Network Working Group L J Blunk
J R Vollbrecht
Internet Draft Merit Network, Inc.
expires September 1995 March 1995
PPP Extensible Authentication Protocol (EAP)
<draft-ietf-pppext-eap-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
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Distribution of this memo is unlimited.
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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 Extensible Authentication Protocol.
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Table of Contents
1. Introduction .......................................... 1
1.1 Specification of Requirements ................... 1
1.2 Terminology ..................................... 2
2. PPP Extensible Authentication Protocol (EAP) .......... 3
2.1 Configuration Option Format ..................... 5
2.2 Packet Format ................................... 6
2.2.1 Request and Response ............................ 7
2.2.2 Success and Failure ............................. 9
3. Initial EAP Request/Response Types .................... 10
3.1 Identity ........................................ 11
3.2 Notification .................................... 12
3.3 Nak ............................................. 13
3.4 Password ........................................ 14
3.5 MD5-Challenge ................................... 15
3.6 MD4-S/Key and MD5-S/Key ......................... 16
3.7 Echoed User Input and Non-echoed User Input ..... 17
3.8 Kerberos V4 and Kerberos V5 ..................... 18
3.9 Vendor specific ................................. 19
REFERENCES ................................................ 21
ACKNOWLEDGEMENTS .......................................... 21
CHAIR'S ADDRESS ........................................... 21
AUTHOR'S ADDRESS .......................................... 21
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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 Extensible Authentication Protocol
(EAP). 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.
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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.
displayable message
This is interpreted to be human readable string of
characters, 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.
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2. PPP Extensible Authentication Protocol (EAP)
The PPP Extensible Authentication Protocol (EAP) is a general
protocol for PPP authentication which supports multiple
authentication mechanisms. EAP does not select a specific
authentication mechanism at Link Control Phase, but rather postpones
this until the Authentication Phase. This allows the authenticator
to request more information before determining the specific
authentication mechanism. This also permits the use of a "back-end"
server which actually implements the various mechanisms while the PPP
authenticator merely passes through the authentication exchange.
1. After the Link Establishment phase is complete, the authenticator
sends one or more Requests to authenticate the peer. The Request
has a type field to indicate what is being requested. Examples of
Request types include "identity", "password", "MD5-challenge",
"generic user input" (for token cards), "s/key", etc. The
"password" and "MD5-challenge" requests correspond closely to the
"PAP" and "CHAP" authentication protocols, respectively.
Typically, the authenticator will send an initial "identity"
Request followed by one or more Requests for authentication
information. However, an initial "identity" Request is not
required, and may be bypassed in cases where the identity is
presumed (leased lines, dedicated dial-ups, etc.).
2. The peer sends a Response packet in reply to each Request. The
Response will vary with each Request type.
3. The authenticator terminates the authentication phase with a
Success or Failure reply. On a Success reply, the authenticator
will proceed to the Network Phase. On a Failure reply, the link
will be terminated.
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Advantages
The EAP protocol can support multiple authentication mechanisms
without having to pre-negotiate a particular one during LCP Phase.
Certain devices (e.g. a NAS) do not necessarily have to understand
each request type and may be able to simply act as a passthrough
agent for a "back-end" server on a host. The device only need look
for the success/failure code to terminate the authentication phase
Disadvantages
EAP does require the addition of a new authentication type to LCP and
thus PPP implementations will need to be modified to use it. It also
strays from the previous PPP authentication model of negotiating a
specific authentication mechanism during LCP.
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2.1. Configuration Option Format
A summary of the Authentication-Protocol Configuration Option format
to negotiate the EAP 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
???? (Hex) for PPP Extensible Authentication Protocol (EAP)
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2.2. Packet Format
Exactly one PPP EAP packet is encapsulated in the Information field
of a PPP Data Link Layer frame where the protocol field indicates
type hex ???? (PPP EAP). A summary of the EAP 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 EAP packet.
EAP Codes are assigned as follows:
1 Request
2 Response
3 Success
4 Failure
Identifier
The Identifier field is one octet and aids in matching responses
with requests.
Length
The Length field is two octets and indicates the length of the EAP
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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2.2.1. Request and Response
Description
The Request packet is sent by the authenticator to the peer. Each
Request has a type field which serves to indicate what is being
requested. The authenticator MUST transmit an EAP packet with the
Code field set to 1 (Request). Additional Request packets MUST be
sent until a valid Response packet is received, or an optional
retry counter expires. Retransmitted Requests MUST be sent with
the same Identifier value in order to distinguish them from new
Requests. The contents of the data field is dependent on the
Request type. The peer MUST send a Response packet in reply to a
Request packet. Responses MUST only be sent in reply to a
received Request and never retransmitted on a timer. The
Identifier field of the Response MUST match that of the Request.
A summary of the Request 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Type-Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
1 for Request;
2 for Response.
Identifier
The Identifier field is one octet. The Identifier field MUST be
the same if a Request packet is retransmitted due to a timeout
while waiting for a Response. Any new Requests MUST modify the
Identifier field. If a duplicate Response is received, it must be
silently discarded.
Type
The Type field is one octet. This field indicates the Type of
Request or Response. Only one Type may be specified per EAP
Request or Response. Normally, the Type field of the Response
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will be the same as the Type of the Request. However, there is a
specific Response Type for indicating that a Request type is
unacceptable. This Response Type will indicate an acceptable
Request type for authentication. An initial specification of
Types follows in a later section of this document.
Type-Data
The Type-Data field varies with the Type of Request and the
associated Response.
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2.2.2. Success and Failure
Description
The Success packet is sent by the authenticator to the peer to
acknowledge successful authentication. The authenticator MUST
transmit an EAP packet with the Code field set to 3 (Success).
If the authenticator cannot authenticate the peer (unacceptable
Responses to one or more Requests) then the implementation MUST
transmit an EAP packet with the Code field set to 4 (Failure). An
authenticator may with to issue multiple Requests before sending a
Failure response in order to allow for human typing mistakes.
Implementation Note: Because the Success and Failure packets
are not acknowledged, they may be potentially lost. A peer
MUST allow for this circumstance. The peer can use a Network
Protocol packet as an alternative indication of Success.
Likewise, the receipt of a LCP Terminate-Request can be taken
as a Failure.
A summary of the Success and Failure 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
3 for Success;
4 for Failure.
Identifier
The Identifier field is one octet and aids in matching replies to
Responses. The Identifier field MUST match the Indentifier field
of the Response packet that it is sent in response to.
Length
4
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3. Initial EAP Request/Response Types
This section defines the initial set of EAP Types used in
Request/Response exchanges. More Types may be defined in follow-on
documents. The Type field is one octet and identifies the structure
of an EAP Request or Response packet. The first 3 Types are
considered special case Types. The remaining Types define
authentication exchanges. The Nak Type is valid only for Response
packets, it may not be sent in a Request. The Nak Type may only be
sent in repsonse to a Request with an authentication Type.
The initial EAP Types are assigned as follows:
1 Identity
2 Notification
3 Nak (Response only)
4 Password
5 MD5-Challenge
6 MD4-S/Key
7 MD5-S/Key
8 Echoed user input
9 Non-echoed user input
10 Kerberos V4
11 Kerboros V5
240-255 Vendor Specific (reserved)
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3.1. Identity
Description
The Identity Type is used to query the identity of the peer.
Generally, the authenticator will issue this as the initial
Request. An optional displayable message may be included to
prompt the peer. A Response MUST be sent to this Request with a
Type of 1 (Identity).
Type
1
Type-Data
This field MAY contain a displayable message in the Request. The
Response uses this field to return the Identity. If the Identity
is unknown, this field should be zero bytes in length. The field
SHOULD NOT be null terminated. The length of this field is
derived from the Length field of the Request/Response packet.
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3.2. Notification
Description
The Notification Type is optionally used to convey a displayable
message from the authenticator to the peer. The peer SHOULD
display this message to the user or log it if it cannot be
displayed. It is intended to provide an acknowledged notification
of some imperative nature. Examples include a password with an
expiration time that is about to expire, an S/Key id which is
nearing 0, an authentication failure warning, etc. In most
circumstances, notification should not be required.
Type
2
Type-Data
The Type-Data field in the Request contains a displayable message
greater than zero octets in length. The length of the message is
determined by Length field of the Request packet. The message
SHOULD NOT be null terminated. A Response MUST be sent in reply
to the Request with a Type field of 2 (Notification). The Type-
Data field of the Response is zero octets in length. The
Response should be sent immediately (independent of how the
message is displayed or logged).
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3.3. Nak
Description
The Nak Type is valid only in Response messages. It is sent in
reply to a Request where the desired authentication Type is
unacceptable. Authentication Types are numbered 4 and above.
The Response contains the authentication Type desired by the peer.
Type
4
Type-Data
This field MUST contain a single octet indicating the desired
authentication type.
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3.4. Password
Description
The Password Type is analagous to the PPP PAP protocol [3]. The
Request may contain an optional displayable message to prompt the
peer. A Repsonse MUST be sent in reply to the Request. The
Response MAY be either of Type 4 (Password) or Type 3 (Nak). The
Nak reply indicates the peer's desired authentication mechanism
Type.
Type
4
Type-Data
This field MAY contain a displayable message in the Request. The
Response uses this field to return the Password. The field SHOULD
NOT be null terminated. The length of this field is derived from
the Length field of the Request/Response packet.
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3.5. MD5-Challenge
Description
The MD5-Challenge Type is analagous to the PPP CHAP protocol [3]
(with MD5 as the specified algorithm). The PPP Authentication
Protocols RFC [3] should be referred to for further implementation
specifics. The Request contains a "challenge" message to the
peer. A Repsonse MUST be sent in reply to the Request. The
Response MAY be either of Type 5 (MD5-Challenge) or Type 3 (Nak).
The Nak reply indicates the peer's desired authentication
mechanism Type.
Type
5
Type-Data
The contents of the Type-Data field is summarized below. For
reference on the use of this fields see the PPP Authentication
Protocols RFC [3].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value-Size | Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Name ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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3.6. MD4-S/Key and MD5-S/Key
Description These protocols are defined in "The S/KEY One-Time
Password System" [4]. The Request contains a displayable message
consisting of the S/Key count and a seed. A Repsonse MUST be sent in
reply to the Request. The Response MAY be either of Type 6 or 7
(MD4-S/key or MD5-S/key) or Type 3 (Nak). The Nak reply indicates
the peer's desired authentication mechanism Type.
Type
6 for MD4-S/key;
7 for MD5-S/key.
Type-Data
The Type-Data field contains the S/Key "challenge" (count and seed)
as a displayable message in the Request. This field is used for the
6 words (displayable text) from the S/Key dictionary [4] in the
Response. The messages SHOULD NOT be null terminated. The length of
the field is derived from the Length field of the Request/Reply
packet.
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3.7. Echoed User Input and Non-echoed User Input
Description
The user Input type are used where a user must provide physical
input. The typical application would be for "Token" cards where a
challenge or time-based token is retrieved from the card and
entered by the user. The non-echoed Type would be used in cases
where the entered data is potentially sensitive and thus should be
hidden. A Repsonse MUST be sent in reply to the Request. The
Response MAY be either of Type 8 or 9 (User Input) or Type 3
(Nak). The Nak reply indicates the peer's desired authentication
mechanism Type.
Type
8 for Echoed User Input;
9 for Non-echoed User Input.
Type-Data
The Type-Data field contains a displayable message in the Request.
The peer must then prompt the user for input in response to the
Request. This field is then used to return the input in the
Response. The message SHOULD NOT be null terminated. The length
of the data is derived from the Length field of the
Request/Response packet.
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3.8. Kerberos V4 and Kerberos V5
These protocols are to be defined in a later document.
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3.9. Vendor specific
Description
These Type field are reserved for Vendor Specific authentication
mechanism.
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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 EAP).
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.
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References
[1] Simpson, W. A., "The Point-to-Point Protocol (PPP)", RFC
1661.
[2] Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1700,
USC/Information Sciences Institute, October 1994.
[3] Lloyd, B., and Simpson, W., "PPP Authentication Protocols",
RFC 1334, October 1992.
[4] Haller, N., "The S/KEY One-Time Password System", RFC 1760,
Bellcore, February 1995.
Acknowledgments
This protocol derives much of its inspiration from Dave Carrel's
AHA draft as well as the PPP CHAP protocol [3]. Bill Simpson
provided much of the boilerplate used throughout this document.
Al Rubens (Merit) also provided valuable feedback.
Chair's Address
The working group can be contacted via the current chair:
Fred Baker
cisco Systems, Inc.
EMail: fbaker@cisco.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
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