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Code Editor : ipsec.html

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<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 &quot;IPsec&quot; (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 &quot;in the background&quot;,
 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 &quot;belt and suspenders&quot; 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 &quot;null encryption&quot;. 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 &quot;targeted
 marketing&quot; 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 &quot;unnecessary&quot; 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 &quot;IPsec&quot; 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 ----------&gt;
           &lt;---------- MR1
       MI2 ----------&gt;
           &lt;---------- MR2
       MI3 ----------&gt;
           &lt;---------- 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 &quot;Proposals&quot;.  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 (&quot;or&quot;) and conjunctions (&quot;and&quot;).

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 &quot;next
 protocol&quot; 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 &quot;experimental&quot;. 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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