Network Working Group                                   S. Bellovin, Ed.
Request for Comments: 3631                              J. Schiller, Ed.
Category: Informational                                  C. Kaufman, Ed.
                                             Internet Architecture Board
                                                           December 2003


                  Security Mechanisms for the Internet

Status 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.

   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 Factors

2.1.  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 exceedingly

   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 distributed

   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.

   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.

2.4.  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 Mechanisms

3.1.  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.

   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.

   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.

3.5.  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.

   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.

3.10.  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 typical

   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 they

   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 permitted

   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.

   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.

8.  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.

   [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", RFC
               2402, 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.

   [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.

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10.  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.com

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