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diff --git a/doc/ipsec.html b/doc/ipsec.html deleted file mode 100644 index 4fb27b92b..000000000 --- a/doc/ipsec.html +++ /dev/null @@ -1,1040 +0,0 @@ -<!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> |