





Network Working Group                                          N. Haller
Request for Comments: 1704                  Bell Communications Research
Category: Informational                                      R. Atkinson
                                               Naval Research Laboratory
                                                            October 1994


                       On Internet Authentication

Status of this Memo

   This document provides information for the Internet community.  This
   memo does not specify an Internet standard of any kind.  Distribution
   of this memo is unlimited.

1. INTRODUCTION

   The authentication requirements of computing systems and network
   protocols vary greatly with their intended use, accessibility, and
   their network connectivity.  This document describes a spectrum of
   authentication technologies and provides suggestions to protocol
   developers on what kinds of authentication might be suitable for some
   kinds of protocols and applications used in the Internet.  It is
   hoped that this document will provide useful information to
   interested members of the Internet community.

   Passwords, which are vulnerable to passive attack, are not strong
   enough to be appropriate in the current Internet [CERT94].  Further,
   there is ample evidence that both passive and active attacks are not
   uncommon in the current Internet [Bellovin89, Bellovin92, Bellovin93,
   CB94, Stoll90].  The authors of this paper believe that many
   protocols used in the Internet should have stronger authentication
   mechanisms so that they are at least protected from passive attacks.
   Support for authentication mechanisms secure against active attack is
   clearly desirable in internetworking protocols.

   There are a number of dimensions to the internetwork authentication
   problem and, in the interest of brevity and readability, this
   document only describes some of them.  However, factors that a
   protocol designer should consider include whether authentication is
   between machines or between a human and a machine, whether the
   authentication is local only or distributed across a network,
   strength of the authentication mechanism, and how keys are managed.








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2. DEFINITION OF TERMS

   This section briefly defines some of the terms used in this paper to
   aid the reader in understanding these suggestions.  Other references
   on this subject might be using slightly different terms and
   definitions because the security community has not reached full
   consensus on all definitions.  The definitions provided here are
   specifically focused on the matters discussed in this particular
   document.

   Active Attack:  An attempt to improperly modify data, gain
          authentication, or gain authorization by inserting false
          packets into the data stream or by modifying packets
          transiting the data stream. (See passive attacks and replay
          attacks.)

   Asymmetric Cryptography:  An encryption system that uses different
          keys, for encryption and decryption.  The two keys have an
          intrinsic mathematical relationship to each other.  Also
          called Public~Key~Cryptography.  (See Symmetric Cryptography)

   Authentication:  The verification of the identity of the source of
          information.

   Authorization:  The granting of access rights based on an
          authenticated identity.

   Confidentiality: The protection of information so that someone not
          authorized to access the information cannot read the
          information even though the unauthorized person might see the
          information's container (e.g., computer file or network
          packet).

   Encryption: A mechanism often used to provide confidentiality.

   Integrity:  The protection of information from unauthorized
          modification.

   Key Certificate: A data structure consisting of a public key, the
          identity of the person, system, or role associated with that
          key, and information authenticating both the key and the
          association between that identity and that public key.  The
          keys used by PEM are one example of a key certificate
          [Kent93].

   Passive Attack:  An attack on an authentication system that inserts
          no data into the stream, but instead relies on being able to
          passively monitor information being sent between other



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          parties.  This information could be used a later time in what
          appears to be a valid session.  (See active attack and replay
          attack.)

   Plain-text:  Unencrypted text.

   Replay Attack:  An attack on an authentication system by recording
          and replaying previously sent valid messages (or parts of
          messages).  Any constant authentication information, such as a
          password or electronically transmitted biometric data, can be
          recorded and used later to forge messages that appear to be
          authentic.

   Symmetric Cryptography: An encryption system that uses the same key
          for encryption and decryption.  Sometimes referred to as
          Secret~Key~Cryptography.

3. AUTHENTICATION TECHNOLOGIES

   There are a number of different classes of authentication, ranging
   from no authentication to very strong authentication.  Different
   authentication mechanisms are appropriate for addressing different
   kinds of authentication problems, so this is not a strict
   hierarchical ordering.

   3.1 No Authentication

      For completeness, the simplest authentication system is not to
      have any.  A non-networked PC in a private (secure) location is an
      example of where no authentication is acceptable.  Another case is
      a stand-alone public workstation, such as "mail reading"
      workstations provided at some conferences,  on which the data is
      not sensitive to disclosure or modification.

   3.2 Authentication Mechanisms Vulnerable to Passive Attacks

      The simple password check is by far the most common form of
      authentication.  Simple authentication checks come in many forms:
      the key may be a password memorized by the user, it may be a
      physical or electronic item possessed by the user, or it may be a
      unique biological feature.  Simple authentication systems are said
      to be "disclosing" because if the key is transmitted over a
      network it is disclosed to eavesdroppers.  There have been
      widespread reports of successful passive attacks in the current
      Internet using already compromised machines to engage in passive
      attacks against additional machines [CERT94].  Disclosing
      authentication mechanisms are vulnerable to replay attacks.
      Access keys may be stored on the target system, in which case a



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      single breach in system security may gain access to all passwords.
      Alternatively, as on most systems, the data stored on the system
      can be enough to verify passwords but not to generate them.

   3.3 Authentication Mechanisms Vulnerable to Active Attacks

      Non-disclosing password systems have been designed to prevent
      replay attacks.  Several systems have been invented to generate
      non-disclosing passwords.  For example, the SecurID Card from
      Security Dynamics uses synchronized clocks for authentication
      information.  The card generates a visual display and thus must be
      in the possession of the person seeking authentication.  The S/Key
      (TM) authentication system developed at Bellcore generates
      multiple single use passwords from a single secret key [Haller94].
      It does not use a physical token, so it is also suitable for
      machine-machine authentication.  In addition there are challenge-
      response systems in which a device or computer program is used to
      generate a verifiable response from a non-repeating challenge.
      S/Key authentication does not require the storage of the user's
      secret key, which is an advantage when dealing with current
      untrustworthy computing systems.  In its current form, the S/Key
      system is vulnerable to a dictionary attack on the secret password
      (pass phrase) which might have been poorly chosen.  The Point-to-
      Point Protocol's CHAP challenge-response system is non-disclosing
      but only useful locally [LS92, Simpson93].  These systems vary in
      the sensitivity of the information stored in the authenticating
      host, and thus vary in the security requirements that must be
      placed on that host.

   3.4 Authentication Mechanisms Not Vulnerable to Active Attacks

      The growing use of networked computing environments has led to the
      need for stronger authentication.  In open networks, many users
      can gain access to any information flowing over the network, and
      with additional effort, a user can send information that appears
      to come from another user.

      More powerful authentication systems make use of the computation
      capability of the two authenticating parties.  Authentication may
      be unidirectional, for example authenticating users to a host
      computer system, or it may be mutual in which case the entity
      logging in is assured of the identity of the host.  Some
      authentication systems use cryptographic techniques and establish
      (as a part of the authentication process) a shared secret (e.g.,
      session key) that can be used for further exchanges.  For example,
      a user, after completion of the authentication process, might be
      granted an authorization ticket that can be used to obtain other
      services without further authentication.  These authentication



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      systems might also provide confidentiality (using encryption) over
      insecure networks when required.

4. CRYPTOGRAPHY

   Cryptographic mechanisms are widely used to provide authentication,
   either with or without confidentiality, in computer networks and
   internetworks.  There are two basic kinds of cryptography and these
   are described in this section.  A fundamental and recurring problem
   with cryptographic mechanisms is how to securely distribute keys to
   the communicating parties.  Key distribution is addressed in Section
   6 of this document.

   4.1 Symmetric Cryptography

      Symmetric Cryptography includes all systems that use the same key
      for encryption and decryption.  Thus if anyone improperly obtains
      the key, they can both decrypt and read data encrypted using that
      key and also encrypt false data and make it appear to be valid.
      This means that knowledge of the key by an undesired third party
      fully compromises the confidentiality of the system.  Therefore,
      the keys used need to be distributed securely, either by courier
      or perhaps by use of a key distribution protocol, of which the
      best known is perhaps that proposed by Needham and Schroeder
      [NS78, NS87].  The widely used Data Encryption Standard (DES)
      algorithm, that has been standardized for use to protect
      unclassified civilian US Government information, is perhaps the
      best known symmetric encryption algorithm [NBS77].

      A well known system that addresses insecure open networks as a
      part of a computing environment is the Kerberos (TM)
      Authentication Service that was developed as part of Project
      Athena at MIT [SNS88, BM91, KN93].  Kerberos is based on Data
      Encryption Standard (DES) symmetric key encryption and uses a
      trusted (third party) host that knows the secret keys of all users
      and services, and thus can generate credentials that can be used
      by users and servers to prove their identities to other systems.
      As with any distributed authentication scheme, these credentials
      will be believed by any computer within the local administrative
      domain or realm.  Hence, if a user's password is disclosed, an
      attacker would be able to masquerade as that user on any system
      which trusts Kerberos.  As the Kerberos server knows all secret
      keys, it must be physically secure.  Kerberos session keys can be
      used to provide confidentiality between any entities that trust
      the key server.






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   4.2 Asymmetric Cryptography

      In the late 1970s, a major breakthrough in cryptology led to the
      availability of Asymmetric Cryptography.  This is different from
      Symmetric Cryptography because different keys are used for
      encryption and decryption, which greatly simplifies the key
      distribution problem.  The best known asymmetric system is based
      on work by Rivest, Shamir, and Adleman and is often referred to as
      "RSA" after the authors' initials [RSA78].

      SPX is an experimental system that overcomes the limitations of
      the trusted key distribution center of Kerberos by using RSA
      Public Key Cryptography [TA91].  SPX assumes a global hierarchy of
      certifying authorities at least one of which is trusted by each
      party.  It uses digital signatures that consist of a token
      encrypted in the private key of the signing entity and that are
      validated using the appropriate public key.  The public keys are
      believed to be correct as they are obtained under the signature of
      the trusted certification authority.  Critical parts of the
      authentication exchange are encrypted in the public keys of the
      receivers, thus preventing a replay attack.

   4.3 Cryptographic Checksums

      Cryptographic checksums are one of the most useful near term tools
      for protocol designers.  A cryptographic checksum or message
      integrity checksum (MIC) provides data integrity and
      authentication but not non-repudiation.  For example, Secure SNMP
      and SNMPv2 both calculate a MD5 cryptographic checksum over a
      shared secret item of data and the information to be authenticated
      [Rivest92, GM93].  This serves to authenticate the data origin and
      is believed to be very difficult to forge.  It does not
      authenticate that the data being sent is itself valid, only that
      it was actually sent by the party that claims to have sent it.
      Crytographic checksums can be used to provide relatively strong
      authentication and are particularly useful in host-to-host
      communications.  The main implementation difficulty with
      cryptographic checksums is key distribution.

   4.4 Digital Signatures

      A digital signature is a cryptographic mechanism which is the
      electronic equivalent of a written signature.  It serves to
      authenticate a piece of data as to the sender.  A digital
      signature using asymmetric cryptography (Public Key) can also be
      useful in proving that data originated with a party even if the
      party denies having sent it; this property is called non-
      repudiation.  A digital signature provides authentication without



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      confidentiality and without incurring some of the difficulties in
      full encryption.  Digital signatures are being used with key
      certificates for Privacy Enhanced Mail [Linn93, Kent93,
      Balenson93, Kaliski93].

5. USER TO HOST AUTHENTICATION

   There are a number of different approaches to authenticating users to
   remote or networked hosts.  Two types of hazard are created by remote
   or networked access: First an intruder can eavesdrop on the network
   and obtain user ids and passwords for a later replay attack. Even the
   form of existing passwords provides a potential intruder with a head
   start in guessing new ones.

   Currently, most systems use plain-text disclosing passwords sent over
   the network (typically using telnet or rlogin) from the user to the
   remote host [Anderson84, Kantor91].  This system does not provide
   adequate protection from replay attacks where an eavesdropper gains
   remote user ids and remote passwords.

   5.1 Protection Against Passive Attack Is Necessary

      Failure to use at least a non-disclosing password system means
      that unlimited access is unintentionally granted to anyone with
      physical access to the network.  For example, anyone with physical
      access to the Ethernet cable can impersonate any user on that
      portion of the network.  Thus, when one has plain-text disclosing
      passwords on an Ethernet, the primary security system is the guard
      at the door (if any exist).  The same problem exists in other LAN
      technologies such as Token-Ring or FDDI.  In some small internal
      Local Area Networks (LANs) it may be acceptable to take this risk,
      but it is an unacceptable risk in an Internet [CERT94].

      The minimal defense against passive attacks, such as
      eavesdropping, is to use a non-disclosing password system.  Such a
      system can be run from a dumb terminal or a simple communications
      program (e.g., Crosstalk or PROCOMM) that emulates a dumb terminal
      on a PC class computer.  Using a stronger authentication system
      would certainly defend against passive attacks against remotely
      accessed systems, but at the cost of not being able to use simple
      terminals.  It is reasonable to expect that the vendors of
      communications programs and non user-programmable terminals (such
      as X-Terminals) would build in non-disclosing password or stronger
      authentication systems if they were standardized or if a large
      market were offered.  One of the advantages of Kerberos is that,
      if used properly, the user's password never leaves the user's
      workstation.  Instead they are used to decrypt the user's Kerberos
      tickets, which are themselves encrypted information which are sent



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      over the network to application servers.

   5.2 Perimeter Defenses as Short Term Tool

      Perimeter defenses are becoming more common.  In these systems,
      the user first authenticates to an entity on an externally
      accessible portion of the network, possibly a "firewall" host on
      the Internet, using a non-disclosing password system. The user
      then uses a second system to authenticate to each host, or group
      of hosts, from which service is desired.  This decouples the
      problem into two more easily handled situations.

      There are several disadvantages to the perimeter defense, so it
      should be thought of as a short term solution.  The gateway is not
      transparent at the IP level, so it must treat every service
      independently.  The use of  double authentication is, in general,
      difficult or impossible for computer-computer communication.  End
      to end protocols, which are common on the connectionless Internet,
      could easily break.  The perimeter defense must be tight and
      complete, because if it is broken, the inner defenses tend to be
      too weak to stop a potential intruder.  For example, if disclosing
      passwords are used internally, these passwords can be learned by
      an external intruder (eavesdropping).  If that intruder is able to
      penetrate the perimeter, the internal system is completely
      exposed.  Finally, a perimeter defense may be open to compromise
      by internal users looking for shortcuts.

      A frequent form of perimeter defense is the application relay.  As
      these relays are protocol specific, the IP connectivity of the
      hosts inside the perimeter with the outside world is broken and
      part of the power of the Internet is lost.

      An administrative advantage of the perimeter defense is that the
      number of machines that are on the perimeter and thus vulnerable
      to attack is small.  These machines may be carefully checked for
      security hazards, but it is difficult (or impossible) to guarantee
      that the perimeter is leak-proof.  The security of a perimeter
      defense is complicated as the gateway machines must pass some
      types of traffic such as electronic mail.  Other network services
      such as the Network Time Protocol (NTP) and the File Transfer
      Protocol (FTP) may also be desirable [Mills92, PR85, Bishop].
      Furthermore, the perimeter gateway system must be able to pass
      without bottleneck the entire traffic load for its security
      domain.







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   5.3 Protection Against Active Attacks Highly Desirable

      In the foreseeable future, the use of stronger techniques will be
      required to protect against active attacks.  Many corporate
      networks based on broadcast technology such as Ethernet probably
      need such techniques.  To defend against an active attack, or to
      provide privacy, it is necessary to use a protocol with session
      encryption, for example Kerberos, or use an authentication
      mechanism that protects against replay attacks, perhaps using time
      stamps.  In Kerberos, users obtain credentials from the Kerberos
      server and use them for authentication to obtain services from
      other computers on the network.  The computing power of the local
      workstation can be used to decrypt credentials (using a key
      derived from the user-provided password) and store them until
      needed.  If the security protocol relies on synchronized clocks,
      then NTPv3 might be useful because it distributes time amongst a
      large number of computers and is one of the few existing Internet
      protocols that includes authentication mechanisms [Bishop,
      Mills92].

      Another approach to remotely accessible networks of computers is
      for all externally accessible machines to share a secret with the
      Kerberos KDC.  In a sense, this makes these machines "servers"
      instead of general use workstations.  This shared secret can then
      be used encrypt all communication between the two machines
      enabling the accessible workstation to relay authentication
      information to the KDC in a secure way.

      Finally, workstations that are remotely accessible could use
      asymmetric cryptographic technology to encrypt communications.
      The workstation's public key would be published and well known to
      all clients.  A user could use the public key to encrypt a simple
      password and the remote system can decrypt the password to
      authenticate the user without risking disclosure of the password
      while it is in transit.  A limitation of this workstation-oriented
      security is that it does not authenticate individual users only
      individual workstations.  In some environments for example,
      government multi-level secure or compartmented mode workstations,
      user to user authentication and confidentiality is also needed.

6. KEY DISTRIBUTION & MANAGEMENT

   The discussion thus far has periodically mentioned keys, either for
   encryption or for authentication (e.g., as input to a digital
   signature function).  Key management is perhaps the hardest problem
   faced when seeking to provide authentication in large internetworks.
   Hence this section provides a very brief overview of key management
   technology that might be used.



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   The Needham & Schroeder protocol, which is used by Kerberos, relies
   on a central key server.  In a large internetwork, there would need
   to be significant numbers of these key servers, at least one key
   server per administrative domain.  There would also need to be
   mechanisms for separately administered key servers to cooperate in
   generating a session key for parties in different administrative
   domains.  These are not impossible problems, but this approach
   clearly involves significant infrastructure changes.

   Most public-key encryption algorithms are computationally expensive
   and so are not ideal for encrypting packets in a network.  However,
   the asymmetric property makes them very useful for setup and exchange
   of symmetric session keys.  In practice, the commercial sector
   probably uses asymmetric algorithms primarily for digital signatures
   and key exchange, but not for bulk data encryption.  Both RSA and the
   Diffie-Hellman techniques can be used for this [DH76].  One advantage
   of using asymmetric techniques is that the central key server can be
   eliminated.  The difference in key management techniques is perhaps
   the primary difference between Kerberos and SPX.  Privacy Enhanced
   Mail has trusted key authorities use digital signatures to sign and
   authenticate the public keys of users [Kent93].  The result of this
   operation is a key certificates which contains the public key of some
   party and authentication that the public key in fact belongs to that
   party.  Key certificates can be distributed in many ways.  One way to
   distribute key certificates might be to add them to existing
   directory services, for example by extending the existing Domain Name
   System to hold each host's the key certificate in a new record type.

   For multicast sessions, key management is harder because the number
   of exchanges required by the widely used techniques is proportional
   to the number of participating parties.  Thus there is a serious
   scaling problem with current published multicast key management
   techniques.

   Finally, key management mechanisms described in the public literature
   have a long history of subtle flaws.  There is ample evidence of
   this, even for well-known techniques such as the Needham & Schroeder
   protocol [NS78, NS87].  In some cases, subtle flaws have only become
   known after formal methods techniques were used in an attempt to
   verify the protocol.  Hence, it is highly desirable that key
   management mechanisms be kept separate from authentication or
   encryption mechanisms as much as is possible.  For example, it is
   probably better to have a key management protocol that is distinct
   from and does not depend upon another security protocol.







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7. AUTHENTICATION OF NETWORK SERVICES

   In addition to needing to authenticate users and hosts to each other,
   many network services need or could benefit from authentication.
   This section describes some approaches to authentication in protocols
   that are primarily host to host in orientation.  As in the user to
   host authentication case, there are several techniques that might be
   considered.

   The most common case at present is to not have any authentication
   support in the protocol.  Bellovin and others have documented a
   number of cases where existing protocols can be used to attack a
   remote machine because there is no authentication in the protocols
   [Bellovin89].

   Some protocols provide for disclosing passwords to be passed along
   with the protocol information.  The original SNMP protocols used this
   method and a number of the routing protocols continue to use this
   method [Moy91, LR91, CFSD88].  This method is useful as a
   transitional aid to slightly increase security and might be
   appropriate when there is little risk in having a completely insecure
   protocol.

   There are many protocols that need to support stronger authentication
   mechanisms.  For example, there was widespread concern that SNMP
   needed stronger authentication than it originally had.  This led to
   the publication of the Secure SNMP protocols which support optional
   authentication, using a digital signature mechanism, and optional
   confidentiality, using DES encryption.  The digital signatures used
   in Secure SNMP are based on appending a cryptographic checksum to the
   SNMP information.  The cryptographic checksum is computed using the
   MD5 algorithm and a secret shared between the communicating parties
   so is believed to be difficult to forge or invert.

   Digital signature technology has evolved in recent years and should
   be considered for applications requiring authentication but not
   confidentiality.  Digital signatures may use a single secret shared
   among two or more communicating parties or it might be based on
   asymmetric encryption technology.  The former case would require the
   use of predetermined keys or the use of a secure key distribution
   protocol, such as that devised by Needham and Schroeder.  In the
   latter case, the public keys would need to be distributed in an
   authenticated manner.  If a general key distribution mechanism were
   available, support for optional digital signatures could be added to
   most protocols with little additional expense.  Each protocol could
   address the key exchange and setup problem, but that might make
   adding support for digital signatures more complicated and
   effectively discourage protocol designers from adding digital



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   signature support.

   For cases where both authentication and confidentiality are required
   on a host-to-host basis, session encryption could be employed using
   symmetric cryptography, asymmetric cryptography, or a combination of
   both.  Use of the asymmetric cryptography simplifies key management.
   Each host would encrypt the information while in transit between
   hosts and the existing operating system mechanisms would provide
   protection within each host.

   In some cases, possibly including electronic mail, it might be
   desirable to provide the security properties within the application
   itself in a manner that was truly user-to-user rather than being
   host-to-host.  The Privacy Enhanced Mail (PEM) work is employing this
   approach [Linn93, Kent93, Balenson93, Kaliski93].  The recent IETF
   work on Common Authentication Technology might make it easier to
   implement a secure distributed or networked application through use
   of standard security programming interfaces [Linn93a].

8. FUTURE DIRECTIONS

   Systems are moving towards the cryptographically stronger
   authentication mechanisms described earlier.  This move has two
   implications for future systems.  We can expect to see the
   introduction of non-disclosing authentication systems in the near
   term and eventually see more widespread use of public key crypto-
   systems.  Session authentication, integrity, and privacy issues are
   growing in importance. As computer-to-computer communication becomes
   more important, protocols that provide simple human interfaces will
   become less important. This is not to say that human interfaces are
   unimportant; they are very important.  It means that these interfaces
   are the responsibility of the applications, not the underlying
   protocol.  Human interface design is beyond the scope of this memo.

   The use of public key crypto-systems for user-to-host authentication
   simplifies many security issues, but unlike simple passwords, a
   public key cannot be memorized.  As of this writing, public key sizes
   of at least 500 bits are commonly used in the commercial world.  It
   is likely that larger key sizes will be used in the future.  Thus,
   users might have to carry their private keys in some electrically
   readable form.  The use of read-only storage, such as a floppy disk
   or a magnetic stripe card provides such storage, but it might require
   the user to trust their private keys to the reading device.  Use of a
   smart card, a portable device containing both storage and program
   might be preferable.  These devices have the potential to perform the
   authenticating operations without divulging the private key they
   contain.  They can also interact with the user requiring a simpler
   form of authentication to "unlock" the card.



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   The use of public key crypto-systems for host-to-host authentication
   appears not to have the same key memorization problem as the user-
   to-host case does.  A multiuser host can store its key(s) in space
   protected from users and obviate that problem.  Single user
   inherently insecure systems, such as PCs and Macintoshes, remain
   difficult to handle but the smart card approach should also work for
   them.

   If one considers existing symmetric algorithms to be 1-key
   techniques, and existing asymmetric algorithms such as RSA to be 2-
   key techniques, one might wonder whether N-key techniques will be
   developed in the future (i.e., for values of N larger than 2).  If
   such N-key technology existed, it might be useful in creating
   scalable multicast key distribution protocols.  There is work
   currently underway examining the possible use of the Core Based Tree
   (CBT) multicast routing technology to provide scalable multicast key
   distribution [BFC93].

   The implications of this taxonomy are clear.  Strong cryptographic
   authentication is needed in the near future for many protocols.
   Public key technology should be used when it is practical and cost-
   effective.  In the short term, authentication mechanisms vulnerable
   to passive attack should be phased out in favour of stronger
   authentication mechanisms.  Additional research is needed to develop
   improved key management technology and scalable multicast security
   mechanisms.

SECURITY CONSIDERATIONS

   This entire memo discusses Security Considerations in that it
   discusses authentication technologies and needs.

ACKNOWLEDGEMENTS

   This memo has benefited from review by and suggestions from the
   IETF's Common Authentication Technology (CAT) working group, chaired
   by John Linn, and from Marcus J. Ranum.

REFERENCES

   [Anderson84]  Anderson, B., "TACACS User Identification Telnet
   Option", RFC 927, BBN, December 1984.

   [Balenson93]  Balenson, D., "Privacy Enhancement for Internet
   Electronic Mail: Part III: Algorithms, Modes, and Identifiers", RFC
   1423, TIS, IAB IRTF PSRG, IETF PEM WG, February 1993.





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   [BFC93]  Ballardie, A., Francis, P., and J. Crowcroft, "Core Based
   Trees (CBT) An Architecture for Scalable Inter-Domain Multicast
   Routing", Proceedings of ACM SIGCOMM93, ACM, San Franciso, CA,
   September 1993, pp. 85-95.

   [Bellovin89]  Bellovin, S., "Security Problems in the TCP/IP Protocol
   Suite", ACM Computer Communications Review, Vol. 19, No. 2, March
   1989.

   [Bellovin92]  Bellovin, S., "There Be Dragons", Proceedings of the
   3rd Usenix UNIX Security Symposium, Baltimore, MD, September 1992.

   [Bellovin93]  Bellovin, S., "Packets Found on an Internet", ACM
   Computer Communications Review, Vol. 23, No. 3, July 1993, pp. 26-31.

   [BM91]  Bellovin S., and M. Merritt, "Limitations of the Kerberos
   Authentication System", ACM Computer Communications Review, October
   1990.

   [Bishop]  Bishop, M., "A Security Analysis of Version 2 of the
   Network Time Protocol NTP: A report to the Privacy & Security
   Research Group", Technical Report PCS-TR91-154, Department of
   Mathematics & Computer Science, Dartmouth College, Hanover, New
   Hampshire.

   [CB94]  Cheswick W., and S. Bellovin, "Chapter 10: An Evening with
   Berferd", Firewalls & Internet Security, Addison-Wesley, Reading,
   Massachusetts, 1994.  ISBN 0-201-63357-4.

   [CERT94]  Computer Emergency Response Team, "Ongoing Network
   Monitoring Attacks", CERT Advisory CA-94:01, available by anonymous
   ftp from cert.sei.cmu.edu, 3 February 1994.

   [CFSD88]  Case, J., Fedor, M., Schoffstall, M., and  J. Davin,
   "Simple Network Management Protocol", RFC 1067, University of
   Tennessee at Knoxville, NYSERNet, Inc., Rensselaer Polytechnic
   Institute, Proteon, Inc., August 1988.

   [DH76]  Diffie W., and M. Hellman, "New Directions in Cryptography",
   IEEE Transactions on Information Theory, Volume IT-11, November 1976,
   pp. 644-654.

   [GM93]  Galvin, J., and K. McCloghrie, "Security Protocols for
   Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC
   1446, Trusted Information Systems, Hughes LAN Systems, April 1993.






Haller & Atkinson                                              [Page 14]

RFC 1704               On Internet Authentication           October 1994


   [Haller94]  Haller, N., "The S/Key One-time Password System",
   Proceedings of the Symposium on Network & Distributed Systems
   Security, Internet Society, San Diego, CA, February 1994.

   [Kaufman93]  Kaufman, C., "Distributed Authentication Security
   Service (DASS)", RFC 1507, Digital Equipment Corporation, September
   1993.

   [Kaliski93]  Kaliski, B., "Privacy Enhancement for Internet
   Electronic Mail: Part IV: Key Certification and Related Services",
   RFC 1424, RSA Laboratories, February 1993.

   [Kantor91]  Kantor, B., "BSD Rlogin", RFC 1258, Univ. of Calif San
   Diego, September 1991.

   [Kent93]  Kent, S., "Privacy Enhancement for Internet Electronic
   Mail: Part II: Certificate-Based Key Management", RFC 1422, BBN, IAB
   IRTF PSRG, IETF PEM, February 1993.

   [KN93]  Kohl, J., and C. Neuman, "The Kerberos Network Authentication
   Service (V5)", RFC 1510, Digital Equipment Corporation,
   USC/Information Sciences Institute, September 1993.

   [Linn93]  Linn, J., "Privacy Enhancement for Internet Electronic
   Mail: Part I: Message Encryption and Authentication Procedures", RFC
   1421, IAB IRTF PSRG, IETF PEM WG, February 1993.

   [Linn93a]  Linn, J., "Common Authentication Technology Overview", RFC
   1511, Geer Zolot Associate, September 1993.

   [LS92]  Lloyd B., and W. Simpson, "PPP Authentication Protocols", RFC
   1334, L&A, Daydreamer, October 1992.

   [LR91]  Lougheed K., and Y. Rekhter, "A Border Gateway protocol 3
   (BGP-3)", RFC 1267, cisco Systems, T.J. Watson Research Center, IBM
   Corp., October 1991.

   [Mills92]  Mills, D., "Network Time Protocol (Version 3) -
   Specification, Implementation, and Analysis", RFC 1305, UDEL, March
   1992.

   [NBS77]  National Bureau of Standards, "Data Encryption Standard",
   Federal Information Processing Standards Publication 46, Government
   Printing Office, Washington, DC, 1977.

   [NS78]  Needham, R., and M. Schroeder, "Using Encryption for
   Authentication in Large Networks of Computers", Communications of the
   ACM, Vol. 21, No. 12, December 1978.



Haller & Atkinson                                              [Page 15]

RFC 1704               On Internet Authentication           October 1994


   [NS87]  Needham, R., and M. Schroeder, "Authentication Revisited",
   ACM Operating Systems Review, Vol. 21, No. 1, 1987.

   [PR85]  Postel J., and J. Reynolds, "File Transfer Protocol", STD 9,
   RFC 959, USC/Information Sciences Institute, October 1985.

   [Moy91]  Moy, J., "OSPF Routing Protocol, Version 2", RFC 1247,
   Proteon, Inc., July 1991.

   [RSA78]  Rivest, R., Shamir, A., and L. Adleman, "A Method for
   Obtaining Digital Signatures and Public Key Crypto-systems",
   Communications of the ACM, Vol. 21, No. 2, February 1978.

   [Rivest92]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
   MIT Laboratory for Computer Science and RSA Data Security, Inc.,
   April 1992.

   [Simpson93]  Simpson, W., "The Point to Point Protocol", RFC 1548,
   Daydreamer, December 1993.

   [SNS88]  Steiner, J., Neuman, C., and J. Schiller, "Kerberos: "An
   Authentication Service for Open Network Systems", USENIX Conference
   Proceedings, Dallas, Texas, February 1988.

   [Stoll90]  Stoll, C., "The Cuckoo's Egg: Tracking a Spy Through the
   Maze of Computer Espionage", Pocket Books, New York, NY, 1990.

   [TA91]  Tardo J., and K. Alagappan, "SPX: Global Authentication Using
   Public Key Certificates", Proceedings of the 1991 Symposium on
   Research in Security & Privacy, IEEE Computer Society, Los Amitos,
   California, 1991. pp.232-244.




















Haller & Atkinson                                              [Page 16]

RFC 1704               On Internet Authentication           October 1994


   AUTHORS' ADDRESSES

   Neil Haller
   Bell Communications Research
   445 South Street  -- MRE 2Q-280
   Morristown, NJ 07962-1910

   Phone: (201) 829-4478
   EMail: nmh@thumper.bellcore.com


   Randall Atkinson
   Information Technology Division
   Naval Research Laboratory
   Washington, DC 20375-5320

   Phone: (DSN) 354-8590
   EMail: atkinson@itd.nrl.navy.mil

































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