rfc1994.txt

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      The Identifier field is one octet and aids in matching challenges,
      responses and replies.

   Length

      The Length field is two octets and indicates the length of the
      CHAP packet including the Code, Identifier, Length and Data
      fields.  Octets outside the range of the Length field should be
      treated as Data Link Layer padding and should be ignored on
      reception.

   Data

      The Data field is zero or more octets.  The format of the Data
      field is determined by the Code field.










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RFC 1994                        PPP CHAP                     August 1996


4.1.  Challenge and Response

   Description

      The Challenge packet is used to begin the Challenge-Handshake
      Authentication Protocol.  The authenticator MUST transmit a CHAP
      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
      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 Notes: Because the Success might be lost, the
         authenticator MUST allow repeated Response packets during the
         Network-Layer Protocol phase 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
         previously returned for that specific Challenge (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.














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RFC 1994                        PPP CHAP                     August 1996


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

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     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.

   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).




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RFC 1994                        PPP CHAP                     August 1996


   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.


4.2.  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.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     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.








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RFC 1994                        PPP CHAP                     August 1996


   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 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.

   The secret SHOULD NOT be the same in both directions.  This allows an
   attacker to replay the peer's challenge, accept the computed
   response, and use that response to authenticate.

   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).  If the same



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   secret was used, PAP would reveal the secret to be used later with
   CHAP.

   Instead, for each user name 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 methods 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.  Such a mechanism is outside the scope of this
   specification.


Acknowledgements

   David Kaufman, Frank Heinrich, and Karl Auerbach used a challenge
   handshake at SDC when designing one of the protocols for a "secure"
   network in the mid-1970s.  Tom Bearson built a prototype Sytek
   product ("Poloneous"?) on the challenge-response notion in the 1982-
   83 timeframe.  Another variant is documented in the various IBM SNA
   manuals.  Yet another variant was implemented by Karl Auerbach in the
   Telebit NetBlazer circa 1991.

   Kim Toms and Barney Wolff provided useful critiques of earlier
   versions of this document.

   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.














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RFC 1994                        PPP CHAP                     August 1996


References

   [1]   Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
         51, RFC 1661, DayDreamer, July 1994.

   [2]   Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
         1700, USC/Information Sciences Institute, October 1994.

   [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.



Contacts

   Comments should be submitted to the [e-mail] mailing list.

   This document was reviewed by the Point-to-Point Protocol Working
   Group of the Internet Engineering Task Force (IETF).  The working
   group can be contacted via the current chair:

      Karl Fox
      Ascend Communications
      3518 Riverside Drive, Suite 101
      Columbus, Ohio 43221

          [e-mail]
          [e-mail]


   Questions about this memo can also be directed to:

      William Allen Simpson
      DayDreamer
      Computer Systems Consulting Services
      1384 Fontaine
      Madison Heights, Michigan  48071

          [e-mail]
          [e-mail] (preferred)










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