Network Working Group L J Blunk
J R Vollbrecht
Internet Draft Merit
expires in six months July 1994
PPP Kerberos version 4 Authentication Protocol (KAPv4)
<draft-ietf-pppext-kapv4-auth-00.txt>
Status of this Memo
This document is the product of the Point-to-Point Protocol
Extensions Working Group of the Internet Engineering Task Force
(IETF). Comments should be submitted to the ietf-ppp@merit.edu
mailing list.
Distribution of this memo is unlimited.
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is not appropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts
Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net
(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
Rim).
Abstract
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 the PPP Kerberos version 4 Authentication
Protocol.
Blunk & Vollbrecht expires in six months [Page i]
DRAFT PPP KAPv4 July 1994
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 the PPP KAPv4 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.
Blunk & Vollbrecht expires in six months [Page 1]
DRAFT PPP KAPv4 July 1994
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.
Blunk & Vollbrecht expires in six months [Page 2]
DRAFT PPP KAPv4 July 1994
2. PPP Kerberos version 4 Authentication Protocol
The PPP Kerberos version 4 Authentication Protocol (KAPv4) is used to
verify the identity of a peer using the Kerberos version 4
Authentication System. Normally, Kerberos version 4 authentication
requires that the peer possess an IP address. However, with PPP, the
peer may not know the IP address prior to authentication. Given this
constraint, the KAPv4 protocol is designed to operate over the PPP
link prior to the IP Network Protocol having been negotiated. It is
assumed that the authenticator has network connectivity to the
Kerberos server(s) to be used for authentication.
1. After the Link Establishment phase is complete, the
authenticator sends a request for the identity of the the peer.
2. The peer replies with its Kerberos principal identity. The
peer may also optionally specify a service name and instance.
It is recommended that ppp-kapv4 be used as the primary name
for the service. An instance should generally not be required
but MUST be configurable. If the peer does not present a
service name, this indicates that it will not check the
identity of service to which it is authenticating.
3. The authenticator then uses the Kerberos Authentication Service
Protocol to request the Kerberos credentials to be used by the
peer. These credentials are then forwarded to the peer along
with a randon challenge. The challenge is used to prevent
replay attacks.
4. The peer decrypts these credentials to obtain a ticket for the
authenticator and a session key. The session key is used to
encrypt the challenge. The peer then sends the ticket together
with the response to the challenge back to the authenticator.
It may optionally include a mutual challenge to verify the
identity of the authenticator.
5. The authenticator then decrypts the ticket and checks the
response to its challenge. If these are valid, the
authentication is acknowledged with a success message;
otherwise the authenticator replies with a failure response.
The authenticator also replies with a response to the peer's
challenge.
Blunk & Vollbrecht expires in six months [Page 3]
DRAFT PPP KAPv4 July 1994
Advantages
KAPv4 provides an authentication mechanism which does not require
passing a user's password in the clear. It also allows PPP
authentication to leverage off of existing Kerberos authentication
servers.
The authenticator is not the repository for any peer secrets. This
is a significant advantage when there are many peers.
This authentication can be mutual. The ability of the peer to
successfully decrypt the Kerberos credentials and generate a response
to the challenge is the assurance to the authenticator that the peer
is authentic. The ability of the authenticator to encrypt a response
to a challenge using the session key is the assurance to the peer
that the authenticator is also authentic.
Disadvantages
KAPv4 depends upon encryption technology which may be subject to
export controls.
Blunk & Vollbrecht expires in six months [Page 4]
DRAFT PPP KAPv4 July 1994
2.1. Configuration Option Format
A summary of the Authentication-Protocol Configuration Option format
to negotiate the KAPv4 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
???? for PPP Kerberos version 4 Authentication Protocol (KAPv4)
Blunk & Vollbrecht expires in six months [Page 5]
DRAFT PPP KAPv4 July 1994
2.2. Packet Format
Exactly one PPP Kerberos version 4 Authentication Protocol packet is
encapsulated in the Information field of a PPP Data Link Layer frame
where the protocol field indicates type hex ???? (PPP Kerberos
version 4 Authentication Protocol). A summary of the KAPv4 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 KAPv4
packet. KAPv4 Codes are assigned as follows:
1 Initiate
2 Principal
3 Credentials
4 Ticket
5 Success
6 Failure
Identifier
The Identifier field is one octet and aids in matching replies
with requests and responses to challenges.
Length
The Length field is two octets and indicates the length of the
KAPv4 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.
Blunk & Vollbrecht expires in six months [Page 6]
DRAFT PPP KAPv4 July 1994
2.2.1. Initiate
Description
The Initiate packet is used to begin the PPP Kerberos version 4
Authentication Protocol. The authenticator MUST transmit a KAPv4
packet with the Code field set to 1 (Initiate). Additional
Initiate packets MUST be sent until a valid Response packet is
received, or an optional retry counter expires.
A summary of the Initiate 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1
Identifier
The Identifier field is one octet. The Identifier field MUST be
changed each time an Initiate is sent.
Blunk & Vollbrecht expires in six months [Page 7]
DRAFT PPP KAPv4 July 1994
2.2.2. Principal
Description
The Principal packet is used to identify the peer to the
authenticator. It is sent in response to an Initiate packet. The
peer MUST transmit a KAPv4 packet with the Code field set to 2
(Principal). The identifier of the Principal packet MUST match the
indentifier in the Initiate packet. The authenticator uses the
supplied information to generate a KRB_AS_REQ message for the
Kerberos Server. The authenticator responds to the Principal
packet with either a Credentials packet or a Failure packet.
A summary of the Principal 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Name-Size | Name ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Instance-Size | Instance ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Realm-Size | Realm ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sname-Size | Service Name ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sinstance-Size| Service Instance ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2
Identifier
The Identifier field is one octet. The Principal Identifier MUST
be copied from the Identifier field of the Initiate which caused
the Response.
Principal fields
The Name, Instance, and Realm fields are used to identify the
Kerberos Principal. The Service Name is optional and specifies
that the peer expects to authenticate to a particular service and
Blunk & Vollbrecht expires in six months [Page 8]
DRAFT PPP KAPv4 July 1994
will check this attribute when examining the credentials. If the
peer does not care about the service identity, it should put a
zero in the Service Name Size field.
Blunk & Vollbrecht expires in six months [Page 9]
DRAFT PPP KAPv4 July 1994
2.2.3. Credentials
Description
The Credentials packet is used to supply the peer with the needed
credentials for authentication. It is sent in response to a
Principal packet and contains a ticket for the authenticator and
a session key. It also contains a random challenge to prevent
replay attacks. The authenticator MUST transmit a KAPv4 packet
with the Code field set to 3 (Credentials). Additional
Credentials packets MUST be sent until a valid Response packet is
received, or an optional retry counter expires.
A summary of the Credentials 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cred-Size | Credentials ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chal-Size | Challenge ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
3
Identifier
The Identifier field is one octet. The Identifier field MUST be
changed each time a Credentials packet is sent.
Credentials
The Credentials field is copied from the KRB_AS_REP packet
received by the authenticator from the Kerberos server. It takes
the form:
{ T_(peer,authenticator), K_(peer, authenticator) }K_peer
Challenge
The Challenge Value is a variable length stream of octets. The
Challenge Value MUST be changed each time a Credentials packet is
Blunk & Vollbrecht expires in six months [Page 10]
DRAFT PPP KAPv4 July 1994
sent.
Blunk & Vollbrecht expires in six months [Page 11]
DRAFT PPP KAPv4 July 1994
2.2.4. Ticket
Description
The Ticket packet is sent by the peer to the authenticator as
verification of its authenticity. It is sent in response to a
Credentials packet and contains a ticket for the authenticator and
a response to the authenticator's challenge. The response for the
challenge is generated by encrypting the challenge with the
session key. The peer may also specicy a Mutual Challenge for the
authenticator. This field should not be included if the peer does
not wish to challenge the authenticator. The peer MUST transmit a
KAPv4 packet with the Code field set to 4 (Ticket).
A summary of the Ticket 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ticket-Size | Ticket ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Response-Size | Response ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MA_Chal-Size | MA_Challenge ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
4
Identifier
The Identifier field is one octet. The Identifier field MUST
match the Indentifier field of the Credentials packet that it is
sent in response to.
Ticket
The Ticket field is copied from the Credentials data received from
the authenticator. It takes the form:
T_(peer,authenticator)
Blunk & Vollbrecht expires in six months [Page 12]
DRAFT PPP KAPv4 July 1994
Response
The response field is generated by encrypting the challenge from
the Credentials packets using the Session key. The DES PCBC
encryption algorithm is used to generate this response. The
Session key is used as the ivec.
MA_Challenge
The MA_Challenge field is a variable length string of octets used
to mutually authenticate the authenticator. The peer should put a
zero in the MA_Challenge size field if it does not with to
challenge the authenticator.
Blunk & Vollbrecht expires in six months [Page 13]
DRAFT PPP KAPv4 July 1994
2.2.5. Success
Description
If the fields received in a Ticket match the expected values AND
the response to the challenge is correct, then the implementation
MUST transmit a KAPv4 packet with the Code field set to 5
(Success). The authenticator must also include a response to the
MA_Challenge if the field was present in the Ticket packet.
A summary of the Success 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message-Size | Message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MA_Resp-Size | MA_Response ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
5
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 Ticket packet 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.
MA_Response
The MA_Response contains the reponse to the peers MA_Challenge (if
present). The MA_Response is the DES PCBC encryption of
MA_Challenge using the Session Key. The Session Key is used as
Blunk & Vollbrecht expires in six months [Page 14]
DRAFT PPP KAPv4 July 1994
the ivec for the encryption.
Blunk & Vollbrecht expires in six months [Page 15]
DRAFT PPP KAPv4 July 1994
2.2.6. Failure
Description
If the fields received in a Ticket do NOT match the expected
values OR the response to the challenge does NOT match the
expected value, then the implementation MUST transmit a KAPv4
packet with the Code field set to 6 (Failure), and SHOULD take
action to terminate the link. A Failure packet may also be sent
in response to a Principal packet. The Failure would be sent in
the case where the pricipal or service identity were unacceptable
or an error was received in response to the authenticator
KBR_AS_REQ message.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message-Size | Message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
6
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 Ticket or Principal packet which caused
this reply.
Message-Size
This 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
Blunk & Vollbrecht expires in six months [Page 16]
DRAFT PPP KAPv4 July 1994
126 decimal. Mechanisms for extension to other character sets are
the topic of future research.
Blunk & Vollbrecht expires in six months [Page 17]
DRAFT PPP KAPv4 July 1994
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.
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 KAPv4). 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 methods under different
circumstances, then distinct user names SHOULD be employed, each of
which identifies exactly one authentication method.
Blunk & Vollbrecht expires in six months [Page 18]
DRAFT PPP KAPv4 July 1994
References
[1] Simpson, W. A., "The Point-to-Point Protocol (PPP)", work in
progress.
[2] Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1340,
USC/Information Sciences Institute, July 1992.
Acknowledgments
Thanks to Bill Simpson for his initial work on this document.
Chair's Address
The working group can be contacted via the current chair:
Fred Baker
Advanced Computer Communications
315 Bollay Drive
Santa Barbara, California, 93111
EMail: fbaker@acc.com
Author's Address
Questions about this memo can also be directed to:
Larry J Blunk John R Vollbrecht
EMail: ljb@merit.edu EMail: jrv@merit.edu
Blunk & Vollbrecht expires in six months [Page 19]
DRAFT PPP KAPv4 July 1994
Table of Contents
1. Introduction .......................................... 1
1.1 Specification of Requirements ................... 1
1.2 Terminology ..................................... 2
2. PPP Kerberos version 4 Authentication Protocol ........ 3
2.1 Configuration Option Format ..................... 5
2.2 Packet Format ................................... 6
2.2.1 Initiate ........................................ 7
2.2.2 Principal ....................................... 8
2.2.3 Credentials ..................................... 10
2.2.4 Ticket .......................................... 12
2.2.5 Success ......................................... 14
2.2.6 Failure ......................................... 16
SECURITY CONSIDERATIONS ...................................... 18
REFERENCES ................................................... 19
ACKNOWLEDGEMENTS ............................................. 19
CHAIR'S ADDRESS .............................................. 19
AUTHOR'S ADDRESS ............................................. 19