remote login sessions, even for end-to-end IPsec tunnels between
- systems in the two offices.</P>
-<H4><A name="fewer">Using fewer tunnels</A></H4>
-<P>It may also help to use fewer tunnels. For example, if all you
- actually need encrypted is connections between:</P>
-<UL>
-<LI>mail servers at branch and head offices</LI>
-<LI>a few branch office users and the head office database server</LI>
-</UL>
-<P>You might build one tunnel per mail server and one per remote
- database user, restricting traffic to those applications. This gives
- the traffic analyst some information, however. He or she can
- distinguish the tunnels by looking at information in the ESP header
- and, given that distinction and the patterns of tunnel usage, might be
- able to figure out something useful. Perhaps not, but why take the
- risk?</P>
-<P>We suggest instead that you build one tunnel per branch office,
- encrypting everything passing from head office to branches. This has a
- number of advantages:</P>
-<UL>
-<LI>it is easier to build and administer</LI>
-<LI>it resists traffic analysis somewhat better</LI>
-<LI>it provides security for whatever you forgot. For example, if some
- user at a remote office browses proprietary company data on some head
- office web page (that the security people may not even know about!),
- then that data is encrypted before it reaches the Internet.</LI>
-</UL>
-<P>Of course you might also want to add additional tunnels. For example,
- if some of the database data is confidential and should not be exposed
- even within the company, then you need protection from the user's
- desktop to the database server. We suggest you do that in whatever way
- seems appropriate -- IPsec, SSH or SSL might fit -- but, whatever you
- choose, pass it between locations via a gateway-to-gateway IPsec tunnel
- to provide some resistance to traffic analysis.</P>
-<H2><A name="primitives">Cryptographic components</A></H2>
-<P>IPsec combines a number of cryptographic techniques, all of them
- well-known and well-analyzed. The overall design approach was
- conservative; no new or poorly-understood components were included.</P>
-<P>This section gives a brief overview of each technique. It is intended
- only as an introduction. There is more information, and links to
- related topics, in our<A href="glossary.html"> glossary</A>. See also
- our<A href="biblio.html"> bibliography</A> and cryptography<A href="web.html#crypto.link">
- web links</A>.</P>
-<H3><A name="block.cipher">Block ciphers</A></H3>
-<P>The<A href="glossary.html#encryption"> encryption</A> in the<A href="glossary.html#ESP">
- ESP</A> encapsulation protocol is done with a<A href="glossary.html#block">
- block cipher</A>.</P>
-<P>We do not implement<A href="glossary.html#DES"> single DES</A>. It is<A
-href="politics.html#desnotsecure"> insecure</A>. Our default, and
- currently only, block cipher is<A href="glossary.html#3DES"> triple DES</A>
-.</P>
-<P>The<A href="glossary.html#rijndael"> Rijndael</A> block cipher has
- won the<A href="glossary.html#AES"> AES</A> competition to choose a
- relacement for DES. It will almost certainly be added to FreeS/WAN and
- to other IPsec implementations.<A href="web.html#patch"> Patches</A>
- are already available.</P>
-<H3><A name="hash.ipsec">Hash functions</A></H3>
-<H4><A name="hmac.ipsec">The HMAC construct</A></H4>
-<P>IPsec packet authentication is done with the<A href="glossary.html#HMAC">
- HMAC</A> construct. This is not just a hash of the packet data, but a
- more complex operation which uses both a hashing algorithm and a key.
- It therefore does more than a simple hash would. A simple hash would
- only tell you that the packet data was not changed in transit, or that
- whoever changed it also regenerated the hash. An HMAC also tells you
- that the sender knew the HMAC key.</P>
-<P>For IPsec HMAC, the output of the hash algorithm is truncated to 96
- bits. This saves some space in the packets. More important, it prevents
- an attacker from seeing all the hash output bits and perhaps creating
- some sort of attack based on that knowledge.</P>
-<H4>Choice of hash algorithm</H4>
-<P>The IPsec RFCs require two hash algorithms --<A href="glossary.html#MD5">
- MD5</A> and<A href="glossary.html#SHA"> SHA-1</A> -- both of which
- FreeS/WAN implements.</P>
-<P>Various other algorithms -- such as RIPEMD and Tiger -- are listed in
- the RFCs as optional. None of these are in the FreeS/WAN distribution,
- or are likely to be added, although user<A href="web.html#patch">
- patches</A> exist for several of them.</P>
-<P>Additional hash algorithms --<A href="glossary.html#SHA-256">
- SHA-256, SHA-384 and SHA-512</A> -- may be required to give hash
- strength matching the strength of<A href="glossary.html#AES"> AES</A>.
- These are likely to be added to FreeS/WAN along with AES.</P>
-<H3><A name="DH.keying">Diffie-Hellman key agreement</A></H3>
-<P>The<A href="glossary.html#DH"> Diffie-Hellman</A> key agreement
- protocol allows two parties (A and B or<A href="glossary.html#alicebob">
- Alice and Bob</A>) to agree on a key in such a way that an eavesdropper
- who intercepts the entire conversation cannot learn the key.</P>
-<P>The protocol is based on the<A href="glossary.html#dlog"> discrete
- logarithm</A> problem and is therefore thought to be secure.
- Mathematicians have been working on that problem for years and seem no
- closer to a solution, though there is no proof that an efficient
- solution is impossible.</P>
-<H3><A name="RSA.auth">RSA authentication</A></H3>
-<P>The<A href="glossary.html#RSA"> RSA</A> algorithm (named for its
- inventors -- Rivest, Shamir and Adleman) is a very widely used<A href="glossary.html#">
- public key</A> cryptographic technique. It is used in IPsec as one
- method of authenticating gateways for Diffie-Hellman key negotiation.</P>
-<H2><A name="structure">Structure of IPsec</A></H2>
-<P>There are three protocols used in an IPsec implementation:</P>
-<DL>
-<DT>ESP, Encapsulating Security Payload</DT>
-<DD>Encrypts and/or authenticates data</DD>
-<DT>AH, Authentication Header</DT>
-<DD>Provides a packet authentication service</DD>
-<DT>IKE, Internet Key Exchange</DT>
-<DD>Negotiates connection parameters, including keys, for the other two</DD>
-</DL>
-<P>The term "IPsec" is slightly ambiguous. In some contexts, it includes
- all three of the above but in other contexts it refers only to AH and
- ESP.</P>
-<H3><A name="IKE.ipsec">IKE (Internet Key Exchange)</A></H3>
-<P>The IKE protocol sets up IPsec (ESP or AH) connections after
- negotiating appropriate parameters (algorithms to be used, keys,
- connection lifetimes) for them. This is done by exchanging packets on
- UDP port 500 between the two gateways.</P>
-<P>IKE (RFC 2409) was the outcome of a long, complex process in which
- quite a number of protocols were proposed and debated. Oversimplifying
- mildly, IKE combines:</P>
-<DL>
-<DT>ISAKMP (RFC 2408)</DT>
-<DD>The<STRONG> I</STRONG>nternet<STRONG> S</STRONG>ecurity<STRONG> A</STRONG>
-ssociation and<STRONG> K</STRONG>ey<STRONG> M</STRONG>anagement<STRONG>
- P</STRONG>rotocol manages negotiation of connections and defines<A href="glossary.html#SA">
- SA</A>s (Security Associations) as a means of describing connection
- properties.</DD>
-<DT>IPsec DOI for ISAKMP (RFC 2407)</DT>
-<DD>A<STRONG> D</STRONG>omain<STRONG> O</STRONG>f<STRONG> I</STRONG>
-nterpretation fills in the details necessary to turn the rather abstract
- ISAKMP protocol into a more tightly specified protocol, so it becomes
- applicable in a particular domain.</DD>
-<DT>Oakley key determination protocol (RFC 2412)</DT>
-<DD>Oakley creates keys using the<A href="glossary.html#DH">
- Diffie-Hellman</A> key agreement protocol.</DD>
-</DL>
-<P>For all the details, you would need to read the four<A href="rfc.html">
- RFCs</A> just mentioned (over 200 pages) and a number of others. We
- give a summary below, but it is far from complete.</P>
-<H4><A name="phases">Phases of IKE</A></H4>
-<P>IKE negotiations have two phases.</P>
-<DL>
-<DT>Phase one</DT>
-<DD>The two gateways negotiate and set up a two-way ISAKMP SA which they
- can then use to handle phase two negotiations. One such SA between a
- pair of gateways can handle negotiations for multiple tunnels.</DD>
-<DT>Phase two</DT>
-<DD>Using the ISAKMP SA, the gateways negotiate IPsec (ESP and/or AH)
- SAs as required. IPsec SAs are unidirectional (a different key is used
- in each direction) and are always negotiated in pairs to handle two-way
- traffic. There may be more than one pair defined between two gateways.</DD>
-</DL>
-<P>Both of these phases use the UDP protocol and port 500 for their
- negotiations.</P>
-<P>After both IKE phases are complete, you have IPsec SAs to carry your
- encrypted data. These use the ESP or AH protocols. These protocols do
- not have ports. Ports apply only to UDP or TCP.</P>
-<P>The IKE protocol is designed to be extremely flexible. Among the
- things that can be negotiated (separately for each SA) are:</P>
-<UL>
-<LI>SA lifetime before rekeying</LI>
-<LI>encryption algorithm used. We currently support only<A href="glossary.html#3DES">
- triple DES</A>. Single DES is<A href="politics.html#desnotsecure">
- insecure</A>. The RFCs say you MUST do DES, SHOULD do 3DES and MAY do
- various others. We do not do any of the others.</LI>
-<LI>authentication algorithms. We support<A href="glossary.html#MD5">
- MD5</A> and<A href="glossary.html#SHA"> SHA</A>. These are the two the
- RFCs require.</LI>
-<LI>choice of group for<A href="glossary.html#DH"> Diffie-Hellman</A>
- key agreement. We currently support Groups 2 and 5 (which are defined
- modulo primes of various lengths) and do not support Group 1 (defined
- modulo a shorter prime, and therefore cryptographically weak) or groups
- 3 and 4 (defined using elliptic curves). The RFCs require only Group 1.</LI>
-</UL>
-<P>The protocol also allows implementations to add their own encryption
- algorithms, authentication algorithms or Diffie-Hellman groups. We do
- not support any such extensions, but there are some<A href="web.html#patch">
- patches</A> that do.</P>
-<P>There are a number of complications:</P>
-<UL>
-<LI>The gateways must be able to authenticate each other's identities
- before they can create a secure connection. This host authentication is
- part of phase one negotiations, and is a required prerequisite for
- packet authentication used later. Host authentication can be done in a
- variety of ways. Those supported by FreeS/WAN are discussed in our<A href="adv_config.html#auto-auth">
- advanced configuration</A> document.</LI>
-<LI>Phase one can be done in two ways.
-<UL>
-<LI>Main Mode is required by the RFCs and supported in FreeS/WAN. It
- uses a 6-packet exzchange.</LI>
-<LI>Aggressive Mode is somewhat faster (only 3 packets) but reveals more
- to an eavesdropper. This is optional in the RFCs, not currently
- supported by FreeS/WAN, and not likely to be.</LI>
-</UL>
-</LI>
-<LI>A new group exchange may take place after phase one but before phase
- two, defining an additional group for use in the<A href="glossary.html#DH">
- Diffie-Hellman</A> key agreement part of phase two. FreeS/WAN does not
- currently support this.</LI>
-<LI>Phase two always uses Quick Mode, but there are two variants of
- that:
-<UL>
-<LI>One variant provides<A href="glossary.html#PFS"> Perfect Forward
- Secrecy (PFS)</A>. An attacker that obtains your long-term host
- authentication key does not immediately get any of your short-term
- packet encryption of packet authentication keys. He must conduct
- another successful attack each time you rekey to get the short-term
- keys. Having some short-term keys does not help him learn others. In
- particular, breaking your system today does not let him read messages
- he archived yestarday, assuming you've changed short-term keys in the
- meanwhile. We enable PFS as the default.</LI>
-<LI>The other variant disables PFS and is therefore slightly faster. We
- do not recommend this since it is less secure, but FreeS/WAN does
- support it. You can enable it with a<VAR> pfs=no</VAR> statement in<A href="manpage.d/ipsec.conf.5.html">
- ipsec.conf(5)</A>.</LI>
-<LI>The protocol provides no way to negotiate which variant will be
- used. If one gateway is set for PFS and the other is not, the
- negotiation fails. This has proved a fairly common source of
- interoperation problems.</LI>
-</UL>
-</LI>
-<LI>Several types of notification message may be sent by either side
- during either phase, or later. FreeS/WAN does not currently support
- these, but they are a likely addition in future releases.</LI>
-<LI>There is a commit flag which may optionally be set on some messages.
- The<A href="http://www.lounge.org/ike_doi_errata.html"> errata</A> page
- for the RFCs includes two changes related to this, one to clarify the
- description of its use and one to block a<A href="glossary.html#DOS">
- denial of service</A> attack which uses it. We currently do not
- implement this feature.</LI>
-</UL>
-<P>These complications can of course lead to problems, particularly when
- two different implementations attempt to interoperate. For example, we
- have seen problems such as:</P>
-<UL>
-<LI>Some implementations (often products crippled by<A href="politics.html#exlaw">
- export laws</A>) have the insecure DES algorithm as their only
- supported encryption method. Other parts of our documentation discuss
- the<A href="politics.html#desnotsecure"> reasons we do not implement
- single DES</A>, and<A href="interop.html#noDES"> how to cope with
- crippled products</A>.</LI>
-<LI>Windows 2000 IPsec tries to negotiate using Aggressive Mode, which
- we don't support. Later on, it uses the commit bit, which we also don't
- support.</LI>
-<LI>Various implementations disable PFS by default, and therefore will
- not talk to FreeS/WAN until you either turn on PFS on their end or turn
- it off in FreeS/WAN with a<VAR> pfs=no</VAR> entry in the connection
- description.</LI>
-<LI>FreeS/WAN's interaction with PGPnet is complicated by their use of
- notification messages we do not yet support.</LI>
-</UL>
-<P>Despite this, we do interoperate successfully with many
- implementations, including both Windows 2000 and PGPnet. Details are in
- our<A href="interop.html"> interoperability</A> document.</P>
-<H4><A name="sequence">Sequence of messages in IKE</A></H4>
-<P>Each phase (see<A href="#phases"> previous section</A>)of IKE
- involves a series of messages. In Pluto error messages, these are
- abbreviated using:</P>
-<DL>
-<DT>M</DT>
-<DD><STRONG>M</STRONG>ain mode, settting up the keying channel (ISAKMP
- SA)</DD>
-<DT>Q</DT>
-<DD><STRONG>Q</STRONG>uick mode, setting up the data channel (IPsec SA)</DD>
-<DT>I</DT>
-<DD><STRONG>I</STRONG>nitiator, the machine that starts the negotiation</DD>
-<DT>R</DT>
-<DD><STRONG>R</STRONG>esponder</DD>
-</DL>
-<P>For example, the six messages of a main mode negotiation, in
- sequence, are labelled:</P>
-<PRE> MI1 ---------->
- <---------- MR1
- MI2 ---------->
- <---------- MR2
- MI3 ---------->
- <---------- MR3</PRE>
-<H4><A name="struct.exchange">Structure of IKE messages</A></H4>
-<P>Here is our Pluto developer explaining some of this on the mailing
- list:</P>
-<PRE>When one IKE system (for example, Pluto) is negotiating with another
-to create an SA, the Initiator proposes a bunch of choices and the
-Responder replies with one that it has selected.
-
-The structure of the choices is fairly complicated. An SA payload
-contains a list of lists of "Proposals". The outer list is a set of
-choices: the selection must be from one element of this list.
-
-Each of these elements is a list of Proposals. A selection must be
-made from each of the elements of the inner list. In other words,
-*all* of them apply (that is how, for example, both AH and ESP can
-apply at once).
-
-Within each of these Proposals is a list of Transforms. For each
-Proposal selected, one Transform must be selected (in other words,
-each Proposal provides a choice of Transforms).
-
-Each Transform is made up of a list of Attributes describing, well,
-attributes. Such as lifetime of the SA. Such as algorithm to be
-used. All the Attributes apply to a Transform.
-
-You will have noticed a pattern here: layers alternate between being
-disjunctions ("or") and conjunctions ("and").
-
-For Phase 1 / Main Mode (negotiating an ISAKMP SA), this structure is
-cut back. There must be exactly one Proposal. So this degenerates to
-a list of Transforms, one of which must be chosen.</PRE>
-<H3><A name="services">IPsec Services, AH and ESP</A></H3>
-<P>IPsec offers two services,<A href="glossary.html#authentication">
- authentication</A> and<A href="glossary.html#encryption"> encryption</A>
-. These can be used separately but are often used together.</P>
-<DL>
-<DT>Authentication</DT>
-<DD>Packet-level authentication allows you to be confident that a packet
- came from a particular machine and that its contents were not altered
- en route to you. No attempt is made to conceal or protect the contents,
- only to assure their integrity. Packet authentication can be provided
- separately using an<A href="glossary.html#AH"> Authentication Header</A>
-, described just below, or it can be included as part of the<A href="glossary.html#ESP">
- ESP</A> (Encapsulated Security Payload) service, described in the
- following section. That service offers encryption as well as
- authentication. In either case, the<A href="glossary.html#HMAC"> HMAC</A>
- construct is used as the authentication mechanism.
-<P>There is a separate authentication operation at the IKE level, in
- which each gateway authenticates the other. This can be done in a
- variety of ways.</P>
-</DD>
-<DT>Encryption</DT>
-<DD>Encryption allows you to conceal the contents of a message from
- eavesdroppers.
-<P>In IPsec this is done using a<A href="glossary.html#block"> block
- cipher</A> (normally<A href="glossary.html#3DES"> Triple DES</A> for
- Linux). In the most used setup, keys are automatically negotiated, and
- periodically re-negotiated, using the<A href="glossary.html#IKE"> IKE</A>
- (Internet Key Exchange) protocol. In Linux FreeS/WAN this is handled by
- the Pluto Daemon.</P>
-<P>The IPsec protocol offering encryption is<A href="glossary.html#ESP">
- ESP</A>, Encapsulated Security Payload. It can also include a packet
- authentication service.</P>
-</DD>
-</DL>
-<P>Note that<STRONG> encryption should always be used with some packet
- authentication service</STRONG>. Unauthenticated encryption is
- vulnerable to<A href="glossary.html#middle"> man-in-the-middle attacks</A>
-. Also note that encryption does not prevent<A href="glossary.html#traffic">
- traffic analysis</A>.</P>
-<H3><A name="AH.ipsec">The Authentication Header (AH)</A></H3>
-<P>Packet authentication can be provided separately from encryption by
- adding an authentication header (AH) after the IP header but before the
- other headers on the packet. This is the subject of this section.
- Details are in RFC 2402.</P>
-<P>Each of the several headers on a packet header contains a "next
- protocol" field telling the system what header to look for next. IP
- headers generally have either TCP or UDP in this field. When IPsec
- authentication is used, the packet IP header has AH in this field,
- saying that an Authentication Header comes next. The AH header then has
- the next header type -- usually TCP, UDP or encapsulated IP.</P>
-<P>IPsec packet authentication can be added in transport mode, as a
- modification of standard IP transport. This is shown in this diagram
- from the RFC:</P>
-<PRE> BEFORE APPLYING AH
- ----------------------------
- IPv4 |orig IP hdr | | |
- |(any options)| TCP | Data |
- ----------------------------
-
- AFTER APPLYING AH
- ---------------------------------
- IPv4 |orig IP hdr | | | |
- |(any options)| AH | TCP | Data |
- ---------------------------------
- ||
- except for mutable fields</PRE>
-<P>Athentication can also be used in tunnel mode, encapsulating the
- underlying IP packet beneath AH and an additional IP header.</P>
-<PRE> ||
-IPv4 | new IP hdr* | | orig IP hdr* | | |
- |(any options)| AH | (any options) |TCP | Data |
- ------------------------------------------------
- ||
- | in the new IP hdr |</PRE>
-<P>This would normally be used in a gateway-to-gateway tunnel. The
- receiving gateway then strips the outer IP header and the AH header and
- forwards the inner IP packet.</P>
-<P>The mutable fields referred to are things like the time-to-live field
- in the IP header. These cannot be included in authentication
- calculations because they change as the packet travels.</P>
-<H4><A name="keyed">Keyed MD5 and Keyed SHA</A></H4>
-<P>The actual authentication data in the header is typically 96 bits and
- depends both on a secret shared between sender and receiver and on
- every byte of the data being authenticated. The technique used is<A href="glossary.html#HMAC">
- HMAC</A>, defined in RFC 2104.</P>
-<P>The algorithms involved are the<A href="glossary.html#MD5"> MD5</A>
- Message Digest Algorithm or<A href="glossary.html#SHA"> SHA</A>, the
- Secure Hash Algorithm. For details on their use in this application,
- see RFCs 2403 and 2404 respectively.</P>
-<P>For descriptions of the algorithms themselves, see RFC 1321 for MD5
- and<A href="glossary.html#FIPS"> FIPS</A> (Federal Information
- Processing Standard) number 186 from<A href="glossary.html#NIST"> NIST</A>
-, the US National Institute of Standards and Technology for SHA.<A href="biblio.html#schneier">
-<CITE> Applied Cryptography</CITE></A> covers both in some detail, MD5
- starting on page 436 and SHA on 442.</P>
-<P>These algorithms are intended to make it nearly impossible for anyone
- to alter the authenticated data in transit. The sender calculates a
- digest or hash value from that data and includes the result in the
- authentication header. The recipient does the same calculation and
- compares results. For unchanged data, the results will be identical.
- The hash algorithms are designed to make it extremely difficult to
- change the data in any way and still get the correct hash.</P>
-<P>Since the shared secret key is also used in both calculations, an
- interceptor cannot simply alter the authenticated data and change the
- hash value to match. Without the key, he or she (or even the dreaded
- They) cannot produce a usable hash.</P>
-<H4><A name="sequence">Sequence numbers</A></H4>
-<P>The authentication header includes a sequence number field which the
- sender is required to increment for each packet. The receiver can
- ignore it or use it to check that packets are indeed arriving in the
- expected sequence.</P>
-<P>This provides partial protection against<A href="glossary.html#replay">
- replay attacks</A> in which an attacker resends intercepted packets in
- an effort to confuse or subvert the receiver. Complete protection is
- not possible since it is necessary to handle legitmate packets which
- are lost, duplicated, or delivered out of order, but use of sequence
- numbers makes the attack much more difficult.</P>
-<P>The RFCs require that sequence numbers never cycle, that a new key
- always be negotiated before the sequence number reaches 2^32-1. This
- protects both against replays attacks using packets from a previous
- cyclce and against<A href="glossary.html#birthday"> birthday attacks</A>
- on the the packet authentication algorithm.</P>
-<P>In Linux FreeS/WAN, the sequence number is ignored for manually keyed
- connections and checked for automatically keyed ones. In manual mode,
- there is no way to negotiate a new key, or to recover from a sequence
- number problem, so we don't use sequence numbers.</P>
-<H3><A name="ESP.ipsec">Encapsulated Security Payload (ESP)</A></H3>
-<P>The ESP protocol is defined in RFC 2406. It provides one or both of
- encryption and packet authentication. It may be used with or without AH
- packet authentication.</P>
-<P>Note that<STRONG> some form of packet authentication should<EM>
- always</EM> be used whenever data is encrypted</STRONG>. Without
- authentication, the encryption is vulnerable to active attacks which
- may allow an enemy to break the encryption. ESP should<STRONG> always</STRONG>
- either include its own authentication or be used with AH
- authentication.</P>
-<P>The RFCs require support for only two mandatory encryption algorithms
- --<A href="glossary.html#DES"> DES</A>, and null encryption -- and for
- two authentication methods -- keyed MD5 and keyed SHA. Implementers may
- choose to support additional algorithms in either category.</P>
-<P>The authentication algorithms are the same ones used in the IPsec<A href="glossary.html#AH">
- authentication header</A>.</P>
-<P>We do not implement single DES since<A href="politics.html#desnotsecure">
- DES is insecure</A>. Instead we provide<A href="glossary.html#3DES">
- triple DES or 3DES</A>. This is currently the only encryption algorithm
- supported.</P>
-<P>We do not implement null encryption since it is obviously insecure.</P>
-<H2><A name="modes">IPsec modes</A></H2>
-<P>IPsec can connect in two modes. Transport mode is a host-to-host
- connection involving only two machines. In tunnel mode, the IPsec
- machines act as gateways and trafiic for any number of client machines
- may be carried.</P>
-<H3><A name="tunnel.ipsec">Tunnel mode</A></H3>
-<P>Security gateways are required to support tunnel mode connections. In
- this mode the gateways provide tunnels for use by client machines
- behind the gateways. The client machines need not do any IPsec
- processing; all they have to do is route things to gateways.</P>
-<H3><A name="transport.ipsec">Transport mode</A></H3>
-<P>Host machines (as opposed to security gateways) with IPsec
- implementations must also support transport mode. In this mode, the
- host does its own IPsec processing and routes some packets via IPsec.</P>
-<H2><A name="parts">FreeS/WAN parts</A></H2>
-<H3><A name="KLIPS.ipsec">KLIPS: Kernel IPsec Support</A></H3>
-<P>KLIPS is<STRONG> K</STRONG>erne<STRONG>L</STRONG><STRONG> IP</STRONG>
-SEC<STRONG> S</STRONG>upport, the modifications necessary to support
- IPsec within the Linux kernel. KILPS does all the actual IPsec
- packet-handling, including</P>
-<UL>
-<LI>encryption</LI>
-<LI>packet authentication calculations</LI>
-<LI>creation of ESP and AH headers for outgoing packets</LI>
-<LI>interpretation of those headers on incoming packets</LI>
-</UL>
-<P>KLIPS also checks all non-IPsec packets to ensure they are not
- bypassing IPsec security policies.</P>
-<H3><A name="Pluto.ipsec">The Pluto daemon</A></H3>
-<P><A href="manpage.d/ipsec_pluto.8.html">Pluto(8)</A> is a daemon which
- implements the IKE protocol. It</P>
-<UL>
-<LI>handles all the Phase one ISAKMP SAs</LI>
-<LI>performs host authentication and negotiates with other gateways</LI>
-<LI>creates IPsec SAs and passes the data required to run them to KLIPS</LI>
-<LI>adjust routing and firewall setup to meet IPsec requirements. See
- our<A href="firewall.html"> IPsec and firewalling</A> document for
- details.</LI>
-</UL>
-<P>Pluto is controlled mainly by the<A href="manpage.d/ipsec.conf.5.html">
- ipsec.conf(5)</A> configuration file.</P>
-<H3><A name="command">The ipsec(8) command</A></H3>
-<P>The<A href="manpage.d/ipsec.8.html"> ipsec(8)</A> command is a front
- end shellscript that allows control over IPsec activity.</P>
-<H3><A name="ipsec.conf">Linux FreeS/WAN configuration file</A></H3>
-<P>The configuration file for Linux FreeS/WAN is</P>
-<PRE> /etc/ipsec.conf</PRE>
-<P>For details see the<A href="manpage.d/ipsec.conf.5.html">
- ipsec.conf(5)</A> manual page .</P>
-<H2><A name="key">Key management</A></H2>
-<P>There are several ways IPsec can manage keys. Not all are implemented
- in Linux FreeS/WAN.</P>
-<H3><A name="current">Currently Implemented Methods</A></H3>
-<H4><A name="manual">Manual keying</A></H4>
-<P>IPsec allows keys to be manually set. In Linux FreeS/WAN, such keys
- are stored with the connection definitions in /etc/ipsec.conf.</P>
-<P><A href="glossary.html#manual">Manual keying</A> is useful for
- debugging since it allows you to test the<A href="glossary.html#KLIPS">
- KLIPS</A> kernel IPsec code without the<A href="glossary.html#Pluto">
- Pluto</A> daemon doing key negotiation.</P>
-<P>In general, however, automatic keying is preferred because it is more
- secure.</P>
-<H4><A name="auto">Automatic keying</A></H4>
-<P>In automatic keying, the<A href="glossary.html#Pluto"> Pluto</A>
- daemon negotiates keys using the<A href="glossary.html#IKE"> IKE</A>
- Internet Key Exchange protocol. Connections are automatically re-keyed
- periodically.</P>
-<P>This is considerably more secure than manual keying. In either case
- an attacker who acquires a key can read every message encrypted with
- that key, but automatic keys can be changed every few hours or even
- every few minutes without breaking the connection or requiring
- intervention by the system administrators. Manual keys can only be
- changed manually; you need to shut down the connection and have the two
- admins make changes. Moreover, they have to communicate the new keys
- securely, perhaps with<A href="glossary.html#PGP"> PGP</A> or<A href="glossary.html#SSH">
- SSH</A>. This may be possible in some cases, but as a general solution
- it is expensive, bothersome and unreliable. Far better to let<A href="glossary.html#Pluto">
- Pluto</A> handle these chores; no doubt the administrators have enough
- to do.</P>
-<P>Also, automatic keying is inherently more secure against an attacker
- who manages to subvert your gateway system. If manual keying is in use
- and an adversary acquires root privilege on your gateway, he reads your
- keys from /etc/ipsec.conf and then reads all messages encrypted with
- those keys.</P>
-<P>If automatic keying is used, an adversary with the same privileges
- can read /etc/ipsec.secrets, but this does not contain any keys, only
- the secrets used to authenticate key exchanges. Having an adversary
- able to authenticate your key exchanges need not worry you overmuch.
- Just having the secrets does not give him any keys. You are still
- secure against<A href="glossary.html#passive"> passive</A> attacks.
- This property of automatic keying is called<A href="glossary.html#PFS">
- perfect forward secrecy</A>, abbreviated PFS.</P>
-<P>Unfortunately, having the secrets does allow an<A href="glossary.html#active">
- active attack</A>, specifically a<A href="glossary.html#middle">
- man-in-the-middle</A> attack. Losing these secrets to an attacker may
- not be quite as disastrous as losing the actual keys, but it is<EM>
- still a serious security breach</EM>. These secrets should be guarded
- as carefully as keys.</P>
-<H3><A name="notyet">Methods not yet implemented</A></H3>
-<H4><A name="noauth">Unauthenticated key exchange</A></H4>
-<P>It would be possible to exchange keys without authenticating the
- players. This would support<A href="glossary.html#carpediem">
- opportunistic encryption</A> -- allowing any two systems to encrypt
- their communications without requiring a shared PKI or a previously
- negotiated secret -- and would be secure against<A href="glossary.html#passive">
- passive attacks</A>. It would, however, be highly vulnerable to active<A
-href="glossary.html#middle"> man-in-the-middle</A> attacks. RFC 2408
- therefore specifies that all<A href="glossary.html#ISAKMP"> ISAKMP</A>
- key management interactions<EM> must</EM> be authenticated.</P>
-<P>There is room for debate here. Should we provide immediate security
- against<A href="glossary.html#passive"> passive attacks</A> and
- encourage widespread use of encryption, at the expense of risking the
- more difficult<A href="glossary.html#active"> active attacks</A>? Or
- should we wait until we can implement a solution that can both be
- widespread and offer security against active attacks?</P>
-<P>So far, we have chosen the second course, complying with the RFCs and
- waiting for secure DNS (see<A href="glossary.html#DNS"> below</A>) so
- that we can do<A href="glossary.html#carpediem"> opportunistic
- encryption</A> right.</P>
-<H4><A name="DNS">Key exchange using DNS</A></H4>
-<P>The IPsec RFCs allow key exchange based on authentication services
- provided by<A href="glossary.html#SDNS"> Secure DNS</A>. Once Secure
- DNS service becomes widely available, we expect to make this the<EM>
- primary key management method for Linux FreeS/WAN</EM>. It is the best
- way we know of to support<A href="glossary.html#carpediem">
- opportunistic encryption</A>, allowing two systems without a common PKI
- or previous negotiation to secure their communication.</P>
-<P>We currently have code to acquire RSA keys from DNS but do not yet
- have code to validate Secure DNS signatures.</P>
-<H4><A name="PKI">Key exchange using a PKI</A></H4>
-<P>The IPsec RFCs allow key exchange based on authentication services
- provided by a<A href="glossary.html#PKI"> PKI</A> or Public Key
- Infrastructure. With many vendors selling such products and many large
- organisations building these infrastructures, this will clearly be an
- important application of IPsec and one Linux FreeS/WAN will eventually
- support.</P>
-<P>On the other hand, this is not as high a priority for Linux FreeS/WAN
- as solutions based on<A href="glossary.html#SDNS"> secure DNS</A>. We
- do not expect any PKI to become as universal as DNS.</P>
-<P>Some<A href="web.html#patch"> patches</A> to handle authentication
- with X.509 certificates, which most PKIs use, are available.</P>
-<H4><A name="photuris">Photuris</A></H4>
-<P><A href="glossary.html#photuris">Photuris</A> is another key
- management protocol, an alternative to IKE and ISAKMP, described in
- RFCs 2522 and 2523 which are labelled "experimental". Adding Photuris
- support to Linux FreeS/WAN might be a good project for a volunteer. The
- likely starting point would be the OpenBSD photurisd code.</P>
-<H4><A name="skip">SKIP</A></H4>
-<P><A href="glossary.html#SKIP">SKIP</A> is yet another key management
- protocol, developed by Sun. At one point it was fairly widely used, but
- it now seems moribund, displaced by IKE. Sun now (as of Solaris 8.0)
- ship an IPsec implementation using IKE. We have no plans to implement
- SKIP. If a user were to implement it, we would almost certainly not
- want to add the code to our distribution.</P>
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