RFC1994 - 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.

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 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:

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. 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
authentication.

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
specification.

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
attacks.

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 |
+-+-+-+-+-+-+-+-+

Type

3

Length

5

Authentication-Protocol

c223 (hex) for Challenge-Handshake Authentication Protocol.

Algorithm

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
implemented:

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 ...
+-+-+-+-+

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.

4.1. Challenge and Response

Description

The Challenge packet is used to begin the Challenge-Handshake
Authentication Protocol. The authenticator MUST transmit a CHAP
packet with the Code field set to 1 (Challenge). Additional
Challenge packets MUST be sent until a valid Response packet is
received, or an optional retry counter expires.

A Challenge packet MAY also be transmitted at any time during the
Network-Layer Protocol phase to ensure that the connection has not
been altered.

The peer SHOULD expect Challenge packets during the Authentication
phase and the Network-Layer Protocol phase. Whenever a Challenge
packet is received, the peer MUST transmit a CHAP packet with the
Code field set to 2 (Response).

Whenever a Response packet is received, the authenticator compares
the Response Value with its own calculation of the expected value.
Based on this comparison, the authenticator MUST send a Success or
Failure packet (described below).

Implementation Notes: Because the Success might be lost, the
authenticator MUST allow repeated Response packets during the
Network-Layer Protocol phase after completing the
Authentication phase. To prevent discovery of alternative
Names and Secrets, any Response packets received having the
current Challenge Identifier MUST return the same reply Code
previously returned for that specific Challenge (the message
portion MAY be different). Any Response packets received
during any other phase MUST be silently discarded.

When the Failure is lost, and the authenticator terminates the
link, the LCP Terminate-Request and Terminate-Ack provide an
alternative indication that authentication failed.

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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value-Size | Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Name ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Code

1 for Challenge;

2 for Response.

Identifier

The Identifier field is one octet. The Identifier field MUST be
changed each time a Challenge is sent.

The Response Identifier MUST be copied from the Identifier field
of the Challenge which caused the Response.

Value-Size

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

Value

The Value field is one or more octets. The most significant octet
is transmitted first.

The Challenge Value is a variable stream of octets. The
importance of the uniqueness of the Challenge Value and its
relationship to the secret is described above. The Challenge
Value MUST be changed each time a Challenge is sent. The length
of the Challenge Value depends upon the method used to generate
the octets, and is independent of the hash algorithm used.

The Response Value is the one-way hash calculated over a stream of
octets consisting of the Identifier, followed by (concatenated
with) the "secret", followed by (concatenated with) the Challenge
Value. The length of the Response Value depends upon the hash
algorithm used (16 octets for MD5).

Name

The Name field is one or more octets representing the
identification of the system transmitting the packet. There are
no limitations on the content of this field. For example, it MAY
contain ASCII character strings or globally unique identifiers in
ASN.1 syntax. The Name should not be NUL or CR/LF terminated.
The size is determined from the Length field.

4.2. Success and Failure

Description

If the Value received in a Response is equal to the expected
value, then the implementation MUST transmit a CHAP packet with
the Code field set to 3 (Success).

If the Value received in a Response is not equal to the expected
value, then the implementation MUST transmit a CHAP packet with
the Code field set to 4 (Failure), and SHOULD take action to
terminate the link.

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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-

Code

3 for Success;

4 for Failure.

Identifier

The Identifier field is one octet and aids in matching requests
and replies. The Identifier field MUST be copied from the
Identifier field of the Response which caused this reply.

Message

The Message field is zero or more octets, and its contents are
implementation dependent. It is intended to be human readable,
and MUST NOT affect operation of the protocol. It is recommended
that the message contain displayable ASCII characters 32 through
126 decimal. Mechanisms for extension to other character sets are
the topic of future research. The size is determined from the
Length field.

Security Considerations

Security issues are the primary topic of this RFC.

The interaction of the authentication protocols within PPP are highly
implementation dependent. This is indicated by the use of SHOULD
throughout the document.

For example, upon failure of authentication, some implementations do
not terminate the link. Instead, the implementation limits the kind
of traffic in the Network-Layer Protocols to a filtered subset, which
in turn allows the user opportunity to update secrets or send mail to
the network administrator indicating a problem.

There is no provision for re-tries of failed authentication.
However, the LCP state machine can renegotiate the authentication
protocol at any time, thus allowing a new attempt. It is recommended
that any counters used for authentication failure not be reset until
after successful authentication, or subsequent termination of the
failed link.

There is no requirement that authentication be full duplex or that
the same protocol be used in both directions. It is perfectly
acceptable for different protocols to be used in each direction.
This will, of course, depend on the specific protocols negotiated.

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

In practice, within or associated with each PPP server, there is a
database which associates "user" names with authentication
information ("secrets"). It is not anticipated that a particular
named user would be authenticated by multiple methods. This would
make the user vulnerable to attacks which negotiate the least secure
method from among a set (such as PAP rather than CHAP). If the same

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

Instead, for each user name there should be an indication of exactly
one method used to authenticate that user name. If a user needs to
make use of different authentication methods under different
circumstances, then distinct user names SHOULD be employed, each of
which identifies exactly one authentication method.

Passwords and other secrets should be stored at the respective ends
such that access to them is as limited as possible. Ideally, the
secrets should only be accessible to the process requiring access in
order to perform the authentication.

The secrets should be distributed with a mechanism that limits the
number of entities that handle (and thus gain knowledge of) the
secret. Ideally, no unauthorized person should ever gain knowledge
of the secrets. Such a mechanism is outside the scope of this
specification.

Acknowledgements

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

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

Special thanks to Dave Balenson, Steve Crocker, James Galvin, and
Steve Kent, for their extensive explanations and suggestions. Now,
if only we could get them to agree with each other.

References

[1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
51, RFC1661, 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., RFC1321, April 1992.

Contacts

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

karl@MorningStar.com
karl@Ascend.com

Questions about this memo can also be directed to:

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

wsimpson@UMich.edu
wsimpson@GreenDragon.com (preferred)

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