Network Working Group M. Nystroem
Internet-Draft RSA Security
Expires: March 30, 2007 September 26, 2006
The Protected One-Time Password Protocol (EAP-POTP)
draft-nystrom-eap-potp-07
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
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 inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 30, 2007.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes a general EAP method suitable for use with
One-Time Password (OTP) tokens, and offers particular advantages for
tokens with direct electronic interfaces to their associated clients.
The method can be used to provide unilateral or mutual
authentication, and key material, in protocols utilizing EAP, such as
PPP, IEEE 802.1X and IKEv2.
Nystroem Expires March 30, 2007 [Page 1]
Internet-Draft EAP-POTP September 2006
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Background . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Rationale behind the design . . . . . . . . . . . . . . . 4
1.4. Relationship with EAP methods in RFC 3748 . . . . . . . . 5
2. Conventions used in this document . . . . . . . . . . . . . . 6
3. Authentication model . . . . . . . . . . . . . . . . . . . . . 7
4. Description of the EAP-POTP method . . . . . . . . . . . . . . 8
4.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Version negotiation . . . . . . . . . . . . . . . . . . . 11
4.3. Cryptographic algorithm negotiation . . . . . . . . . . . 12
4.4. Session resumption . . . . . . . . . . . . . . . . . . . . 13
4.5. Key derivation and session identifiers . . . . . . . . . . 14
4.6. Error handling and result indications . . . . . . . . . . 15
4.7. Use of the EAP Notification method . . . . . . . . . . . . 16
4.8. Protection against brute-force attacks . . . . . . . . . . 16
4.9. MAC calculations in EAP-POTP . . . . . . . . . . . . . . . 17
4.10. EAP-POTP packet format . . . . . . . . . . . . . . . . . . 19
4.11. EAP-POTP TLV objects . . . . . . . . . . . . . . . . . . . 21
4.11.1. Version TLV . . . . . . . . . . . . . . . . . . . . . 21
4.11.2. Server-Info TLV . . . . . . . . . . . . . . . . . . . 23
4.11.3. OTP TLV . . . . . . . . . . . . . . . . . . . . . . . 25
4.11.4. NAK TLV . . . . . . . . . . . . . . . . . . . . . . . 34
4.11.5. New PIN TLV . . . . . . . . . . . . . . . . . . . . . 36
4.11.6. Confirm TLV . . . . . . . . . . . . . . . . . . . . . 38
4.11.7. Vendor-Specific TLV . . . . . . . . . . . . . . . . . 42
4.11.8. Resume TLV . . . . . . . . . . . . . . . . . . . . . 44
4.11.9. User Identifier TLV . . . . . . . . . . . . . . . . . 47
4.11.10. Token Key Identifier TLV . . . . . . . . . . . . . . 48
4.11.11. Time Stamp TLV . . . . . . . . . . . . . . . . . . . 49
4.11.12. Counter TLV . . . . . . . . . . . . . . . . . . . . . 50
4.11.13. Challenge TLV . . . . . . . . . . . . . . . . . . . . 51
4.11.14. Keep-Alive TLV . . . . . . . . . . . . . . . . . . . 52
4.11.15. Protected TLV . . . . . . . . . . . . . . . . . . . . 53
4.11.16. Crypto Algorithm TLV . . . . . . . . . . . . . . . . 54
5. EAP Key Management Framework considerations . . . . . . . . . 58
6. Security considerations . . . . . . . . . . . . . . . . . . . 59
6.1. Security claims . . . . . . . . . . . . . . . . . . . . . 59
6.2. Passive and active attacks . . . . . . . . . . . . . . . . 59
6.3. Denial of service attacks . . . . . . . . . . . . . . . . 61
6.4. The use of pepper . . . . . . . . . . . . . . . . . . . . 61
6.5. The race attack . . . . . . . . . . . . . . . . . . . . . 61
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 63
7.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 63
7.2. Cryptographic algorithm identifier octets . . . . . . . . 63
8. Intellectual property considerations . . . . . . . . . . . . . 64
Nystroem Expires March 30, 2007 [Page 2]
Internet-Draft EAP-POTP September 2006
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 65
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 66
10.1. Normative references . . . . . . . . . . . . . . . . . . . 66
10.2. Informative references . . . . . . . . . . . . . . . . . . 66
Appendix A. Profile of EAP-POTP for RSA SecurID . . . . . . . . . 68
Appendix B. Examples of EAP-POTP exchanges . . . . . . . . . . . 69
B.1. Basic mode, unilateral authentication . . . . . . . . . . 69
B.2. Basic mode, session resumption . . . . . . . . . . . . . . 69
B.3. Mutual authentication without session resumption . . . . . 70
B.4. Mutual authentication with transfer of pepper . . . . . . 72
B.5. Failed mutual authentication . . . . . . . . . . . . . . . 73
B.6. Session resumption . . . . . . . . . . . . . . . . . . . . 75
B.7. Failed session resumption . . . . . . . . . . . . . . . . 76
B.8. Mutual authentication, and new PIN requested. . . . . . . 78
B.9. Use of next OTP mode . . . . . . . . . . . . . . . . . . . 81
Appendix C. Use of the MPPE-Send/Receive-Key RADIUS attributes . 84
C.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 84
C.2. MPPE key attribute population . . . . . . . . . . . . . . 84
Appendix D. Key strength considerations . . . . . . . . . . . . . 85
D.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 85
D.2. Example 1: 6-digit One-Time Passwords . . . . . . . . . . 85
D.3. Example 2: 8-digit One-Time Passwords . . . . . . . . . . 85
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 87
Intellectual Property and Copyright Statements . . . . . . . . . . 88
Nystroem Expires March 30, 2007 [Page 3]
Internet-Draft EAP-POTP September 2006
1. Introduction
1.1. Scope
This document describes an Extensible Authentication Protocol (EAP)
[1] method suitable for use with One-Time Password (OTP) tokens, and
offers particular advantages for tokens that are electronically
connected to a user's computer, e.g. through a USB interface. The
method can be used to provide unilateral or mutual authentication,
and key material, in protocols utilizing EAP, such as PPP [10], IEEE
802.1X [11] and IKEv2 [12].
1.2. Background
A One-Time Password (OTP) token may be a handheld hardware device, a
hardware device connected to a personal computer through an
electronic interface such as USB, or a software module resident on a
personal computer, which generates one-time passwords that may be
used to authenticate a user towards some service. This document
describes an EAP method intended to meet the needs of organizations
wishing to use OTP tokens in an interoperable manner to authenticate
users over EAP. The method is designed to be independent of
particular OTP algorithms and to meet the requirements on modern EAP
methods (see e.g. [13]).
The basic variant of this method provides client authentication only.
This mode is only to be used within a secured tunnel. A more
advanced variant provides mutual authentication, integrity protection
of the exchange, protection against eavesdroppers, and establishment
of authenticated keying material. Both variants allow for fast
session resumption.
While this document also includes a profile of the general method for
the RSA SecurID(TM) mechanism, it is described in terms of general
constructions. It is therefore intended that the document will serve
as a framework for use also by other OTP algorithms.
Note: The term "OTP" as used herein shall not be confused with the
EAP OTP method defined in [1].
1.3. Rationale behind the design
EAP-POTP has been designed with the intent that its messages and data
elements be easily parsed by EAP implementations. This makes it
easier to programmatically use the EAP method in the peer and the
authenticator, reducing the need for user interactions and allowing
for local generation of user prompts, when needed. In contrast, the
Generic Token Card (GTC) method from [1], which uses text strings
Nystroem Expires March 30, 2007 [Page 4]
Internet-Draft EAP-POTP September 2006
generated by the EAP server, is intended to be interpreted and acted
upon by humans. Furthermore, EAP-POTP allows for mutual
authentication and establishment of keying material, which GTC does
not. To retain the generic nature of GTC, the EAP-POTP method has
been designed to support a wide range of OTP algorithms, with
profiling expected for specific such algorithms. This document
provides a profile of EAP-POTP for RSA SecurID tokens.
1.4. Relationship with EAP methods in RFC 3748
The EAP OTP method defined in [1], which builds on [14], is an
example of a particular OTP algorithm and is not related to the EAP
method defined in this document other than that a profile of EAP-POTP
may be created for the OTP algorithm from [14].
The Generic Token Card EAP method defined in [1] is intended to work
with a variety of OTP algorithms. The same is true for EAP-POTP, the
EAP method defined herein. Advantages of profiling a particular OTP
algorithm for use with EAP-POTP compared to using EAP GTC are
described in Section 1.3.
Nystroem Expires March 30, 2007 [Page 5]
Internet-Draft EAP-POTP September 2006
2. Conventions used in this document
The key words "MUST", "MUST NOT", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", and "MAY", in this document are to be
interpreted as described in RFC 2119 [2].
Nystroem Expires March 30, 2007 [Page 6]
Internet-Draft EAP-POTP September 2006
3. Authentication model
The EAP-POTP method provides user authentication as defined below.
Additionally, it may provide mutual authentication (authenticating
the EAP server to the EAP client) and establish keying material.
There are basically three entities in the authentication method
described here:
o A client, or "peer", using EAP terminology, acting on behalf of a
user possessing an OTP token;
o A server, or "authenticator", using EAP terminology, to which the
user needs to authenticate; and
o A backend authentication server, providing an authentication
service to the authenticator.
The term "EAP server" is used here with the same meaning as in [1].
Any protocol used between the authenticator and the backend
authentication server is outside the scope of this document, although
RADIUS [15] is a typical choice. It is assumed that the EAP client
and the peer are located on the same host, and hence only the term
"peer" is used in the following for these entities.
The EAP-POTP method assumes the use of a shared secret key, or
"seed", which is known both by the user and the backend
authentication server. The secret seed is stored on an OTP token
that the user possesses, as well as on the authentication server.
In its most basic variant, the EAP-POTP method provides only one
service, namely user authentication where the user provides
information to the authentication server, so that the server can
authenticate the user. A more advanced variant provides mutual
authentication, protection against eavesdropping and establishment of
authenticated keying material.
Nystroem Expires March 30, 2007 [Page 7]
Internet-Draft EAP-POTP September 2006
4. Description of the EAP-POTP method
4.1. Overview
Note: Since the EAP-POTP method is general in nature, the term
"POTP-X" is used below as a placeholder for an EAP method type
identifier, identifying the use of a particular OTP algorithm with
EAP-POTP. As an example, in the case of using RSA SecurID tokens
within EAP-POTP, the EAP method type shall be 32 (see Appendix A).
A typical EAP-POTP authentication is performed as follows (Appendix B
provides more detailed examples):
a. The optional EAP Identity Request/Response is exchanged, as per
RFC 3748 [1]. An identity provided here may alleviate the need
for a "User Identifier" or a "Token Key Identifier" triplet
("TLV") (defined below) later in the exchange.
b. The EAP server sends an EAP-Request of type POTP-X with a Version
TLV. The Version TLV indicates the highest and lowest version of
this method supported by the server. The EAP server typically
also includes an OTP TLV in the EAP-Request. The OTP TLV
instructs the peer to respond with the current OTP (possibly in
protected form), and may contain a challenge and some other
information, like server policies. The EAP server should also
include a Server-Info TLV in the request, and must do so if it
supports session resumption. The Server-Info TLV identifies the
authentication server, contains an identifier for this (new)
session, and may be used by the peer to find an already existing
session with the EAP server.
c. The peer responds with an EAP-Response of type Nak (3) if it does
not support POTP-X or if it does not support a version of this
method that is also supported by the server, as indicated in the
server's Version TLV.
If the peer supports a version of this method that is also
supported by the EAP server, the peer generates an EAP-Response
of type POTP-X as follows:
* First, it generates a Version TLV which indicates the peer's
highest supported version within the range of versions offered
by the server. This Version TLV will be part of the EAP-
Response to the EAP server.
* Next, if the peer's highest supported version equals that of
the EAP server, and the EAP server sent a Server-Info TLV, the
peer checks if it has a saved session with the EAP server. If
Nystroem Expires March 30, 2007 [Page 8]
Internet-Draft EAP-POTP September 2006
an existing session with the server is found, and session
resumption is possible (the Server-Info TLV may explicitly
disallow it) the peer calculates new session keys (if the
session is a protected-mode session) and responds with a
Resume TLV and the Version TLV.
* Otherwise, if the peer's highest supported version equals that
of the EAP server, and the received EAP-Request message
contains an OTP TLV, the peer requests (possibly through user
interaction) the OTP token to calculate a one-time password
based on the information in the received EAP-Request message
(which could, for example, carry a challenge), the current
token state (e.g. token time), a shared secret (the "seed"),
and a user-provided PIN (note that, depending on the OTP token
type, some of the information in the EAP-Request may not be
used in the OTP calculation, and the PIN may be optional too).
If the received OTP TLV has the P bit set (see below), the
peer then combines the token-provided OTP with other
information, and provides the combined data to a key
derivation function. The key derivation function generates
several keys, of which one is used to calculate a MAC on the
received message together with some other information. The
resulting MAC together with some additional information is
then placed in an OTP TLV (with the P bit set) that is sent in
a response to the EAP server together with the Version TLV.
If the P bit is not set in the received OTP TLV, the peer
instead inserts the calculated OTP value directly in an OTP
TLV, which then is sent to the EAP server together with the
Version TLV.
* Finally, if the peer's highest supported version differs from
the server's, or if the server did not provide any TLVs
besides the Version TLV in its initial request, the peer just
sends back the generated Version TLV as an EAP-Response to the
EAP server.
d. If the EAP server receives an EAP-Response of type Nak (3) the
session negotiation failed and the EAP server may try with
another EAP method. Otherwise, the EAP server checks the peer's
supported version. If the peer did not support the highest
version supported by the server, the server will send a new EAP-
Request with TLVs adjusted for that version. Otherwise, and
assuming the EAP server did send additional TLVs in its initial
EAP-Request, the EAP server will attempt to authenticate the peer
based on the response provided in c). Depending on the result of
this authentication, the EAP server may either:
Nystroem Expires March 30, 2007 [Page 9]
Internet-Draft EAP-POTP September 2006
* send a new EAP-Request of type POTP-X to the peer indicating
that session resumption was not possible, and ask for a new
OTP (this would be the case when the peer responded with a
Resume TLV and the session indicated in the Resume TLV was not
valid),
* send a new EAP-Request of type POTP-X to the peer (e.g. to ask
for the next OTP),
* accept the authentication (and send an EAP-Request message
containing a Confirm TLV to the peer if the received response
has the P bit set or was a successful attempt at a protected-
mode session resumption, or otherwise send an EAP-Success
message to the peer), or
* fail the authentication (and send an EAP-Failure message -
possibly preceded by an EAP-Request message of type
Notification (2) - to the peer).
e. If the peer receives an EAP-Success or an EAP-Failure message the
protocol run is finished. If the peer receives an EAP-Request of
type Notification it responds as specified by RFC 3748 [1]. If
the peer receives an EAP-Request of type POTP-X with a Confirm
TLV it attempts to authenticate the EAP server using the provided
data. If the authentication is successful the peer responds with
an EAP-Response of type POTP-X with a Confirm TLV. If it is
unsuccessful, the peer responds with an empty EAP-Response of
type POTP-X. If the peer receives an EAP-Request of type POTP-X
containing some other TLVs it continues as specified in c) above
(though no version negotiation will take place in this case) or
as described for those TLVs.
f. When an EAP server, which has sent an EAP-Request of type POTP-X
with a Confirm TLV receives an EAP-Response of type POTP-X with a
Confirm TLV present, it can proceed in one of two ways: If it has
detected that there is a need to send additional EAP-Requests of
type POTP-X, it shall enter a "protected state" where from now on
all POTP-X TLVs must be encrypted and integrity protected before
being sent (at this point the parties shall have calculated a
master session key as described in Section 4.5). One reason to
continue the POTP-X conversation after exchange of the Confirm
TLV could be that the user needs to update her OTP PIN, and hence
the EAP server needs to send a New PIN TLV at which point the
handshake is back at step c) above (save for the version
negotiation and that all TLVs shall be protected). If there is
no need to send additional EAP-Request packets, the EAP server
shall instead send an EAP-Success method to the peer to indicate
successful protocol completion. The EAP server may not continue
Nystroem Expires March 30, 2007 [Page 10]
Internet-Draft EAP-POTP September 2006
the conversation unless it indicate its intent to do so in the
Confirm TLV.
An EAP server, which has sent an EAP-Request of type POTP-X with
a Confirm TLV and receives an EAP-Response of type POTP-X which
is empty (i.e. does not contain any TLVs), shall respond with an
EAP-Failure and terminate the handshake.
As implied by the description, steps c) through f) may be carried out
a number of times before completion of the exchange. One example of
this is when the authentication server initially requests an OTP,
accepts the response from the peer, performs an (intermediary)
Confirm TLV exchange, requests the peer to select a new PIN, and
finally asks the peer to authenticate with an OTP based on the new
PIN (which again will be followed with a final Confirm TLV exchange).
4.2. Version negotiation
The EAP-POTP method provides a version negotiation mechanism that
enables implementations to be backward compatible with previous
versions of the protocol. This specification documents the EAP-POTP
protocol version 1. Version negotiation proceeds as follows:
a. In the first EAP-Request of type POTP-X, the EAP server MUST send
a Version TLV in which it sets the "Highest supported" version
field to its highest supported version number, and the "Lowest
supported" version field to its lowest supported version number.
The EAP server MAY include other TLV triplets as described below
and compatible with the "Highest" supported version number to
optimize the number of round-trips in the case of a peer
supporting the server's "Highest" version number.
b. If the peer supports a version of the protocol that falls within
the range of versions indicated by the EAP server, it MUST
respond with an EAP-Response of type POTP-X, and containing a
Version TLV with the "Highest supported" version field set to the
highest version supported by the peer. The peer MUST also
respond to any TLV triplets included in the EAP-Request, if it
supported the "Highest supported" version indicated in the
server's Version TLV.
The EAP peer MUST respond with an EAP-Response of type Nak (3) if
it does not support a version that falls within the range of
versions indicated by the EAP server. This will allow the EAP
server to use another EAP method for peer authentication.
c. When the EAP server receives an EAP-Response containing a Version
TLV from the peer, but the "Highest supported" version field in
Nystroem Expires March 30, 2007 [Page 11]
Internet-Draft EAP-POTP September 2006
the TLV differs from the "Highest supported" version field sent
by the EAP server, or when the version is the same as the one
originally proposed by the EAP server, but the EAP server did not
include any TLV triplets in the initial request, the EAP server
sends a new EAP-Request of type POTP-X with the negotiated
version and TLV triplets as desired and described herein.
The version negotiation procedure guarantees that the EAP peer and
server will agree to the highest version supported by both parties.
If version negotiation fails, use of EAP-POTP will not be possible,
and another mutually acceptable EAP method will need to be negotiated
if authentication is to proceed.
The EAP-POTP version field may be modified in transit by an attacker.
It is therefore important that EAP entities only accept EAP-POTP
versions according to an explicit policy.
4.3. Cryptographic algorithm negotiation
Cryptographic algorithms are negotiated through the use of the Crypto
Algorithm TLV. EAP-POTP provides a default digest algorithm (SHA-
256) [3], a default encryption algorithm (AES-CBC) [4] , and a
default MAC algorithm (HMAC) [5], and these algorithms MUST be
supported by all EAP-POTP implementations. An EAP server that does
not want to make use of any other algorithms than the default ones
need not send a Crypto Algorithm TLV. An EAP server that does want
to negotiate use of some other algorithms MUST send the Crypto
Algorithm TLV in the initial EAP-Request of type POTP-X that also
contains an OTP TLV with the P bit set. The TLV MUST NOT be present
in any other EAP-Request in the session (the two exception to this
are if the client attempted a session resumption which failed and
therefore did not evaluate a sent Crypto Algorithm TLV, or if the
Crypto Algorithm TLV was part of the initial message from the EAP
server and the client negotiated another EAP-POTP version than the
highest one supported by the EAP server. When either of these cases
apply, the server MUST include the Crypto Algorithm TLV in the first
EAP-Request that also contains an OTP TLV with the P bit set
subsequent to the failed session resumption/protocol version
negotiation). In the Crypto Algorithm TLV, the EAP server suggests
some combination of digest, encryption, and MAC algorithms (if the
server only wants to negotiate a particular class of algorithms then
suggestions for the other classes need not be present, since the
default applies).
The peer MUST include a Crypto Algorithm TLV in an EAP-Response if
and only if an EAP-Request of type POTP-X has been received
containing a Crypto Algorithm TLV, it was legal for that EAP-Request
to contain a Crypto Algorithm TLV, the peer does not try to resume an
Nystroem Expires March 30, 2007 [Page 12]
Internet-Draft EAP-POTP September 2006
existing session, and the peer and the EAP server agrees on at least
one algorithm not being the default one. If the peer does not supply
a value for a particular class of algorithms in a responding Crypto
Algorithm TLV, then the default algorithm applies for that class.
When resuming an existing session (see the next Section), there is no
need for the peer to negotiate since the session already is
associated with a set of algorithms. Servers MUST fail a session
(i.e. send an EAP-Failure) if they receive an EAP-Response TLV
containing both a Resume TLV and a Crypto Algorithm TLV).
Clearly, EAP servers and peers MUST NOT suggest any other algorithms
than the ones their policy allows them to use. Policies may also
restrict what combinations of cryptographic algorithms are
acceptable.
4.4. Session resumption
This method makes use of session identifiers and server identifiers
to allow for improved efficiency in the case where a peer repeatedly
attempts to authenticate to an EAP server within a short period of
time. This capability is particularly useful for support of wireless
roaming.
In order to help the peer find a session associated with the EAP
server, an EAP server that supports session resumption MUST send a
Server-Info TLV containing a server identifier in its initial EAP-
Request of type POTP-X that also contains an OTP TLV. The identifier
may then be used by the peer for lookup purposes.
It is left to the peer whether to attempt to continue a previous
session, thus shortening the negotiation, or not. Typically the
peer's decision will be made based on the time elapsed since the
previous authentication attempt to that EAP server. If the peer
decides to attempt to resume a session with the EAP server, it sends
a Resume TLV identifying the chosen session and other contents as
described below to the EAP server.
Based on the session identifier chosen by the peer, and the time
elapsed since the previous authentication, the EAP server will decide
whether to allow the session resumption, or whether to continue with
a new session.
o If the EAP server is willing to resume a previously established
session, it MUST authenticate the peer based on the contents of
the Resume TLV. If the authentication succeeds, the handshake
will continue in one of two ways:
Nystroem Expires March 30, 2007 [Page 13]
Internet-Draft EAP-POTP September 2006
* If the session is a protected-mode session, then the server
MUST respond with a request containing a Confirm TLV. If the
Confirm TLV authenticates the EAP server then the peer responds
with an empty Confirm TLV, to which the EAP server responds
with an EAP-Success message. If the Confirm TLV does not
authenticate the EAP server, the peer responds with an empty
EAP-Response of type POTP-X.
* If the session is not a protected-mode session, i.e. it is a
session created from a basic-mode peer authentication, then the
server MUST respond with an EAP-Success message.
If the authentication of the peer fails, the EAP server SHOULD
send another EAP-Request containing an OTP TLV and a Server-Info
TLV with the N bit set to indicate that no session resumption is
possible. The EAP server MAY also send an EAP-Failure message,
possibly preceded by an EAP-Request of type Notification (2), in
which case the EAP run will terminate.
o If the EAP server is not willing or able to resume a previously
established session, it will respond with another EAP-Request
containing an OTP TLV and a Server-Info TLV with the N bit set
(indicating no session resumption).
Sessions SHOULD NOT be maintained longer than the security of the
exchange which created the session permits. E.g. if it is estimated
that an attacker could be successful in brute-force searching for the
OTP in 24 hours, then EAP-POTP session lifetimes should be clearly
less than this value.
4.5. Key derivation and session identifiers
The EAP-POTP method described herein makes use of a key derivation
function denoted "PBKDF2". PBKDF2 is described in [6], Section 5.2.
The PBKDF2 PRF SHALL be set to the negotiated MAC algorithm. The
default MAC algorithm, which MUST be supported, is HMAC-SHA256. HMAC
is defined in [5] and SHA-256 is defined in [3]. HMAC-SHA256 is the
HMAC construct from [5] with SHA-256 as the hash function H. The
output length of HMAC-SHA256 when used as a PRF for PBKDF2 shall be
32 octets (i.e. the full output length).
The output from PBKDF2 as described here will consist of five keys:
o K_MAC, a MAC key used for mutual authentication and integrity
protection,
o K_ENC, an encryption key used to protect certain data during the
authentication,
Nystroem Expires March 30, 2007 [Page 14]
Internet-Draft EAP-POTP September 2006
o SRK, a session resumption key only used for session resumption
purposes,
o MSK, a Master Session Key as defined in [1], and
o EMSK, an Extended Master Session Key, also as defined in [1].
For the default algorithms, K_MAC, K_ENC, and SRK SHALL be 16 octets.
For other cases, the key lengths will be as determined by the
negotiated algorithms. The MSK and the EMSK SHALL each be 64 octets,
in conformance with [1]. The "dkLen" parameter from Section 5.2 of
[6] SHALL therefore in the case of default algorithms be set to 176
(the combined length of K_MAC, K_ENC, SRK, MSK, and EMSK).
[1] and [16] define usage of the MSK and the EMSK . For a particular
use case, see also Appendix C.
4.6. Error handling and result indications
EAP does not allow for the sending of an EAP-Response of type Nak (3)
within a method after the initial EAP-Request and EAP-Response pair
of that particular method has been exchanged (see [1], Section 2.1).
Instead, when a peer is unable to continue an EAP-POTP session, the
peer MAY respond to an outstanding EAP-Request by sending an empty
EAP-Response of type POTP-X rather than immediately terminating the
conversation. This allows the EAP server to log the cause of the
error.
To ensure that the EAP server receives the empty EAP-Response, the
peer SHOULD wait for the EAP server to reply before terminating the
conversation. The EAP server MUST reply with an EAP-Failure.
When EAP-POTP is run in protected mode, the exchange of the Confirm
TLV (Section 4.11.6) serves as a success result indication - when the
peer receives a Confirm TLV it knows that the EAP server has
successfully authenticated it. Similarly, when the EAP server
receives the Confirm TLV response from the peer it knows that the
peer has authenticated it. In protected mode, the peer will not
accept an EAP-Success packet unless it has received and validated a
Confirm TLV. The Confirm TLV sent from the EAP server to the peer is
a "protected result indication" as defined in [1], as it is integrity
protected and cannot be replayed. The Confirm TLV sent from the peer
to the EAP server is however not a protected result indication. An
empty EAP-POTP response sent from the peer to the EAP server serves
as a failure result indication.
Nystroem Expires March 30, 2007 [Page 15]
Internet-Draft EAP-POTP September 2006
4.7. Use of the EAP Notification method
Except where explicitly allowed in the following, the EAP
Notification method MUST NOT be used within an EAP-POTP session. The
EAP Notification method MAY be used within an EAP-POTP session in the
following situations:
o The EAP server MAY send an EAP-Request of type Notification (2)
when it has received an EAP-Response containing an OTP TLV and is
unable to authenticate the user. In this case, once the EAP-
Response of type Notification is received, the EAP server MAY
retry the authentication and send a new EAP-Request containing an
OTP TLV, or it MAY fail the session and send an EAP-Failure
message.
o The EAP server MAY send an EAP-Request of type Notification (2)
when it has received an unacceptable New PIN TLV. In this case,
once the EAP-Response of type Notification is received, the EAP
server MAY retry the PIN update and send a new EAP-Request with a
New PIN TLV, or it MAY fail the session and send an EAP-Failure
message.
4.8. Protection against brute-force attacks
Since OTPs may be relatively short, it is important to slow down an
attacker sufficiently so that it is economically unattractive to
brute-force search for an OTP given an observed EAP-POTP handshake in
protected mode. One way to do this is to do a high number of
iterated hashes in the PBKDF2 function. Another is for the client to
include a value ("pepper") unknown to the attacker in the hash
computation. Whereas a traditional "salt" value normally is sent in
the clear, this "pepper" value will not be sent in the clear, but may
instead be transferred to the EAP server in encrypted form. In
practice, the procedure is as follows:
a. The EAP server indicates in its OTP TLV whether it supports
pepper searching. Additionally, it may indicate to the peer that
a new pepper shall be chosen.
b. If the peer supports the use of pepper, the peer checks whether
it already has established a shared pepper with this server:
If it does have a pepper stored for this server, and the server
did not indicate that a new pepper shall be generated, then it
uses the existing pepper value as specified in Section 4.11.3
below to calculate an OTP TLV response. In this case the
iteration count shall be kept to a minimum as the security of the
scheme is provided through the pepper and efficiency otherwise is
Nystroem Expires March 30, 2007 [Page 16]
Internet-Draft EAP-POTP September 2006
lost.
If the peer does not have a pepper stored for this server, but
the server indicated support for pepper searching, or the server
indicated that a new pepper shall be generated, then the peer
generates a random and uniformly distributed pepper of sufficient
length (the maximum length supported by the server is provided in
the server's OTP TLV), and includes the new pepper in the PBKDF2
computation.
If the peer does not have a pepper stored for this server, and
the server did not indicate support for pepper searching, then a
pepper will not be used in the response computation.
Clearly, if the peer itself does not support the use of pepper
then a pepper will not be used in the response computation.
c. The EAP server may, in its subsequent Confirm TLV, provide a
pepper to the peer for later use. In this case, the pepper will
be substantially longer than a peer-chosen pepper, and encrypted
with a key derived from the PBKDF2 computation.
The above procedure allows for pepper updates to be initiated by
either side, e.g. based on policy. Since the pepper can be seen as a
MAC key, its lifetime should be limited.
An EAP server which is not capable of storing pepper values for each
user it is authenticating may still support the use of pepper - the
cost for this will be the extra computation time to do pepper
searches. This cost is still substantially lower than the cost for
an attacker, however, since the server already knows the underlying
OTP.
4.9. MAC calculations in EAP-POTP
4.9.1. Introduction
In protected mode, EAP-POTP uses MACs for authentication purposes as
well as to ensure the integrity of protocol sessions. This section
defines how the MACs are calculated and the rationale for the design.
4.9.2. MAC calculation
In protected mode, and when resuming a previous session, rather than
sending authenticating credentials such as one-time passwords or
shared keys directly, evidence of knowledge of the credentials is
sent. This evidence is a MAC on the hash of (certain parts of) EAP-
POTP messages exchanged so far in a session using a key K_MAC:
Nystroem Expires March 30, 2007 [Page 17]
Internet-Draft EAP-POTP September 2006
mac = MAC(K_MAC, msg_hash(msg_1, msg_2, ..., msg_n))
where
"MAC" is the negotiated MAC algorithm, "K_MAC" is a key derived as
specified in Section 4.5, and "msg_hash(msg_1, msg_2, ..., msg_n)" is
the message hash defined below of messages msg_1, msg_2, ..., msg_n.
4.9.3. Message hash algorithm
To compute a message hash for the MAC, given a sequence of EAP
messages msg_1, msg_2, ..., msg_n, the following operations shall be
carried out:
a. Re-transmissions messages are removed from the sequence of
messages.
b. The contents (i.e. starting with the EAP "Type" field and
excluding the EAP "Code", "Identifier", and "Length" fields) of
each message msg_1, msg_2, ..., msg_n is concatenated together.
c. User identifier TLVs MUST NOT be included in the hash (this is to
allow for a back-end service that does not know about individual
user names), i.e. any such TLV is removed from the message which
it appeared in.
d. The resulting string is hashed using the negotiated hash
algorithm.
4.9.4. Design rationale
The reason for excluding the "Identifier" field is that the actual,
transmitted, "Identifier" field is not always known to the EAP method
layer. The reason for excluding the "Length" field is to allow the
possibility for an intermediary to remove or replace a Username TLV
(e.g. for anonymity or service reasons) before passing a received
response on to an authentication server. While this on the surface
may appear as bad security practice, it may in practice only result
in denial of service, something which always may be achieved by an
attacker able to modify messages in transit. By excluding the "Code"
field, the hash is simply calculated on applicable sent and received
message contents. Excluding the "Code" field is regarded as harmless
since the hash is to be made on the set of POTP-X messages, all
having the same (known) Code value.
4.9.5. Implementation considerations
To save on storage space, each EAP entity may partially hash messages
Nystroem Expires March 30, 2007 [Page 18]
Internet-Draft EAP-POTP September 2006
as they are sent and received (e.g. HashInit(); HashUpdate(message
1); ...; HashUpdate(message n-1); HashFinal(message n)). This
reduces the amount of state needed for this purpose to the internal
state required for the negotiated hash algorithm.
4.10. EAP-POTP packet format
A summary of the EAP-POTP 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 | Reserved | TLV-based EAP-POTP message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 - Request
2 - Response
Identifier
The Identifier field is one octet and aids in matching responses
with requests. For a more detailed description of this field, and
how to use it, see [1].
Length
The Length field is two octets and indicates the length of the EAP
packet including the Code, Identifier, Length, Type, Version,
Flags, and TLV-based EAP-POTP message fields.
Type
Identifies use of a particular OTP algorithm with EAP-POTP.
Reserved
This octet is reserved for future use. It SHALL be set to zero
for this version. Recipients SHALL ignore this octet for this
version of EAP-POTP.
TLV-based EAP-POTP message
Nystroem Expires March 30, 2007 [Page 19]
Internet-Draft EAP-POTP September 2006
This field will contain 0, 1, or more Type-Length-Value triplets
defined as follows (this is similar to the EAP-TLV TLVs defined in
PEAPv2 [17], and the explanation of the generic fields is borrowed
from that document).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
1 - Mandatory TLV
The TLVs within EAP POTP-X are used to carry parameters between
the EAP peer and the EAP server. An EAP peer may not
necessarily implement all the TLVs supported by an EAP server,
and to allow for interoperability, a special TLV allows an EAP
server to discover if a TLV is supported by the EAP peer.
The mandatory bit in a TLV indicates that if the peer or server
does not support the TLV, it MUST send a NAK TLV in response;
and all the other TLVs in the message MUST be ignored. If an
EAP peer or server finds an unsupported TLV which is marked as
non-mandatory (i.e. optional), it MUST NOT send a NAK TLV on
this ground only.
The mandatory bit does not imply that the peer or server is
required to understand the contents of the TLV. The
appropriate response to a supported TLV with content that is
not understood is defined by the specification of the
particular TLV.
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this
version of the EAP-POTP.
TLV Type
The following TLV types are defined for use with EAP-POTP:
Nystroem Expires March 30, 2007 [Page 20]
Internet-Draft EAP-POTP September 2006
0 - Reserved for future use
1 - Version
2 - Server-Info
3 - OTP
4 - NAK
5 - New PIN
6 - Confirm
7 - Vendor-Specific
8 - Resume
9 - User Identifier
10 - Token Key Identifier
11 - Time Stamp
12 - Counter
13 - Keep-Alive
14 - Protected
15 - Crypto Algorithm
16 - Challenge
These TLVs are defined in the following. With the exception of
the NAK TLV, a particular TLV type MUST NOT appear more than
once in a message of type POTP-X.
Length
The length of the Value field in octets.
Value
The value of the TLV.
4.11. EAP-POTP TLV objects
4.11.1. Version TLV
The Version TLV carries information about the supported EAP-POTP
method version.
This TLV MUST be present in the initial EAP-Request of type POTP-X
from the EAP server and in the initial response of type POTP-X from
the peer. It MUST NOT be present in any subsequent EAP-Request or
EAP-Response in the session. The Version TLV MUST be supported by
all peers and all EAP servers conforming to this specification and
MUST NOT be responded to with a NAK TLV. The version negotiation
procedure is described in detail in Section 4.2
Nystroem Expires March 30, 2007 [Page 21]
Internet-Draft EAP-POTP September 2006
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Highest | Lowest |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
1
Length
3 in EAP-Requests, 2 in EAP-Responses
Reserved
Reserved for future use. This octet MUST be set to zero for this
version. Recipients SHALL ignore this octet for this version of
EAP-POTP.
Highest
This field SHALL be interpreted as an unsigned integer in network
byte order representing the highest protocol version supported by
the sender. If a value provided by a peer to an EAP server falls
between the server's "Highest" and "Lowest" supported version
(inclusive) then that value will be the negotiated version for the
authentication session.
Lowest
This field SHALL be interpreted as an unsigned integer in network
byte order representing the lowest version acceptable by the EAP
server. The field MUST be present in an EAP-Request. The field
MUST NOT be present in an EAP-Response. A peer SHALL respond to
an EAP-Request of type POTP-X with an EAP-Response of type Nak (3)
Nystroem Expires March 30, 2007 [Page 22]
Internet-Draft EAP-POTP September 2006
if the peer's highest supported version is lower than the value of
this field.
This document defines version 1 of the protocol. EAP server
implementations conforming to this document SHALL therefore set the
Highest field to 1. Peer implementations conforming to this document
SHALL set the Highest field to 1.
4.11.2. Server-Info TLV
The Server-Info TLV carries information about the EAP server and the
session (when applicable). It provides one piece in the framework
for fast session resumption.
This TLV SHOULD always be present in an EAP-Request of type POTP-X
that also carries an OTP TLV, as long as the peer has not been
authenticated, and MUST be present in such a request if the server
supports session resumption. It MUST NOT be present in any other
EAP-Request of type POTP-X or in any EAP-Response packets. This TLV
type MUST be supported by all peers conforming to this specification
and MUST NOT be responded to with a NAK TLV (this is not to say that
all peers need to support session resumption, only that they cannot
respond to this TLV with a NAK TLV).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |N| Session Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session Identifier (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sess.Id (cont.)| Nonce ... (16 octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Server Identifier ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
Nystroem Expires March 30, 2007 [Page 23]
Internet-Draft EAP-POTP September 2006
TLV Type
2
Length
>25
Reserved
Reserved for future use. All 7 bits MUST be set to zero for this
version. Recipients SHALL ignore this bit for this version of
EAP-POTP.
N
The N bit signals that the peer MUST NOT attempt to resume any
session it has stored associated with this server.
Session Identifier
An eight-octet identifier for the session about to be negotiated.
Note that, in the case of session resumption, this session
identifier will not be used (the session identifier for the
resumed session will continue to be used).
Nonce
A sixteen-octet nonce chosen by the server. During session
resumption, this nonce is used when calculating new K_ENC, K_MAC,
SRK, MSK, and EMSK keys as specified below.
Server Identifier
An identifier for the authentication server. The peer MAY use
this identifier to search for a stored session associated with
this server, or to associate the session to be negotiated with the
server. The value of the identifier SHOULD be chosen so as to
reduce the risk of collisions with other EAP server identifiers as
much as possible. One possibility is to use the DNS name of the
EAP server. The identifier MAY also be used by the peer to select
a suitable key on the OTP token (when there are multiple keys
available).
The identifier MUST NOT be longer than 128 octets. The identifier
SHALL be a UTF-8 [7] encoded string of printable characters
(without any terminating NULL character).
Nystroem Expires March 30, 2007 [Page 24]
Internet-Draft EAP-POTP September 2006
4.11.3. OTP TLV
In an EAP-Request, the OTP TLV is used to request an OTP (or a value
derived from an OTP) from the peer. In an EAP-Response, the OTP TLV
carries an OTP or a value derived from an OTP.
This TLV type MUST be supported by all peers and all EAP servers
conforming to this specification and MUST NOT be responded to with a
NAK TLV. The OTP TLV MUST NOT be present in an EAP-Request of type
POTP-X which contains a New PIN TLV. Further, the OTP TLV MUST NOT
be present in an EAP-Response of type POTP-X unless the preceding
EAP-Request of type POTP-X contained an OTP TLV and it was valid for
it to do so. Finally, an OTP TLV MUST NOT be present in an EAP-
Response of type POTP-X that also contains a Resume TLV. The OTP TLV
is defined as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |A|P|C|N|T|E|R| Pepper Length |Iteration Count|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Iteration Count (cont.) | Auth. Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication Data (cont.) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
3
Length
7 + length of Authentication Data field
Reserved
Nystroem Expires March 30, 2007 [Page 25]
Internet-Draft EAP-POTP September 2006
Reserved for future use. All nine bits SHALL be set to zero (0)
for this version. Recipients SHALL ignore these bits for this
version of EAP-POTP.
A
The A bit MUST be set in an EAP-Request if and only if the request
immediately follows an EAP-Response of type POTP-X containing a
New PIN TLV (see Section 4.11.5), and the new PIN in the response
was accepted by the EAP server. In this case, the A bit signals
that the EAP-server has accepted the PIN, and that the peer SHALL
use the newly established PIN when calculating the response (when
applicable). The A bit MUST NOT be set if the R bit is set. A
peer SHALL regard a request where both the R bit and the A bit is
set as invalid and return an empty POTP-X EAP-Response message.
In an EAP-Response, the A bit, when set, indicates that the OTP
was calculated with the use of the newly selected user PIN. The A
bit MUST be set in a response if and only if the EAP-Request which
triggered the response contained an OTP TLV with the A bit set.
P
In an EAP-Request, the P bit indicates that the OTP in the
response MUST be protected. Use of this bit also indicates that
mutual authentication will take place as well as generation of
keying material. It is RECOMMENDED to always set the P bit. If a
peer receives an EAP-Request with an OTP TLV that does not have
the P bit set, and the peer's policy dictates protected mode, the
peer MUST respond with an empty POTP-X EAP-Response message. All
peers MUST support protected mode.
In an EAP-Response, this bit indicates that the provided OTP has
been protected (see below). The P bit MUST be set in a response
(and hence the OTP MUST be protected) if and only if the EAP-
Request which triggered the response contained an OTP TLV with the
P bit set.
In an 802.1x EAP over LAN (EAPOL) environment (this includes
wireless LAN environments), the P bit MUST be set, or,
alternatively, the EAP-POTP method MUST be carried out inside an
authenticated tunnel that provides a cryptographic binding with
inner EAP methods such as the one provided by PEAPv2 ([17]).
C
Nystroem Expires March 30, 2007 [Page 26]
Internet-Draft EAP-POTP September 2006
The C bit carries meaning only when the OTP algorithm in question
makes use of server challenges. For other OTP algorithms, the C
bit SHALL always be set to zero.
In an EAP-Request, the C bit ("Combine") indicates that the OTP
SHALL be calculated using both the provided challenge and internal
state (e.g. current token time). The OTP SHALL be calculated
based only on the provided challenge (and the shared secret) if
the C bit is not set and a challenge is present. The returned OTP
SHALL always be calculated based on the peer's current state (and
the shared secret) if no challenge is present. If the C bit is
set but no challenge is provided, the peer SHALL regard the
request as invalid, and return an empty POTP-X EAP-Response
message.
In an EAP response, this bit indicates that the provided OTP has
been calculated using a provided challenge and the token state.
The C bit MUST be set in a response if and only if the EAP-Request
which triggered the response contained an OTP TLV with the C bit
set and a challenge.
N
In an EAP-Request, the N bit, when set, indicates that the OTP to
calculate SHALL be based on the next token "state", and not the
current one. As an example, for a time-based token, this means
the next time slot. For an event-based token, this could mean the
next counter value, if counter values are used. This bit will
normally not be set in initial EAP-Request messages, but may be
set in subsequent ones. Further, the N bit carries no meaning in
an EAP-Request if a challenge is present and the C bit is not set,
and SHALL be set to 0 in this case. A peer SHALL regard a request
containing a challenge and where the N bit is set but not the C
bit as invalid, and return an empty POTP-X EAP-Response message.
Note that setting the N bit in an EAP-Request will normally
advance the internal state of the token.
In an EAP-Response, the N bit, when set, indicates that the OTP
was calculated based on the next token "state" (as explained
above), and not the current one. The N bit MUST be set in a
response if and only if the EAP-Request which triggered the
response contained an OTP TLV with the N bit set.
T
The T bit only carries meaning for OTP methods normally
incorporating a user PIN in the OTP computation.
Nystroem Expires March 30, 2007 [Page 27]
Internet-Draft EAP-POTP September 2006
In an EAP-Request, the T bit, when set, indicates that the OTP to
calculate MUST NOT include a user PIN.
In an EAP-Response, the T bit, when set, indicates that the OTP
was calculated without the use of a user PIN. The T bit MUST be
set in a response if and only if the EAP-Request which triggered
the response contained an OTP TLV with the T bit set. Note that
client policy may prohibit PIN-less calculations and in these
cases the client MAY respond with an empty POTP-X EAP response
message.
E
In an EAP-Request, the E bit, when set, indicates that the peer
MUST NOT use any stored pepper value associated with this server
in the PBKDF2 computation. Rather, it MUST generate a new pepper
(if supported by the peer) and/or use the iteration count
parameter to protect the OTP (if the server's Max Pepper Length is
0, then the peer MUST rely on the iteration count only to protect
the OTP). This bit will usually not be set in initial EAP-Request
messages, but may be set in subsequent ones, e.g. if the server
upon receipt of an OTP TLV with a pepper identifier detects that
it does not have a pepper with that identifier in storage. This
bit carries no meaning, and MUST be set to zero, when the P bit is
not set. A peer SHALL regard a request where the E bit is set but
not the P bit as invalid, and return an empty POTP-X EAP-Response
message.
In an EAP-Response, the E bit indicates that the response has been
calculated without use of any stored pepper value.
R
In an EAP-Request, the R bit ("Repeat"), when set, indicates that
the peer SHOULD calculate its response based on the same OTP value
as was used for the preceding response. This bit MAY be set when
the EAP server has received an OTP TLV from the peer protected
with a pepper which the server no longer is in possession of.
Since the server has not attempted validation of the provided
data, there is no need for the EAP peer to retrieve a new OTP
value. This bit carries no meaning, and MUST be set to zero, when
the E bit is not set. A peer SHALL regard a request where the R
bit is set but not the E bit as invalid, and return an empty
POTP-X EAP-Response message. Further, the R bit MUST NOT be set
when the A bit also is set, see above.
In an EAP-Response, the R bit is never set.
Nystroem Expires March 30, 2007 [Page 28]
Internet-Draft EAP-POTP September 2006
Pepper Length
This octet SHALL be present if and only if the P bit is set. When
present, it SHALL be interpreted as an unsigned integer in
network-byte order, having a value between 0...255 (inclusive).
In an EAP-Request, the integer represents the maximum length (in
bits) of a client-generated pepper the server is prepared to
search for. Peers MUST NOT generate peppers longer than this
value. If the value is set to zero, it means the peer MUST NOT
generate a pepper for the PBKDF2 calculation. In an EAP-Response,
it indicates the length of the used pepper.
Iteration Count
These four octets SHALL be present if and only if the P bit is
set. When present, they SHALL be interpreted as an unsigned,
four-octet integer in network-byte order. In an EAP-Request, the
integer represents the maximum iteration count the peer may use in
the PBKDF2 computation. Peers MUST NOT use iteration counts
higher than this value. In an EAP-Response, it indicates the
actual iteration count used.
Note regarding the Pepper Length and Iteration Count parameters: A
peer MUST compare these policy parameters provided by the EAP server
with local policy and MUST NOT continue the handshake if use of the
EAP server's suggested parameters would result in a lower security
than the client's acceptable policy. If the security given by the
EAP server's provided policy parameters surpasses the security level
given by the peer's local policy the client SHOULD use the server's
parameters (subject to reason - active attackers could otherwise
mount simple denial-of-service attacks against peers or servers, e.g.
by providing unreasonably high values for the iteration count). Note
that the server-provided parameters only applies to the case where
the peer cannot use or does not have a previously provided server-
provided pepper. If a peer cannot continue the handshake due to the
server's policy being unacceptable, it MUST return an empty POTP-X
EAP-Response message.
Authentication Data
EAP-Request: In an EAP-Request, the Authentication Data, when
present, contains an optional "challenge". The challenge is an
optional octet string that SHOULD be uniquely generated for each
request it is present in (i.e. it is a "nonce"), and SHOULD be
eight octets or longer when present. To avoid fragmentation (i.e.
EAP messages longer than the minimum EAP MTU size, see [1]), the
challenge MUST NOT be longer than 64 octets. When the challenge
is not present, the OTP will be calculated on the current token
Nystroem Expires March 30, 2007 [Page 29]
Internet-Draft EAP-POTP September 2006
state only. The peer MAY ignore a provided challenge if and only
if the OTP token the peer is interacting with is not capable of
including a challenge in the OTP calculation. In this case, EAP
server policies will determine whether to accept a provided OTP
value or not.
EAP-Response: The following applies to the Authentication Data field
in an EAP-Response:
* When the P bit is not set, the peer SHALL directly place the
OTP value calculated by the token in the Authentication Data
field. In this case, the EAP server MUST NOT send a Confirm
TLV upon successful authentication of the peer (instead, it
sends an EAP-Success message).
* When the P bit is set, the peer SHALL populate this field as
follows. After the token has calculated the OTP value, the
peer SHALL compute:
K_MAC | K_ENC | MSK | EMSK | SRK = PBKDF2(otp, salt | pepper
| auth_id, iteration_count, key_length)
where
"|" denotes concatenation,
"otp" is the already computed OTP value,
"salt" is a sixteen-octet nonce,
"pepper" is an optional nonce (at most 255 bits long, and if
necessary padded to be a multiple of 8 bits long, see below)
included to complicate the task of finding a matching "otp"
value for an attacker,
"auth_id" is an identifier (at most 255 octets in length)
for the authenticator (i.e. the network access server) as
reported by lower layers and as specified below,
"iteration_count" is an iteration count chosen such that the
computation time on the peer is acceptable (based on the
server's indicated policy and the peer's local policy),
while an attacker, having observed the response and
initiating a search for a matching OTP will be sufficiently
slowed down. The "iteration_count" value MUST be chosen to
provide a suitable level of protection (e.g. at least
100,000) unless a server-provided pepper is being used, in
which case it SHOULD be 1.
Nystroem Expires March 30, 2007 [Page 30]
Internet-Draft EAP-POTP September 2006
"key_length" is the combined length of the desired key
material, in octets. When default algorithms are used,
key_length SHALL be 176.
The "pepper" values are only included in PBKDF2 calculations
and are never sent to EAP servers (though the peers do send
their length, in bits). The purpose of the pepper values
are, as mentioned above, to slow down an attacker's search
for a matching OTP, while not slowing down the peer (which
iterated hashes do). If the pepper has been generated by
the peer and the chosen pepper length in bits is not a
multiple of 8 then the pepper value SHALL be padded to the
left with '0' bits to the nearest multiple of 8 before being
used in the PBKDF2 calculation. This is to ensure the input
to the calculation consists only of whole octets. As an
example, if the chosen pepper length is four, the pepper
value will be padded to the left with four '0' bits to form
an octet before being used in the PBKDF2 calculation.
When pepper is used, it is RECOMMENDED that the length of
the pepper and the iteration count are chosen in such a way
that it is computationally infeasible/unattractive for an
attacker to brute-force search for the given OTP within
lifetime of that OTP.
As mentioned previously, a peer MUST NOT include a newly
generated pepper value in the PBKDF2 computation if the
server did not indicate its support for pepper searching in
this session. If the server did not indicate support for
pepper searching, then the PBKDF2 computation MUST be
carried out with a sufficiently higher number of iterations
so as to compensate for the lack of pepper (see further
Appendix D).
A server may, in an earlier session, have transferred a
pepper value to the peer in a Confirm TLV (see below). When
this is the case, and the peer still has that pepper value
stored for this server, the peer MUST NOT generate a new
pepper but MUST instead use this transferred pepper value in
the PBKDF2 calculations. The only exception to this is when
a local policy (e.g. timer) dictates that the peer must
switch to a new pepper (and the server indicated support for
pepper searching).
The following applies to the auth_id component:
- For dial-up, "auth_id" SHALL either be the empty string
or the phone number called by the peer. The phone number
Nystroem Expires March 30, 2007 [Page 31]
Internet-Draft EAP-POTP September 2006
SHALL be specified in the form of a URL conformant with
RFC 2806 ([8]), e.g. "tel:+16175550101". Processing of
received phone numbers SHALL be conformant with RFC 2806
(this assumes that "tel" URIs will be shorter than 256
octets, which would normally be the case).
- For use with IEEE 802.1X, "auth_id" SHALL either be the
empty string or the MAC address of the authenticator in
binary format (six octets).
- For IP-based EAP, "auth_id" SHALL either be the empty
string or the IPv4 or IPv6 address of the authenticator
as seen by the peer and in binary format (4 respectively
16 octets). As an example, the IPv4 address "192.0.2.5"
would be represented as (in hex) C0 00 02 05, whereas the
IPv6 address "2001:DB8::101" would be represented as (in
hex) 20 01 0D B8 00 00 00 00 00 00 00 00 00 00 01 01.
Note: Use of the authenticator's identifying information
within the computation aids in protection against man-in-
the-middle attacks where a rogue authenticator seeks to
intercept and forward the Authentication Data in order to
impersonate the peer at a legitimate authenticator (but see
also the discussion around spoofed authenticator addresses
in Section 6). For these reasons, a peer SHOULD NOT set the
auth_id component to the empty string unless it is unable to
learn the identifying information of the authenticator. In
these cases, the EAP server's policy will determine whether
the session may continue or not.
As an example, when otp = "12345678", salt =
0x54434534543445435465768789099880, pepper is not used,
auth_id = "192.0.2.5", iteration_count = 2000, and
key_length = 176, the input to the PBKDF2 calculation will
be (first two parameters in hex, line wrap for readability):
(3132333435363738, 54434534543445435465768789099880 |
c0000205, 2000, 176)
As described, when the default algorithms are used, K_MAC is
the first 16 octets of the output from PBKDF2, K_ENC the
next 16 octets, MSK the following 64 octets, EMSK the next
64 octets, and SRK the final 16 octets. Using K_MAC, the
peer calculates:
mac = MAC(K_MAC, msg_hash(msg_1, msg_2, ..., msg_n))
Nystroem Expires March 30, 2007 [Page 32]
Internet-Draft EAP-POTP September 2006
as specified in Section 4.9 and where msg_1, msg_2, ...,
msg_n is a sequence of all EAP messages of type POTP-X
exchanged so far in this session, as sent and received by
the peer (for the peer's initial MAC it will typically be
just one message, the EAP server's initial EAP-Request of
type POTP-X).
The peer then places the first 16 octets of "mac" in the
Authentication Data field, followed by the "salt" value,
followed by one octet representing the length of the
"auth_id" value in octets, followed by the actual "auth_id"
value in binary form, optionally followed by a pepper
identifier (only when the peer made use of a pepper value
previously provided by the EAP server). Pepper identifiers,
when present, are always four octets. All variables SHALL
be present in the form they were input to the PBKDF2
algorithm. This will result in the Authentication Data
field being 33 + (length of auth_id in octets) + (4, for
pepper identifier, when present) octets in length.
Continuing the previous example, the Authentication Data
field will be populated with (in hex, line wrap for
readability):
< 16 octets of mac > | 54434534543445435465768789099880 | 04
| c0000205
Note: Since in this case (i.e. when the P bit is set)
successful authentication of the peer by the EAP server will
be followed by the transmission of an EAP-Request of type
POTP-X containing a Confirm TLV for mutual authentication,
the peer MUST save either all the input parameters to the
PBKDF2 computation or the keys K_MAC, K_ENC, SRK, MSK, and
EMSK (recommended, since they will be used later). This is
because the peer cannot be guaranteed to be able to generate
the same OTP value again. For the same reason (the Confirm-
TLV from the EAP server), the peer MUST also store either
the hash of the contents of the sent EAP-Response or the
EAP-Response itself (but see the note above about not
including any User Identifier TLVs in the hash computation).
Given a set of possible OTP values, the authentication
server verifies an authentication request from the peer by
computing
K_MAC' | K_ENC' | MSK' | EMSK' | SRK' = PBKDF2(otp', salt |
pepper' | auth_id, iteration_count, 176)
Nystroem Expires March 30, 2007 [Page 33]
Internet-Draft EAP-POTP September 2006
for each possible OTP value otp', and each possible pepper
value pepper' and the provided values for salt,
authenticator identity, and iteration count. Note that the
EAP server may accept more than one OTP value at a given
time, e.g. due to clock drift in the token. If the given
pepper length is not a multiple of eight, each tested pepper
value will be padded to the left to the nearest multiple of
eight, in the same manner as was done by the peer. If the
server already shares a secret pepper value with this peer
then obviously there will only be one possible pepper value,
and the server will find it based on the pepper_identifier
provided by the peer. The server SHALL send a new EAP-
Request of type POTP-X with an OTP TLV with the E bit set if
the peer provided a pepper identifier unknown to the server.
For each K_MAC', the EAP server computes
mac' = MAC(K_MAC', msg_hash(msg_1', msg_2', ..., msg_n'))
where MAC is the negotiated MAC algorithm, msg_hash is the
message hash algorithm defined in Section 4.9, and msg_1',
msg_2', ... msg_n' are the same messages as the peer
calculated its message hash on, but this time as sent and
received by the EAP server. If the first 16 octets of mac'
matches the first 16 octets in the Authentication Data field
of the EAP-Response in question, and the provided
authenticator identity is acceptable (e.g. matches the EAP
server's view of the authenticator's identity) then the peer
is authenticated.
If the authentication was successful, the authentication
server then attempts to authenticate itself to the peer by
use of the Confirm TLV (see below). If the authentication
fails, the EAP server MAY send another EAP-Request of type
POTP-X containing an OTP TLV to the peer, or it MAY send an
EAP-Failure message (in both cases possibly preceded by an
EAP-Request of type Notification).
4.11.4. NAK TLV
Presence of this TLV indicates that the peer did not support a
received TLV with the M bit set. This TLV may occur 0, 1, or more
times in an EAP-Response of type POTP-X. Each occurrence flags the
non-support of a particular received TLV.
The NAK TLV MUST be supported by all peers and all EAP servers
conforming to this specification and MUST NOT be responded to with a
NAK TLV. Receipt of a NAK TLV by an EAP server MAY cause an
Nystroem Expires March 30, 2007 [Page 34]
Internet-Draft EAP-POTP September 2006
authentication to fail, and the EAP server to send an EAP-Failure
message to the peer.
Note: The definition of the NAK TLV herein matches the definition
made in [17], and has the same type number. Field descriptions are
copied from that document, with some minor modifications.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NAK-Type | TLVs ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
4
Length
>= 6
Vendor-Id
The Vendor-Id field is four octets, and contains the Vendor-Id of
the TLV that was not supported. The high-order octet is 0 and the
low-order 3 octets are the SMI Network Management Private
Enterprise Code of the Vendor in network byte order. The
Vendor-Id field MUST be zero for TLVs that are not Vendor-Specific
TLVs. For Vendor-Specific TLVs, the Vendor-ID MUST be set to the
SMI code.
NAK-Type
Nystroem Expires March 30, 2007 [Page 35]
Internet-Draft EAP-POTP September 2006
The type of the unsupported TLV. The TLV MUST have been included
in the most recently received EAP message.
TLVs
This field contains a list of TLVs, each of which MUST NOT have
the mandatory bit set. These optional TLVs can be used in the
future to communicate why the offending TLV was determined to be
unsupported.
4.11.5. New PIN TLV
In an EAP-Request, the New PIN TLV is used to request a new user PIN
from the peer. The EAP server MAY provide a new PIN as described
below. In an EAP-Response, the New PIN TLV carries a chosen new user
PIN. This TLV may be used by an EAP server when policy dictates that
the peer (user) needs to change a PIN associated with the OTP Token.
This TLV type SHOULD be supported by peers and EAP servers conforming
to this specification. The New PIN TLV MUST NOT be sent by an EAP
server unless the peer has been authenticated. If the peer was
authenticated in protected mode, then the New PIN TLV MUST NOT be
present in an EAP-Request until after the exchange of the Confirm TLV
(i.e. until after mutual authentication has occurred and keys are in
place to protect the TLV). The New PIN TLV MUST be sent by a peer if
and only if the EAP-Request which triggered the response contained a
New PIN TLV, it was valid for the EAP server to send such a TLV in
that request, and the TLV is supported by the peer.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Q|A| PIN Length | PIN ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Min. PIN Length|Max. PIN Length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
Nystroem Expires March 30, 2007 [Page 36]
Internet-Draft EAP-POTP September 2006
TLV Type
5
Length
>=2 (the PIN may or may not be present.)
Reserved
Reserved for future use. All six bits SHALL be set to zero for
this version. Recipients SHALL ignore these bits for this version
of EAP-POTP.
Q
The Q bit, when set in an EAP-Request, indicates that an
accompanying PIN is required, i.e. the peer (user) is not free to
choose another PIN. When the Q bit is set, there MUST be an
accompanying PIN and the provided PIN MUST be used in subsequent
OTP generations. A peer SHALL respond with an empty POTP-X EAP-
Response message if the Q bit is set but there is not any
accompanying PIN. When the Q bit is not set, any provided PIN is
suggested only, and the peer is free to choose another PIN,
subject to local policy.
The Q bit carries no meaning, and SHALL be set to zero, in an EAP-
Response.
A
This bit allows methods that distinguish between two different PIN
types (e.g., decimal vs. alphanumeric) to designate whether the
augmented set is to be used (when set) or not (when not set). The
A bit carries no meaning, and SHALL be set to zero, in an EAP-
Response.
PIN Length
This field SHALL be interpreted as an unsigned integer in network
byte order representing the length of the provided PIN (this
implies that the maximum length of a PIN will be 255 octets).
PIN
In an EAP-Request, subject to the setting of the Q bit, the PIN
field MAY be empty. If empty, the peer (user) will need to choose
a PIN subject to local and (any) provided policy. When the PIN
Nystroem Expires March 30, 2007 [Page 37]
Internet-Draft EAP-POTP September 2006
field is not empty, it MUST consist of UTF-8 encoded printable
characters without a terminating NULL character.
In an EAP-Response, the PIN value SHALL consist of a UTF-8 encoded
string of printable characters without a terminating NULL
character.
The peer accepts a PIN suggested by the EAP server by replying
with the same PIN, but MAY replace it with another one, depending
on the server's setting of the Q bit. The length of the PIN is
application-dependent as are any other requirements for the PIN,
e.g., allowed characters. The peer MUST be prepared to receive a
repeated request for a new PIN as described above if the EAP
server for some reason does not accept the received PIN. Such a
request MAY be preceded by an EAP-Request of type Notification (2)
providing information to the user about the reason for the
rejection. Mechanisms for transferring knowledge about PIN
requirements from the EAP server to the peer (beyond those
specified for this TLV, such as maximal and minimal PIN length)
are outside the scope of this document. However, some information
MAY be provided in notification messages transferred from the EAP
server to the peer as per above.
Min. PIN Length
This field MAY be present in an EAP-Request. This field MUST NOT
be present in an EAP-Response. It SHALL be interpreted as an
unsigned integer in network byte order representing the minimum
length allowed for a new PIN.
Max. PIN Length
This field MUST NOT be present in an EAP-Request unless the Min.
PIN Length field is present, in which case it MAY be present. The
field MUST NOT be present in an EAP-Response. It SHALL be
interpreted as an unsigned integer in network byte order
representing the maximum length allowed for a new PIN. The value
of this field, when present, MUST be equal to, or larger, than the
value of the Min. PIN Length field.
4.11.6. Confirm TLV
Presence of this TLV in a request indicates that the EAP server has
successfully authenticated the peer and now attempts to authenticate
itself to the peer. Presence of this TLV in a response indicates
that the peer successfully authenticated the EAP server, and that
calculated keys (K_MAC, K_ENC, MSK, EMSK, and SRK) now become
available for use.
Nystroem Expires March 30, 2007 [Page 38]
Internet-Draft EAP-POTP September 2006
The Confirm TLV MUST NOT appear together with any other TLV in an
EAP-Request message of type POTP-X and MUST NOT be sent unless the
peer has been authenticated through an OTP TLV with the P bit set or
through a Resume TLV for which the underlying session was established
in protected mode. The Confirm TLV MUST be present in an EAP-
Response if and only if the request that triggered the response
contained a Confirm TLV, it was legal for it to do so, and the
Confirm TLV authenticated the EAP server to the peer. If the peer
was not able to authenticate the server, then it MUST send an empty
(i.e. no TLVs present) EAP-Response of type POTP-X.
An EAP server MUST send an EAP-Success message after receiving an
EAP-Response of type POTP-X containing a valid Confirm TLV, sent in
response to an EAP-Request containing a Confirm TLV where the C bit
was not set. A peer MUST NOT accept an EAP-Success message when it
has sent an OTP TLV with the P bit set unless it has received an
acceptable Confirm TLV from the EAP server.
This TLV type MUST be supported by all peers and EAP servers
conforming to this specification and MUST NOT be responded to with a
NAK TLV.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |C| Authentication Data ... (16 octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pepper Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IV ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encrypted Pepper ... (16 octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
Nystroem Expires March 30, 2007 [Page 39]
Internet-Draft EAP-POTP September 2006
6
Length
17 or 37 + length of IV in requests, 1 in responses.
Reserved
Reserved for future use. These seven bits SHALL be set to zero
(0) for this version. Recipients SHALL ignore these bits for this
version of EAP-POTP.
C
The C bit, when set in an EAP-Request, indicates that the EAP
server intends to send more EAP-Requests of type POTP-X in this
session, after receipt of a Confirm TLV from the peer.
The C bit carries no meaning in EAP-Responses, and MUST NOT be set
within them.
Note: An EAP-Response containing a Confirm TLV, sent in response
to an EAP-Request containing a Confirm TLV that did not have the C
bit set, MUST be followed by an EAP-Success message from the EAP
server concluding the handshake. The response MAY however be
followed by another EAP-Request from the EAP server, containing
e.g. a New PIN TLV (wrapped in a Protected TLV), if the C bit was
set in the EAP-Request. Therefore, peers MUST NOT assume that the
only EAP message following an EAP-Response of type POTP-X
containing a Confirm TLV is EAP-Success. The C bit gives EAP
servers a way to indicate their intent to follow the Confirm TLV
with more requests, and allows the peer's state machine to adapt
to this.
Authentication Data
EAP-Request:
In a request, this field consists of the first 16 octets of
(see also Section 4.11.3):
mac_a = MAC(K_MAC', msg_hash(trig_msg))
where
Nystroem Expires March 30, 2007 [Page 40]
Internet-Draft EAP-POTP September 2006
MAC is the negotiated MAC algorithm,
"K_MAC'" has been calculated as described in Section 4.11.3 or
(in the case of session resumption) Section 4.11.8, and
"msg_hash" is the message hash algorithm defined in
Section 4.9, and "trig_msg" the latest EAP-Response of type
POTP-X received from the peer (the one which triggered this
request).
Given a saved or recomputed value for K_MAC, the peer
authenticates the EAP server by computing
mac'' = MAC(K_MAC, msg_hash(trig_msg'))
where "msg_hash(trig_msg')" is the peer's hash of the same EAP-
Response as the EAP server calculated its message hash on, but
this time as it was sent by the peer. If the first 16 octets
of mac'' matches the first 16 octets in the Authentication Data
field of the EAP-Request in question, then the EAP server is
authenticated.
EAP-Response:
Not used in this version, and SHALL NOT be present in EAP-
Responses.
Pepper Identifier
In an EAP-Request, the truncated MAC MAY optionally be followed by
an encrypted pepper and its identifier. This initial, four-octet
field identifies a pepper generated by the server.
For this version of EAP-POTP, this field SHALL NOT be present in
EAP-Responses.
IV (Initialization Vector)
An initialization vector for the encryption. The length of the
vector is dependent on the negotiated encryption algorithm. E.g.
for AES-CBC it SHALL be 16 octets. The IV is only present if a
pepper is present and the negotiated encryption algorithm makes
use of IVs. This field SHALL NOT be present in EAP-Response
messages for this version of EAP-POTP.
Encrypted Pepper
Nystroem Expires March 30, 2007 [Page 41]
Internet-Draft EAP-POTP September 2006
When present in an EAP-Request, this will be a uniformly
distributed and randomly chosen sixteen-octet pepper generated by
the EAP server and encrypted with the negotiated encryption
algorithm, using K_ENC as the encryption key and possibly
(depending on the encryption algorithm) using an IV (stored in the
IV field). This field MUST be present if and only if the Pepper
Identifier field is present.
EAP servers are RECOMMENDED to include a freshly generated
encrypted pepper (and a corresponding Pepper Identifier) in every
Confirm TLV.
This field SHALL NOT be present in EAP-Response messages for this
version of EAP-POTP.
When a new pepper was generated by the server and transferred in
encrypted form to the peer, then this new pepper value will be stored
in the EAP server upon receipt of the Confirm TLV from the peer, and
SHOULD be stored with its identifier and associated with the EAP
server and the current user in the peer upon receipt of the EAP-
Success message. If the peer already had a pepper stored for the EAP
server it SHALL replace it with the newly received one.
4.11.7. Vendor-Specific TLV
The Vendor-Specific TLV is available to allow vendors to support
their own extended attributes not suitable for general usage. A
Vendor-Specific-TLV can contain one or more inner TLVs, referred to
as Vendor TLVs. The TLV-type of a Vendor-TLV will be defined by the
vendor. All the Vendor TLVs inside a single Vendor-Specific TLV
SHALL belong to the same vendor.
This TLV type MAY be sent by EAP servers as well as by peers and MUST
be supported by all entities conforming to this specification.
Conforming implementations may not support specific Vendor TLVs
inside a Vendor-Specific TLV however, and MAY in this case respond to
the Vendor TLVs with a NAK TLV containing the appropriate Vendor-ID
and Vendor TLV type.
The presence of a Vendor-Specific TLV in an EAP-Request or EAP-
Response of type POTP-X MUST NOT violate any existing rules for co-
existence of TLVs in such requests or responses. If it does, then it
will result in an EAP-Failure (when the peer made the violation) or
an empty EAP-POTP response (when the EAP-server made the violation).
It is left to the definition of specific Vendor-Specific TLVs to
further constrain when they are allowed to appear. In particular,
EAP-POTP implementations may have policies that completely disallow
use of the Vendor-Specific TLV before protected mode mutual
Nystroem Expires March 30, 2007 [Page 42]
Internet-Draft EAP-POTP September 2006
authentication has occurred (since the Protected TLV, Section 4.11.15
then can be used to protect all TLVs).
Note: This TLV type has the same definition and TLV type number as
the Vendor-Specific TLV in [17], and the description of it is largely
borrowed from that document.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor TLVs ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
7
Length
>4
Vendor-ID
The Vendor-Id field is four octets. The high-order octet SHALL be
set to 0 and the low-order 3 octets SHALL be set to the SMI
Network Management Private Enterprise Code (see [18]) of the
Vendor in network byte order. The Vendor-Id MUST be zero for TLVs
that are not Vendor-Specific TLVs. For Vendor-Specific TLVs, the
Vendor-ID MUST be set to the SMI code.
Vendor TLVs
Nystroem Expires March 30, 2007 [Page 43]
Internet-Draft EAP-POTP September 2006
This field shall contain vendor-specific TLVs, in a format defined
by the vendor. To avoid fragmentation (i.e. EAP messages longer
than the minimum EAP MTU size), the field SHOULD NOT be longer
than 256 octets.
To ensure interoperability when a peer from vendor A sends a vendor-
specific TLV that is not understood by the recipient, the vendor A
peer SHALL, upon receipt of the NAK TLV from the recipient, refrain
from usage of the vendor-specific TLV in question for the rest of the
handshake, and MUST NOT fail the session due to the receipt of the
NAK TLV for the Vendor TLV (i.e., the peer SHALL continue as if the
vendor-specific TLV had not been sent). Additionally, all
implementations conformant with this document SHOULD allow use of
vendor-specific extensions to be turned off via configuration.
4.11.8. Resume TLV
The Resume TLV MAY be sent by a peer to an authentication server to
attempt session resumption.
This TLV type MUST only be sent in response to an EAP-Request of type
POTP-X containing a Server-Info TLV allowing session resumption. The
Resume TLV MUST be supported by all EAP servers that send a Server-
Info TLV allowing session resumption.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Session Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session Identifier (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Sess.Id (cont.)| Authentication Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication Data (cont.) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
Nystroem Expires March 30, 2007 [Page 44]
Internet-Draft EAP-POTP September 2006
TLV Type
8
Length
45
Reserved
Reserved for future use. This octet SHALL be set to zero (0) for
this version. Recipients SHALL ignore this octet for this version
of EAP-POTP.
Session Identifier
An eight-octet identifier for the session the peer is trying to
resume.
Authentication Data
Upon receipt of the Server-Info TLV, and if the N bit is not set,
the peer searches for any stored sessions associated with the
server identified by the Server Name field. If a stored session
is found, the peer generates a random, sixteen-octet nonce,
"c_nonce", and calculates:
K_MAC | K_ENC | MSK | EMSK | SRK = PBKDF2(base_key, c_nonce |
s_nonce, iteration_count, key_length)
where
"|" denotes concatenation,
"base_key" is either the current SRK for the session (if the
session was created in protected mode) or the OTP used when the
session was created (if the session was created in basic mode),
"c_nonce" is the generated 16-octet nonce,
"s_nonce" the server nonce from the Server-Info TLV,
"iteration_count" is the iteration count as determined by local
policy, and
"key_length" is the combined length of the desired key material,
in octets. When default algorithms are being used, key_length
SHALL be 176.
Nystroem Expires March 30, 2007 [Page 45]
Internet-Draft EAP-POTP September 2006
The iteration count need only be 1 (one) when resuming a session
established in protected mode, but MUST be chosen to provide a
suitable level of protection when resuming a session established
in basic mode (see also Section 4.11.3).
Note: Session resumption for basic mode MUST only be carried out
in a server-authenticated and protected tunnel that also provides
a cryptographic binding for inner EAP methods.
The peer then calculates:
MAC = MAC(K_MAC, msg_hash(resume_req))
where
"MAC" is the negotiated MAC algorithm, and
"msg_hash(resume_req) is the message hash algorithm defined in
Section 4.9 applied on resume_req, the EAP server's EAP-Request of
type POTP-X containing the Server-Info TLV which allowed session
resumption.
The peer then places the first 16 octets of the MAC followed by
the c_nonce value followed by the iteration count value (as a
4-byte unsigned integer in network byte order) in the
Authentication Data field. As an example, when c_nonce =
0x2b3b1b12babdebebfb43bd7bdfbeb8df and iteration_count = 1, the
Authentication Data field will be populated with (in hex, line
wrap for readability):
< 16 octets of mac > | 2b3b1b12babdebebfb43bd7bdfbeb8df | 00000001
The server authenticates the peer by performing the corresponding
calculations. If the authentication is successful, the server
MUST send an EAP-Request of type POTP-X containing a Confirm TLV
to the peer. If the authentication fails, the server MUST send
either an EAP-Request of type POTP-X containing an OTP TLV and a
Server-Info TLV where the Server-Info TLV indicates that session
resumption is not possible, or send an EAP-Failure.
When resuming in basic mode, all calculated keys SHALL be
discarded after the MAC has been calculated and verified. When
resuming in protected mode, the new SRK will replace the stored
SRK, and the new MSK and EMSK will be exported upon successful
completion of the method.
Nystroem Expires March 30, 2007 [Page 46]
Internet-Draft EAP-POTP September 2006
4.11.9. User Identifier TLV
The User Identifier TLV carries an identifier, typically the
username, for the holder of the OTP token used to generate the OTP.
At least one of the User Identifier TLV and the Token Key Identifier
TLV SHOULD be present in the session's first EAP-Response of type
POTP-X that also carries an OTP TLV unless a suitable identity has
been provided in a preceding EAP-Response of type Identity (1) or is
determined by some other means (see [1], Section 2). Use of the User
Identifier TLV and/or the Token Key Identifier TLV is RECOMMENDED
even when an EAP-Response of type Identity (1) has been sent. If a
peer sends both a User Identifier TLV and a Token Key Identifier TLV
then the EAP server SHALL interpret the Token Key Identifier TLV as
specifying a particular token key for the given user. The EAP server
MUST respond with an EAP-Failure if it cannot find a token key for
the provided user.
This TLV type is sent by peers and MUST be supported by all EAP
servers conforming to this specification. The User Identifier TLV
MUST NOT be present in a response that does not also carry an OTP
TLV.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| User Identifier ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
9
Length
Nystroem Expires March 30, 2007 [Page 47]
Internet-Draft EAP-POTP September 2006
>=1
User Identifier
The value SHALL be an UTF-8 encoded string representing the holder
of the token (MUST NOT be NULL-terminated). The string MUST be
less than 128 octets in length.
4.11.10. Token Key Identifier TLV
The Token Key Identifier TLV carries an identifier for the token key
used to generate the OTP.
At least one of the User Identifier TLV and the Token Key Identifier
TLV SHOULD be present in the session's first EAP-Response of type
POTP-X which also carries the OTP TLV unless a suitable identity has
been provided in a preceding EAP-Response of type Identity (1) or is
determined by some other means (see [1], Section 2). Use of the User
Identifier TLV and/or the Token Key Identifier TLV is RECOMMENDED
even when an EAP-Response of type Identity (1) has been sent. If a
peer sends both a User Identifier TLV and a Token Key Identifier TLV
then the EAP server SHALL interpret the Token Key Identifier TLV as
specifying a particular token key for the given user. The EAP server
MUST respond with an EAP-Failure if it cannot find a token key
corresponding to the provided token key identifier.
This TLV type is sent by peers and MUST be supported by all EAP
servers conforming to this specification. The Token Key Identifier
TLV MUST NOT be present in a response that does not also carry an OTP
TLV.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Token Key Identifier ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
Nystroem Expires March 30, 2007 [Page 48]
Internet-Draft EAP-POTP September 2006
TLV Type
10
Length
>=1
Token Key Identifier
An identifier for the OTP token key used to generate the OTP. The
field MUST be less than 128 octets in length.
4.11.11. Time Stamp TLV
The Time Stamp TLV MAY be sent by peers to simplify authentications.
When present, it carries the time as reported by the OTP Token.
An EAP server conformant with this specification SHOULD support (i.e.
recognize) this TLV, but need not be able to process or act on it.
An EAP server that does not support this TLV but receives an EAP-
Response with the TLV present MAY ignore the value. The Time Stamp
TLV MUST NOT be present in any EAP-Responses of type POTP-X other
than those that also carries an OTP TLV.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time Stamp ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
11
Length
Nystroem Expires March 30, 2007 [Page 49]
Internet-Draft EAP-POTP September 2006
>= 20 (depending on precision)
Time Stamp
The time, as reported by the OTP token, at which the OTP used for
the accompanying OTP TLV was calculated. The field SHALL contain
a UTF-8 encoded value of the XML simple type "dateTime" with time
zone information and precision down to at least seconds. E.g.
"2004-06-16T15:20:02Z".
4.11.12. Counter TLV
The Counter TLV MAY be sent by peers to simplify authentications.
When present, it carries the token counter value, as reported by the
OTP Token.
An EAP server conformant with this specification SHOULD support (i.e.
recognize) this TLV, but need not be able to process or act on it.
An EAP server that does not support this TLV but receives an EAP-
Response with the TLV present MAY ignore the value. The Counter TLV
MUST NOT be present in any EAP-Responses of type POTP-X other than
those that also carries an OTP TLV.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Counter ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
12
Length
Nystroem Expires March 30, 2007 [Page 50]
Internet-Draft EAP-POTP September 2006
>= 1 (depending on precision)
Counter
The counter value, as reported by the OTP token, at which the OTP
used for the accompanying OTP TLV was calculated. The counter
value SHALL be represented as an unsigned integer in network-byte
order. E.g. a counter value of 1030 may be sent as the two octets
(in hex) 04 06.
4.11.13. Challenge TLV
The Challenge TLV carries the challenge used by the token to
calculate the OTP, as reported by the token to the peer. In The
Challenge TLV MUST be sent by a peer if and only if the challenge
otherwise would be unknown to the EAP server (e.g. the token or peer
modified a received challenge or generated its own challenge).
An EAP server conformant with this specification SHOULD support (i.e.
recognize) this TLV, but need not be able to process or act on it.
An EAP server that does not support this TLV but receives an EAP-
Response with the TLV present MAY ignore the value. The Challenge
TLV MUST NOT be present in any EAP-Responses of type POTP-X other
than those that also carries an OTP TLV.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Challenge ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
16
Length
Nystroem Expires March 30, 2007 [Page 51]
Internet-Draft EAP-POTP September 2006
>= 1
Challenge
The challenge value that was used to calculate the OTP used for
the accompanying OTP TLV.
4.11.14. Keep-Alive TLV
The Keep-Alive is used to avoid EAP-POTP timeouts.
The Keep-Alive TLV MAY be sent by a peer to avoid time-outs when the
peer has received an EAP-Request containing an OTP TLV or a New PIN
TLV and is waiting for a response from the user.
An EAP-Request containing a Keep-Alive TLV MUST be sent by an EAP
server when the server receives an EAP-Response containing a Keep-
Alive TLV, and the server has an outstanding request which did not
contain a Keep-Alive TLV. In this situation, the server does not
need to re-transmit its latest outstanding request, but due to the
synchronous nature of EAP it needs to send another request. Re-
transmission of the latest outstanding request could be confusing for
the peer since the request would get a new Identifier value. The
Keep-Alive TLV MAY also be sent by an EAP server when the server
detects that its processing time will exceed some locally configured
threshold and may cause a network timeout. In this case, the peer
MUST respond with an EAP-Response containing a Keep-Alive TLV.
This TLV type MUST be supported by all peers and all EAP servers
conforming to this specification and MUST NOT be responded to with a
NAK TLV. The Keep-Alive TLV MUST NOT be sent in any other situations
than the ones described above. The Keep-Alive TLV MUST NOT be sent
together with any other TLVs defined herein. Implementations SHOULD
also follow recommendations made in Section 4.3 of [1].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Nystroem Expires March 30, 2007 [Page 52]
Internet-Draft EAP-POTP September 2006
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
13
Length
0
4.11.15. Protected TLV
The Protected TLV SHALL be used to encrypt individual or multiple
TLVs after successful exchange of the Confirm TLV (i.e. as soon as
calculated keys have been confirmed). The Protected TLV therefore
wraps "ordinary" TLVs.
This TLV type may be sent by EAP servers as well as by peers and MUST
be supported by all peers conforming to this specification. It
SHOULD be supported by all EAP servers conforming to this
specification (it need not be supported if a server never will have a
need to continue a POTP-X conversation after exchange of the Confirm
TLV).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Authentication Code ... (16 octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IV ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encrypted TLVs ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
Nystroem Expires March 30, 2007 [Page 53]
Internet-Draft EAP-POTP September 2006
TLV Type
14
Length
>32
Message Authentication Code (MAC)
This field integrity-protects the TLV. The MAC SHALL be
calculated over the IV and the Encrypted TLVs field in the
following manner:
mac = MAC(K_MAC, iv | encrypted_tlvs)
where
MAC is the negotiated MAC algorithm, "iv" is the IV field's value,
and "encrypted_tlvs" is the value of the Encrypted TLVs field.
The first 16 octets of the MAC is placed in the Message
Authentication Code field.
Recipients MUST verify the MAC. If the verification fails, the
conversation SHALL be terminated (i.e. peers send an empty POTP-X
EAP-Response message, EAP servers send an EAP-Failure message
possibly preceded by an EAP-Request of type Notification).
IV
An initialization vector for the encryption, see below. The
length of the vector is dependent on the negotiated encryption
algorithm. E.g. for AES-CBC it shall be 16 octets. For some
encryption algorithms there may not be any initialization vector.
IVs, when present, shall be randomly chosen and non-predictable.
Encrypted TLVs
This field SHALL contain one or more encrypted POTP-X TLVs. The
encryption algorithm SHALL be as negotiated, use K_ENC as the
encryption key, and use the IV field as the initialization vector
(when applicable).
4.11.16. Crypto Algorithm TLV
The Crypto Algorithm TLV allows for negotiation of cryptographic
algorithms. Cryptographic Algorithm negotiation is described in
detail in Section 4.3.
Nystroem Expires March 30, 2007 [Page 54]
Internet-Draft EAP-POTP September 2006
This TLV MUST be present in the initial EAP-Request of type POTP-X
that also carries an OTP TLV indicating protected mode, assuming the
EAP server wants to negotiate use of any other algorithms than the
default ones. It MAY also be present in an EAP-Request of type
POTP-X that carries an OTP TLV that is sent as a result of a failed
session resumption (in this case, the peer has not yet responded to
this TLV), or when the Crypto Algorithm TLV was part of the initial
message from the EAP server and the client negotiated another EAP-
POTP version than the highest one supported by the EAP server. The
Crypto Algorithm TLV MUST NOT be present in any other EAP-Requests.
Further, the Crypto Algorithm TLV MUST NOT be present in an EAP-
Response of type POTP-X unless the preceding EAP-Request also
contained it and it was legal for it to do so. This TLV MUST be
supported by all peers and all EAP servers conforming to this
specification and MUST NOT be responded to with a NAK TLV.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Hash Alg.Length| Hash Algorithms ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Encr.Alg.Length| Encryption Algorithms ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|MAC Alg. Length| MAC Algorithms ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved for future use. This bit SHALL be set to zero (0) for
this version. Recipients SHALL ignore this bit for this version
of EAP-POTP.
TLV Type
15
Length
>=4 (at least one class of algorithms and one algorithm for that
class needs to be present)
Reserved
Nystroem Expires March 30, 2007 [Page 55]
Internet-Draft EAP-POTP September 2006
Reserved for future use. This octet MUST be set to zero for this
version. Recipients SHALL ignore this octet for this version of
EAP-POTP.
Hash Alg. Length
The length of the Hash Algorithms field in octets.
Hash Algorithms
Each octet pair of this field represents a hash algorithm as
follows. An EAP server MAY supply several suggestions for hash
algorithms. Each algorithm MUST appear only once. The algorithms
SHALL be supplied in order of priority. Peers MUST supply at most
one algorithm (if none is present the default applies). The
defined values are:
Value Hash algorithm
------------------------------
0x00 0x00 Reserved
0x00 0x01 SHA-1
0x00 0x02 SHA-224
0x00 0x03 SHA-256 (default)
0x00 0x04 SHA-384
0x00 0x05 SHA-512
0x80 - Vendor-specific (or experimental)
As indicated, values 0x8000 and higher are for proprietary vendor-
specific algorithms. Values in the range 0x0006 - 0x7fff are to
be assigned through IANA, see Section 7.
Encr Alg. Length
The length of the Encryption Algorithms field in bytes.
Encryption Algorithms
Each octet pair of this field represents an encryption algorithm
as follows. An EAP server MAY supply several suggestions for
encryption algorithms. Each algorithm MUST appear only once. The
algorithms SHALL be supplied in order of priority. Peers MUST
supply at most one algorithm (if none is present the default
applies). The defined values are:
Nystroem Expires March 30, 2007 [Page 56]
Internet-Draft EAP-POTP September 2006
Value Encryption algorithm
------------------------------
0x00 0x00 Reserved
0x00 0x01 AES-CBC (default) with 128-bit keys and 16-octet IVs
0x00 0x02 3DES-CBC with 112-bit keys and 8-octet IVs
0x80 - Vendor-specific
As indicated, values 0x8000 and higher are for vendor-specific
proprietary algorithms. Values in the range 0x0003 - 0x7fff are
to be assigned through IANA, see Section 7.
MAC Alg. Length
The length of the MAC Algorithms field in bytes.
MAC Algorithms
Each octet pair of this field represents a MAC algorithm as
follows. An EAP server MAY supply several suggestions for MAC
algorithms. Each algorithm MUST appear only once. The algorithms
SHALL be supplied in order of priority. Peers MUST supply at most
one algorithm (if none is present the default applies). The
defined values are:
Oct.1 Oct.2 MAC Algorithm
------------------------------
0x00 0x00 Reserved
0x00 0x01 HMAC (default)
0x80 - Vendor-specific
As indicated, values 0x8000 and higher are for vendor-specific
proprietary algorithms. Values in the range 0x0002 - 0x7fff are
to be assigned through IANA, see Section 7.
When HMAC is negotiated, the hash algorithm used for HMAC SHALL be
the negotiated hash algorithm.
Nystroem Expires March 30, 2007 [Page 57]
Internet-Draft EAP-POTP September 2006
5. EAP Key Management Framework considerations
In line with recommendations made in [16], EAP-POTP defines the
following identifiers to be associated with generated key material:
Peer-ID: The combined contents of the User Identifier TLV and the
Token Key Identifier TLV.
Server-ID: The contents of the Server Identifier field of the
Server-Info TLV.
Method-ID: The identifier of the established session (i.e. the
contents of the Session Identifier field of the Server-Info TLV
that defined the session).
Nystroem Expires March 30, 2007 [Page 58]
Internet-Draft EAP-POTP September 2006
6. Security considerations
6.1. Security claims
In conformance with RFC 3748 [1], the following security claims are
made for the EAP-POTP method:
Authentication mechanism: Generic OTP
Ciphersuite negotiation: Yes (No in basic variant)
Mutual authentication: Yes (No in basic variant)
Integrity protection: Yes (No in basic variant)
Replay protection: Yes (see below)
Confidentiality: Only in the OTP protection variant, and
then only OTP values and any information
sent after exchange of the Confirm TLV
Key derivation: Yes (No in basic variant)
Key strength: Depends on size of OTP value, strength of
underlying shared secret, strength and
characteristics of OTP algorithm, pepper
length, iteration count, and whether the
method is used within a tunnel such as
PEAPv2. For some illustrative examples,
and a further discussion of this, see
Appendix D.
Dictionary attack prot.: N/A (Human-selected passwords not used)
Fast reconnect: Yes
Crypt. binding: N/A (EAP-POTP is not a tunnel method)
Session independence: Yes
Fragmentation: N/A (Packets shall not exceed MTU of 1020)
Channel binding: Yes (No in basic variant)
Acknowledged S/F: Yes
State Synchronization: Yes (No in basic variant)
6.2. Passive and active attacks
The basic variant (i.e. when the protection of OTPs and mutual
authentication is not used) of this EAP method does not provide
session privacy, session integrity, server authentication or
protection from active attacks. In particular, man-in-the-middle
attacks, where an attacker acts as an authenticator in order to
acquire a valid OTP are possible.
Similarly, the basic variant of this EAP method does not protect
against session hijacking taking place after authentication. Nor
does it in itself protect against replay attacks, where the attacker
gains access by replaying a previous, valid request, but see also the
next subsection. When PIN codes are transmitted, they are sent
without protection and are also subject to replay attacks.
Nystroem Expires March 30, 2007 [Page 59]
Internet-Draft EAP-POTP September 2006
In order to protect against these attacks, the peer MUST only use the
basic variant of this method over a server-authenticated and
confidentiality-protected connection. This can be achieved via use
of, e.g., PEAPv2 [17].
When the OTP protection variant is used however, the EAP method
provides privacy for OTPs and new PINs, negotiation of cryptographic
algorithms, mutual authentication, and protection against replay
attacks and protocol version downgrades. It also provides protection
against man-in-the-middle attacks, not due to the infeasibility for a
man-in-the-middle to solve for a valid OTP given an OTP TLV, but due
to the computational expense of finding the OTP in the limited time
period during which it is valid (this is mainly true for tokens
including the current time in their OTP calculations or when a sent
challenge has a certain lifetime). It should be noted, however, that
a retrieved OTP, even if "old" and invalid, still may divulge some
information about the user's PIN. Clearly this is also true for the
basic variant. Implementations of this EAP method where user PINs
are sent with OTPs are therefore RECOMMENDED to ensure regular user
PIN changes, regardless of whether the protected variant or the basic
variant is employed.
It should also be noted, that while it is possible for a rogue access
point e.g. to clone MAC addresses, and hence mount a man-in-the-
middle attack, such an access point will not be able to calculate the
session keys MSK and EMSK. This demonstrates the importance of using
the derived key material properly to protect a subsequent session.
Protected mode protects against version downgrade attacks due to the
HMAC both parties transmit in this mode. As described, each party
calculates the HMAC on sent and received EAP-POTP handshake messages.
If an attacker were to modify a Version TLV, this would be reflected
in a difference between the calculated MACs (since the recipient of
the Version TLV received a different value than the sender sent).
Unless the attacker knows K_MAC, he cannot calculate the correct MAC,
and hence the difference will be detected.
The OTP protection variant also protects against session hijacking,
if the derived key material is used (directly or indirectly) to
protect a subsequent session. For these reasons, use of the OTP
protection variant is RECOMMENDED.
It should be noted that not even the OTP protection variant provides
privacy for user names and/or token key identifiers however. EAP-
POTP MUST be used within a secure tunnel such as the one provided by
PEAPv2 ([17]) if privacy for these parameters is required.
When resuming sessions created in the basic variant (which MUST only
Nystroem Expires March 30, 2007 [Page 60]
Internet-Draft EAP-POTP September 2006
take place within a protected tunnel), the peer is authenticated by
demonstrating knowledge of not just a valid session identifier but
also of the OTP used when the session was created. Server nonces
prevents replay attacks but there still remains some likelihood of an
attacker guessing the correct combination of session identifier and
OTP value. Assuming OTPs with entropy about 32 bits this means that
the likelihood of succeeding with such an attack is about 1/2^48 due
to the birthday paradox. Servers allowing session resumption for the
basic variant MUST protect against such attacks, e.g. by keeping
track of the rate of failed resumption attempts.
6.3. Denial of service attacks
An active attacker may replace the iteration count value in OTP TLVs
sent by the peer to slow down an authentication server.
Authentication servers SHOULD protect against this, e.g. by
disregarding OTP TLVs with an iteration count value higher than some
pre- or dynamically- (depending on load) set number.
6.4. The use of pepper
As described in Section 4.8, the use of pepper will slow down an
attacker's search for a matching OTP. The ability to transfer a
pepper value in encrypted form from the EAP server to the peer means
that, even though there may be an initial computational cost for the
EAP server to authenticate the peer, subsequent authentications will
be efficient, while at the same time more secure, since a pre-shared,
128 bits long, pepper value will not be easily found by an attacker.
An attacker observing an EAP-Request containing an OTP TLV calculated
using a pepper chosen by the peer may however, depending on available
resources, be able to successfully attack that particular EAP-POTP
session, since it most likely will be based on a relatively short
pepper value or only an iteration count. Once the correct OTP has
been found, eavesdropping on the EAP server's Confirm TLV will
potentially give the attacker access to the longer, server-provided
pepper for the remaining lifetime of that pepper value. For this
reason, initial exchanges with EAP servers SHOULD occur in a secure
environment (e.g. in a PEAPv2 tunnel offering cryptographic binding
with inner EAP methods), and if not, the iteration count MUST be
significantly higher than for messages where a pre-shared pepper is
used. The lifetime of the shared pepper must also be calculated with
this in mind. Finally, the pepper value MUST be securely stored by
the peer and the EAP server, associated with the user.
6.5. The race attack
In the case of fragmentation of EAP messages, it is possible (in the
basic variant of this method) for an attacker to listen to most of an
Nystroem Expires March 30, 2007 [Page 61]
Internet-Draft EAP-POTP September 2006
OTP, guess the remainder, and then race the legitimate user to
complete the authentication. Conforming backend authentication
server implementations MUST protect against this race condition. One
defense against this attack is outlined below and borrowed from [14];
implementations MAY use this approach or MAY select an alternative
defense. Note that the described defense relies on the user
providing the identity in response to an initial Identity EAP-
Request.
One possible defense is to prevent a user from starting multiple
simultaneous authentication sessions. This means that once the
legitimate user has initiated authentication, an attacker would be
blocked until the first authentication process has completed. In
this approach, a timeout is necessary to thwart a denial of service
attack.
Nystroem Expires March 30, 2007 [Page 62]
Internet-Draft EAP-POTP September 2006
7. IANA considerations
7.1. General
This document is a description of a general EAP method for OTP
tokens. It also defines EAP method 32 as a profile of the general
method. Extending the set of EAP-POTP TLVs or the set of EAP-POTP
cryptographic algorithms shall be seen as revisions of the protocol
and hence requiring an RFC that updates, or obsoletes this document.
7.2. Cryptographic algorithm identifier octets
A new registry for EAP-POTP cryptographic algorithm identifier octets
shall be created. The initial contents of this registry shall be as
specified in Section 4.11.16.
Assignment of new values for hash algorithms, encryption algorithms,
and MAC algorithms in the Crypto Algorithm TLV MUST be done through
IANA with "Specification Required" and "IESG Approval" (see [9] for
the meaning of these terms).
Nystroem Expires March 30, 2007 [Page 63]
Internet-Draft EAP-POTP September 2006
8. Intellectual property considerations
RSA Security has filed an IETF IPR Disclosure for IPR related to this
document. The IETF IPR Disclosure number is 569 and it can be found
at the IETF IPR Disclosure page. RSA Security makes no
representations regarding intellectual property claims by other
parties. Such determination is the responsibility of the user.
RSA, RSA Security and SecurID are either registered trademarks or
trademarks of RSA Security Inc. in the United States and/or other
countries. The names of other products and services mentioned may be
the trademarks of their respective owners.
Nystroem Expires March 30, 2007 [Page 64]
Internet-Draft EAP-POTP September 2006
9. Acknowledgments
This document was improved by comments from, and discussion with, a
number of RSA Security employees. Simon Josefsson drafted the
initial versions of an RSA SecurID EAP method while working for RSA
Laboratories. The inspiration for the TLV-type of information
exchange comes from [17]. Special thanks to Oliver Tavakoli of Funk
Software who has provided numerous useful comments and suggestions,
Randy Chou of Aruba Networks for good suggestions in the session
resumption area, and Jim Burns of Meetinghouse who provided
inspiration for the Protected TLV. Thanks also to the IESG
reviewers, Pasi Eronen, David Black, and Uri Blumenthal, for
insightful comments that helped to improve the document.
Nystroem Expires March 30, 2007 [Page 65]
Internet-Draft EAP-POTP September 2006
10. References
10.1. Normative references
[1] Blunk, L., Vollbrecht, J., Aboba, B., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[3] National Institute of Standards and Technology, "Secure Hash
Standard", FIPS 180-2, February 2004.
[4] National Institute of Standards and Technology, "Specification
for the Advanced Encryption Standard (AES)", FIPS 197,
November 2001.
[5] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, February 1997.
[6] RSA Laboratories, "Password-Based Cryptography Standard",
PKCS #5 v2.0, March 1999.
[7] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
RFC 2279, January 1998.
[8] Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806,
April 2000.
[9] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
10.2. Informative references
[10] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",
RFC 1661, July 1994.
[11] The Institute of Electrical and Electronics Engineers, Inc.,
"IEEE Standard for Local and metropolitan area networks --
Port-Based Network Access Control", IEEE 802.1X-2001,
July 2001.
[12] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[13] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for Wireless
Nystroem Expires March 30, 2007 [Page 66]
Internet-Draft EAP-POTP September 2006
LANs", RFC 4017, March 2005.
[14] Haller, N., Metz, C., Nesser, P., and M. Straw, "A One-Time
Password System", RFC 2289, February 1998.
[15] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote
Dial In User Service (RADIUS)", RFC 2865, June 2000.
[16] Aboba, B., Simon, D., Eronen, P., and H. Levkowetz, Ed.,
"Extensible Authentication Protocol (EAP) Key Management
Framework", Work in progress draft-ietf-eap-keying-14.txt,
June 2006.
[17] Palekar, A., Simon, D., Zorn, G., Salowey, J., Zhou, H., and S.
Josefsson, "Protected EAP Protocol (PEAP) Version 2", Work in
progress draft-josefsson-pppext-eap-tls-eap-10.txt,
October 2004.
[18] Internet Assigned Numbers Authority, "Private Enterprise
Numbers", January 2005.
[19] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes",
RFC 2548, March 1999.
Nystroem Expires March 30, 2007 [Page 67]
Internet-Draft EAP-POTP September 2006
Appendix A. Profile of EAP-POTP for RSA SecurID
Note: The RSA SecurID product is a hardware token card (or software
emulation thereof) produced by RSA Security Inc., which is used for
end-user authentication.
The EAP method type identifier for the RSA SecurID profile of EAP-
POTP SHALL be 32.
Peers and EAP servers implementing the SecurID profile of EAP-POTP
SHALL conform to all EAP-POTP normative requirements in this
document, and, in addition, the New PIN TLV and the Protected TLV
MUST be supported by peers.
Nystroem Expires March 30, 2007 [Page 68]
Internet-Draft EAP-POTP September 2006
Appendix B. Examples of EAP-POTP exchanges
In the examples, "V1","V2","V3", etc. stand for arbitrary values of
the correct type.
B.1. Basic mode, unilateral authentication
This mode should only be used within a secured tunnel. The peer
identifies itself with a User Identifier TLV.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
OTP TLV:
P=0,C=0,N=0,T=0,E=0,R=0
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
OTP TLV:
P=0,C=0,N=0,T=0,E=0,R=0
Authentication Data=V1
User Identifier TLV:
User Identifier=V2
<- EAP-Success
B.2. Basic mode, session resumption
This example illustrates successful resumption of a basic mode
session. It must be carried out only in a protected tunnel.
Nystroem Expires March 30, 2007 [Page 69]
Internet-Draft EAP-POTP September 2006
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
OTP TLV:
P=0,C=0,N=0,T=0,E=0,R=0
Server-Info TLV:
N=0
Session Identifier=V1
Server Identifier=V2
Nonce=V3
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
Resume TLV:
Session Identifier=V4 (indicating earlier, basic mode, session)
Authentication Data=V5
<- EAP-Success
B.3. Mutual authentication without session resumption
In this case, the peer uses the token key identifier in addition to
the user identifier. The initial EAP-Identity exchange may also
provide user information, or may be restricted to only general domain
information. Pepper is not used, but will be used in a subsequent
session since the server provides the peer with an encrypted pepper
in its Confirm TLV. Absence of the Crypto Algorithm TLV indicates
use of default cryptographic algorithms.
Peer EAP server
<- EAP-Request
Type=Identity
Nystroem Expires March 30, 2007 [Page 70]
Internet-Draft EAP-POTP September 2006
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
Server-Info TLV:
N=0
Session Identifier=V1
Server Identifier=V2
Nonce=V3
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=0
Iteration Count=V4
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=0
Iteration Count=V4
Authentication Data=V5
User Identifier TLV:
User Identifier=V6
Token Key Identifier TLV:
Token Key Identifier=V7
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=0
Authentication Data=V8
Pepper Identifier=V9
Encrypted Pepper=V10
EAP-Response ->
Nystroem Expires March 30, 2007 [Page 71]
Internet-Draft EAP-POTP September 2006
Type=OTP-X
Confirm TLV:
(no data)
<- EAP-Success
B.4. Mutual authentication with transfer of pepper
The difference between this example and the previous one is that the
peer makes use of an existing pepper in the PBKDF2 computation. The
EAP server provides a new pepper to the peer in the Confirm TLV.
Note that the peer had not been able to use a pepper in the response
calculation unless it had found the existing pepper, since the server
specified a maximum (new) pepper length of zero.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
Server-Info TLV:
N=0
Session Identifier=V1
Server Identifier=V2
Nonce=V3
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=0
Iteration Count=V4
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
OTP TLV:
Nystroem Expires March 30, 2007 [Page 72]
Internet-Draft EAP-POTP September 2006
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V5
Iteration Count=V6
Authentication Data=V7
(includes a pepper identifier)
User Identifier TLV:
User Identifier=V8
Token Key Identifier TLV:
Token Key Identifier=V9
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=0
Authentication Data=V10
Pepper Identifier=V11
Encrypted Pepper=V12
EAP-Response ->
Type=OTP-X
Confirm TLV:
(no data)
<- EAP-Success
B.5. Failed mutual authentication
This example differs from the previous one in that the peer is not
able to authenticate the server. It therefore sends an empty EAP-
Response of type POTP-X, which the EAP server acknowledges by
responding with an EAP-Failure. Pepper is not used.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Nystroem Expires March 30, 2007 [Page 73]
Internet-Draft EAP-POTP September 2006
Highest=0,Lowest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V1
Iteration Count=V2
Server-Info TLV:
N=0
Session Identifier=V3
Server Identifier=V4
Nonce=V5
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V1
Iteration Count=V2
Authentication Data=V6
User Identifier TLV:
User Identifier=V7
Token Key Identifier TLV:
Token Key Identifier=V8
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=0
Authentication Data=V9
EAP-Response ->
Type=OTP-X
(no data)
<- EAP-Failure
Nystroem Expires March 30, 2007 [Page 74]
Internet-Draft EAP-POTP September 2006
B.6. Session resumption
This example illustrates successful session resumption.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V1
Iteration Count=V2
Server-Info TLV:
N=0
Session Identifier=V3
Server Identifier=V4
Nonce=V5
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
Resume TLV:
Session Identifier=V6 (indicating earlier, protected mode, session)
Authentication Data=V7
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=0
Authentication Data=V8
EAP-Response ->
Type=OTP-X
Nystroem Expires March 30, 2007 [Page 75]
Internet-Draft EAP-POTP September 2006
Confirm TLV:
(no data)
<- EAP-Success
B.7. Failed session resumption
This example illustrates a failed session resumption, followed by a
complete mutual authentication. The user is identified through the
User Identifier TLV. The client is able to re-use an older pepper.
The server sends a new pepper for subsequent use in its Confirm TLV.
The server suggests some non-default cryptographic algorithms but the
client only supports the default ones.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V1
Iteration Count=V2
Server-Info TLV:
N=0
Session Identifier=V3
Server Identifier=V4
Nonce=V5
Crypto Algorithm TLV:
Hash Alg. Length=V6
Hash Algorithms=V7
Encr. Alg. Length=V8
Encr. Algorithms=V9
MAC Alg. Length=V10
MAC Algorithms=V11
EAP-Response ->
Nystroem Expires March 30, 2007 [Page 76]
Internet-Draft EAP-POTP September 2006
Type=OTP-X
Version TLV:
Highest=0
Resume TLV:
Session Identifier=V12 (indicating earlier session)
Authentication Data=V13
<- EAP-Request
Type=OTP-X
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V14
Iteration Count=V15
Server-Info TLV:
N=1 (no resumption)
Session Identifier=V3
Server Identifier=V4
Nonce=V16
EAP-Response ->
Type=OTP-X
OTP TLV:
P=1,C=0,N=1,T=1,E=0,R=0
Pepper Length=V17
Iteration Count=V18
Authentication Data=V19 (with pepper identifier)
User Identifier TLV:
User Identifier=V20
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=0
Authentication Data=V21
Pepper Identifier=V22
Encrypted Pepper=V23
EAP-Response ->
Type=OTP-X
Confirm TLV:
(no data)
Nystroem Expires March 30, 2007 [Page 77]
Internet-Draft EAP-POTP September 2006
<- EAP-Success
B.8. Mutual authentication, and new PIN requested.
In this example, the user is also requested to select a new PIN. The
new PIN is allowed to be alphanumeric, and must be at least 6
characters long. The user selects another PIN than the one suggested
by the server. The token key is identified through a combination of
the user identifier and the token key identifier. While waiting for
the user input, to avoid network timeouts, the peer sends an EAP-
Response containing a Keep-Alive TLV to the EAP server. The EAP
server responds by sending an EAP-Request containing a Keep-Alive TLV
back to the peer. Note that all TLVs exchanged after the Confirm TLV
exchange are wrapped in the Protected TLV. Absence of the Crypto
Algorithm TLV indicates use of default cryptographic algorithms.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V1
Iteration Count=V2
Server-Info TLV:
N=0
Session Identifier=V3
Server Identifier=V4
Nonce=V5
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
OTP TLV:
Nystroem Expires March 30, 2007 [Page 78]
Internet-Draft EAP-POTP September 2006
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V6
Iteration Count=V7
Authentication Data=V8 (with pepper identifier)
User Identifier TLV:
User Identifier=V9
Token Key Identifier TLV:
Token Key Identifier=V10
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=1
Authentication Data=V11
EAP-Response ->
Type=OTP-X
Confirm TLV:
(no data)
<- EAP-Request
Type=OTP-X
Protected TLV:
MAC=V12
IV=V13
Encrypted TLVs=V14
(Contains:
New PIN TLV:
Q=0,A=1
PIN=V15
Min. PIN Length=6)
EAP-Response ->
Type=OTP-X
Protected TLV:
MAC=V16
IV=V17
Encrypted TLVs=V18
(Contains:
Keep-Alive TLV:
(no data))
Nystroem Expires March 30, 2007 [Page 79]
Internet-Draft EAP-POTP September 2006
<- EAP-Request
Type=OTP-X
Protected TLV:
MAC=V19
IV=V20
Encrypted TLVs=V21
(Contains:
Keep-Alive TLV:
(no data))
EAP-Response ->
Type=OTP-X
Protected TLV:
MAC=V22
IV=V23
Encrypted TLVs=V24
(Contains:
New PIN TLV:
Q=0,A=0
PIN=V25)
<- EAP-Request
Type=OTP-X
Protected TLV:
MAC=V26
IV=V27
Encrypted TLVs=V28
(Contains:
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V1
Iteration Count=V2)
EAP-Response ->
Type=OTP-X
Protected TLV
MAC=V29
IV=V30
Encrypted TLVs=V31
(Contains:
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V6
Iteration Count=V7
Nystroem Expires March 30, 2007 [Page 80]
Internet-Draft EAP-POTP September 2006
Authentication Data=V31)
<- EAP-Request
Type=OTP-X
Protected TLV
MAC=V32
IV=V33
Encrypted TLVs=V34
(Contains:
Confirm TLV:
C=0
Authentication Data=V35)
EAP-Response ->
Type=OTP-X
Protected TLV
MAC=V36
IV=V37
Encrypted TLVs=V38
(Contains:
Confirm TLV:
(no data))
<- EAP-Success
B.9. Use of next OTP mode
In this example, the peer is requested to provide a second OTP to the
EAP server.
Peer EAP server
<- EAP-Request
Type=Identity
EAP-Response ->
Type=Identity
<- EAP-Request
Type=OTP-X
Version TLV:
Highest=0,Lowest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Nystroem Expires March 30, 2007 [Page 81]
Internet-Draft EAP-POTP September 2006
Pepper Length=V1
Iteration Count=V2
Server-Info TLV:
N=0
Session Identifier=V3
Server Identifier=V4
Nonce=V5
EAP-Response ->
Type=OTP-X
Version TLV:
Highest=0
OTP TLV:
P=1,C=0,N=0,T=0,E=0,R=0
Pepper Length=V6
Iteration Count=V7
Authentication Data=V8
User Identifier TLV:
User Identifier=V9
<- EAP-Request
Type=OTP-X
OTP TLV:
P=1,C=0,N=1,T=1,E=0,R=0
Pepper Length=V1
Iteration Count=V2
EAP-Response ->
Type=OTP-X
OTP TLV:
P=1,C=0,N=1,T=1,E=0,R=0
Pepper Length=V6
Iteration Count=V7
Authentication Data=V10
<- EAP-Request
Type=OTP-X
Confirm TLV:
C=0
Authentication Data=V11
Nystroem Expires March 30, 2007 [Page 82]
Internet-Draft EAP-POTP September 2006
EAP-Response ->
Type=OTP-X
Confirm TLV:
(no data)
<- EAP-Success
Nystroem Expires March 30, 2007 [Page 83]
Internet-Draft EAP-POTP September 2006
Appendix C. Use of the MPPE-Send/Receive-Key RADIUS attributes
C.1. Introduction
This section describes how to populate the MPPE-Send-Key and the
MPPE-Receive-Key RADIUS attributes defined in [19] using an MSK
established in EAP-POTP.
C.2. MPPE key attribute population
Once the EAP-POTP MSK has been generated, it is used as follows to
populate the MPPE-Send-Key and the MPPE-Receive-Key attributes:
Use the initial 32 octets of the MSK as the value for the "Key" sub-
field in the plaintext "String" field of the MPPE-Send-Key attribute,
and use the final 32 octets of the MSK as the "Key" sub-field in the
plaintext "String" field of the MPPE-Receive-Key attribute (Note:
"Send" and "Receive" here refers to the Authenticator, for the peer
they are reversed).
Nystroem Expires March 30, 2007 [Page 84]
Internet-Draft EAP-POTP September 2006
Appendix D. Key strength considerations
D.1. Introduction
As described in Section 6, the strength of keys generated in EAP-POTP
protected mode depends on a number of factors. This appendix
provides examples of actual key strengths achieved under various
assumptions.
It should be noted, that while some of the examples indicate that the
strength of generated keys is relatively weak, the strength applies
only to those EAP-POTP sessions between a peer and an EAP server that
do not share a pepper. Once a pepper, provided by an EAP server to a
peer has been established, future sessions using this pepper will
provide full-strength keys.
D.2. Example 1: 6-digit One-Time Passwords
In this example we assume the following:
OTPs are six decimal digits long;
4-digit PINs are added to generated OTPs; and
OTP hardening (iteration count and pepper searching combined)
effectively adds 10 bits of entropy. One way of achieving this
without use of pepper searching is to have the iteration count in
PBKDF2 set to 1,000,000.
The effective key strength then becomes roughly:
log_2(10**6) + log_2(10**4) + log_2(2**10) = 43 bits
The above assumes the entopy of the underlying shared secret is >43
bits and that there are no other weaknesses in the OTP algorithm.
D.3. Example 2: 8-digit One-Time Passwords
In this example we assume the following:
OTPs are six decimal digits long;
4-character alphanumeric PINs are added to generated OTPs; and
OTP hardening (iteration count and pepper searching combined)
effectively adds 10 bits of entropy.
The effective key strength then becomes roughly:
Nystroem Expires March 30, 2007 [Page 85]
Internet-Draft EAP-POTP September 2006
log_2(10**8) + log_2(26**4) + log_2(2**10) = 55 bits
The above assumes the entopy of the underlying shared secret is >55
bits and that there are no other weaknesses in the OTP algorithm.
Nystroem Expires March 30, 2007 [Page 86]
Internet-Draft EAP-POTP September 2006
Author's Address
Magnus Nystroem
RSA Security
Email: magnus@rsasecurity.com
Nystroem Expires March 30, 2007 [Page 87]
Internet-Draft EAP-POTP September 2006
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Nystroem Expires March 30, 2007 [Page 88]
