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| author | Rene Mayrhofer <rene@mayrhofer.eu.org> | 2006-05-22 05:12:18 +0000 |
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| committer | Rene Mayrhofer <rene@mayrhofer.eu.org> | 2006-05-22 05:12:18 +0000 |
| commit | aa0f5b38aec14428b4b80e06f90ff781f8bca5f1 (patch) | |
| tree | 95f3d0c8cb0d59d88900dbbd72110d7ab6e15b2a /doc/ipsec.html | |
| parent | 7c383bc22113b23718be89fe18eeb251942d7356 (diff) | |
| download | vyos-strongswan-aa0f5b38aec14428b4b80e06f90ff781f8bca5f1.tar.gz vyos-strongswan-aa0f5b38aec14428b4b80e06f90ff781f8bca5f1.zip | |
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diff --git a/doc/ipsec.html b/doc/ipsec.html new file mode 100644 index 000000000..4fb27b92b --- /dev/null +++ b/doc/ipsec.html @@ -0,0 +1,1040 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd"> +<HTML> +<HEAD> +<TITLE>Introduction to FreeS/WAN</TITLE> +<META HTTP-EQUIV="Content-Type" CONTENT="text/html; CHARSET=iso-8859-1"> +<STYLE TYPE="text/css"><!-- +BODY { font-family: serif } +H1 { font-family: sans-serif } +H2 { font-family: sans-serif } +H3 { font-family: sans-serif } +H4 { font-family: sans-serif } +H5 { font-family: sans-serif } +H6 { font-family: sans-serif } +SUB { font-size: smaller } +SUP { font-size: smaller } +PRE { font-family: monospace } +--></STYLE> +</HEAD> +<BODY> +<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> +<HR> +<A HREF="toc.html">Contents</A> +<A HREF="politics.html">Previous</A> +<A HREF="mail.html">Next</A> +</BODY> +</HTML> |