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RFC 1994 - PPP Challenge Handshake Authentication Protocol (CHAP)

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Network Working Group                                         W. Simpson
Request for Comments: 1994                                    DayDreamer
Obsoletes: 1334                                              August 1996
Category: Standards Track

         PPP Challenge Handshake Authentication Protocol (CHAP)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.


   The Point-to-Point Protocol (PPP) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.

   PPP also defines an extensible Link Control Protocol, which allows
   negotiation of an Authentication Protocol for authenticating its peer
   before allowing Network Layer protocols to transmit over the link.

   This document defines a method for Authentication using PPP, which
   uses a random Challenge, with a cryptographically hashed Response
   which depends upon the Challenge and a secret key.

Table of Contents

     1.     Introduction ..........................................    1
        1.1       Specification of Requirements ...................    1
        1.2       Terminology .....................................    2
     2.     Challenge-Handshake Authentication Protocol ...........    2
        2.1       Advantages ......................................    3
        2.2       Disadvantages ...................................    3
        2.3       Design Requirements .............................    4
     3.     Configuration Option Format ...........................    5
     4.     Packet Format .........................................    6
        4.1       Challenge and Response ..........................    7
        4.2       Success and Failure .............................    9
     SECURITY CONSIDERATIONS ......................................   10
     ACKNOWLEDGEMENTS .............................................   11
     REFERENCES ...................................................   12
     CONTACTS .....................................................   12

1.  Introduction

   In order to establish communications over a point-to-point link, each
   end of the PPP link must first send LCP packets to configure the data
   link during Link Establishment phase.  After the link has been
   established, PPP provides for an optional Authentication phase before
   proceeding to the Network-Layer Protocol phase.

   By default, authentication is not mandatory.  If authentication of
   the link is desired, an implementation MUST specify the
   Authentication-Protocol Configuration Option during Link
   Establishment phase.

   These authentication protocols are intended for use primarily by
   hosts and routers that connect to a PPP network server via switched
   circuits or dial-up lines, but might be applied to dedicated links as
   well.  The server can use the identification of the connecting host
   or router in the selection of options for network layer negotiations.

   This document defines a PPP authentication protocol.  The Link
   Establishment and Authentication phases, and the Authentication-
   Protocol Configuration Option, are defined in The Point-to-Point
   Protocol (PPP) [1].

1.1.  Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.

   MUST      This word, or the adjective "required", means that the
             definition is an absolute requirement of the specification.

   MUST NOT  This phrase means that the definition is an absolute
             prohibition of the specification.

   SHOULD    This word, or the adjective "recommended", means that there
             may exist valid reasons in particular circumstances to
             ignore this item, but the full implications must be
             understood and carefully weighed before choosing a
             different course.

   MAY       This word, or the adjective "optional", means that this
             item is one of an allowed set of alternatives.  An
             implementation which does not include this option MUST be
             prepared to interoperate with another implementation which
             does include the option.

1.2.  Terminology

   This document frequently uses the following terms:

             The end of the link requiring the authentication.  The
             authenticator specifies the authentication protocol to be
             used in the Configure-Request during Link Establishment

   peer      The other end of the point-to-point link; the end which is
             being authenticated by the authenticator.

   silently discard
             This means the implementation discards the packet without
             further processing.  The implementation SHOULD provide the
             capability of logging the error, including the contents of
             the silently discarded packet, and SHOULD record the event
             in a statistics counter.

2.  Challenge-Handshake Authentication Protocol

   The Challenge-Handshake Authentication Protocol (CHAP) is used to
   periodically verify the identity of the peer using a 3-way handshake.
   This is done upon initial link establishment, and MAY be repeated
   anytime after the link has been established.

   1.    After the Link Establishment phase is complete, the
         authenticator sends a "challenge" message to the peer.

   2.    The peer responds with a value calculated using a "one-way
         hash" function.

   3.    The authenticator checks the response against its own
         calculation of the expected hash value.  If the values match,
         the authentication is acknowledged; otherwise the connection
         SHOULD be terminated.

   4.    At random intervals, the authenticator sends a new challenge to
         the peer, and repeats steps 1 to 3.

2.1.  Advantages

   CHAP provides protection against playback attack by the peer through
   the use of an incrementally changing identifier and a variable
   challenge value.  The use of repeated challenges is intended to limit
   the time of exposure to any single attack.  The authenticator is in
   control of the frequency and timing of the challenges.

   This authentication method depends upon a "secret" known only to the
   authenticator and that peer.  The secret is not sent over the link.

   Although the authentication is only one-way, by negotiating CHAP in
   both directions the same secret set may easily be used for mutual

   Since CHAP may be used to authenticate many different systems, name
   fields may be used as an index to locate the proper secret in a large
   table of secrets.  This also makes it possible to support more than
   one name/secret pair per system, and to change the secret in use at
   any time during the session.

2.2.  Disadvantages

   CHAP requires that the secret be available in plaintext form.
   Irreversably encrypted password databases commonly available cannot
   be used.

   It is not as useful for large installations, since every possible
   secret is maintained at both ends of the link.

      Implementation Note: To avoid sending the secret over other links
      in the network, it is recommended that the challenge and response
      values be examined at a central server, rather than each network
      access server.  Otherwise, the secret SHOULD be sent to such
      servers in a reversably encrypted form.  Either case requires a
      trusted relationship, which is outside the scope of this

2.3.  Design Requirements

   The CHAP algorithm requires that the length of the secret MUST be at
   least 1 octet.  The secret SHOULD be at least as large and
   unguessable as a well-chosen password.  It is preferred that the
   secret be at least the length of the hash value for the hashing
   algorithm chosen (16 octets for MD5).  This is to ensure a
   sufficiently large range for the secret to provide protection against
   exhaustive search attacks.

   The one-way hash algorithm is chosen such that it is computationally
   infeasible to determine the secret from the known challenge and
   response values.

   Each challenge value SHOULD be unique, since repetition of a
   challenge value in conjunction with the same secret would permit an
   attacker to reply with a previously intercepted response.  Since it
   is expected that the same secret MAY be used to authenticate with
   servers in disparate geographic regions, the challenge SHOULD exhibit
   global and temporal uniqueness.

   Each challenge value SHOULD also be unpredictable, least an attacker
   trick a peer into responding to a predicted future challenge, and
   then use the response to masquerade as that peer to an authenticator.

   Although protocols such as CHAP are incapable of protecting against
   realtime active wiretapping attacks, generation of unique
   unpredictable challenges can protect against a wide range of active

   A discussion of sources of uniqueness and probability of divergence
   is included in the Magic-Number Configuration Option [1].

3.  Configuration Option Format

   A summary of the Authentication-Protocol Configuration Option format
   to negotiate the Challenge-Handshake Authentication Protocol is shown
   below.  The fields are transmitted from left to right.

   |     Type      |    Length     |     Authentication-Protocol   |
   |   Algorithm   |






      c223 (hex) for Challenge-Handshake Authentication Protocol.


      The Algorithm field is one octet and indicates the authentication
      method to be used.  Up-to-date values are specified in the most
      recent "Assigned Numbers" [2].  One value is required to be

         5       CHAP with MD5 [3]

4.  Packet Format

   Exactly one Challenge-Handshake Authentication Protocol packet is
   encapsulated in the Information field of a PPP Data Link Layer frame
   where the protocol field indicates type hex c223 (Challenge-Handshake
   Authentication Protocol).  A summary of the CHAP packet format is
   shown below.  The fields are transmitted from left to right.

   |     Code      |  Identifier   |            Length             |
   |    Data ...


      The Code field is one octet and identifies the type of CHAP
      packet.  CHAP Codes are assigned as follows:

         1       Challenge
         2       Response
         3       Success
         4       Failure


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


      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


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

4.1.  Challenge and Response


      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.

   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 ...


      1 for Challenge;

      2 for Response.


      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.


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


      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


      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  ...


      3 for Success;

      4 for Failure.


      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.


      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

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

   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


   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.


   [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.


   Comments should be submitted to the ietf-ppp@merit.edu 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


   Questions about this memo can also be directed to:

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

          wsimpson@GreenDragon.com (preferred)


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