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rfc:rfc1334

Network Working Group B. Lloyd Request for Comments: 1334 L&A

                                                            W. Simpson
                                                            Daydreamer
                                                          October 1992
                    PPP Authentication Protocols

Status of this Memo

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

Abstract

 The Point-to-Point Protocol (PPP) [1] provides a standard method of
 encapsulating Network Layer protocol information 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 two protocols for Authentication: the Password
 Authentication Protocol and the Challenge-Handshake Authentication
 Protocol.  This memo is the product of the Point-to-Point Protocol
 Working Group of the Internet Engineering Task Force (IETF).
 Comments on this memo should be submitted to the ietf-ppp@ucdavis.edu
 mailing list.

Table of Contents

 1.  Introduction ...............................................    2
 1.1 Specification Requirements .................................    2
 1.2 Terminology ................................................    3
 2. Password Authentication Protocol ............................    3
 2.1 Configuration Option Format ................................    4
 2.2 Packet Format ..............................................    5
 2.2.1 Authenticate-Request .....................................    5
 2.2.2 Authenticate-Ack and Authenticate-Nak ....................    7
 3. Challenge-Handshake Authentication Protocol..................    8
 3.1 Configuration Option Format ................................    9
 3.2 Packet Format ..............................................   10
 3.2.1 Challenge and Response ...................................   11
 3.2.2 Success and Failure ......................................   13

Lloyd & Simpson [Page 1] RFC 1334 PPP Authentication October 1992

 SECURITY CONSIDERATIONS ........................................   14
 REFERENCES .....................................................   15
 ACKNOWLEDGEMENTS ...............................................   16
 CHAIR'S ADDRESS ................................................   16
 AUTHOR'S ADDRESS ...............................................   16

1. Introduction

 PPP has three main components:
    1. A method for encapsulating datagrams over serial links.
    2. A Link Control Protocol (LCP) for establishing, configuring,
       and testing the data-link connection.
    3. A family of Network Control Protocols (NCPs) for establishing
       and configuring different network-layer protocols.
 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 the PPP authentication protocols.  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 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.

Lloyd & Simpson [Page 2] RFC 1334 PPP Authentication October 1992

 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 should 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:
 authenticator
    The end of the link requiring the authentication.  The
    authenticator specifies the authentication protocol to be used in
    the Configure-Request during Link Establishment phase.
 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. Password Authentication Protocol

 The Password Authentication Protocol (PAP) provides a simple method
 for the peer to establish its identity using a 2-way handshake.  This
 is done only upon initial link establishment.
 After the Link Establishment phase is complete, an Id/Password pair
 is repeatedly sent by the peer to the authenticator until
 authentication is acknowledged or the connection is terminated.
 PAP is not a strong authentication method.  Passwords are sent over
 the circuit "in the clear", and there is no protection from playback

Lloyd & Simpson [Page 3] RFC 1334 PPP Authentication October 1992

 or repeated trial and error attacks.  The peer is in control of the
 frequency and timing of the attempts.
 Any implementations which include a stronger authentication method
 (such as CHAP, described below) MUST offer to negotiate that method
 prior to PAP.
 This authentication method is most appropriately used where a
 plaintext password must be available to simulate a login at a remote
 host.  In such use, this method provides a similar level of security
 to the usual user login at the remote host.
    Implementation Note: It is possible to limit the exposure of the
    plaintext password to transmission over the PPP link, and avoid
    sending the plaintext password over the entire network.  When the
    remote host password is kept as a one-way transformed value, and
    the algorithm for the transform function is implemented in the
    local server, the plaintext password SHOULD be locally transformed
    before comparison with the transformed password from the remote
    host.

2.1. Configuration Option Format

 A summary of the Authentication-Protocol Configuration Option format
 to negotiate the Password Authentication Protocol 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |     Authentication-Protocol   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
    3
 Length
    4
 Authentication-Protocol
    c023 (hex) for Password Authentication Protocol.
 Data
    There is no Data field.

Lloyd & Simpson [Page 4] RFC 1334 PPP Authentication October 1992

2.2. Packet Format

 Exactly one Password Authentication Protocol packet is encapsulated
 in the Information field of a PPP Data Link Layer frame where the
 protocol field indicates type hex c023 (Password Authentication
 Protocol).  A summary of the PAP 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             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Data ...
 +-+-+-+-+
 Code
    The Code field is one octet and identifies the type of PAP packet.
    PAP Codes are assigned as follows:
       1       Authenticate-Request
       2       Authenticate-Ack
       3       Authenticate-Nak
 Identifier
    The Identifier field is one octet and aids in matching requests
    and replies.
 Length
    The Length field is two octets and indicates the length of the PAP
    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.

2.2.1. Authenticate-Request

 Description
    The Authenticate-Request packet is used to begin the Password
    Authentication Protocol.  The link peer MUST transmit a PAP packet

Lloyd & Simpson [Page 5] RFC 1334 PPP Authentication October 1992

    with the Code field set to 1 (Authenticate-Request) during the
    Authentication phase.  The Authenticate-Request packet MUST be
    repeated until a valid reply packet is received, or an optional
    retry counter expires.
    The authenticator SHOULD expect the peer to send an Authenticate-
    Request packet.  Upon reception of an Authenticate-Request packet,
    some type of Authenticate reply (described below) MUST be
    returned.
       Implementation Note: Because the Authenticate-Ack might be
       lost, the authenticator MUST allow repeated Authenticate-
       Request packets after completing the Authentication phase.
       Protocol phase MUST return the same reply Code returned when
       the Authentication phase completed (the message portion MAY be
       different).  Any Authenticate-Request packets received during
       any other phase MUST be silently discarded.
       When the Authenticate-Nak 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 Authenticate-Request 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             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Peer-ID Length|  Peer-Id ...
 +-+-+-+-+-+-+-+-+-+-+-+-+
 | Passwd-Length |  Password  ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+
 Code
    1 for Authenticate-Request.
 Identifier
    The Identifier field is one octet and aids in matching requests
    and replies.  The Identifier field MUST be changed each time an
    Authenticate-Request packet is issued.

Lloyd & Simpson [Page 6] RFC 1334 PPP Authentication October 1992

 Peer-ID-Length
    The Peer-ID-Length field is one octet and indicates the length of
    the Peer-ID field.
 Peer-ID
    The Peer-ID field is zero or more octets and indicates the name of
    the peer to be authenticated.
 Passwd-Length
    The Passwd-Length field is one octet and indicates the length of
    the Password field.
 Password
    The Password field is zero or more octets and indicates the
    password to be used for authentication.

2.2.2. Authenticate-Ack and Authenticate-Nak

 Description
    If the Peer-ID/Password pair received in an Authenticate-Request
    is both recognizable and acceptable, then the authenticator MUST
    transmit a PAP packet with the Code field set to 2 (Authenticate-
    Ack).
    If the Peer-ID/Password pair received in a Authenticate-Request is
    not recognizable or acceptable, then the authenticator MUST
    transmit a PAP packet with the Code field set to 3 (Authenticate-
    Nak), and SHOULD take action to terminate the link.
 A summary of the Authenticate-Ack and Authenticate-Nak 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             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Msg-Length   |  Message  ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-
 Code
    2 for Authenticate-Ack;

Lloyd & Simpson [Page 7] RFC 1334 PPP Authentication October 1992

    3 for Authenticate-Nak.
 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 Authenticate-Request which caused this
    reply.
 Msg-Length
    The Msg-Length field is one octet and indicates the length of the
    Message field.
 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.

3. 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.
 After the Link Establishment phase is complete, the authenticator
 sends a "challenge" message to the peer.  The peer responds with a
 value calculated using a "one-way hash" function.  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.
 CHAP provides protection against playback attack 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.
 This method is most likely used where the same secret is easily
 accessed from both ends of the link.

Lloyd & Simpson [Page 8] RFC 1334 PPP Authentication October 1992

    Implementation Note: CHAP requires that the secret be available in
    plaintext form.  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.
 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.
 The challenge value SHOULD satisfy two criteria: uniqueness and
 unpredictability.  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
 attacks.
 A discussion of sources of uniqueness and probability of divergence
 is included in the Magic-Number Configuration Option [1].

3.1. 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.

Lloyd & Simpson [Page 9] RFC 1334 PPP Authentication October 1992

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |     Authentication-Protocol   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Algorithm   |
 +-+-+-+-+-+-+-+-+
 Type
    3
 Length
    5
 Authentication-Protocol
    c223 (hex) for Challenge-Handshake Authentication Protocol.
 Algorithm
    The Algorithm field is one octet and indicates the one-way hash
    method to be used.  The most up-to-date values of the CHAP
    Algorithm field are specified in the most recent "Assigned
    Numbers" RFC [2].  Current values are assigned as follows:
       0-4     unused (reserved)
       5       MD5 [3]

3.2. 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.
  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             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Data ...
 +-+-+-+-+

Lloyd & Simpson [Page 10] RFC 1334 PPP Authentication October 1992

 Code
    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
 Identifier
    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.

3.2.1. 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

Lloyd & Simpson [Page 11] 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.

Lloyd & Simpson [Page 12] RFC 1334 PPP Authentication October 1992

 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.

Lloyd & Simpson [Page 13] RFC 1334 PPP Authentication October 1992

  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

Lloyd & Simpson [Page 14] RFC 1334 PPP Authentication October 1992

    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.

Lloyd & Simpson [Page 15] RFC 1334 PPP Authentication October 1992

 [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: (916) 676-1147
    EMail: brian@lloyd.com

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: Bill.Simpson@um.cc.umich.edu

Lloyd & Simpson [Page 16]

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