One document matched: draft-bersani-eap-psk-01.txt
Differences from draft-bersani-eap-psk-00.txt
Internet Draft Florent Bersani
File: draft-bersani-eap-psk-01.txt France Telecom R&D
Expires: August 2004 February 2004
The EAP PSK Protocol
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document specifies an Extensible Authentication Protocol (EAP)
method for authentication and session key derivation using a pre-
shared key (PSK). This PSK is used by a unique underlying
cryptographic primitive, a block cipher, which is instantiated with
AES-128.
EAP-PSK performs mutual authentication and session key derivation. It
provides identity protection and shall provide fast reconnect in
future versions. It provides a secure communication channel within
EAP in case the authentication is successful. This secure channel can
be used to allow for instance protected result indications.
EAP-PSK is designed to be itself extensible.
EAP-PSK is intended to be easy to deploy and well-suited for
authentication over insecure networks such as IEEE 802.11.
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Table of Contents
1. Introduction...................................................4
1.1 Design goals for EAP-PSK....................................4
1.2 Caveats.....................................................4
1.3 Specification of requirements...............................5
1.4 Terminology.................................................5
1.5 Conventions.................................................7
1.6 Related work................................................7
2. Protocol overview..............................................8
2.1 Cryptographic design of EAP-PSK.............................8
2.1.1 The authentication...................................8
2.1.2 The key derivation...................................9
2.1.3 The protected channel...............................10
2.2 EAP-PSK key hierarchy......................................11
2.2.1 The PSK.............................................11
2.2.2 The TEK.............................................11
2.2.3 The MSK.............................................12
2.2.4 The EMSK............................................12
2.2.5 The IV..............................................12
2.3 EAP-PSK message flow.......................................12
2.3.1 EAP-PSK basic message flow..........................12
2.3.2 EAP-PSK advanced message flows......................13
2.4 Retry behavior.............................................14
2.5 Fragmentation..............................................14
3. EAP-PSK state machines........................................15
3.1 EAP-PSK peer state machine.................................15
3.1.1 The Init state......................................15
3.1.2 The ID_sent state...................................16
3.1.3 The MAC_sent state..................................17
3.1.4 The P_Channel state.................................17
3.1.5 The Failure state...................................18
3.1.6 The Success state...................................18
3.2 EAP-PSK server state machine...............................18
3.2.1 The Init state......................................19
3.2.2 The ID_req state....................................19
3.2.3 The MAC_req state...................................20
3.2.4 The P_channel state.................................21
3.2.5 The Failure state...................................21
3.2.6 The Success state...................................21
4. EAP-PSK message format........................................22
4.1 Table of the Type field of the different attributes........22
4.2 Format of the different attributes.........................22
4.2.1 AT_IDREQ............................................22
4.2.2 AT_IDRES............................................22
4.2.3 AT_MAC..............................................23
4.2.4 AT_PCHANNEL.........................................24
4.2.5 AT_RAND.............................................25
4.2.6 AT_STATUS...........................................25
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5. IANA considerations...........................................26
6. Security considerations.......................................26
6.1 Identity protection........................................26
6.2 Mutual authentication......................................27
6.3 Key derivation.............................................27
6.4 Dictionary attacks.........................................28
6.5 Protected channel..........................................28
6.6 Negotiation attacks........................................28
6.7 Fast reconnect.............................................28
6.8 Man-in-the-middle attacks..................................28
6.9 Generating random numbers..................................29
7. Security claims...............................................29
8. Intellectual Property Right Notice............................29
9. Acknowledgements..............................................30
10. References...................................................30
10.1 Normative.................................................30
10.2 Informative...............................................31
11. Authors' Addresses...........................................33
12. Full Copyright Statement.....................................33
Annex A: Work still to be done on this document..................33
1. Editorial...................................................33
2. Security....................................................34
3. Technical...................................................34
Annex B: Guidance for PSK generation from a password.............35
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1. Introduction
1.1 Design goals for EAP-PSK
The Extensible Authentication Protocol, [EAP], provides a standard
mechanism for support of additional authentication methods within
[PPP]. EAP is also used within IEEE 802 networks through the [IEEE
802.1X] framework.
EAP supports many authentication mechanisms usually called EAP
methods. This document specifies an EAP method that uses a pre-shared
key (PSK).
Design goals for this method were:
1. Simplicity: It should be easy to implement and to deploy without
any pre-existing infrastructure.
2. Wide applicability: It should be possible to use this method to
authenticate over any network. In particular, it should be
suitable for [IEEE 802.11] wireless LANs and comply to [IEEE
802REQ]
3. Security: It should be conservative in its cryptographic design
and enjoy security proofs
4. Extensibility: It should be possible to add to this method the
required extensions as their need appears
5. Patent-avoidance: It should be free of any Intellectual Property
Right claims
Thus, EAP-PSK relies on a single cryptographic primitive, [AES], and
performs mutual authentication and session key derivation. It also
provides identity protection and shall provide fast reconnect in
future versions. It provides a secure communication channel within
EAP in case the authentication is successful. This secure channel can
be used to allow for instance protected result indications.
It uses a Type-Length-Value design to ensure that it will be easy to
extend. This design is quite analogous to the attribute format used
in [EAP-SIM] or the TLV suggested in [EAP-TLV]. Some of these TLVs
may be encapsulated in a protected channel (which is itself
implemented as a dedicated TLV), quite similarly to the ideas
presented in [PEAP-TLV].
1.2 Caveats
Since PSK are of frequent use in security protocols, because a PSK
simply means a cryptographic key in the symmetric cryptographic
setting (see e.g. [HAC] for an introduction to symmetric vs.
asymmetric cryptography), attention should be paid not to confuse
EAP-PSK with any other protocols that may also refer to a PSK, for
instance [WPA] when used in its PSK mode.
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EAP-PSKÆs PSK should also not be confused with the PSKs possibly used
by other protocols relying on PSKs: EAP-PSKÆs PSK should be
cryptographically separated from any other PSK or else the security
of EAP-PSK might be voided. It is a rule of the thumb in cryptography
to use different keys for different applications.
The generation of the PSK used by EAP-PSK is outside of the scope of
this document. The PSK SHOULD be generated by a good source of
randomness (see [RFC 1750]). In particular, a PSK should not be
confused with a password. However, in case one wants to generate the
PSK from a password, although this is strongly discouraged, guidance
to do so is provided in annex B.
The definition of the repository of the PSK used by EAP-PSK is also
outside of the scope of this document. In particular, nothing
prevents from storing EAP-PSK's PSK on a tamper-resistant device such
as a smart card rather than having it memorized or written down on a
sheet of paper. Indeed such a storage on a smart card might allow to
choose stronger credentials and to avoid their duplication (that is
to say, for instance, preventing a user from easily sharing its PSK
with somebody else).
1.3 Specification of requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2219].
1.4 Terminology
TBC
This document frequently uses the following terms:
AES
Advanced Encryption Standard, a block cipher, please refer
to [AES] for more details
Backend Authentication Server
A backend authentication server is an entity that provides
an authentication service to an authenticator. When used,
this server typically executes EAP Methods for the
Authenticator. [This terminology is also used in
[IEEE 802.1X.].]
EAP Authenticator (or simply Authenticator)
The end of the EAP link initiating the EAP authentication
methods. [Note: This terminology is also used in
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[IEEE 802.1X.], and has the same meaning in this
document].
EAP Peer (or simply Peer)
The end of the EAP Link that responds to the
authenticator. [Note: In [IEEE 802.1X], this end is
known as the Supplicant.]
EAP Server (or simply Server)
The entity that terminates the EAP authentication with the
peer. In the case where there is no Backend Authentication
Server, this term refers to the EAP Authenticator. Where
the EAP Authenticator operates in pass-through, it refers
to the Backend Authentication Server.
MAC
Message Authentication Code. Informally, the purpose of a
MAC is to facilitate, without the use of any additional
mechanisms, assurances regarding both the source of a
message and its integrity, please refer to [HAC] for more
details
MAP
Mutual Authentication Protocol. The name of a family of
protocols that allow mutual authentication and that are
specified in [EAKD]
OMAC
One Key CBC-MAC, a method to generate a message
authentication code, please refer to [OMAC] for more
details
PSK
Pre-shared key. A pre-shared key is a cryptographic key in
the symmetric setting that is derived by some out-of-band
mechanism. It is merely a sequence of binary digits of
given length that should have been chosen at random.
PRF
Pseudo-random function. Please refer for instance to [EAX]
for a precise cryptographic definition of this term
PRP
Pseudo-random permutation. Please refer for instance to
[EAX] for a precise cryptographic definition of this term
Temporary Identity
A temporary identity, contrary to a permanent identity, is
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a NAI that the user uses a pseudonym for one EAP dialog.
This features is used for identity protection. It is the
server's responsibility to provide the peer with temporary
identities in case identity protection is required.
1.5 Conventions
All numbers involved in cryptographic calculations are considered in
network-byte order.
|| denotes concatenation of strings.
[String] denotes the MAC of String calculated as specified by the
context.
** denotes integer exponentiation.
ôiö denotes the unsigned binary representation on 128 bits of the
integer I in network byte order. Therefore this notation only makes
sense when i is between 0 and 2**128-1.
1.6 Related work
There exist other EAP methods which also somehow rely on PSKs:
[EAP-SIM] and [EAP-AKA] are two of them, which do not directly
compete with EAP-PSK since they are designed to take advantage of the
GSM and UMTS infrastructures. There are therefore not easy to deploy
in case one does not dispose of such infrastructures.
EAP-MD5, described in [EAP], is another one, which has however been
deprecated for security reasons: it is not safe to use it over
insecure networks.
EAP-OTP, described in [EAP], and other One-Time Password methods such
as [EAP-SecurID] may also rely on PSKs but they also do not directly
compete with EAP-PSK since they require additional elements (e.g. the
One-Time password generator and server) on the client and on the
server side.
[LEAP] (Lightweight EAP) also known as EAP-Cisco Wireless is quite
similar to EAP-PSK. However it is a proprietary protocol that has
been shown to bear cryptographic weaknesses (see for instance
[LEAPVUL]).
[EAP-SKE] is quite similar to EAP-PSK. This method is still work in
progress and has put emphasis on network efficiency in roaming
situations. Work could be done to merge EAP-PSK and EAP-SKE.
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[EAP-Archie] is very similar to EAP-PSK. This method is still work in
progress and has very much inspired this work. EAP-PSK makes
amendments to the cryptographic parts specified in EAP-Archie and
provides the protected channel and the TLV approach as new features.
Work could be done to merge EAP-PSK and EAP-Archie.
[EAP-SRP] is quite different of EAP-PSK except since it uses both
symmetric and asymmetric cryptography and it is subject to IPR claims
by Stanford university.
2. Protocol overview
2.1 Cryptographic design of EAP-PSK
EAP-PSK rely on a single cryptographic primitive, a block cipher,
which is instantiated with AES-128. This instantiation has been
chosen because:
1. AES-128 is standardized and its implementations are widely
available
2. AES-128 has been carefully reviewed by the community and is
believed to be secure
For a description of AES-128, please refer to [AES].
However, in case it should be needed, new instantiations of EAP-PSK
could easily be proposed as it does not intricately depend on the
chosen block cipher. The only parameters of the block cipher that
EAP-PSK depends on are its block size (128 bits) and its key size
(128 bits).
EAP-PSK uses three cryptographic parts:
1. An authentication protocol to mutually authenticate the
communicating parties, that follows the design presented in
[EAKD] under the name MAP1 instantiated with [OMAC] and [AES]
2. A key derivation protocol to derive keying material according to
the EAP Key Management Framework [EKMF], that uses the modified
counter mode presented in [SOBMMO]
3. An authenticated encryption protocol with associated data to
provide a protected channel for both mutually authenticated
parties to communicate securely within the method, that uses the
[EAX] mode of operation
2.1.1 The authentication
The authentication protocol used by EAP-PSK is the Mutual
Authentication Protocol 1.
The Mutual Authentication Protocol 1 (MAP1) is described in [EAKD].
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It consists of a one and half round trip exchange:
Bob Alice
| |
| RA |
|<---------------------------------------------------------|
| |
| [B||A||RA||RB] |
|--------------------------------------------------------->|
| |
| [A||RB] |
|<---------------------------------------------------------|
where:
1. RA and RB are random numbers chosen respectively by Alice and
Bob
2. A and B are Alice's and Bob's respective identities
EAP-PSK instantiates this protocol with:
1. RA and RB 128 bit random numbers chosen respectively by Alice
and Bob
2. A and B are Alice's and BobÆs respective permanent full NAIs. It
was chosen to include their permanent full NAIs in the
calculation of the MAC (and not the temporary NAIs they may be
using in case identity protection is enabled) to avoid getting
out of the setting of the security proof of [EAKD]. Choosing to
use the permanent full NAIs may possibly endanger identity
protection (though there doesn't seem to be any trivial attack
taking advantage of this) but since mutual authentication has a
security proof and identity protection currently does not, it
seems reasonable to preserve the existing security proof. [EAKD]
was chosen for historical reasons (since EAP-PSK was very much
influenced by EAP-Archie, please refer to section 9) and because
this scheme enjoyed a security proof.
3. The MAC algorithm we use is OMAC1 with AES-128 using a 128-bit
portion of the PSK called AK (see section 2.2) and producing a
tag length of 128 bits. OMAC was chosen because of its handling
of arbitrary length messages and its design simplicity (though
some care must be taken in its implementation to avoid side
channel attacks, please refer to [EAX]). It also enjoys a
security proof, which has so far not been found to be flawed and
is believed to have been extensively reviewed by the
cryptographic community.
2.1.2 The key derivation
The key derivation is realized using the modified counter mode.
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The modified counter mode is described in [SOBMMO]. This mode was
chosen because it seems to be one of the rare simple key derivation
schemes that relies on a block cipher and has a proof of its
security.
EAP-PSK instantiates this modified counter mode with all rotation
values (the ri following [SOBMMO] Figure 3 notation) taken equal to
zero (no rotations) and the counter values (the ci following [SOBMMO]
Figure 3 notation) taken respectively equal to the first t integers
(that is ci=i, starting with i=1).
The parameter t is taken equal to 9.
The underlying block cipher used by this counter mode is AES-128.
The input block to the key derivation is taken to be: [B||A||RA|RB]
as per section 2.1.1 notation.
For what regards the output blocks:
1. The first output block of the key derivation is taken to be the
TEK
2. The second to fifth output blocks of the key derivation are
taken to be the MSK
3. The sixth to ninth output blocks of the key derivation are taken
to be the EMSK
2.1.3 The protected channel
To provide a protected channel within EAP-PSK in case of a successful
authentication, EAP-PSK uses the EAX mode of operation described in
[EAX].
EAX is instantiated with AES-128 as the underlying block cipher keyed
with the TEK.
EAX is instantiated within EAP-PSK with a tag length of 128 bits.
The nonce N used by EAX (following [EAX] Figure 3 notation) is a
counter starting with ô0ö and incremented by the one at each
subsequent EAP-PSK message (except retransmissions of course) within
one EAP-PSK dialog. Thus, N can be considered a replay counter.
The message M used by EAX (following [EAX] Figure 3 notation)
consists of the message that one party wishes to send to the other
over the protected channel (i.e a concatenation of EAP-PSK
attributes).
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The header H used by EAX (following [EAX] Figure 3 notation) is
currently unused and thus taken to be the empty string.
EAX was mainly chosen because it strongly relied on OMAC in its
design and OMAC had already been chosen in EAP-PSK for the
authentication part. It was also chosen because of the simplicity of
its design and the security proof it enjoys. It should however be
understood that there are currently many other proposed modes for
authenticated encryption with associated data (including IPR free
ones, like CCM, CWC or GCM, please refer to [MOSKBC] for more
details) and that the complexity and novelty of EAX security proof
may be a concern.
2.2 EAP-PSK key hierarchy
This section instantiates the EAP Key hierarchy described in [EKMF]
for EAP-PSK.
2.2.1 The PSK
EAP-PSK uses a long-lived 256-bit secret shared between the EAP Peer
and the EAP Server called the PSK.
The PSK has an internal structure. It consists of two 128-bit
subkeys, respectively called the authentication key (AK) and the key-
derivation key (KDK). The protocol uses the AK to mutually
authenticate the EAP Peer and the EAP Server. The protocol uses the
KDK to derive keying material between the EAP Peer and the EAP
Server. It should be emphasized that EAP-PSK assumes that AK and KDK
are cryptographically separated and that its security proof rely on
this assumption. In case, EAP-PSK's PSK is drawn at random from the
set of possible keys, this assumption is verified.
EAP-PSK assumes that the PSK is known only to the EAP Peer and EAP
Server, and the security properties of the protocol may be
compromised if it has wider distribution.
The protocol also assumes the EAP Server and EAP Peer identify the
correct PSK to use with the other by their respective [NAI]s.
2.2.2 The TEK
EAP-PSK allows for TEK derivation from a MAC exchanged during
authentication and the KDK.
The TEK is a 128 bit key that is used to set a protected channel for
both mutually authenticated parties to communicate securely within
the method.
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2.2.3 The MSK
EAP-PSK allows for MSK derivation from a MAC exchanged during
authentication and the KDK.
As specified in [EKMF], the MSK is 64 bytes long.
2.2.4 The EMSK
EAP-PSK allows for EMSK derivation from a MAC exchanged during
authentication and the KDK.
As specified in [EKMF], the EMSK is 64 bytes long.
2.2.5 The IV
EAP-PSK does not derive any IV.
2.3 EAP-PSK message flow
2.3.1 EAP-PSK basic message flow
Basically, EAP-PSK is comprised of six messages (i.e. three round
trips):
1. The first message is sent by the server to the peer to request
its identity
2. The second message is sent by the peer to the server to answer
the identity request: the peer indicates its identity (temporary
or permanent) to the server
3. The third message is sent by the server to the peer and starts
the mutual authentication procedure: it essentially consists of
a random value chosen by the server
4. The fourth message is sent by the peer to the server and
contains a random value chosen by the peer and an authentication
tag calculated by the peer over both random values as well as
the peer and serverÆs permanent full NAIs to prove the identity
of the peer to the server
5. The fifth message is sent by the server to the peer and contains
an authentication tag calculated over the random value chosen by
the peer and the serverÆs permanent full NAI to prove the
identity of the server to the peer. This message may also
contain data encapsulated in a protected channel that has just
been set up as a result of the authentication procedure. This
data at least serves to make sure that the protected channel was
correctly set up and to give indications on the probable future
decision of the server. It may also be used to give to the peer
the next temporary identity it should use.
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6. The sixth message is sent by the peer to the server and may also
contain data encapsulated in a protected channel that has just
been set up as a result of the authentication procedure
Peer Server
| EAP-PSK/AT_IDREQ |
|<---------------------------------------------------------|
| |
| EAP-PSK/AT_IDRES |
|--------------------------------------------------------->|
| |
| EAP-PSK/AT_Rand |
|<---------------------------------------------------------|
| |
| EAP-PSK/AT_Rand, AT_MAC |
|--------------------------------------------------------->|
| |
| EAP-PSK/AT_MAC, AT_PCHANNEL |
|<---------------------------------------------------------|
| |
| EAP-PSK/AT_PCHANNEL |
|--------------------------------------------------------->|
| |
This basic message flow could be comprised of only three messages,
were it not the request/response nature of EAP that prevents the
third message to be the last one. We take advantage of this situation
by mandating the setup of a protected channel.
The basic message flow also include a fresh query by the server of
the peer's identity, since the current revision of EAP ([EAPbis])
recommends to do so in its section 5.1.
2.3.2 EAP-PSK advanced message flows
EAP-PSK provides different advanced features that may lead to
different message flows:
1. Identity protection. This feature might add an additional
roundtrip to the basic message flow when used. This additional
round trip is inserted after the second message in the basic
flow when the peer and the server fall out of synchronization on
the pseudonym the peer uses to protect its identity. Hence, when
the server does not recognize the identity provided by the peer
in response to EAP-PSK/AT_IDREQ, the server sends a message to
the peer asking it to disclose its permanent identity. The peer
MAY respond to this message if its security policy allows him to
do so by sending its permanent identity or by resending its
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temporary identity or it MAY not respond to this request. The
basic message flow then proceeds.
2. Fast reconnect. TBC
3. Conversation over the protected channel. This feature might add
some round trips to the basic message flow when used. These
additional round trips are inserted after the sixth message in
the basic message flow. They consist in exchanging data between
the peer and the server over the protected channel that has been
set up thanks to the authentication. This protected data
exchange might for instance be of some use if the peerÆs account
is pre paid and is charged on a per packet or temporal basis: in
case the peer wants to top it up, it can do so, e.g. by making a
financial transaction with the server. This protected data
exchange might also be used to check the identity of the claimed
Authenticator that the peer has connected to. In each message
exchanged over the protected channel, both parties indicate
whether the exchange has to continue, may continue or is done
for them and what their current decision is (unconditional
success, conditional success or failure). These indications
mimic the variables presented in [EAP-SM]
2.4 Retry behavior
EAP-PSK complies to the EAP retry behavior described in [EAP], that
is: the EAP Server is responsible for retry behavior. This means that
if the EAP Server does not receive a reply from the peer, it MUST
resend the EAP-Request for which it has not yet received an EAP-
Response. However, the EAP Peer MUST NOT resend EAP-Response messages
without first being prompted by the EAP Server.
As a result, it is possible that an EAP Peer will receive duplicate
EAP-Request messages, and may send duplicate EAP-Responses. Both the
EAP Peer and the EAP Server should be engineered to handle this
possibility.
2.5 Fragmentation
EAP-PSK does not support fragmentation or reassembly.
Therefore it is to be used either over networks which MTU is enough
to convey encapsulated EAP-PSK packets without fragmentation or
encapsulated in other protocols which take care of fragmentation and
reassembly.
However, in case it is needed, fragmentation support might well be
provided for communication over the protected channel thanks to, for
instance, an AT_Fragment attribute.
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3. EAP-PSK state machines
This section is TBC
The state diagrams given below are simplistic. Their goal is to give
a compact description of EAP-PSK.
Due to its TLV design (i.e. attribute usage), this document only
specifies the attributes that MUST be included in the requests or
responses. Though it does not forbid adding other non-mandatory
attributes, the peer and the server SHOULD for now ignore any non-
mandatory attribute that are not encapsulated in the AT_PCHANNEL
attribute.
3.1 EAP-PSK peer state machine
+-----------+ +-----------+ +-----------+
| | | |<--+ | |
--->| Init |--->| ID_sent | | | Failure |<----+
| | | |---+-->| | |
+-----------+ +-----------+ | +-----------+ |
| |
| |
+-----------+ | |
| |<--+ |
+--------------------| MAC_sent | |
| | |-------------------------+
| +-----------+ |
| |
+--------------------------+-------------------------------+
| |
| +-----------+ | +-----------+
| | | | | |
+-->| P_Channel |----------+------------>| Success |
| | | |
+-----------+ +-----------+
3.1.1 The Init state
The peer starts in the Init state: in this state it awaits the first
EAP-PSK request of the EAP server. This first request MUST be an EAP-
PSK packet containing at least one attribute: the AT_IDREQ attribute.
The peer MUST silently discard any EAP-PSK packet that does not
contain the AT_IDREQ attribute. If identity protection is a concern,
the server SHOULD NOT request the peer's permanent identity.
Upon reception of an EAP-PSK packet that contains the AT_IDREQ
attribute, depending on the flag set in this attribute, the
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identities the peer currently has and its security policy the peer
has three options:
1. Provide the requested identity if it is able to do so (e.g. if
it hasn't lost its current temporary identity) and if its
security policy doesn't prevent him from doing so (e.g. if the
server directly request the peer's permanent identity, which the
peer is configured never to disclose)
2. Provide its permanent identity if its security policy allows him
to do so, whatever the identity requested by the server was
3. Silently discard the received packet if the peer cannot or
doesn't want to answer until either the peer or the server fails
(for instance due to time out or retry counter or retry counter
limit) or it receives a packet it can and wants to answer to.
If the peer chooses option #1 or option #2, it sends the
corresponding response (i.e. an EAP-PSK packet that MUST contain the
AT_IDRES attribute) and its EAP-PSK method state moves to ID_send
3.1.2 The ID_sent state
In this state the peer MAY receive two different types of EAP-PSK
packets:
1. An EAP-PSK packet that contains an AT_IDREQ attribute
2. An EAP-PSK packet that contains an AT_RAND attribute
For now the two situations described above SHOULD be considered to be
mutually exclusive (i.e. if the peer receives an EAP-PSK packet that
contains both AT_IDREQ and AT_RAND attributes), it SHOULD silently
discard it. However, this point, as much of this document is open to
discussion and may evolve (see Annex A).
In case it receives an EAP-PSK packet that contains an AT_IDREQ
attribute, the peer has the same options as when it received an EAP-
PSK packet containing and AT_IDREQ attribute while in the Init state.
The only difference, is that the peer knows that the identity it has
already sent seems not to have been recognized by the server. [Note:
the state machine diagram however indicates that in case the peer
wants then to take option #3 of the Init state while in the ID_sent
state, it also can go directly to the Failure state without waiting
for time outs or retry counter limits, this behavior is open to
discussion, see annex A]. In this situation the peer either stays in
the ID_sent state or moves to the Failure state.
In case it receives an EAP-PSK packet that contains an AT_RAND
attribute, the peer builds an EAP-PSK packet in response that MUST
contain an AT_MAC attribute and an AT_RAND attribute and moves on to
the MAC_sent state. To build this AT_MAC, it begins by choosing
himself a 128 random value, which will be the value of the AT_RAND
attribute that the peer MUST include in its response. The value of
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the AT_MAC attribute is then computed according to the following
formula (please see also section 2.1.1):
[B||A||RA||RB]
where:
o B stands for the ASCII encoding of the permanent full NAI of the
peer
o A stands for the ASCII encoding of the permanent full NAI of the
server
o RA stands for the value of the AT_RAND the peer has just
received
o RB stands for the random value the peer has just chosen to build
the AT_RAND it must include in its response
3.1.3 The MAC_sent state
In this state, the peer MUST receive an EAP-PSK packet that contains
an AT_MAC attribute and an AT_PCHANNEL attribute.
Upon reception of an EAP-PSK packet that contains an AT_MAC attribute
and an AT_PCHANNEL attribute, the peer MUST start by processing the
AT_MAC attribute. It MUST verify that the value of this attribute is
equal to the value given by the following formula (please see also
section 2.1.1):
[A||RB]
where:
o A stands for the ASCII encoding of the permanent full NAI of the
server
o RB stands for the value of the AT_RAND attribute that the peer
has just sent to the server
In case, this verification succeeds the peer moves to the P_channel
state to process the AT_PCHANNEL attribute. In case, it fails the
peer moves to the Failure state.
3.1.4 The P_Channel state
In this state, the peer only processes EAP-PSK messages that contain
an AT_PCHANNEL attribute. Furthermore, the AT_PCHANNEL MUST be the
only attribute that the peer processes.
While processing an AT_PCHANNEL attribute, the peer MUST first check
the replay counter (see section 2.1.3). If the replay counter
verifies (that is to say is equal to the value of the last counter
sent incremented by one starting with a counter value of 0), the peer
then decrypts the value of the attribute AT_PCHANNEL and processes
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the attributes encapsulated within. Any EAP-PSK packet that fails to
verify the replay counter or decrypt the value of the AT_PCHANNEL
MUST be discarded without any further processing.
The AT_PCHANNEL attribute MUST contain an AT_STATUS attribute that
both indicates (adopting EAP state machine terminology, please refer
to [EAP-SM]):
o The server's current decision regarding the peer (FAIL,
COND_SUCC, UNCOND_SUCC)
o The server's current decision regarding the continuation of the
method (CONT, MAY_CONT, DONE)
If the peer receives a valid AT_PCHANNEL attribute, it must answer
with an EAP-PSK packet that also contains a valid AT_PCHANNEL
attribute.
While in this state the peer, depending on its policy, its current
STATUS variables and the STATUS variables it receives from the
server,
o Stay in this state
o Advance to Failure state
o Advance to Success state
3.1.5 The Failure state
In this state, the peer has decided that the current EAP-PSK dialog
has failed and MUST discard any incoming EAP-PSK packet corresponding
to that dialog. It waits for time outs, retry counter limits or an
EAP-Failure packet. This state is an EAP-PSK state and must not be
confused with a possible EAP state (e.g. Failure state in Figure 3 of
[EAP-SM]).
3.1.6 The Success state
In this state, the peer has decided that the current EAP-PSK dialog
has succeeded and MUST discard any incoming EAP-PSK packet
corresponding to that dialog. This state is an EAP-PSK state and must
not be confused with a possible EAP state (e.g. Success state in
Figure 3 of [EAP-SM]).
3.2 EAP-PSK server state machine
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+-----------+ +-----------+ +-----------+
| | | |<--+ | |
--->| Init |--->| ID_req | | | Failure |<----+
| | | |---+-->| | |
+-----------+ +-----------+ | +-----------+ |
| |
| |
+-----------+ | |
| |<--+ |
+--------------------| MAC_req | |
| | |-------------------------+
| +-----------+ |
| |
+--------------------------+-------------------------------+
| |
| +-----------+ | +-----------+
| | | | | |
+-->| P_Channel |----------+------------>| Success |
| | | |
+-----------+ +-----------+
3.2.1 The Init state
The server starts in the Init state: in this state, it builds an EAP-
PSK packet that contains an AT_IDREQ attribute. It sets the flags of
the AT_IDREQ (see section 4) according to its policy. To provide
identity protection, the server SHOULD not set any of these flags in
the first request it issues to the peer, that is to say, that the
server SHOULD accept temporary identities.
After sending this EAP-PSK packet, the server advances to the ID_req
state.
3.2.2 The ID_req state
In this state, the server waits for the response of the peer. The
response of the peer MUST be an EAP-PSK packet that contains an
AT_IDRES attribute.
If the server does not recognize the identity sent by the peer, it
can either:
o Send another EAP-PSK packet that contains an AT_IDREQ attribute,
with flag settings that MAY differ from the first EAP-PSK packet
containing an AT_IDREQ attribute that was issued
o Move to Failure state
If the server ever issues an EAP-PSK packet with the permanent
identity flag of the AT_IDREQ attribute set, it MUST send this
attribute again within the EAP-PSK protected channel at the end of
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the EAP-PSK dialog to allow the detection of attacks against the
identity protection.
If the server recognizes the identity sent by the peer, it chooses at
random a 128 bit value and sends an EAP-PSK packet that MUST contain
an AT_RAND which value is the random number it has just chosen. It
then moves to the MAC_req state.
3.2.3 The MAC_req state
In this state, the server waits for an EAP-PSK packet that contains
an AT_MAC and an AT_RAND attribute.
Upon reception of an EAP-PSK packet that contains an AT_MAC attribute
and an AT_RAND attribute, the server MUST verify that the value of
the AT_MAC attribute is equal to the value given by the following
formula (please see also section 2.1.1):
[B||A||RA||RB]
where:
o B stands for the ASCII encoding of the permanent full NAI of the
peer
o A stands for the ASCII encoding of the permanent full NAI of the
server
o RA stands for the value of the AT_RAND the server sent before
entering the MAC_req state
o RB stands for the value of the AT_RAND the server has just
received
In case, this verification succeeds the server send an EAP-PSK that
contains an AT_MAC and an AT_PCHANNEL attribute and moves to the
P_channel state. The value of the AT_MAC attribute sent by the server
is given by the following formula (please see also section 2.1.1):
[A||RB]
where:
o A stands for the ASCII encoding of the permanent full NAI of the
server
o RB stands for the value of the AT_RAND attribute that the peer
has just sent to the server
The AT_PCHANNEL attribute MUST contain an AT_STATUS attribute and is
calculated according to the procedure specified in section 2.1.3 and
4. If the server and the peer care about identity protection, it
SHOULD also contain an AT_IDRES attribute which value gives the peer
its next temporary identity to use with EAP-PSK.
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In case, the verification of the AT_MAC sent by the peer fails the
server moves to the Failure state.
3.2.4 The P_channel state
In this state, the server only processes EAP-PSK messages that
contain an AT_PCHANNEL attribute. Furthermore, the AT_PCHANNEL MUST
be the only attribute that the server processes.
While processing an AT_PCHANNEL attribute, the server MUST first
check the replay counter (see section 2.1.3). If the replay counter
verifies (that is to say is equal to the value of the last counter
sent incremented by one starting with a counter value of 0), the
server then decrypts the value of the attribute AT_PCHANNEL and
processes the attributes encapsulated within. Any EAP-PSK packet that
fails to verify the replay counter or decrypt the value of the
AT_PCHANNEL MUST be discarded without any further processing.
The AT_PCHANNEL attribute MUST contain an AT_STATUS attribute that
both indicates (adopting EAP state machine terminology, please refer
to [EAP-SM]):
o The server's current decision regarding the peer (FAIL,
COND_SUCC, UNCOND_SUCC)
o The server's current decision regarding the continuation of the
method (CONT, MAY_CONT, DONE)
While in this state the server, depending on its policy, its current
STATUS variables and the STATUS variables it receives from the peer,
o Stay in this state
o Advance to Failure state
o Advance to Success state
3.2.5 The Failure state
In this state, the server has decided that the current EAP-PSK dialog
has failed and MUST discard any incoming EAP-PSK packet corresponding
to that dialog. The logical next step is the server sending an EAP
Failure packet. This state is an EAP-PSK state and must not be
confused with a possible EAP state (e.g. Failure state in Figure 4 of
[EAP-SM]).
3.2.6 The Success state
In this state, the server has decided that the current EAP-PSK dialog
has succeeded and MUST discard any incoming EAP-PSK packet
corresponding to that dialog. This state indicates to EAP that EAP-
PSK has succeeded. This state is an EAP-PSK state and must not be
confused with a possible EAP state (e.g. Success state in Figure 4 of
[EAP-SM]).
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4. EAP-PSK message format
4.1 Table of the Type field of the different attributes
TBC
Attribute Name Type Field Value
AT_IDREQ 1
AT_IDRES 2
AT_RAND 3
AT_MAC 4
AT_PCHANNEL 5
AT_STATUS 6
4.2 Format of the different attributes
This section presents the respective formats of the different
attributes listed in alphabetical order.
Within the different attribute formats, reserved bytes are specified.
These reserved bytes are essentially here for word alignment on 32
bit boundaries. They are set to zero when sending and ignored on
reception.
4.2.1 AT_IDREQ
The format of the AT_IDREQ attribute is shown below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_IDREQ | Length = 1 |P| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The flag P is set to one when the permanent identity of the peer is
requested and to 0 otherwise.
The use of the AT_IDREQ is defined in section 3.
4.2.2 AT_IDRES
The format of the AT_IDRES attribute is shown below.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_IDRES | Length | Actual Identity Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Identity :
: . :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The use of the AT_IDRES is defined in Section 3.
The value field of this attribute begins with 2-byte actual identity
length, which specifies the length of the identity in bytes. This
field is followed by the subscriber identity of the indicated actual
length.
The identity does not include any terminating null characters.
Because the length of the attribute must be a multiple of 4 bytes,
the sender pads the identity with zero bytes when necessary.
4.2.3 AT_MAC
The AT_MAC attribute is used for authentication within EAP-PSK.
The value field of the AT_MAC attribute contains two reserved bytes
followed by a 128 bit keyed message authentication code (MAC). The
MAC is calculated over message-specific data.
The contents of the message-specific data that are MACed are
specified separately for each EAP/PSK message in Section 2.1.1.
The format of the AT_MAC attribute is shown below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_MAC | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MAC |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
When the AT_MAC attribute is expected to be included in an EAP-PSK
message, the recipient MUST process the AT_MAC attribute before
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looking at any other attributes (apart from the attributes that may
be necessary to the verification of the MAC i.e AT_RANDs).
If the message authentication code is absent or invalid, then the
recipient MUST ignore all other attributes in the message and operate
as specified in Section 3.
4.2.4 AT_PCHANNEL
AT_PCHANNEL_is used to transmit information between the peer and
server over a protected channel, that is to say a channel that
provides confidentiality, data origin authentication and replay
protection.
The format of the AT_PCHANNEL attribute is shown below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_PCHANNEL | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Nonce |
| |
| |
|---------------------------------------------------------------|
| |
| Tag |
| |
| |
|---------------------------------------------------------------|
| |
| Payload |
: :
: :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value field of the AT_PCHANNEL attribute consists of one reserved
bytes followed by a 4 byte nonce, a 4 byte tag and a variable length
payload.
The nonce is calculated as specified in section 2.1.3.
The tag is 128 bit long. It is calculated as specified in 2.1.3 and
in [EAX].
The payload consists in the cipher text resulting from the encryption
in the EAX mode of operation of the information that the peer and the
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server wish to exchange over the protected channel under Nonce and
the derived TEK (see Section 2.1). The information that the peer and
the server may exchange over the protected channel consists of a
concatenation of EAP-PSK attributes.
4.2.5 AT_RAND
The format of the AT_RAND attribute is shown below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_RAND | Length = 5 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| RAND |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value field of the AT_RAND attribute contains two reserved bytes
followed by a 16 byte random number generated either by the peer or
the server freshly for this EAP-PSK authentication exchange. The
random numbers are used as a mean to authenticate and indirectly as a
seed value for the new keying material.
The server MUST choose a fresh RAND value and send it to the peer at
the beginning of the EAP-PSK exchange. The peer MUST also choose a
fresh RAND value and send it to the server at the beginning of the
EAP-PSK exchange.
The randomness of the RAND values are critical for the security of
EAP-PSK.
4.2.6 AT_STATUS
The format of the AT_STATUS attribute is shown below.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_STATUS | Length |Dec|Con| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
There MUST be exactly one AT_STATUS encapsulated in each AT_PCHANNEL.
The two bit Dec flag is set to:
o 00 if the current decision is FAIL
o 01 if the current decision is COND_SUCC
o 10 if the current decision is UNCOND_SUCC
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The two bit Con flag is set to:
o 00 if the current continuation state is CONT
o 01 if the current continuation state is MAY_CONT
o 10 if the current continuation state is DONE
5. IANA considerations
This document introduces one new IANA consideration.
It requires IANA to allocate a new EAP Type for EAP-PSK.
6. Security considerations
The EAP base protocol [EAPbis] highlights several attacks that are
possible against the EAP protocol as there is no inherent security
mechanisms provided. This section discusses the claimed security
properties of EAP-PSK as well as vulnerabilities and security
recommendations.
6.1 Identity protection
EAP-PSK includes optional identity privacy support that protects the
privacy of the peer identity against passive eavesdropping.
The peer and the server SHOULD store the pseudonym in a non-volatile
memory so that it can be maintained across reboots.
An active attacker that impersonates the server may use the AT-IDREQ
attribute to attempt to learn the peer's permanent identity by
setting the permanent identity flag.
However, it is a matter of policy for the peer to accept to respond
to such requests or not: the peer can refuse to send its permanent
identity if it believes that the server should be able to recognize
its temporary identity. If identity protection is a concern then the
peer MUST NOT send its permanent identity. Any other policy allows
identity protection compromise.
If the peer and server cannot guarantee that the pseudonym will be
maintained reliably and identity privacy is required then additional
protection from an external security mechanism such as Protected
Extensible Authentication Protocol (PEAP) [PEAP] may be used. If an
external security mechanism is in use the identity privacy features
of EAP-PSK may not be useful. The security considerations of using
an external security mechanism with EAP-PSK are beyond the scope of
this document.
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It should also be kept in mind that the layers below EAP-PSK may
disclose elements that can lead to identity protection compromise
(e.g. the peerÆs [IEEE 802.3] Medium Access Control Address).
6.2 Mutual authentication
EAP-PSK provides mutual authentication.
The server believes the peer is authentic because it can calculate a
valid MAC and the peer believes that the server is authentic because
it can calculate a correct MAC.
The authentication protocol used in EAP-PSK, MAP1, enjoys a security
proof in the provable security paradigm, see [EAKD].
The MAC algorithm used in the instantiation of MAP1 within EAP-PSK,
OMAC1, also enjoys a security proof in the provable security
paradigm, see [OMAC].
The underlying block cipher used, AES-128, is widely believed to be a
secure block cipher.
Finally, the key used for mutual authentication, AK, is only used for
that purpose, making thus this part cryptographically independent of
the other parts.
6.3 Key derivation
EAP-PSK supports key derivation.
The key hierarchy is specified in Section 2.2.
The mechanism used for key derivation is the modified counter mode.
The instantiation of the modified counter in EAP-PSK (i.e. the
selected ri and ci) comply with the conditions stated in [SOBMMO] so
that the security proof in the provable security paradigm of [SOBMMO]
holds.
The underlying block cipher used, AES-128, is widely believed to be a
secure block cipher.
The key derivation mechanism uses a dedicated key, the KDK.
The input block to the key derivation is taken to be: [B||A||RA|RB].
The reasons that motivated such a choice are:
o This value is exactly 128 bit long since the tag length chosen
for OMAC is 128 bits
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o This value is believed to be fresh for each new EAP-PSK dialog
since it depends on RA and RB which are two values chosen at
random by the peer and the server through OMAC which is believed
to be a PRF
o This value is believed not to be under the control either of the
peer or the server since it is produced by OMAC which is
believed to be a PRF and it takes into account both the inputs
of the peer and the server
The key derivation scheme is believed to be secure since the modified
counter mode is proved to be a PRF if the underlying block cipher is
a secure PRP, hence the output blocks are cryptographically
separated.
6.4 Dictionary attacks
Because EAP-PSK is not a password protocol, it is not vulnerable to
dictionary attacks: EAP-PSKÆs PSK MUST NOT be derived from a
password. Derivation of EAP-PSKÆs PSK may lead to dictionary attacks.
In case, EAP-PSKÆs PSK is however derived from a password, guidance
is provided in Annex B how to do so. It is also believed that MAP1
makes dictionary attacks harder, since the first value that is MACed
should not be predictable neither for the peer nor the server or
anybody else.
6.5 Protected channel
EAP-PSK provides a protected channel over which the peer and the
server can securely exchange information, in case of a successful
authentication.
This protected channel provides confidentiality, data origin
authentication, replay protection and confirmation of the end of the
conversation.
6.6 Negotiation attacks
EAP-PSK does not protect from negotiation attacks since it currently
does not provide version negotiation as only one version is
specified.
6.7 Fast reconnect
EAP-PSK shall provide fast reconnect. TBC.
6.8 Man-in-the-middle attacks
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Due to the use of symmetric cryptography and the security proofs of
its cryptographic components, EAP-PSK is believed not to be
vulnerable to man-in-the-middle attacks.
There are man-in-the-middle attacks associated with the use of any
EAP method within a tunneled protocol such as PEAP, or within a
sequence of EAP methods followed by each other (see [MTAP]). This
specification does not address these attacks. If EAP-PSK is used with
a tunneling protocol or as part of a sequence of methods, there
should be cryptographic binding provided between the protocols and
EAP-PSK to prevent man-in-the-middle attacks. However the mechanism
how the binding is provided is beyond the scope of this document.
6.9 Generating random numbers
An EAP-PSK implementation SHOULD use a good source of randomness to
generate the random numbers required in the protocol. Please see [RFC
1750] for more information on generating random numbers for security
applications.
7. Security claims
This section provides the security claims required by [EAPbis].
[a] Intended use. EAP-PSK is intended for use over both physically
insecure networks and physically or otherwise secure networks.
Applicable media include but are not limited to PPP, IEEE 802 wired
networks and IEEE 802.11.
[b] Mechanism. EAP-PSK is based on symmetric cryptography and uses a
256 bit Pre-shared Key
[c] Security claims. The security properties of the method are
discussed in Section 6.
[d] Key strength. EAP-PSK supports key derivation with 128-bit
effective key strength.
[e] Description of key hierarchy. Please see Section 2.2.
[f] Indication of vulnerabilities. Vulnerabilities are discussed in
Section 6.
8. Intellectual Property Right Notice
The author neither has, nor is of aware of, any patents or pending
patents relevant to material included in this draft.
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9. Acknowledgements
This EAP method has been inspired by [EAP-SIM] and [EAP-Archie]. It
also considerably reused extracts of these documents. Many thanks to
their respective authors and especially: Jesse Walker, Russ Housley
Henry Haverinen and Joseph Salowey.
Many thanks to Aur‰lien Magniez, Henri Gilbert, Laurent Butti, J‰rŸme
Razniewski and Olivier Charles for their feedback on this document.
10. References
10.1 Normative
[AES] Federal Information Processing Standards (FIPS)
Publication 197, " Specification for the Advanced
Encryption Standard (AES)", National Institute of
Standards and Technology, November 26, 2001.
[EAP] Blunk, L. and Vollbrecht, J., "PPP Extensible
Authentication Protocol (EAP)", RFC 2284, March 1998.
[EAPbis] Blunk, L. et al., "Extensible Authentication Protocol
(EAP)", Internet-Draft (work in progress), February
2004, http://ietf.levkowetz.com/drafts/eap/rfc2284bis/
draft-ietf-eap-rfc2284bis-09.txt
[EAP-SM] Vollbrecht, J. et al., "State Machines for EAP Peer and
Authenticator", Internet-Draft (work in progress),
October 2003, draft-ietf-eap-statemachine-01
[EAKD] Bellare, M, and P. Rogaway, "Entity Authentication and
Key Distribution", CRYPTO 93, LNCS 773, pp232-249,
Springer-Verlag, Berlin, 1994.
[EAX] Bellare, M. et al., "The EAX mode of operation",
January 2004,
http://www.cs.ucsd.edu/users/mihir/papers/eax.pdf
[EKMF] Aboba, B. et al., "EAP Key Management Framework",
Internet-Draft (work in progress), October 2003, draft-
ietf-eap-keying-01.txt
[NAI] Aboba, B., and M. Beadles, "The Network Access
Identifier", RFC 2486, January 1999.
[OMAC] Iwata, T., and Kurosawa., K., ôOMAC: One-Key CBC MACö
Fast Software Encryption, FSE 2003, LNCS, Springer-
Verlag.
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[PPP] Simpson, W., Editor, "The Point-to-Point Protocol
(PPP)", STD 51, RFC 1661, July 1994.
[RFC1750] Eastlake, D. et al., "Randomness Recommendations for
Security", RFC 1750, December 1994.
[RFC 2219] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SOBMMO] Gilbert, H., ôThe Security of One-Block-to-Many Modes
of Operationö, Fast Software Encryption, FSE 2003,
LNCS, Springer-Verlag.
10.2 Informative
[EAP-AKA] Arkko, J. Haverinen, H., ôEAP AKA Authenticationö,
Internet-Draft (work in progress), October 2003,
draft-arkko-pppext-eap-aka-11.txt
[EAP-Archie] Walker, J. and Housley, R., ôThe EAP Archie Protocolö,
Internet-Draft (work in progress), June 2003, draft-
jwalker-eap-archie-01.txt,
[EAP-SecurID] Josefsson, S., ôThe EAP SecurID(r) Mechanismö,
Internet-Draft (work in progress), February 2002,
draft-josefsson-eap-securid-01.txt
[EAP-SIM] Haverinen, H. Salowey, J., "EAP SIM Authentication",
Internet-Draft (work in progress),October 2003, draft-
haverinen-pppext-eap-sim-12.txt
[EAP-SKE] Salgarelli, L., ôEAP SKE authentication and key
exchange protocolö, Internet-Draft (work in progress),
May 2003, draft-salgarelli-pppext-eap-ske-03.txt
[EAP-SRP] Carlson, J. et al., ôEAP SRP-SHA1 Authentication
Protocolô, Internet-Draft (work in progress), draft-
ietf-pppext-eap-srp-04.txt
[EAP-TLV] Hiller, T. et al., "A Container Type for the Extensible
Authentication Protocol (EAP)", Internet-Draft (work in
progress), May 2003, draft-hiller-eap-tlv-01.txt
[HAC] Menezes, A. et al., ôHandbook of Applied Cryptographyö,
CRC Press, 1996.
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[IEEE 802.1X] IEEE STD 802.1X, Standards for Local and Metropolitan
Area Networks: Port Based Access Control, June 14, 2001
[IEEE 802.3] Institute of Electrical and Electronics Engineers,
"Information Technology - IEEE Standard for Information
technology--Telecommunications and information exchange
between systems--Local and metropolitan area networksù
Specific requirements--Part 3: Carrier Sense Multiple
Access with Collision Detection (CSMA/CD) Access Method
and Physical Layer Specificationsö
[IEEE 802.11] Institute of Electrical and Electronics Engineers,
"Information Technology - Telecommunications and
Information Exchange between Systems - Local and
Metropolitan Area Network - Specific Requirements ¡
Part 11: Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) Specifications", IEEE Standard
802.11
[IEEE 802REQ] Stanley, Dorothy et al., ôEAP Method Requirements for
Wireless LANsö, Internet-Draft (work in progress),
January 2004, draft-walker-ieee802-req-00.txt
[LEAP] Macnally, C., ôCisco LEAP protocol descriptionö,
September 2001
[LEAPVUL] Wright, J., ôWeaknesses in LEAP Challenge/Responseö,
Defcon 2003
[MOSKBC] National Institute of Standards and Technology, "Modes
of operation for symmetric key block ciphers",
http://www.csrc.nist.gov/CryptoToolkit/modes/
[MTAP] Asokan, N. et al., ôMan-in-the-middle in Tunnelled
Authentication Protocolsö,
http://eprint.iacr.org/2002/163
[PEAP] Palekar, A. et al., ôProtected EAP Protocol (PEAP)ö,
Internet-Draft (work in progress), October 2003, draft-
josefsson-pppext-eap-tls-eap-07.txt
[PEAP-TLV] Salowey, J., "Protected EAP TLV", Internet-Draft (work
in progress), June 2003, draft-salowey-eap-
protectedtlv-02.txt
[PKCS5] RSA laboratories, ôPKCS #5 v2.0: Password-Based
Cryptography Standardö
[PWD] National Institute of Standards and Technology (NIST).
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ôFIPS PUB 112: Password Usageö. May 30, 1985.
[WPA] Wi-Fi Alliance, ôWi-Fi Protected Accessö, version 2.0,
April 2003
11. Authors' Addresses
Florent Bersani florent.bersani@francetelecom.com
France Telecom R&D
38, rue du General Leclerc
92794 Issy Les Moulineaux Cedex 9
France
12. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS 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."
Annex A: Work still to be done on this document
1. Editorial
1. Clarify the conventions used for the cryptographic calculations.
2. Make this draft more self-contained.
3. Provide test vectors
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4. Complete the cross-references in the text (page, section,
reference numbers)
5. Harmonize the terminology (e. g. octets or bytes)
6. Complete the terminology section (e.g. with temporary identity)
7. Number the figures
2. Security
1. Study alternative ways to produce the input blocks to the key
derivation procedure.
2. Validate the mechanism to prevent abrupt ending of the
conversation on the protected channel.
3. Discuss use of other cryptographic algorithms
4. Discuss the choice of AES-128 and OMAC1 and a tag length of 128
bits (e.g. why don't we use a truncated tag)
5. Should we use the authentication capacity of associated data of
EAX that is currently left aside?
6. Discuss identity protection
7. Specify the how long the same protected channel may be used
without security compromise (a pretty long time I bet but it
should be explicitly evaluated and stated)
8. Specify how long the same protected channel may be used without
security compromise (a pretty long time I bet but it should be
explicitly evaluated and stated)
9. Study DOS attacks resistance
3. Technical
1. Discuss possibility to enhance network efficiency for instance
by including the AT_IDREQ and AT_RAND attribute in the first
packet.
2. Should the peer be allowed to move directly from the Init state
to the failure state to avoid lengthy time outs or retries?
3. Specify how fast reconnect should be implemented
4. Introduce version negotiation
5. Is it desirable to have all attributes aligned on 32 bit
boundaries?
6. Harmonize with other standards or draft standards (e. g. EAP and
EAP Key management framework)
7. Should the AT_STATUS remain a separate attribute from
AT_PCHANNEL?
8. Should an intermediary state be included in the state machine
that would handle the entry in the P_Channel state?
9. Specify all that remain TBC, e.g. the different notification
messages.
10. Make sure that the interface of this EAP method with the EAP
state machine works well
11. Define more attributes
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Annex B: Guidance for PSK generation from a password
It is formally discouraged to use a password to generate a PSK, since
this commonly lead to exhaustive search or dictionary attacks that
would not otherwise be possible.
However, we provide guidance on how to generate the PSK from a
password.
Guidance on how passwords should be selected is provided in [PWD].
The technique presented herein is drawn from [PKCS5]. It is intended
to mitigate the risks associated with password usage in cryptography,
typically dictionary attacks.
If the binary representation in ASCII of the password is strictly
fewer than 128 bit long (which by the way means that the chosen
password is probably weak because it is too short) then its is padded
to 128 bits with zeroes.
If the binary representation in ASCII of the password is strictly
more than 128 bit long, then it is hashed down to exactly 128 bit
using the Matyas-Meyer-Oseas hash (please refer to [HAC] for a
description of this hash) with IV=0x0123456789ABCDEFFEDCBA9876543210
(this value has been arbitrarily selected).
We now assume that we have a 128 bit number derived from the initial
password (that can be the password itself if its binary
representation in ASCII is exactly 128 bit long). We shall call this
number P128.
EAP-PSKÆs PSK is derived thanks to PBKDF2 instantiated with
(following the notations in [PKCS5]):
1. P128 as P
2. The first 96 bits of the binary ASCII representation of the xor
of the peerÆs and the server's NAI as Salt.
3. 5000 as c
4. 48 as dkLen
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