rfc1334.txt

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      packet with the Code field set to 1 (Challenge).  Additional
      Challenge packets MUST be sent until a valid Response packet is
      received, or an optional retry counter expires.

      A Challenge packet MAY also be transmitted at any time during the
      Network-Layer Protocol phase to ensure that the connection has not
      been altered.

      The peer SHOULD expect Challenge packets during the Authentication
      phase and the Network-Layer Protocol phase.  Whenever a Challenge
      packet is received, the peer MUST transmit a CHAP packet with the
      Code field set to 2 (Response).

      Whenever a Response packet is received, the authenticator compares



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RFC 1334                   PPP Authentication               October 1992


      the Response Value with its own calculation of the expected value.
      Based on this comparison, the authenticator MUST send a Success or
      Failure packet (described below).

         Implementation Note: Because the Success might be lost, the
         authenticator MUST allow repeated Response packets after
         completing the Authentication phase.  To prevent discovery of
         alternative Names and Secrets, any Response packets received
         having the current Challenge Identifier MUST return the same
         reply Code returned when the Authentication phase completed
         (the message portion MAY be different).  Any Response packets
         received during any other phase MUST be silently discarded.

         When the Failure is lost, and the authenticator terminates the
         link, the LCP Terminate-Request and Terminate-Ack provide an
         alternative indication that authentication failed.

   A summary of the Challenge and Response packet format is shown below.
   The fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Value-Size   |  Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Name ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Code

      1 for Challenge;

      2 for Response.

   Identifier

      The Identifier field is one octet.  The Identifier field MUST be
      changed each time a Challenge is sent.

      The Response Identifier MUST be copied from the Identifier field
      of the Challenge which caused the Response.

   Value-Size

      This field is one octet and indicates the length of the Value
      field.



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   Value

      The Value field is one or more octets.  The most significant octet
      is transmitted first.

      The Challenge Value is a variable stream of octets.  The
      importance of the uniqueness of the Challenge Value and its
      relationship to the secret is described above.  The Challenge
      Value MUST be changed each time a Challenge is sent.  The length
      of the Challenge Value depends upon the method used to generate
      the octets, and is independent of the hash algorithm used.

      The Response Value is the one-way hash calculated over a stream of
      octets consisting of the Identifier, followed by (concatenated
      with) the "secret", followed by (concatenated with) the Challenge
      Value.  The length of the Response Value depends upon the hash
      algorithm used (16 octets for MD5).

   Name

      The Name field is one or more octets representing the
      identification of the system transmitting the packet.  There are
      no limitations on the content of this field.  For example, it MAY
      contain ASCII character strings or globally unique identifiers in
      ASN.1 syntax.  The Name should not be NUL or CR/LF terminated.
      The size is determined from the Length field.

      Since CHAP may be used to authenticate many different systems, the
      content of the name field(s) may be used as a key to locate the
      proper secret in a database of secrets.  This also makes it
      possible to support more than one name/secret pair per system.

3.2.2.  Success and Failure

   Description

      If the Value received in a Response is equal to the expected
      value, then the implementation MUST transmit a CHAP packet with
      the Code field set to 3 (Success).

      If the Value received in a Response is not equal to the expected
      value, then the implementation MUST transmit a CHAP packet with
      the Code field set to 4 (Failure), and SHOULD take action to
      terminate the link.

   A summary of the Success and Failure packet format is shown below.
   The fields are transmitted from left to right.




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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Message  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   Code

      3 for Success;

      4 for Failure.

   Identifier

      The Identifier field is one octet and aids in matching requests
      and replies.  The Identifier field MUST be copied from the
      Identifier field of the Response which caused this reply.

   Message

      The Message field is zero or more octets, and its contents are
      implementation dependent.  It is intended to be human readable,
      and MUST NOT affect operation of the protocol.  It is recommended
      that the message contain displayable ASCII characters 32 through
      126 decimal.  Mechanisms for extension to other character sets are
      the topic of future research.  The size is determined from the
      Length field.

Security Considerations

      Security issues are the primary topic of this RFC.

      The interaction of the authentication protocols within PPP are
      highly implementation dependent.  This is indicated by the use of
      SHOULD throughout the document.

      For example, upon failure of authentication, some implementations
      do not terminate the link.  Instead, the implementation limits the
      kind of traffic in the Network-Layer Protocols to a filtered
      subset, which in turn allows the user opportunity to update
      secrets or send mail to the network administrator indicating a
      problem.

      There is no provision for re-tries of failed authentication.
      However, the LCP state machine can renegotiate the authentication
      protocol at any time, thus allowing a new attempt.  It is



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      recommended that any counters used for authentication failure not
      be reset until after successful authentication, or subsequent
      termination of the failed link.

      There is no requirement that authentication be full duplex or that
      the same protocol be used in both directions.  It is perfectly
      acceptable for different protocols to be used in each direction.
      This will, of course, depend on the specific protocols negotiated.

      In practice, within or associated with each PPP server, there is a
      database which associates "user" names with authentication
      information ("secrets").  It is not anticipated that a particular
      named user would be authenticated by multiple methods.  This would
      make the user vulnerable to attacks which negotiate the least
      secure method from among a set (such as PAP rather than CHAP).
      Instead, for each named user there should be an indication of
      exactly one method used to authenticate that user name.  If a user
      needs to make use of different authentication method under
      different circumstances, then distinct user names SHOULD be
      employed, each of which identifies exactly one authentication
      method.

      Passwords and other secrets should be stored at the respective
      ends such that access to them is as limited as possible.  Ideally,
      the secrets should only be accessible to the process requiring
      access in order to perform the authentication.

      The secrets should be distributed with a mechanism that limits the
      number of entities that handle (and thus gain knowledge of) the
      secret.  Ideally, no unauthorized person should ever gain
      knowledge of the secrets.  It is possible to achieve this with
      SNMP Security Protocols [4], but such a mechanism is outside the
      scope of this specification.

      Other distribution methods are currently undergoing research and
      experimentation.  The SNMP Security document also has an excellent
      overview of threats to network protocols.

References

   [1] Simpson, W., "The Point-to-Point Protocol (PPP)", RFC 1331,
       Daydreamer, May 1992.

   [2] Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1340,
       USC/Information Sciences Institute, July 1992.






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

   [4] Galvin, J., McCloghrie, K., and J. Davin, "SNMP Security
       Protocols", Trusted Information Systems, Inc., Hughes LAN
       Systems, Inc., MIT Laboratory for Computer Science, RFC 1352,
       July 1992.

Acknowledgments

   Some of the text in this document is taken from RFC 1172, by Drew
   Perkins of Carnegie Mellon University, and by Russ Hobby of the
   University of California at Davis.

   Special thanks to Dave Balenson, Steve Crocker, James Galvin, and
   Steve Kent, for their extensive explanations and suggestions.  Now,
   if only we could get them to agree with each other.

Chair's Address

   The working group can be contacted via the current chair:

      Brian Lloyd
      Lloyd & Associates
      3420 Sudbury Road
      Cameron Park, California 95682

      Phone: [phone]

      EMail: [e-mail]

Author's Address

   Questions about this memo can also be directed to:

      William Allen Simpson
      Daydreamer
      Computer Systems Consulting Services
      P O Box 6205
      East Lansing, MI  48826-6205

      EMail: [e-mail]








Lloyd & Simpson                                                [Page 16]