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Network Working Group                                   S. Bellovin, Ed.Request for Comments: 3631                              J. Schiller, Ed.Category: Informational                                  C. Kaufman, Ed.                                             Internet Architecture Board                                                           December 2003Security Mechanisms for the InternetStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2003).  All Rights Reserved.Abstract   Security must be built into Internet Protocols for those protocols to   offer their services securely.  Many security problems can be traced   to improper implementations.  However, even a proper implementation   will have security problems if the fundamental protocol is itself   exploitable.  Exactly how security should be implemented in a   protocol will vary, because of the structure of the protocol itself.   However, there are many protocols for which standard Internet   security mechanisms, already developed, may be applicable.  The   precise one that is appropriate in any given situation can vary.  We   review a number of different choices, explaining the properties of   each.1.  Introduction   Internet Security compromises can be divided into several classes,   ranging from Denial of Service to Host Compromise.  Denial of Service   attacks based on sheer volume of traffic are beyond the scope of this   document, though they are the subject of much ongoing discussion and   research.  It is important to note that many such attacks are made   more difficult by good security practices.  Host Compromise (most   commonly caused by undetected Buffer Overflows) represent flaws in   individual implementations rather than flaws in protocols.   Nevertheless, carefully designed protocols can make such flaws less   likely to occur and harder to exploit.Bellovin, et al.             Informational                      [Page 1]

RFC 3631          Security Mechanisms for the Internet     December 2003   However, there are security compromises that are facilitated by the   very protocols that are in use on the Internet.  If a security   problem is inherent in a protocol, no manner of implementation will   be able to prevent the problem.   It is therefore vitally important that protocols developed for the   Internet provide this fundamental security.   Exactly how a protocol should be secured depends on the protocol   itself as well as the security needs of the protocol.  However, we   have developed a number of standard security mechanisms in the IETF.   In many cases appropriate application of these mechanisms can provide   the necessary security for a protocol.   A number of possible mechanisms can be used to provide security on   the Internet.  Which one should be selected depends on many different   factors.  We attempt here to provide guidance, spelling out the   factors and the currently-standardized (or about-to-be-standardized)   solutions, as discussed at the IAB Security Architecture Workshop   [RFC2316].   Security, however, is an art, not a science.  Attempting to follow a   recipe blindly can lead to disaster.  As always, good taste in   protocol design should be exercised.   Finally, security mechanisms are not magic pixie dust that can be   sprinkled over completed protocols.  It is rare that security can be   bolted on later.  Good designs -- that is, secure, clean, and   efficient designs -- occur when the security mechanisms are crafted   along with the protocol.  No conceivable exercise in cryptography can   secure a protocol with flawed semantic assumptions.2.  Decision Factors2.1.  Threat Model   The most important factor in choosing a security mechanism is the   threat model.  That is, who may be expected to attack what resource,   using what sorts of mechanisms?  A low-value target, such as a Web   site that offers public information only, may not merit much   protection.  Conversely, a resource that if compromised could expose   significant parts of the Internet infrastructure, say, a major   backbone router or high-level Domain Name Server, should be protected   by very strong mechanisms.  The value of a target to an attacker   depends on the purpose of the attack.  If the purpose is to access   sensitive information, all systems that handle this information or   mediate access to it are valuable.  If the purpose is to wreak havoc,   systems on which large parts of the Internet depend are exceedinglyBellovin, et al.             Informational                      [Page 2]

RFC 3631          Security Mechanisms for the Internet     December 2003   valuable.  Even if only public information is posted on a web site,   changing its contents can cause embarrassment to its owner and could   result in substantial damage.  It is difficult when designing a   protocol to predict what uses that protocol will someday have.   All Internet connected systems require a minimum amount of   protection.  Starting in 2000 and continuing to the present, we have   witnessed the advent of a new type of Internet security attack: an   Internet "worm" program that seeks out and automatically attacks   systems that are vulnerable to compromise via a number of attacks   built into the worm program itself.  These worm programs can   compromise literally thousands of systems within a very short period   of time.  Note that the first Internet Worm was the "Morris" worm of   1988.  However, it was not followed up with similar programs for over   12 years!   As of the writing of this document, all of these worms have taken   advantage of programming errors in the implementation of otherwise   reasonably secure protocols.  However, it is not hard to envision an   attack that targets a fundamental security flaw in a widely deployed   protocol.  It is therefore imperative that we strive to minimize such   flaws in the protocols we design.   The value of a target to an attacker may depend on where it is   located.  A network monitoring station that is physically on a   backbone cable is a major target, since it could easily be turned   into an eavesdropping station.  The same machine, if located on a   stub net and used for word processing, would be of much less use to a   sophisticated attacker, and hence would be at significantly less   risk.   One must also consider what sorts of attacks may be expected.  At a   minimum, eavesdropping must be seen as a serious threat; there have   been very many such incidents since at least 1993.  Often, active   attacks, that is, attacks that involve insertion or deletion of   packets by the attacker, are a risk as well.  It is worth noting that   such attacks can be launched with off-the-shelf tools, and have in   fact been observed "in the wild".  Of particular interest is a form   of attack called "session hijacking", where someone on a link between   the two communicating parties wait for authentication to complete and   then impersonate one of the parties and continue the connection with   the other.   One of the most important tools available to us for securing   protocols is cryptography.  Cryptography permits us to apply various   kinds of protection to data as it traverses the network, without   having to depend on any particular security properties of the network   itself.  This is important because the Internet, by its distributedBellovin, et al.             Informational                      [Page 3]

RFC 3631          Security Mechanisms for the Internet     December 2003   management and control, cannot be considered a trustworthy media in   and of itself.  Its security derives from the mechanisms that we   build into the protocols themselves, independent of the underlying   media or network operators.   Finally, of course, there is the cost to the defender of using   cryptography.  This cost is dropping rapidly; Moore's Law, plus the   easy availability of cryptographic components and toolkits, makes it   relatively easy to use strong protective techniques.  Although there   are exceptions, public key operations are still expensive, perhaps   prohibitively so if the cost of each public-key operation is spread   over too few transactions, careful engineering design can generally   let us spread this cost over many transactions.   In general, the default today should be to use the strongest   cryptography available in any protocol.  Strong cryptography often   costs no more, and sometimes less, then weaker cryptography.  The   actual performance cost of an algorithm is often unrelated to the   security it provides.  Depending on the hardware available,   cryptography can be performed at very high rates (1+Gbps), and even   in software its performance impact is shrinking over time.2.2.  A Word about Mandatory Mechanisms   We have evolved in the IETF the notion of "mandatory to implement"   mechanisms.  This philosophy evolves from our primary desire to   ensure interoperability between different implementations of a   protocol.  If a protocol offers many options for how to perform a   particular task, but fails to provide for at least one that all must   implement, it may be possible that multiple, non-interoperable   implementations may result.  This is the consequence of the selection   of non-overlapping mechanisms being deployed in the different   implementations.   Although a given protocol may make use of only one or a few security   mechanisms, these mechanisms themselves often can make use of several   cryptographic systems.  The various cryptographic systems vary in   strength and performance.  However, in many protocols we need to   specify a "mandatory to implement" to ensure that any two   implementations will eventually be able to negotiate a common   cryptographic system between them.   There are some protocols that were originally designed to be run in a   very limited domain.  It is often argued that the domain of   implementation for a particular protocol is sufficiently well defined   and secure that the protocol itself need not provide any security   mechanisms.Bellovin, et al.             Informational                      [Page 4]

RFC 3631          Security Mechanisms for the Internet     December 2003   History has shown this argument to be wrong.  Inevitably, successful   protocols - even if developed for limited use - wind up used in a   broader environment, where the initial security assumptions do not   hold.   To solve this problem, the IETF requires that *ALL* protocols provide   appropriate security mechanisms, even when their domain of   application is at first believed to be very limited.   It is important to understand that mandatory mechanisms are mandatory   to *implement*.  It is not necessarily mandatory that end-users   actually use these mechanisms.  If an end-user knows that they are   deploying a protocol over a "secure" network, then they may choose to   disable security mechanisms that they believe are adding insufficient   value as compared to their performance cost.  (We are generally   skeptical of the wisdom of disabling strong security even then, but   that is beyond the scope of this document.)   Insisting that certain mechanisms are mandatory to implement means   that those end-users who need the protocol provided by the security   mechanism have it available when needed.  Particularly with security   mechanisms, just because a mechanism is mandatory to implement does   not imply that it should be the default mechanism or that it may not   be disabled by configuration.  If a mandatory to implement algorithm   is old and weak, it is better to disable it when a stronger algorithm   is available.2.3.  Granularity of Protection   Some security mechanisms can protect an entire network.  While this   economizes on hardware, it can leave the interior of such networks   open to attacks from the inside.  Other mechanisms can provide   protection down to the individual user of a timeshared machine,   though perhaps at risk of user impersonation if the machine has been   compromised.   When assessing the desired granularity of protection, protocol   designers should take into account likely usage patterns,   implementation layers (see below), and deployability.  If a protocol   is likely to be used only from within a secure cluster of machines   (say, a Network Operations Center), subnet granularity may be   appropriate.  By contrast, a security mechanism peculiar to a single   application is best embedded in that application, rather than inside   TCP; otherwise, deployment will be very difficult.Bellovin, et al.             Informational                      [Page 5]

RFC 3631          Security Mechanisms for the Internet     December 20032.4.  Implementation Layer   Security mechanisms can be located at any layer.  In general, putting   a mechanism at a lower layer protects a wider variety of higher-layer   protocols, but may not be able to protect them as well.  A link-layer   encryptor can protect not just IP, but even ARP packets.  However,   its reach is just that one link.  Conversely, a signed email message   is protected even if sent through many store-and-forward mail   gateways, can identify the actual sender, and the signature can be   verified long after the message is delivered.  However, only that one   type of message is protected.  Messages of similar formats, such as   some Netnews postings, are not protected unless the mechanism is   specifically adapted and then implemented in the news-handling   programs.3.  Standard Security Mechanisms3.1.  One-Time Passwords   One-time password schemes, such as that described in [RFC2289], are   very much stronger than conventional passwords.  The host need not   store a copy of the user's password, nor is it ever transmitted over   the network.  However, there are some risks.  Since the transmitted   string is derived from a user-typed password, guessing attacks may   still be feasible.  (Indeed, a program to launch just this attack is   readily available.)  Furthermore, the user's ability to login   necessarily expires after a predetermined number of uses.  While in   many cases this is a feature, an implementation most likely needs to   provide a way to reinitialize the authentication database, without   requiring that the new password be sent in the clear across the   network.   There are commercial hardware authentication tokens.  Apart from the   session hijacking issue, support for such tokens (especially   challenge/response tokens, where the server sends a different random   number for each authentication attempt) may require extra protocol   messages.3.2.  HMAC   HMAC [RFC2104] is the preferred shared-secret authentication   technique.  If both sides know the same secret key, HMAC can be used   to authenticate any arbitrary message.  This includes random   challenges, which means that HMAC can be adapted to prevent replays   of old sessions.Bellovin, et al.             Informational                      [Page 6]

RFC 3631          Security Mechanisms for the Internet     December 2003   An unfortunate disadvantage of using HMAC for connection   authentication is that the secret must be known in the clear by both   parties, making this undesirable when keys are long-lived.   When suitable, HMAC should be used in preference to older techniques,   notably keyed hash functions.  Simple keyed hashes based on MD5   [RFC1321], such as that used in the BGP session security mechanism   [RFC2385], are especially to be avoided in new protocols, given the   hints of weakness in MD5.   HMAC can be implemented using any secure hash function, including MD5   and SHA-1 [RFC3174].  SHA-1 is preferable for new protocols because   it is more frequently used for this purpose and may be more secure.   It is important to understand that an HMAC-based mechanism needs to   be employed on every protocol data unit (aka packet).  It is a   mistake to use an HMAC-based system to authenticate the beginning of   a TCP session and then send all remaining data without any   protection.   Attack programs exist that permit a TCP session to be stolen.  An   attacker merely needs to use such a tool to steal a session after the   HMAC step is performed.3.3.  IPsec   IPsec [RFC2401],[RFC2402],[RFC2406],[RFC2407],[RFC2411] is the   generic IP-layer encryption and authentication protocol.  As such, it   protects all upper layers, including both TCP and UDP.  Its normal   granularity of protection is host-to-host, host-to-gateway, and   gateway-to-gateway.  The specification does permit user-granularity   protection, but this is comparatively rare.  As such, IPsec is   currently inappropriate when host-granularity is too coarse.   Because IPsec is installed at the IP layer, it is rather intrusive to   the networking code.  Implementing it generally requires either new   hardware or a new protocol stack.  On the other hand, it is fairly   transparent to applications.  Applications running over IPsec can   have improved security without changing their protocols at all.  But   at least until IPsec is more widely deployed, most applications   should not assume they are running atop IPsec as an alternative to   specifying their own security mechanisms.  Most modern operating   systems have IPsec available; most routers do not, at least for the   control path.  An application using TLS is more likely to be able to   assert application-specific to take advantage of its authentication.Bellovin, et al.             Informational                      [Page 7]

RFC 3631          Security Mechanisms for the Internet     December 2003   The key management for IPsec can use either certificates or shared   secrets.  For all the obvious reasons, certificates are preferred;   however, they may present more of a headache for the system manager.   There is strong potential for conflict between IPsec and NAT   [RFC2993].  NAT does not easily coexist with any protocol containing   embedded IP address; with IPsec, every packet, for every protocol,   contains such addresses, if only in the headers.  The conflict can   sometimes be avoided by using tunnel mode, but that is not always an   appropriate choice for other reasons.  There is ongoing work to make   IPsec pass through NAT more easily [NATIKE].   Most current IPsec usage is for virtual private networks.  Assuming   that the other constraints are met, IPsec is the security protocol of   choice for VPN-like situations, including the remote access scenario   where a single machine tunnels back into its home network over the   internet using IPsec.3.4.  TLS   TLS [RFC2246] provides an encrypted, authenticated channel that runs   on top of TCP.  While TLS was originally designed for use by Web   browsers, it is by no means restricted to such.  In general, though,   each application that wishes to use TLS will need to be converted   individually.   Generally, the server side is always authenticated by a certificate.   Clients may possess certificates, too, providing mutual   authentication, though this is rarely deployed.  It's an unfortunate   reality that even server side authentication it not as secure in   practice as the cryptography would imply because most implementations   allow users to ignore authentication failures (by clicking OK to a   warning) and most users routinely do so [Bell98].  Designers should   thus be wary of demanding plaintext passwords, even over TLS-   protected connections.  (This requirement can be relaxed if it is   likely that implementations will be able to verify the authenticity   and authorization of the server's certificate.)   Although application modification is generally required to make use   of TLS, there exist toolkits, both free and commercial, that provide   implementations.  These are designed to be incorporated into the   application's code.  An application using TLS is more likely to be   able to assert application specific certificate policies than one   using IPsec.Bellovin, et al.             Informational                      [Page 8]

RFC 3631          Security Mechanisms for the Internet     December 20033.5.  SASL   SASL [RFC2222] is a framework for negotiating an authentication and   encryption mechanism to be used over a TCP stream.  As such, its   security properties are those of the negotiated mechanism.   Specifically, unless the negotiated mechanism authenticates all of   the subsequent messages or underlying protection protocol such as TLS   is used, TCP connections are vulnerable to session stealing.   If you need to use TLS (or IPSec) under SASL, why bother with SASL in   the first place? Why not simply use the authentication facilities of   TLS and be done with it?   The answer here is subtle.  TLS makes extensive use of certificates   for authentication.  As commonly deployed, only servers have   certificates, whereas clients go unauthenticated (at least by the TLS   processing itself).   SASL permits the use of more traditional client authentication   technologies, such as passwords (one-time or otherwise).  A powerful   combination is TLS for underlying protection and authentication of   the server, and a SASL-based system for authenticating clients.  Care   must be taken to avoid man-in-the-middle vulnerabilities when   different authentication techniques are used in different directions.3.6.  GSS-API   GSS-API [RFC2744] provides a framework for applications to use when   they require authentication, integrity, and/or confidentiality.   Unlike SASL, GSS-API can be used easily with UDP-based applications.   It provides for the creation of opaque authentication tokens (aka   chunks of memory) which may be embedded in a protocol's data units.   Note that the security of GSS-API-protected protocols depends on the   underlying security mechanism; this must be evaluated independently.   Similar considerations apply to interoperability, of course.3.7.  DNSSEC   DNSSEC [RFC2535] digitally signs DNS records.  It is an essential   tool for protecting against DNS cache contamination attacks [Bell95];   these in turn can be used to defeat name-based authentication and to   redirect traffic to or past an attacker.  The latter makes DNSSEC an   essential component of some other security mechanisms, notably IPsec.   Although not widely deployed on the Internet at the time of the   writing of this document, it offers the potential to provide a secure   mechanism for mapping domain names to IP protocol addresses.  It may   also be used to securely associate other information with a DNS name.Bellovin, et al.             Informational                      [Page 9]

RFC 3631          Security Mechanisms for the Internet     December 2003   This information may be as simple as a service that is supported on a   given node, or a key to be used with IPsec for negotiating a secure   session.  Note that the concept of storing general purpose   application keys in the DNS has been deprecated [RFC3445], but   standardization of storing keys for particular applications - in   particular IPsec - is proceeding.3.8.  Security/Multipart   Security/Multiparts [RFC1847] are the preferred mechanism for   protecting email.  More precisely, it is the MIME framework within   which encryption and/or digital signatures are embedded.  Both S/MIME   and OpenPGP (see below) use Security/Multipart for their encoding.   Conforming mail readers can easily recognize and process the   cryptographic portions of the mail.   Security/Multiparts represents one form of "object security", where   the object of interest to the end user is protected, independent of   transport mechanism, intermediate storage, etc.  Currently, there is   no general form of object protection available in the Internet.   For a good example of using S/MIME outside the context of email, see   Session Initiation Protocol [RFC 3261].3.9.  Digital Signatures   One of the strongest forms of challenge/response authentication is   based on digital signatures.  Using public key cryptography is   preferable to schemes based on secret key ciphers because no server   needs a copy of the client's secret.  Rather, the client has a   private key; servers have the corresponding public key.   Using digital signatures properly is tricky.  A client should never   sign the exact challenge sent to it, since there are several subtle   number-theoretic attacks that can be launched in such situations.   The Digital Signature Standard [DSS] and RSA [RSA] are both good   choices; each has its advantages.  Signing with DSA requires the use   of good random numbers [RFC1750].  If the enemy can recover the   random number used for any given signature, or if you use the same   random number for two different documents, your private key can be   recovered.  DSS has much better performance than RSA for generating   new private keys, and somewhat better performance generating   signatures, while RSA has much better performance for verifying   signatures.Bellovin, et al.             Informational                     [Page 10]

RFC 3631          Security Mechanisms for the Internet     December 20033.10.  OpenPGP and S/MIME   Digital signatures can be used to build "object security"   applications which can be used to protect data in store and forward   protocols such as electronic mail.   At this writing, two different secure mail protocols, OpenPGP   [OpenPGP] and S/MIME [S/MIME], have been proposed to replace PEM   [PEM].  It is not clear which, if either, will succeed.  While   specified for use with secure mail, both can be adapted to protect   data carried by other protocols.  Both use certificates to identify   users; both can provide secrecy and authentication of mail messages;   however, the certificate formats are very different.  Historically,   the difference between PGP-based mail and S/MIME-based mail has been   the style of certificate chaining.  In S/MIME, users possess X.509   certificates; the certification graph is a tree with a very small   number of roots.  By contrast, PGP uses the so-called "web of trust",   where any user can sign anyone else's certificate.  This   certification graph is really an arbitrary graph or set of graphs.   With any certificate scheme, trust depends on two primary   characteristics.  First, it must start from a known-reliable source,   either an X.509 root, or someone highly trusted by the verifier,   often him or herself.  Second, the chain of signatures must be   reliable.  That is, each node in the certification graph is crucial;   if it is dishonest or has been compromised, any certificates it has   vouched for cannot be trusted.  All other factors being equal (and   they rarely are), shorter chains are preferable.   Some of the differences reflect a tension between two philosophical   positions represented by these technologies.  Others resulted from   having separate design teams.   S/MIME is designed to be "fool proof".  That is, very little end-user   configuration is required. Specifically, end-users do not need to be   aware of trust relationships, etc.  The idea is that if an S/MIME   client says, "This signature is valid", the user should be able to   "trust" that statement at face value without needing to understand   the underlying implications.   To achieve this, S/MIME is typically based on a limited number of   "root" Certifying Authorities (CAs).  The goal is to build a global   trusted certificate infrastructure.   The down side to this approach is that it requires a deployed public   key infrastructure before it will work.  Two end-users may not be   able to simply obtain S/MIME-capable software and begin communicating   securely.  This is not a limitation of the protocol, but a typicalBellovin, et al.             Informational                     [Page 11]

RFC 3631          Security Mechanisms for the Internet     December 2003   configuration restriction for commonly available software.  One or   both of them may need to obtain a certificate from a mutually trusted   CA; furthermore, that CA must already be trusted by their mail   handling software.  This process may involve cost and legal   obligations.  This ultimately results in the technology being harder   to deploy, particularly in an environment where end-users do not   necessarily appreciate the value received for the hassle incurred.   The PGP "web of trust" approach has the advantage that two end-users   can just obtain PGP software and immediately begin to communicate   securely.  No infrastructure is required and no fees and legal   agreements need to be signed to proceed.  As such PGP appeals to   people who need to establish ad-hoc security associations.   The down side to PGP is that it requires end-users to have an   understanding of the underlying security technology in order to make   effective use of it.  Specifically it is fairly easy to fool a naive   users to accept a "signed" message that is in fact a forgery.   To date PGP has found great acceptance between security-aware   individuals who have a need for secure e-mail in an environment   devoid of the necessary global infrastructure.   By contrast, S/MIME works well in a corporate setting where a secure   internal CA system can be deployed.  It does not require a lot of   end-user security knowledge.  S/MIME can be used between institutions   by carefully setting up cross certification, but this is harder to do   than it seems.   As of this writing a global certificate infrastructure continues to   elude us.  Questions about a suitable business model, as well as   privacy considerations, may prevent one from ever emerging.3.11.  Firewalls and Topology   Firewalls are a topological defense mechanism.  That is, they rely on   a well-defined boundary between the good "inside" and the bad   "outside" of some domain, with the firewall mediating the passage of   information.  While firewalls can be very valuable if employed   properly, there are limits to their ability to protect a network.   The first limitation, of course, is that firewalls cannot protect   against inside attacks.  While the actual incidence rate of such   attacks is not known (and is probably unknowable), there is no doubt   that it is substantial, and arguably constitutes a majority of   security problems.  More generally, given that firewalls require a   well-delimited boundary, to the extent that such a boundary does not   exist, firewalls do not help.  Any external connections, whether theyBellovin, et al.             Informational                     [Page 12]

RFC 3631          Security Mechanisms for the Internet     December 2003   are protocols that are deliberately passed through the firewall,   links that are tunneled through, unprotected wireless LANs, or direct   external connections from nominally-inside hosts, weaken the   protection.  Firewalls tend to become less effective over time as   users tunnel protocols through them and may have inadequate security   on the tunnel endpoints.  If the tunnels are encrypted, there is no   way for the firewall to censor them.  An oft-cited advantage of   firewalls is that they hide the existence of internal hosts from   outside eyes.  Given the amount of leakage, however, the likelihood   of successfully hiding machines is rather low.   In a more subtle vein, firewalls hurt the end-to-end model of the   Internet and its protocols.  Indeed, not all protocols can be passed   safely or easily through firewalls.  Sites that rely on firewalls for   security may find themselves cut off from new and useful aspects of   the Internet.   Firewalls work best when they are used as one element of a total   security structure.  For example, a strict firewall may be used to   separate an exposed Web server from a back-end database, with the   only opening the communication channel between the two.  Similarly, a   firewall that permitted only encrypted tunnel traffic could be used   to secure a piece of a VPN.  On the other hand, in that case the   other end of the VPN would need to be equally secured.3.12.  Kerberos   Kerberos [RFC1510] provides a mechanism for two entities to   authenticate each other and exchange keying material.  On the client   side, an application obtains a Kerberos "ticket" and "authenticator".   These items, which should be considered opaque data, are then   communicated from client to server.  The server can then verify their   authenticity.  Both sides may then ask the Kerberos software to   provide them with a session key which can be used to protect or   encrypt data.   Kerberos may be used by itself in a protocol.  However, it is also   available as a mechanism under SASL and GSSAPI.  It has some known   vulnerabilities [KRBATTACK] [KRBLIM] [KRB4WEAK], but it can be used   securely.3.13.  SSH   SSH provides a secure connection between client and server.  It   operates very much like TLS; however, it is optimized as a protocol   for remote connections on terminal-like devices.  One of its more   innovative features is its support for "tunneling" other protocols   over the SSH-protected TCP connection.  This feature has permittedBellovin, et al.             Informational                     [Page 13]

RFC 3631          Security Mechanisms for the Internet     December 2003   knowledgeable security people to perform such actions as reading and   sending e-mail or news via insecure servers over an insecure network.   It is not a substitute for a true VPN, but it can often be used in   place of one.4.  Insecurity Mechanisms   Some common security mechanisms are part of the problem rather than   part of the solution.4.1.  Plaintext Passwords   Plaintext passwords are the most common security mechanism in use   today.  Unfortunately, they are also the weakest.  When not protected   by an encryption layer, they are completely unacceptable.  Even when   used with encryption, plaintext passwords are quite weak, since they   must be transmitted to the remote system.  If that system has been   compromised or if the encryption layer does not include effective   authentication of the server to the client, an enemy can collect the   passwords and possibly use them against other targets.   Another weakness arises because of common implementation techniques.   It is considered good form [MT79] for the host to store a one-way   hash of the users' passwords, rather than their plaintext form.   However, that may preclude migrating to stronger authentication   mechanisms, such as HMAC-based challenge/response.   The strongest attack against passwords, other than eavesdropping, is   password-guessing.  With a suitable program and dictionary (and these   are widely available), 20-30% of passwords can be guessed in most   environments [Klein90].4.2.  Address-Based Authentication   Another common security mechanism is address-based authentication. At   best, it can work in highly constrained environments.  If your   environment consists of a small number of machines, all tightly   administered, secure systems run by trusted users, and if the network   is guarded by a router that blocks source-routing and prevents   spoofing of your source addresses, and you know there are no wireless   bridges, and if you restrict address-based authentication to machines   on that network, you are probably safe.  But these conditions are   rarely met.Bellovin, et al.             Informational                     [Page 14]

RFC 3631          Security Mechanisms for the Internet     December 2003   Among the threats are ARP-spoofing, abuse of local proxies,   renumbering, routing table corruption or attacks, DHCP, IP address   spoofing (a particular risk for UDP-based protocols), sequence number   guessing, and source-routed packets.  All of these can be quite   potent.4.3.  Name-Based Authentication   Name-based authentication has all of the problems of address-based   authentication and adds new ones: attacks on the DNS [Bell95] and   lack of a one to one mapping between addresses and names.  At a   minimum, a process that retrieves a host name from the DNS should   retrieve the corresponding address records and cross-check.   Techniques such as DNS cache contamination can often negate such   checks.   DNSSEC provides protection against this sort of attack.  However, it   does nothing to enhance the reliability of the underlying address.   Further, the technique generates a lot of false alarms.  These   lookups do not provide reliable information to a machine, though they   might be a useful debugging tool for humans and could be useful in   logs when trying to reconstruct how and attack took place.5.  Security Considerations   No security mechanisms are perfect.  If nothing else, any network-   based security mechanism can be thwarted by compromise of the   endpoints.  That said, each of the mechanisms described here has its   own limitations.  Any decision to adopt a given mechanism should   weigh all of the possible failure modes.  These in turn should be   weighed against the risks to the endpoint of a security failure.6.  IANA Considerations   There are no IANA considerations regarding this document.7.  Acknowledgements   Brian Carpenter, Tony Hain, and Marcus Leech made a number of useful   suggestions.  Much of the substance comes from the participants in   the IAB Security Architecture Workshop.Bellovin, et al.             Informational                     [Page 15]

RFC 3631          Security Mechanisms for the Internet     December 20038.  Informative References   [Bell95]    "Using the Domain Name System for System Break-Ins".               Proc.  Fifth Usenix Security Conference, 1995.   [Bell98]    "Cryptography and the Internet", S.M. Bellovin, in               Proceedings of CRYPTO '98, August 1998.   [DSS]       "Digital Signature Standard".  NIST.  May 1994.  FIPS               186.   [Klein90]   "Foiling the Cracker: A Survey of, and Implications to,               Password Security". D. Klein. Usenix UNIX Security               Workshop, August 1990.   [KRBATTACK] "A Real-World Analysis of Kerberos Password Security".               T. Wu. Network and Distributed System Security Symposium               (NDSS '99).  January 1999.   [KRBLIM]    "Limitations of the Kerberos Authentication System".               Proceedings of the 1991 Winter USENIX Conference, 1991.   [KRB4WEAK]  "Misplaced trust: Kerberos 4 session keys".  Proceedings               of the Internet Society Network and Distributed Systems               Security Symposium, March 1997.   [MT79]      "UNIX Password Security", R.H. Morris and K.  Thompson,               Communications of the ACM. November 1979.   [NATIKE]    Kivinen, T., et al., "Negotiation of NAT-Traversal in the               IKE", Work in Progress, June 2002.   [RFC1321]   Rivest, R., "The MD5 Message-Digest Algorithm",RFC 1321,               April 1992.   [RFC1510]   Kohl, J. and C. Neuman, "The Kerberos Network               Authentication Service (V5)",RFC 1510, September 1993.   [RFC1750]   Eastlake, D., Crocker, S. and J. Schiller, "Randomness               Recommendations for Security",RFC 1750, December 1994.   [RFC1847]   Galvin, J., Murphy, S., Crocker, S. and N. Freed,               "Security Multiparts for MIME: Multipart/Signed and               Multipart/Encrypted",RFC 1847, October 1995.   [RFC2104]   Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:  Keyed-               Hashing for Message Authentication",RFC 2104, February               1997.Bellovin, et al.             Informational                     [Page 16]

RFC 3631          Security Mechanisms for the Internet     December 2003   [RFC2222]   Myers, J., "Simple Authentication and Security Layer               (SASL)",RFC 2222, October 1997.   [RFC2246]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",RFC 2246, January 1999.   [RFC2289]   Haller, N., Metz, C., Nesser, P. and M. Straw, "A One-               Time Password System", STD 61,RFC 2289, February 1998.   [RFC2316]   Bellovin, S., "Report of the IAB Security Architecture               Workshop",RFC 2316, April 1998.   [RFC2385]   Hefferman, A., "Protection of BGP Sessions via the TCP               MD5 Signature Option",RFC 2385, August 1998.   [RFC2401]   Kent, S. and R. Atkinson, "Security Architecture for the               Internet Protocol",RFC 2401, November 1998.   [RFC2402]   Kent, S. and R. Atkinson, "IP Authentication Header",RFC2402, November 1998.   [RFC2406]   Kent, S. and R. Atkinson, "IP Encapsulating Security               Payload (ESP)",RFC 2406, November 1998.   [RFC2407]   Piper, D., "The Internet IP Security Domain of               Interpretation for ISAKMP",RFC 2407, November 1998.   [RFC2411]   Thayer, R., Doraswamy, N. and R. Glenn, "IP Security               Document Roadmap",RFC 2411, November 1998.   [RFC2535]   Eastlake, D., "Domain Name System Security Extensions",RFC 2535, March 1999.   [RFC2744]   Wray, J., "Generic Security Service API Version 2:  C-               bindings",RFC 2744, January 2000.   [RFC2993]   Hain, T., "Architectural Implications of NAT",RFC 2993,               November 2000.   [RFC3174]   Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1               (SHA1)",RFC 3174, September 2001.   [RFC3261]   Rosenberg, J., Schulzrinne, H., Camarillo, R., Johnston,               A., Peterson, J., Sparks, R., Handley, M. and E.               Schooler, "SIP:  Session Initiation Protocol",RFC 3261,               June 2002.Bellovin, et al.             Informational                     [Page 17]

RFC 3631          Security Mechanisms for the Internet     December 2003   [RFC3445]   Massey, D. and S. Rose, "Limiting the Scope of the KEY               Resource Record (RR)",RFC 3445, December 2002.   [RSA]       Rivest, R., Shamir, A. and L. Adleman, "A Method for               Obtaining Digital Signatures and Public-Key               Cryptosystems", Communications of the ACM, February 1978.9.  Intellectual Property Statement   The IETF takes no position regarding the validity or scope of any   intellectual property or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; neither does it represent that it   has made any effort to identify any such rights.  Information on the   IETF's procedures with respect to rights in standards-track and   standards-related documentation can be found inBCP-11.  Copies of   claims of rights made available for publication and any assurances of   licenses to be made available, or the result of an attempt made to   obtain a general license or permission for the use of such   proprietary rights by implementors or users of this specification can   be obtained from the IETF Secretariat.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights which may cover technology that may be required to practice   this standard.  Please address the information to the IETF Executive   Director.Bellovin, et al.             Informational                     [Page 18]

RFC 3631          Security Mechanisms for the Internet     December 200310.  Author Information   This document is a publication of the Internet Architecture Board.   Internet Architecture Board Members at the time this document was   completed were:   Bernard Aboba   Harald Alvestrand   Rob Austein   Leslie Daigle, Chair   Patrik Faltstrom   Sally Floyd   Jun-ichiro Itojun Hagino   Mark Handley   Geoff Huston   Charlie Kaufman   James Kempf   Eric Rescorla   Michael StJohns   Internet Architecture Board   EMail: iab@iab.org   Steven M. Bellovin, Editor   EMail: bellovin@acm.org   Jeffrey I. Schiller, Editor   EMail: jis@mit.edu   Charlie Kaufman, Editor   EMail: charliek@microsoft.comBellovin, et al.             Informational                     [Page 19]

RFC 3631          Security Mechanisms for the Internet     December 200311.  Full Copyright Statement   Copyright (C) The Internet Society (2003).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assignees.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Bellovin, et al.             Informational                     [Page 20]