Network Working Group N. Cam-Winget
Internet-Draft D. McGrew
Category: Informational J. Salowey
Expires: April 25, 2005 H. Zhou
Cisco Systems
October 25, 2004
EAP Flexible Authentication via Secure Tunneling (EAP-FAST)
draft-cam-winget-eap-fast-01.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines the Extensible Authentication Protocol (EAP)
based Flexible Authentication via Secure Tunneling (EAP-FAST)
protocol. EAP-FAST is an EAP method that enables secure
communication between a client and a server by using the Transport
Layer Security (TLS) to establish a mutually authenticated tunnel.
However, unlike current existing tunneled authentication protocols,
EAP-FAST also enables the establishment of a mutually authenticated
tunnel by means of symmetric cryptography. Furthermore, within the
secure tunnel, EAP encapsulated methods can ensue to either
facilitate further provision of credentials, authentication or
authorization policies by the server to the client.
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Table of Contents
1. Introduction..................................................3
1.1 Specification Requirements................................4
1.2 Terminology...............................................4
2. Protocol Overview.............................................6
3. Architectural Model...........................................7
4. Protocol Layering Model.......................................7
5. Protected Access Credential (PAC) for EAP-FAST Authentication.8
6. EAP-FAST Authentication.......................................9
6.1 EAP-FAST Authentication Phase 1: Tunnel Establishment.....9
6.2 EAP-FAST Authentication Phase 1: Key Derivations.........10
6.3 EAP-FAST Authentication Phase 2: Tunneled Authentication.11
6.4 Protected EAP Conversation...............................12
6.5 Protected Termination and Acknowledged Result Indication.13
6.6 EAP-FAST Authentication Phase 2: Key Derivations.........14
6.7 Cryptographic Binding: Computing the Compound MAC........15
6.8 EAP-FAST Authentication: Session Key Generation..........15
6.9 PAC Distribution and Refreshing..........................16
7. Version Negotiation..........................................17
8. Error Handling...............................................17
8.1 Error Alerts.............................................18
9. Session Resume...............................................19
10. Fragmentation...............................................20
11. EAP-FAST Detailed Description...............................21
11.1 EAP-FAST Packet Format..................................21
11.2 EAP-FAST TLV Format.....................................23
11.3 TLV format..............................................24
11.4 Result TLV..............................................25
11.5 NAK TLV.................................................26
11.6 Crypto-Binding TLV......................................27
11.7 EAP Payload TLV.........................................28
11.8 Intermediate Result TLV.................................29
11.9 PAC TLV.................................................30
11.9.1 Formats for PAC TLV attributes.....................31
11.9.2 PAC-Key............................................32
11.9.3 PAC-Opaque.........................................32
11.9.4 PAC-Info...........................................33
11.9.5 PAC-Acknowledgement TLV............................35
12. Security Considerations.....................................35
12.1 Mutual Authentication and Integrity Protection..........36
12.2 Method Negotiation......................................36
12.3 Separation of the EAP Server and the Authenticator......37
12.4 Separation of Phase 1 and Phase 2 Servers...............37
12.5 Mitigation of Known Vulnerabilities and Protocol
Deficiencies.................................................38
12.5.1 User Identity Protection and Verification..........39
12.5.2 Dictionary Attack Resistance.......................39
12.5.3 Protection against MitM Attacks....................40
12.5.4 PAC Validation with User Credentials...............41
12.6 PAC Storage Considerations..............................41
12.7 Protecting against Forged Clear Text EAP Packets........42
12.8 Implementation..........................................43
12.9 Security Claims.........................................43
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13. IANA Considerations.........................................44
14. References..................................................44
14.1 Normative...............................................44
14.2 Informative.............................................45
15. Acknowledgments.............................................46
16. Author's Addresses..........................................46
Appendix A: Examples...........................................47
17..............................................................47
17.1 Successful Authentication...............................47
17.2 Failed Authentication...................................48
18. Appendix B: EAP-FAST PRF (T-PRF)............................49
19. Appendix C: Test Vectors....................................50
19.1 Key derivation..........................................50
19.2 Crypto-Bind MIC:........................................51
20. Intellectual Property Statement.............................52
21. Disclaimer of Validity......................................52
22. Copyright Statement.........................................52
23. Expiration Date.............................................52
1. Introduction
The need to provide user friendly and easily deployable network
access solutions has heightened the need to enable strong mutual
authentication protocols that internally use weak user credentials.
While several such authentication protocols [PEAP] [EAP-TTLS] exist
today, they are encumbered by the use of asymmetric cryptographic
operations that often render such protocols prohibitive on very low
end peer devices.
Like [TLS-PSK], EAP-FAST employs symmetric cryptography to allay
the PKI requirements of [PEAP] or [EAP-TTLS]. Additionally, EAP-
FAST employs the TLS client_hello extension [RFC3546] as a further
optimization to minimize the state maintained by the server. EAP-
FAST's design motivations included:
* Mutual Authentication: an AS must verify the identity and
authenticity of the peer, and the peer must verify the
authenticity of an AS.
* Immunity to passive dictionary attacks: as many authentication
protocols require the password to be explicitly provided (either
in the clear or hashed) by the peer to the AS; at minimum, the
communication of the weak credential (e.g. password) must be
immune from eavesdropping
* Immunity to man-in-the-middle (MitM) attacks: in establishing a
mutually authenticated protected tunnel, the protocol must
prevent adversaries from successfully interjecting the
conversation between peer and AS.
* Flexibility to enable support for most password authentication
interfaces: as many different password interfaces (e.g. MSCHAP,
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LDAP, OTP, etc) exist to authenticate a peer, the protocol must
provide this support seamlessly.
* Efficiency: specifically when using wireless media, peers will
be limited in computational and power resources. The protocol
must enable the network access communication to be
computationally lightweight.
With these motivational goals defined, further secondary design
criteria are imposed:
* Flexibility to extend the communications inside the tunnel:
with the growing complexity in network infrastructures the need
to gain authentication, authorization and accounting is also
evolving. For instance, there may be instances in which multiple
(already existent) authentication protocols are required to
achieve mutual authentication. Similarly, different protected
conversations may be required to achieve the proper authorization
once a peer has successfully authenticated. This capability is
similar to [PEAP].
* Minimize the authentication server's per user authentication
state requirements: with large deployments, it is typical to have
many servers acting as the AS for many peers. It is also highly
desirable for a peer to use the same shared secret to secure a
tunnel much the same way it uses the username and password to
gain access to the network. The protocol must facilitate the
use of a single strong shared secret by the peer while enabling
the servers to minimize the per user and device state it must
cache and manage.
1.1 Specification 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 [RFC2119].
1.2 Terminology
Some of the following terms are taken from RFC 2284bis:
EAP 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 authenticator. Where the authenticator
operates in pass-through, it refers to the backend
authentication server.
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Authenticator
The end of the link initiating EAP authentication. The term
Authenticator is used in [IEEE-802.1X], and authenticator has
the same meaning in this document.
Peer
The end of the link that responds to the authenticator. In
[IEEE-802.1X], this end is known as the Supplicant.
Supplicant
The end of the link that responds to the authenticator in [IEEE-
802.1X]. In this document, this end of the link is called the
peer.
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].
Master Session Key (MSK)
Keying material exported by an EAP method.
Man in the Middle (MitM)
An adversary that can successfully inject itself between a peer
and EAP server. The MitM succeeds by impersonating itself as a
valid peer, authenticator or authentication server.
Message Authentication Code (MAC)
A MAC is a function that takes a variable length input and a key
to produce a fixed-length output to carry authentication and
integrity protection of data.
Message Integrity Check (MIC)
A keyed hash function used for authentication and integrity
protection of data. This is usually called a Message
Authentication Code (MAC), but IEEE 802 specifications (and this
document) use the acronym MIC to avoid confusion with Medium
Access Control.
Protected Access Credential (PAC)
Credentials distributed to users for future optimized network
authentication, which always consists of a secret part and an
opaque part. The secret part is secret key material that can be
used in future transactions. The opaque part is presented when
the client wishes to obtain access to network resources. It
aids the server in validating that the client possesses the
secret part.
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Silently Discard
This means the implementation discards the packet without
further processing. The implementation SHOULD provide the
capability of logging the event, including the contents of the
silently discarded packet, and SHOULD record the event in a
statistics counter.
Successful Authentication
In the context of this document, "successful authentication" is
an exchange of EAP messages, as a result of which the
authenticator decides to allow access by the peer, and the peer
decides to use this access. The authenticator's decision
typically involves both authentication and authorization
aspects; the peer may successfully authenticate to the
authenticator but access may be denied by the authenticator due
to policy reasons.
2. Protocol Overview
EAP-FAST is an extensible framework that enables mutual
authentication by using a pre-shared secret to establish a
protected tunnel. Like [PEAP], the protocol is based on TLS;
however, enhancements are made to TLS to enable EAP-FAST to
initiate the tunnel establishment exchange using symmetric
cryptography while minimizing server state. The tunnel is then used
to protect weaker authentication methods, typically based on
passwords.
The pre-shared secret used in EAP-FAST is referred to as the
Protected Access Credential key (or PAC-Key); the PAC-Key is used
to mutually authenticate the client and server when securing a
tunnel. Furthermore, the PAC-Key is refreshed and managed as part
of the EAP-FAST protocol. EAP-FAST allays server state by the use
of a PAC-Opaque, which contains the PAC-Key encrypted by a strong
key only known to the server and sent to the server with the TLS
ClientHello. With the use of PAC-Opaque, EAP-FAST alleviates the
server's need to store per user PAC and state.
The EAP-FAST conversation is used to establish or resume an
existing session to typically establish network connectivity
between a peer and the network. A peer and AS achieve mutual
authentication by invoking a symmetric authenticated key agreement
to protect the communications that further authenticates and
authorizes the client to use the network. A successful result is a
mutual derivation of strong session keys which can then be
provisioned (by the AS) to the network access server (NAS,
typically in 802.11 these are the access points or 802.1X
authenticators).
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3. Architectural Model
The network architectural model for EAP-FAST usage is shown below:
+----------+ +----------+ +----------+ +----------+
| | | | | | | Inner |
| Peer |<---->| Authen- |<---->| EAP-FAST |<---->| Method |
| | | ticator | | server | | server |
| | | | | | | |
+----------+ +----------+ +----------+ +----------+
The entities depicted above are logical entities and may or may not
correspond to separate network components. For example, the EAP-
FAST server and Inner Method server might be a single entity; the
authenticator and EAP-FAST server might be a single entity; or,
indeed, the functions of the authenticator, EAP-FAST server and
Inner Method server might be combined into a single physical
device. For example, typical 802.11 deployments place the
Authenticator in an access point (AP) while a Radius Server may
provide the EAP-FAST and Inner Method server components. The above
diagram illustrates the division of labor among entities in a
general manner and shows how a distributed system might be
constructed; however, actual systems might be realized more simply.
The security considerations section (12) provides an additional
discussion of the implications of separating EAP-FAST from the
inner method.
4. Protocol Layering Model
EAP-FAST packets are encapsulated within EAP, and EAP in turn,
requires a carrier protocol for transport. EAP-FAST packets
encapsulate TLS, which is then used to encapsulate user
authentication information. Thus, EAP-FAST messaging can be
described using a layered model, where each layer encapsulates the
layer beneath it. The following diagram clarifies the relationship
between protocols:
+---------------------------------------------------------------+
| | |
| Lower | TLV Encapsulation (TLVs) |
| to |---------------------------------------------------|
| Upper | TLS |
| Layer |---------------------------------------------------|
| | EAP-FAST |
| |---------------------------------------------------|
| | EAP |
| |---------------------------------------------------|
| | Carrier Protocol (EAPOL, RADIUS, Diameter, etc.) |
+---------------------------------------------------------------+
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The TLV method is a payload with standard Type-Length-Value (TLV)
objects. The TLV objects are used to carry arbitrary parameters
between an EAP peer and an EAP server. All conversations in the
EAP-FAST protected tunnel must be encapsulated in a TLV method.
When the user authentication protocol is itself EAP, the layering
is as follows:
+---------------------------------------------------------------+
| | Inner EAP Method |
| |---------------------------------------------------|
| Lower | TLV Encapsulation (TLVs) |
| to |---------------------------------------------------|
| Upper | TLS |
| Layer |---------------------------------------------------|
| | EAP-FAST |
| |---------------------------------------------------|
| | EAP |
| |---------------------------------------------------|
| | Carrier Protocol (EAPOL, RADIUS, Diameter, etc.) |
+---------------------------------------------------------------+
Methods for encapsulating EAP within carrier protocols are already
defined. For example, 802.1X EAPOL may be used to transport EAP
between client and access point; RADIUS or Diameter are used to
transport EAP between authenticator and EAP-FAST server.
5. Protected Access Credential (PAC) for EAP-FAST Authentication
A pre-shared secret mutually and uniquely shared between the peer
and AS is used to secure a tunnel during EAP-FAST Authentication.
EAP-FAST uses a Protected Access Credential (PAC) to facilitate the
use of a single shared secret by the peer and minimize the per user
state management on the AS. The PAC is a security credential
provided by the AS to a peer and comprised of:
1. PAC-Key: this is a 32-octet key used by the peer to establish
the EAP-FAST Phase 1 tunnel. This key maps as the TLS pre-
master-secret. The PAC-Key is randomly generated by the AS to
produce a strong entropy 32-octet key.
2. PAC-Opaque: this is a variable length field that is sent to
the AS during the EAP-FAST Phase 1 tunnel establishment. The
PAC-Opaque can only be interpreted by the AS to recover the
required information for the server to validate the peer's
identity and authentication. For example, the PAC-Opaque may
include the PAC-Key and the PAC's peer identity. The PAC-
Opaque format and contents are specific to the issuing PAC
server.
3. PAC-Info: this is a variable length field used to provide at
minimum, the authority identity or PAC issuer. Other useful
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but not mandatory information, such as the PAC-Key lifetime,
may also be conveyed by the AS to the peer during PAC
provisioning or refreshment.
As the focus of this draft is to define the EAP-FAST protected
tunneling and authentication mechanism, it does not address
provisioning. That is, provisioning of the PAC may be achieved
using the same mechanisms as the provisioning of any other
credential such as certificates or username/password credential
types.
6. EAP-FAST Authentication
To establish a new session, EAP-FAST employs the PAC to invoke an
authenticated key agreement exchange to establish a protected
tunnel. Once the tunnel is established, the peer and AS can ensue
in further conversations to establish the required authentication
and authorization policies. Part of the authorization policy is
the generation of the Master Session Keys (MSKs). Portions of the
MSKs may be distributed to the NAS using the RADIUS MS-MPPE
[RFC2548] attribute. Finally, the server may also update the PAC
as part of the EAP-FAST protocol conclusion. This section
describes the two phases of EAP-FAST Authentication: Phase 1, the
tunnel establishment and Phase 2, the tunneled authentication.
6.1 EAP-FAST Authentication Phase 1: Tunnel Establishment
This conversation is similar to establishing a new EAP-TLS session
except it uses new EAP type (EAP-FAST).
The initial conversation begins with the authenticator and the peer
negotiating EAP. The authenticator will typically send an EAP-
Request/Identity packet to the peer, and the peer will respond with
an EAP-Response/Identity packet to the authenticator, containing
the username. If the client desires to protect its identity, it
may use an anonymous username.
Once the initial Identity Request/Response exchange is completed,
while the EAP conversation typically occurs between the
authenticator and the peer, the authenticator may act as a pass-
through device, with the EAP packets received from the peer being
encapsulated for transmission to a backend authentication server.
In the discussion that follows, the term "EAP server" or "server"
is used to denote the ultimate endpoint conversing with the peer.
Once having received the peer's Identity, and determined that EAP-
FAST Authentication is to occur, the EAP server must respond with a
EAP-FAST/Start packet, which is an EAP-Request packet with EAP-
Type=EAP-FAST and the Start (S) bit set. The EAP-FAST/Start packet
shall also include an authority identity (A-ID) TLV to better
inform the peer the server's identity. Assuming that the peer
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supports EAP-FAST, the EAP-FAST conversation will then begin, with
the peer sending an EAP-Response packet with EAP-Type=EAP-FAST.
The data field of the EAP-Response packet contains an EAP-FAST
encapsulated TLS ClientHello handshake message.
The ClientHello message contains the peer's challenge (also called
the client_random) and PAC-Opaque. As there may be different EAP-
FAST servers a peer may encounter, a peer may be provisioned with
unique PACs uniquely identified by the A-ID corresponding to the
EAP-FAST server. A peer may choose to cache the different PACs and
determine based on the A-ID the corresponding PAC to employ. While
EAP-FAST is capable of supporting any ciphersuite, in this version,
the ClientHello uses the TLS_RSA_WITH_RC4_128_SHA ciphersuite. As
EAP-FAST uses the PAC to establish the keys, the RSA key exchange
is not executed, but the specification of RC4 and SHA signals the
EAP server that the tunnel must be protected using 128bit RC4 for
confidentiality and SHA1 for authenticity.
The EAP server will then respond with an EAP-Request packet with
EAP-Type=EAP-FAST. The data field of this packet will encapsulate
three TLS records, ServerHello, ChangeCipherSpec and Finished
messages. The ServerHello will contain a server_random and
ChangeCipherSpec. The TLS Finished message, sent immediately after
the ChangeCipherSpec message, contains the first protected message
with the negotiated algorithm, keys, and secrets.
The server generates the master_secret prior to composing the EAP-
FAST TLS ServerHello message to properly generate the TLS Finished
message contents. The server must compute the Tunnel Keys as
described in Section 6.2 at this time to properly respond and
generate its TLS Finished message.
The peer in turn, must consume the ServerHello to extract the
server_random before it can generate the master_secret and Tunnel
Keys, as described in Section 6.2.
After verifying the server Finished message, the peer responds back
with two TLS records, a ChangeCipherSpec and the peer's TLS
Finished message. At this state, the client is ready to receive
and transmit protected messages with the server.
Upon verifying the peer's Finished message, the EAP server
establishes the tunnel and is ready for the receiving and
transmitting protected messages with the peer. The messages are
protected using the Tunnel Keys described in Section 6.2.
6.2 EAP-FAST Authentication Phase 1: Key Derivations
The EAP-FAST Authentication tunnel key is calculated similarly to
the TLS key calculation with an additional 40 octets (referred to,
as the session_key_seed) generated. The additional
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session_key_seed is used in the Session Key calculation in the EAP-
FAST Tunneled Authentication conversation.
An EAP-FAST specific PRF function, T-PRF described in Appendix B
(Section 18) is used to generate a fresh master_secret from the
specified client_random, server_random and PAC-Key.
The PRF function used to generate keying material is defined by
[RFC 2246].
Since a PAC may be used as a credential for other applications
beyond EAP-FAST, the PAC-Key is further hashed using T-PRF to
generate a fresh TLS master_secret. Additionally, the hash of
PAC-Key is required to stretch it to the required 48 octet
master_secret:
master_secret = T-PRF(PAC-Key, "PAC to master secret label hash",
server_random + client_random, 48)
To generate the key material required for EAP-FAST Authentication,
the following TLS construction is used:
key_block = PRF(master_secret, "key expansion", server_random +
client_random)
where '+' denotes concatenation.
Since this version of EAP-FAST Authentication employs 128bit RC4
and SHA1, the key_block is partitioned as follows:
client_write_MAC_secret[hash_size=20]
server_write_MAC_secret[hash_size=20]
client_write_key[Key_material_length=16]
server_write_key[key_material_length=16]
client_write_IV[IV_size=0]
server_write_IV[IV_size=0]
session_key_seed[seed_size= 40]
The client_write_MAC_secret and server_write_MAC_secret are the
keys used by the client and server to authenticate subsequent
messages respectively. Similarly, the client_write_key and
client_write_IV are used by the client to provide message
confidentiality while the server uses the server_write_key and
server_write_IV to achieve confidentiality. The session_key_seed
is later used by the EAP-FAST Authentication Phase 2 conversation
to both cryptographically bind the inner method(s) to the tunnel as
well as generate the resulting EAP-FAST session keys.
6.3 EAP-FAST Authentication Phase 2: Tunneled Authentication
The second portion of the EAP-FAST Authentication conversation
consists of at least one complete EAP conversation occurring within
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the TLS session negotiated in EAP-FAST Authentication Phase 1;
ending with protected termination using the Result-TLV and Crypto-
Binding TLV. All EAP messages are encapsulated in the EAP Message
TLV.
EAP-FAST Phase 2 will occur only if establishment of a new TLS
session in Phase 1 is successful or a TLS session is successfully
resumed in Phase 1.
Phase 2 must not occur if the EAP Peer or EAP Server fails
authentication during Phase 1. That is, if the tunnel
establishment fails and a TLS alert is provided prior to a
cleartext EAP failure.
Additionally, Phase 2 must not occur if a protected EAP-Failure has
been sent by the EAP Server to the peer, terminating the
conversation. Since all packets sent within the EAP-FAST Phase 2
conversation occur after TLS session establishment, they are
protected using the negotiated TLS cipher suite. For example, if
the cipher suite negotiated is TLS_RSA_WITH_RC4_128_SHA, all EAP-
TLV packets of the conversation in Phase 2 including the EAP-TLV
header are protected using 128bit RC4 and SHA1 as defined by the
TLS protocol [RFC 2246].
6.4 Protected EAP Conversation
Phase 2 of the EAP-FAST Authentication conversation consists of at
least one protected EAP authentication, typically using the peer's
credentials (typically username and password). This entire EAP
conversation including the user identity and EAP type are protected
from eavesdropping and modification by the tunnel encapsulation. A
hacker cannot readily determine the EAP method used (except perhaps
by traffic analysis) nor can the hacker inject/modify packets to
subvert the authentication.
Phase 2 of the EAP-FAST conversation begins with the EAP server
sending an EAP-Request/Identity packet to the peer, protected by
the TLS ciphersuite negotiated in EAP-FAST Phase 1. The peer
responds with an EAP-Response/Identity packet to the EAP server,
containing the peer's userID.
After the protected Identity exchange, the EAP server will send an
EAP-Request with the supported EAP type, for example, EAP-Type=EAP-
GTC. EAP-FAST enables the use of any (EAP) method to ensue inside
the tunnel, the EAP-GTC type is used in this specification as an
example.
The EAP conversation within the TLS protected session may involve
zero or more EAP authentication methods, including the EAP-TLV
method; and completes with protected termination shown in Section
6.5.
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After any EAP method, the EAP-FAST server or peer may request the
EAP-FAST peer or server respectively, to prove that it has
participated in the sequence of authentications successfully
completed until that point. The server also concludes the EAP-FAST
Phase 2 conversation by invoking a final Result TLV with a Crypto-
Binding TLV. The Crypto-Binding TLV is sent in the protected TLS
channel. If the EAP-FAST server sends a valid Crypto-Binding TLV
to the EAP-FAST peer, the peer MUST respond with a Crypto-Binding
TLV in an EAP Response. If the Crypto-Binding TLV is invalid, it
MUST be considered failed authentication by EAP-FAST client and a
Result TLV with a failure status should follow. If the peer does
not respond with an EAP-FAST packet containing the crypto-binding
TLV, it MUST be considered failed authentication by the EAP-FAST
server. Once the EAP-FAST peer and EAP-FAST server considers them
as failed authentications, they are the last packets inside the
protected tunnel.
6.5 Protected Termination and Acknowledged Result Indication
The EAP-FAST server and EAP-FAST peer indicate success/failure of a
conversation ensued inside the TLS tunnel. Either an Intermediate
Result TLV is used if further conversations will occur, or a final
Result TLV if it is the concluding success/failure indication. The
inclusion of a Crypto-Binding TLV exchange is used to prove that
both peers participated in the sequence of authentications
(specifically the TLS session and inner authentication methods that
generate keys). The Crypto-Binding-TLV exchange is only needed
with a Success Result TLV to verify the integrity of the tunnel. If
the inner EAP method fails, then no Crypto-Binding-TLV exchange is
needed.
If the PAC needs to be updated, the Crypto-Binding TLV must precede
the final Result TLV as the final Result TLV exchange also includes
the distribution of the PAC in a PAC TLV. Following a successful
Intermediate Result TLV and Crypto-Binding TLV exchange, the Result
TLV will be the next subsequent EAP-TLV exchange that also includes
a PAC TLV to update the PAC.
Both Intermediate and final Result TLVs are sent protected within
the TLS channel. The EAP-FAST peer then replies with a
corresponding Intermediate or final Result TLV inside protected
channel. The conversation concludes with a final Result TLV
exchange followed by the EAP-FAST server sending a cleartext EAP-
Success/Failure indication.
The only outcome which should be considered as a successful EAP-
FAST Authentication is when the final Result TLV of Status=Success
and a valid concluding Crypto-Binding TLV, is answered by a final
Result TLV of Status=Success and a valid Crypto-Binding-TLV.
All other combinations of the (request, response) Result TLVs such
as (Failure, Success), (Failure, Failure), (no Result TLV exchange,
no Crypto-Binding TLVs or where the Crypto-Binding TLV validation
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is not successful) MUST be considered failed authentications, both
by the EAP-FAST peer and EAP-FAST server. Once the EAP-FAST peer
and EAP-FAST server considers them as failed authentications, they
are the last packets inside the protected tunnel. These are
considered failed authentications regardless of whether a cleartext
EAP Success or EAP Failure packet is subsequently sent.
In support for session resumption, an EAP-FAST server may send the
success indication and Crypto-Binding TLV, without initiating any
EAP conversation in EAP-FAST Phase 2. The EAP-FAST client is
allowed to refuse to accept a success message from the EAP-FAST
server since the client's policy may require completion of certain
authentication methods. If session resume is not invoked and the
EAP-FAST server has sent Result-TLV with Status=Success; and the
response from the EAP peer is Status=Failure, then the server MUST
continue with the EAP-FAST Phase 2 authentication conversation.
6.6 EAP-FAST Authentication Phase 2: Key Derivations
Keying material resulting from all successful conversations ensued
in both phases of EAP-FAST Authentication are used to both prove
tunnel integrity and generate session keys. A base compound key is
the resulting key generated as follows:
EAP-FAST session_key_seed(SKS) is a 40 octet value obtained from
the EAP-FAST Authentication Phase 1 described in Section 6.2.
The inner authentication method(s) provide session keys: ISK1..ISKn
corresponding to inner methods 1 through n. Only the MSKs from the
inner methods are required. If the inner method (i) does not
generate an ISK, then ISKi is set to zero (e.g. ISKi = 32 octets of
0x00s). If the inner method generates keying material, EAP-FAST
presumes that a minimum of 32 octets are provided. Otherwise, the
resulting ISK is padded with zeroes to generate a 32 octet value.
Thus, the first 32 octets generated as the encryption keying
material by the inner method is used and assigned as the ISK. For
example, if EAP-TLS [RFC 2716] is used as an inner method, the
resulting first 32 bytes described as the "peer encryption key" in
Section 3.5 of [RFC 2716] is assigned as the ISK.
The algorithm uses the EAP-FAST T-PRF as described in Appendix B
(Section 18) to generate the following:
S-IMCK = SKS 0
For j = 1 to n-1 do
IMCK[j] = T-PRF(S-IMCK[j-1], "Inner Methods Compound Keys", ISK[j],
60);
Where S-IMCK[j] are the first 40 octets of IMCK[j]
ICMK[j] may generate up to 60 octets of keying material. The first
40 octets are used as the key input to the succeeding ICMK[j+1]
derivation and the latter 20 octets are used as the key, CMK[j],
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used to generate the intermediate Crypto-Binding Compound MAC
value.
6.7 Cryptographic Binding: Computing the Compound MAC
For authentication methods that generate keying material, further
protection against man-in-the-middle attacks are mitigated through
the enforcement of cryptographically binding keying material
established by both EAP-FAST Phase 1 and EAP-FAST Phase 2
conversations.
For a successful EAP-FAST Authentication, inner methods are
cryptographically combined to generate a compound session key, CMK,
used to generate an authentication tag referred to as a Compound
MAC and transported in a Crypto-Binding TLV. The Crypto-Binding
TLV is used to assure that the same peers invoked all methods in
EAP-FAST.
EAP-FAST optionally enables the server or client to invoke
Intermediate Result-TLV request/response exchanges with Crypto-
Binding TLVs to verify the integrity of the tunnel between methods
inside the EAP-FAST Phase 2 conversation.
Similarly, EAP-FAST enforces a mandatory inclusion of a Crypto-
Binding TLV after a final method has completed. In both instances,
a Crypto-Binding TLV is included when either an Intermediate Result
TLV or a final Result TLV is used. The Crypto-Binding TLV includes
a 20 octet authentication tag that represents the HMAC-SHA1 hash of
the entire Crypto-Binding TLV. The Compound MAC field is zeroed
out prior to the computation of the HMAC-SHA1 and subsequently
populated with the resulting hash value.
The requesting server shall provide a 32-octet random server_nonce
with its last bit set to 0 and compute the Compound MAC field as
follows:
HMAC-SHA1( CMK, [Crypto-Binding TLV with Compound MAC field=
zeroes])
The responding peer shall respond with the same 32-octet
server_nonce value provided by the requestor with its last bit set
to 1 and computes the responding Compound MAC field as described
above.
6.8 EAP-FAST Authentication: Session Key Generation
EAP-FAST Authentication assures the master session keys are a
result of all conversations ensued by generating a compound session
key (IMCK). The IMCK is mutually derived by the peer and server
using the T-PRF; the IMCK calculation is defined in Section 6.6.
The resulting master session key, MSK, is generated as part of the
IMCKn key hierarchy. Where the S-IMCKn is used to generate the
session keys as follows:
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MSK = T-PRF(S-IMCKn, "Session Key Generating Function",
OutputLength)
The first version of EAP-FAST generates 64 octets to serve as the
successful EAP-FAST authentication master session keys.
Interpretation and assignment of these 64 octets of the master
session key is specific to each link layer ciphersuite.
EAP-FAST implementations MAY generate EMSK as follows:
EMSK = T-PRF(S-IMCKn, "Extended Session Key Generating Function",
64)
The Extended Master Session Key (EMSK) is only known to the EAP-
FAST peer and server and is not provided to a third party.
6.9 PAC Distribution and Refreshing
The server may distribute or refresh a peer's PAC after a
successful EAP-FAST Authentication. A PAC TLV is created to
facilitate the distribution and update. A fresh PAC may be
distributed after a successful Intermediate Result TLV and Crypto-
Binding TLV exchange. A successful EAP-FAST authentication,
including a successful Crypto-Binding exchange must ensue before an
EAP-FAST server can distribute a fresh PAC. A PAC TLV should not
be accepted if it is not TLS tunnel-encapsulated. The fresh PAC is
encapsulated in a PAC TLV containing the PAC-Key, PAC-Opaque and
PAC-Info TLVs. The PAC-Key is the shared secret key the peer uses
to mutually authenticate with the server and establish the tunnel.
The PAC-Opaque contains data that is opaque to the recipient, the
peer is not the intended consumer of PAC-Opaque and thus should not
attempt to interpret it. A peer that has been issued a PAC-Opaque
by a server must store that data, and present it back to the server
as is, in the TLS ClientHello extension [RFC3546]. PAC-Info
provides the peer information about the PAC, at minimum, it
provides the information about the authority identity issuing the
PAC.
Once the EAP-FAST peer receives a PAC TLV, it needs to securely
save the new PAC-Key, PAC-Opaque and optionally, the PAC-Info.
Additionally, upon receipt of a new PAC, the peer must respond with
a successful PAC-Acknowledgement TLV. If the peer responds with a
PAC-Acknowledgement failure, the EAP-FAST server may invoke another
Result TLV failure resulting in a failed EAP-FAST authentication.
The server may refresh a PAC only after a successful exchange of
the concluding Intermediate Result TLV and Crypto-Binding TLV. The
peer must use the new PAC-Key and PAC-Info in subsequent EAP-FAST
Authentication sessions.
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N.B. In-band PAC refreshing is enforced by server policy. The
server, based on the PAC-Opaque information, may determine not to
refresh a peer's PAC through the PAC TLV mechanism even if the PAC-
Key has expired.
7. Version Negotiation
EAP-FAST packets contain a three bit version field, following the
TLS Flags field, which enables EAP-FAST implementations to be
backward compatible with previous versions of the protocol.
Version negotiation proceeds as follows:
In the first EAP-Request sent with EAP type=EAP-FAST, the EAP
server must set the version field to the highest supported version
number.
If the EAP client supports this version of the protocol, it MUST
respond with an EAP-Response of EAP type=EAP-FAST, and the version
number proposed by the EAP-FAST server.
If the EAP-FAST client does not support this version, it responds
with an EAP-Response of EAP type=EAP-FAST and the highest supported
version number.
If the EAP-FAST server does not support the version number proposed
by the EAP-FAST client, it terminates the conversation.
The version negotiation procedure guarantees that the EAP-FAST
client and server will agree to the latest version supported by
both parties. If version negotiation fails, then use of EAP-FAST
will not be possible, and another mutually acceptable EAP method
will need to be negotiated if authentication is to proceed.
The EAP-FAST version is not protected by TLS; and hence can be
modified in transit. In order to detect modification of EAP-FAST
version and specifically downgrade of a EAP-FAST version
negotiated, the peers MUST exchange information on the EAP-FAST
version negotiated using the Crypto-Binding TLV. The concluding
Intermediate or final Result TLV comes with a mandatory Crypto-
Binding TLV that includes the EAP-FAST version which MUST be
consistent to that specified in the EAP-FAST Start message.
8. Error Handling
The EAP-FAST protocol uses TLS alert messages to communicate and
handle error conditions in all phases of EAP-FAST. Errors during
the tunnel establishment or protection in EAP-FAST Authentication
are handled via TLS alert messages, while errors during the
protected tunnel are expected to be handled by the individual EAP
methods. Intermediate Result TLVs are also used as status
indications of the individual EAP methods in EAP-FAST Phase 2.
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If the EAP-FAST server detects an error at any point in the EAP-
FAST conversation, the EAP-FAST server should send an EAP-Request
packet with EAP-Type=EAP-FAST, encapsulating a TLS record
containing the appropriate TLS alert message.
The EAP-FAST server should send a TLS alert message rather than
immediately terminating the conversation so as to allow the peer to
inform the user of the cause of the failure and possibly allow for
a restart of the conversation. To ensure that the peer receives
the TLS alert message, the EAP server must wait for the peer to
reply with an EAP-Response packet before terminating the
connection.
The EAP-Response packet sent by the peer may encapsulate a TLS
client_hello handshake message, in which case the EAP-FAST server
MAY allow the EAP-FAST conversation to be restarted, or it MAY
contain an EAP-Response packet with EAP-Type=EAP-FAST and Flags and
Version fields without any additional data , in which case the EAP-
FAST Server MUST send an EAP-Failure packet, and terminate the
conversation.
It is up to the EAP-FAST server whether to allow restarts, and if
so, how many times the conversation can be restarted. An EAP-FAST
Server implementing restart capability SHOULD impose a limit on the
number of restarts, so as to protect against denial of service
attacks.
If the EAP-FAST client detects an error at any point in the EAP-
FAST conversation, the EAP-FAST client should send an EAP-Response
packet with EAP-Type=EAP-FAST, encapsulating a TLS record
containing the appropriate TLS alert message. The EAP-FAST server
may restart the conversation by sending an EAP-Request packet
encapsulating the TLS server_hello handshake message, in which case
the EAP-FAST client may allow the EAP-FAST conversation to be
restarted; or terminate the conversation.
If during the EAP-FAST Authentication Phase 1 session
establishment, EAP-FAST servers cannot obtain or verify the PAC,
the server should send an EAP-Request packet with EAP-Type=EAP-
FAST, encapsulating a TLS record containing the appropriate TLS
alert message, before terminating the conversation. The EAP-FAST
peer should inform the use of the mismatching PAC and terminating
the conversation.
8.1 Error Alerts
EAP-FAST uses TLS-Alert to handle errors in the EAP-TLS handshake.
EAP-FAST employs the standard TLS error alerts described in TLS
Protocol Specification [RFC 2246]. In addition, it reuses the
following TLS alert to support EAP-FAST specific error conditions:
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bad certificate
Server cannot find an acceptable PAC-Opaque in the ClientHello
message. This can be a result of either the peer not sending the
PAC-Opaque or the PAC-Opaque provided cannot be decrypted by the
server or expired. This message is always a fatal error.
decrypt_error
If PAC-Key doesn't match between the peer and server and either
peer or server fails to validate Finished message, decrypt_error
Alert should be used. This message is always a fatal error.
9. Session Resume
EAP-FAST offers a means to bypass further conversations such as
inner EAP authentication methods when a peer has an established
session identified by Session ID. This enables a peer to
optimally generate fresh master session keys without having to re-
invoke the inner EAP authentication method in EAP-FAST
Authentication Phase 2. Applications that require user
intervention for the inner authentication method (e.g. OTP) can
benefit from this feature when service has been established but
believes it must refresh its master session keys.
EAP-FAST session resumption is achieved in the same manner TLS
achieves session resume. Session Resume is achieved by the peer
responding with a known Session ID in its ClientHello record. The
EAP-FAST Authentication Phase 1 conversation proceeds in a similar
fashion as described in Section 6 with the exception of the use of
the PAC-Opaque in the ClientHello. That is, a session resumption
is distinguished by the client's indication of a valid (e.g. non-
zero) SessionID and omission of the PAC-Opaque in its ClientHello
message. To support session resumption, the server must minimally
cache the client's SessionID, master_secret and CipherSpec. If
the server finds a match for the SessionID and is willing to
establish a new connection using the specified session state, the
server will respond with the same SessionID and proceed with the
EAP-FAST Authentication Phase 1 tunnel establishment described in
Section 6.1. The key derivations used in the EAP-FAST
Authentication Phase 1 employ the corresponding SessionID's
master_secret in accordance to the TLS [RFC 2246] session
resumption specification.
After a successful conclusion of the EAP-FAST Authentication Phase
1 conversation, the server then decides whether to honor session
resumption based on the Session ID value. It may reject and
initiate the inner EAP authentication method to signal the start of
a full EAP-FAST Authentication Phase 2 conversation. The server
may accept a session resumption based on the Session ID specified
by the peer as well as the time elapsed since the previous
authentication.
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The server may accept a session resumption and bypass the inner EAP
authentication method by immediately requesting a final Result TLV
without a Crypto-Binding TLV. The concluding Result TLV exchange
is the same as that described in Section 6.5. The EAP-FAST master
session keys are generated as described in Section 6.8, with the
exception that S-IMCK[n] is SKS without going through the compound
key derivation, as in this case no inner EAP method has run.
Even if the session is successfully resumed, the peer and EAP-FAST
server must not assume that either will skip inner EAP methods. The
peer may have roamed to a network which may use the same EAP
server, but may require conformance with a different authentication
policy. After a session is successfully resumed, the EAP-Server may
start a full Phase 2 of the EAP-FAST Authentication conversation.
10. Fragmentation
A single TLS record may be up to 16384 octets in length, but a TLS
message may span multiple TLS records, and a TLS certificate
message may in principle be as long as 16MB. The group of EAP-FAST
messages sent in a single round may thus be larger than the PPP MTU
size, the maximum RADIUS packet size of 4096 octets, or even the
Multilink Maximum Received Reconstructed Unit (MRRU). As described
in [2], the multilink MRRU is negotiated via the Multilink MRRU LCP
option, which includes an MRRU length field of two octets, and thus
can support MRRUs as large as 64 KB.
However, note that in order to protect against reassembly lockup
and denial of service attacks, it may be desirable for an
implementation to set a maximum size for one such group of TLS
messages. Since a typical certificate chain is rarely longer than a
few thousand octets, and no other field is likely to be anywhere
near as long, a reasonable choice of maximum acceptable message
length might be 64 KB.
If this value is chosen, then fragmentation can be handled via the
multilink PPP fragmentation mechanisms described in [RFC1990].
While this is desirable, EAP methods are used in other applications
such as [IEEE80211] and there may be cases in which multilink or
the MRRU LCP option cannot be negotiated. As a result, an EAP-FAST
implementation MUST provide its own support for fragmentation and
reassembly.
Since EAP is an ACK-NAK protocol, fragmentation support can be
added in a simple manner. In EAP, fragments that are lost or
damaged in transit will be retransmitted, and since sequencing
information is provided by the Identifier field in EAP, there is no
need for a fragment offset field as is provided in IPv4.
EAP-FAST fragmentation support is provided through addition of flag
bits within the EAP-Response and EAP-Request packets, as well as a
TLS Message Length field of four octets. Flags include the Length
included (L), More fragments (M), and EAP-FAST Start (S) bits. The
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L flag is set to indicate the presence of the four octet TLS
Message Length field, and MUST be set for the first fragment of a
fragmented TLS message or set of messages. The M flag is set on all
but the last fragment. The S flag is set only within the EAP-FAST
start message sent from the EAP server to the peer. The TLS Message
Length field is four octets, and provides the total length of the
TLS message or set of messages that is being fragmented; this
simplifies buffer allocation.
When an EAP-FAST peer receives an EAP-Request packet with the M bit
set, it MUST respond with an EAP-Response with EAP-Type=EAP-FAST
and no data. This serves as a fragment ACK. The EAP server must
wait until it receives the EAP-Response before sending another
fragment. In order to prevent errors in processing of fragments,
the EAP server MUST increment the Identifier field for each
fragment contained within an EAP-Request, and the peer must include
this Identifier value in the fragment ACK contained within the EAP-
Response. Retransmitted fragments will contain the same Identifier
value.
Similarly, when the EAP-FAST server receives an EAP-Response with
the M bit set, it must respond with an EAP-Request with EAP-
Type=EAP-FAST and no data. This serves as a fragment ACK. The EAP
peer MUST wait until it receives the EAP-Request before sending
another fragment. In order to prevent errors in the processing of
fragments, the EAP server MUST increment the Identifier value for
each fragment ACK contained within an EAP-Request, and the peer
MUST include this Identifier value in the subsequent fragment
contained within an EAP-Response.
11. EAP-FAST Detailed Description
11.1 EAP-FAST Packet Format
A summary of the EAP-FAST Request/Response packet format is shown
below.
The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Flags | Ver | TLS Message Length +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLS Message Length | TLS Data... +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Code
1 - Request
2 - Response
Identifier
The Identifier field is one octet and aids in matching responses
with requests. The Identifier field MUST be changed on each
Request packet.
Length
The Length field is two octets and indicates the length of the
EAP packet including the Code, Identifier, Length, Type, and Data
fields. Octets outside the range of the Length field should be
treated as Data Link Layer padding and should be ignored on
reception.
Type
43 - EAP-FAST
Flags
0 1 2 3 4
+-+-+-+-+-+
|L M S R R|
+-+-+-+-+-+
L = Length included
M = More fragments
S = EAP-FAST start
R = Reserved (must be zero)
The L bit (length included) is set to indicate the presence of the
four octet TLS Message Length field, and MUST be set for the first
fragment of a fragmented TLS message or set of messages. The M bit
(more fragments) is set on all but the last fragment. The S bit
(EAP-FAST Start) is set in an EAP-FAST Start message. This
differentiates the EAP-FAST Start message from a fragment
acknowledgement.
Version
0 1 2
+-+-+-+
|R|R|1|
+-+-+-+
R = Reserved (must be zero)
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TLS Message Length
The TLS Message Length field is four octets, and is present only
if the L bit is set. This field provides the total length of the
TLS message or set of messages that is being fragmented.
TLS data
The TLS data consists of the encapsulated packet in TLS record
format. An EAP-FAST packet with Flags and Version fields but with
empty data field to used to indicate EAP-FAST acknowledgement for
either TLS Alert or TLS Finished.
11.2 EAP-FAST TLV Format
The EAP-FAST TLV is a payload with standard Type-Length-Value (TLV)
objects similar to those defined by [PEAP]. The TLV objects could
be used to carry arbitrary parameters between EAP peer and EAP
server. Possible uses for TLV objects include: language and
character set for Notification messages; cryptographic binding;
IPv6 Binding Update.
The EAP peer may not necessarily implement all the TLVs supported
by the EAP server; and hence to allow for interoperability, the TLV
method allows an EAP server to discover if a TLV is supported by
the EAP peer, using the NAK TLV.
The mandatory bit in a TLV indicates that the peer must understand
the TLV. A peer can determine that a TLV is unknown when it does
not support the TLV; or when the TLV is corrupted. The mandatory
bit does not indicate that the peer successfully applied the value
of the TLV. The specification of a TLV could define additional
conditions under which the TLV can be determined to be unknown.
If an EAP peer finds an unknown TLV which is marked as mandatory;
it must indicate a failure to the EAP server using the NAK TLV; and
all the other TLVs in the message MUST be ignored.
If an EAP peer finds an unknown TLV which is marked as optional;
then it must ignore the TLV. The EAP peer is not required to inform
the EAP server of unknown TLVs which are marked as optional. If the
EAP peer finds that the packet has no TLVs, then it must send a
response with EAP-TLV Response Packet. The Response packet may
contain no TLVs.
If an EAP server finds an unknown TLV which is marked as mandatory;
the other TLVs in the message MUST be ignored. The EAP server can
drop the connection or send an EAP-TLV request packet with NAK-TLV
to the EAP client.
An EAP-FAST TLV packet can be sequenced before or after any other
EAP method. The packet does not have to contain any mandatory TLVs.
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Compliant EAP-FAST implementations must support the EAP-FAST TLV
exchange, including processing of mandatory/optional settings on
the TLV, the NAK TLV, the Crypto-Binding TLV, EAP Payload TLV, PAC
TLV, Intermediate Result TLV and the Result TLV.
The EAP-TLV Request and Response packets shown below are included
in this specification to serve as information only. The actual EAP-
FAST inner method packets inside the TLS tunnel are all
encapsulated using the EAP-TLV TLV format, instead of the EAP-TLV
format. The EAP-TLV header are not needed and thus omitted, since
all inner method packets are encapsulated in EAP-TLV.
11.3 TLV format
EAP-FAST TLVs are 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
A 14-bit field, denoting the TLV type. Allocated Types include:
0 - Reserved
1 - Reserved
2 - Reserved
3 - Result_TLV
4 - NAK_TLV
5 - Reserved
6 - Reserved
7 - Reserved
8 - Reserved
9 - EAP Payload TLV
10 - Intermediate Result TLV
11 - PAC TLV
12 - Crypto-Binding TLV
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Length
The length of the Value field in octets.
Value
The value of the TLV.
11.4 Result TLV
The Result TLV provides support for acknowledged Success and
Failure messages within EAP-FAST. It 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
3 - Result TLV
Length
2
Status
The status field is two octets. Values include:
1 - Success
2 - Failure
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11.5 NAK TLV
The NAK TLV allows a peer to detect when TLVs that are not
supported by the other peer. It 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NAK-Type | TLVs...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
4 - NAK TLV
Length
Variable
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
The NAK-Type field is two octets. The field contains the Type of
the TLV that was not supported. A TLV of this Type MUST have
been included in the previous packet.
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
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future to communicate why the offending TLV was determined to be
unsupported.
11.6 Crypto-Binding TLV
The Crypto-Binding TLV is used to prove that both peers
participated in the sequence of authentications (specifically the
TLS session and inner EAP methods that generate keys).
Both the Binding Request and Binding Response use the same packet
format; with the SubType field indicating whether it is a request
or response.
The Crypto-Binding TLV can be used to perform Cryptographic Binding
after each EAP method in a sequence of EAP methods completes within
the EAP-FAST Authentication Phase 2. The Crypto-Binding TLV MUST be
used once during or before a Protected Termination along with a
Result or Intermediate TLV.
This message format is used for the Binding Request and also the
Binding Response. This uses TLV type CRYPTO_BINDING_TLV. The format
is as given below, with fields 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Version | Received Ver. | SubType |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ NONCE ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Compound MAC ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
12 - Crypto-Binding TLV.
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Length
56
Reserved
The Reserved field is a single octet and must be set to all
zeros.
Version
The Version field is a single octet, which is set to the version
of Crypto Binding TLV. For the crypto-binding TLV defined in
this specification, it is set to 1.
Received Version
The Received Version field is a single octet and MUST be set to
the EAP-FAST version number received during version negotiation.
SubType
The SubType field is 1 octet.
0 - Binding Request
1 - Binding Response
Nonce
The Nonce field is 32 octets. It contains a 256 bit random number
generated by the server on request. The peer responds with the
server nonce incremented by 1.
Compound MAC
The Compound MAC field is 20 octets. It contains an
authentication tag for this TLV. It is calculated over entire
Crypto-binding TLV with Compound MAC field filled with zero.
11.7 EAP Payload TLV
EAP Payload TLV is used to encapsulate all the EAP messages. It 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EAP Packet...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLVs...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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M
1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
9
Length
>=0
EAP Packet
This field contains a complete EAP packet, including the EAP
header (Code, Identifier, Length, Type) fields. The length of
this field is determined by the Length field of the encapsulated
EAP packet.
TLVs...
This (optional) field contains a list of TLVs associated with the
EAP packet field. The TLVs utilize the same format described
Section 4.3, and MUST NOT have the mandatory bit set. The total
length of this field is equal to the Length field of the EAP
Payload TLV, minus the Length field in the EAP header of the EAP
packet field.
11.8 Intermediate Result TLV
The Intermediate Result TLV provides support for acknowledged
intermediate Success and Failure messages within EAP-FAST. EAP-FAST
implementations MUST support this TLV; and this TLV cannot be
responded to with a NAK TLV. It 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status | TLVs...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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M
1 - Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
10
Length
>=2
Status
The Status field is two octets. Values include:
1 - Success
2 - Failure
TLVs
This (optional) field is of indeterminate length, and contains
the TLVs associated with the Intermediate Result TLV, in the same
format as described in Section 4.3. The TLVs in this field MUST
NOT have the mandatory bit set.
11.9 PAC TLV
The PAC TLV provides support for acknowledged refreshing from the
server side within EAP-FAST. A consistent PAC format will allow it
to be used by multiple applications beyond EAP-FAST. A general PAC
TLV format 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PAC Attributes...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 - Non-mandatory TLV
1 - Mandatory TLV
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R
Reserved, set to zero (0)
TLV Type
11- PAC TLV:
Length
The length of the PAC Attributes field in octets.
PAC Attributes
A list of PAC attributes in the TLV format.
A PAC attribute is comprised of three general PAC fields
encapsulated in a common format. The contents of these fields are
described in succeeding sections. The PAC TLV contains all of the
required information to appropriately distribute the client with a
PAC.
11.9.1 Formats for PAC TLV attributes
A common encapsulating format is used to convey the different
fields that comprise a PAC attribute. The common encapsulation 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
The type field is two octets, denoting the attribute type.
Allocated Types include:
1 - PAC-Key
2 - PAC-Opaque
3 - CRED_LIFETIME
4 - A-ID
5 - I-ID
6 - SERVER_PROTECTED_DATA
7 - A-ID-Info
8 - PAC-Acknowledgement
9 - PAC-Info
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Length
The Length filed is two octets, which contains the length of the
Value field in octets.
Value
The value of the PAC Attribute.
11.9.2 PAC-Key
The PAC-Key is distributed as an attribute of type PAC-Key
(Type=1). The key is a randomly generated octet string. The key
is represented as an octet string whose length is determined by the
length field. The generator of this key is the issuer of the
credential, identified by the A-ID.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Key ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
1 - PAC-Key
Length
The Length filed is two octets. For this version of EAP-FAST,
PAC-Key is 32 octets.
Key
The Key field contains the PAC-Key.
11.9.3 PAC-Opaque
The PAC-Opaque section contains data that is opaque to the
recipient, the peer is not the intended consumer of PAC-Opaque and
thus should not attempt to interpret it. A peer that has been
issued a PAC-Opaque by a server MUST store that data, and present
it back to the server as is, in the ClientHello extension field
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[RFC3546]. If a client has opaque data issued to it by multiple
servers, then it MUST store the data issued by each server
separately. This requirement allows the client to maintain and use
each opaque data as an independent PAC pairing, with a PAC-Key
mapping to a PAC-Opaque identified by the A-ID. As there is a one
to one correspondence between PAC-Key and PAC-Opaque, a peer must
determine the PAC-Key and corresponding PAC-Opaque based on the A-
ID provided in the EAP-FAST/Start message and the A-ID provided in
the PAC-Info when it was provisioned with a PAC-Opaque. Each client
must not parse any PAC-Opaque data given to it.
As the PAC-Opaque is server specific, its contents and definition
are specific to the issuer of the PAC, e.g. the PAC server.
The PAC-Opaque field is embedded as part of the PAC TLV when the
server has determined that the PAC must be refreshed and sends a
new PAC.
The PAC-Opaque field format is summarized 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
2 - PAC-Opaque
Length
The Length filed is two octets, which contains the length of the
value field in octets.
Value
The Value field contains the actual data for PAC-Opaque
The PAC-Opaque field is also passed from the peer to the server
during the EAP-FAST Authentication Phase 1 conversation to enable
the server to validate and recreate the PAC-Key. When it is passed
from the peer, it is encapsulated as defined above in the
ClientHello.
11.9.4 PAC-Info
PAC-Info is comprised of a set of PAC attributes. At minimum, the
A-ID TLV is required to convey the issuing identity to the peer.
Other optional fields may be included in the PAC to provide more
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information to the peer. It is a container attribute for various
types of information each of which is encoded in conformance to the
PAC TLV field format as defined in Section 11.3.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attributes...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
9 - PAC-Info
Length
The Length filed is two octets, which contains the length of the
Attributes field in octets.
Attributes
The Attributes field contains a list of PAC Attributes.
Each mandatory and optional field type is defined as follows:
CRED_LIFETIME (type 3)
This is a 4 octet quantity representing the expiration time of the
credential in UNIX UTC time. This is a mandatory field contained
in the PAC-Opaque field to enable the server to validate the PAC.
This field may also be optionally provided to the peer as part of
PAC-Info.
A-ID (type 4)
Authority identifier is the name of the authority that issued the
token. The A-ID is intended to be unique across all issuing
servers to avoid namespace collisions. Server implementations
should use measures to ensure the A-ID used is globally unique to
avoid name collisions. The A-ID is used by the peer to determine
which PAC to employ. Similarly, the server uses the A-ID to both
authenticate the PAC-Opaque and determine which master key was used
to issue the PAC. This field is mandatory and included in both the
PAC-Opaque and as the first TLV comprising PAC-Info.
I-ID (type 5)
Initiator identifier is the peer identity associated with the
credential. The server employs the I-ID in the EAP-FAST Phase 2
conversation to validate that the same peer identity used to
execute EAP-FAST Phase 1 is also used in at minimum one inner EAP
method in EAP-FAST Phase 2. This field is a mandatory field in
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PAC-Opaque and may optionally be included in the PAC-Info. If the
AS is enforcing the I-ID validation on inner EAP method, then I-ID
is mandatory in PAC-Info, to enable the client to also enforce a
unique PAC for each unique user. If I-ID is missing from the PAC-
Info, it is assumed that the PAC can be used for multiple users and
client will not enforce the unique PAC per user policy.
A-ID-Info (type 7)
Authority Identifier Information is a mandatory TLV intended to
provide a user-friendly name for the A-ID. It may contain the
enterprise name and server name in a more human-readable format.
This TLV serves as an aid to the peer to better inform the end-user
about the A-ID. This field is a mandatory field in the PAC-Info.
11.9.5 PAC-Acknowledgement TLV
The PAC-Acknowledgement TLV is used to acknowledge the receipt of
the PAC TLV by the peer. Peer sends this TLV in response to the PAC
TLV to indicate the result of the retrieving and storing of the new
PAC.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Result |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
8 - PAC-Acknowledgement
Length
2
Result
1 - Success
2 - Failure
12. Security Considerations
EAP-FAST is designed with a focus on wireless media, where the
medium itself is inherent to eavesdropping. Whereas in wired
media, an attacker would have to gain physical access to the wired
medium; wireless media enables anyone to capture information as it
is transmitted over the air, enabling passive attacks. Thus,
physical security can not be assumed and security vulnerabilities
are far greater.
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The threat model used for the security evaluation of EAP-FAST is
that defined in the RFC 2284bis [EAP].
12.1 Mutual Authentication and Integrity Protection
EAP-FAST as a whole, provides message and integrity protection by
establishing a secure tunnel for protecting the authentication
method(s). The confidentiality and integrity protection is that
defined by TLS [RFC 2246] and provides the same security strengths
afforded by TLS employing a strong entropy shared master secret.
When EAP-FAST is invoked for enabling network access, mutual
authentication is first achieved by proof of a mutually shared
unique PAC-Key during the tunnel establishment. Further, the
Result TLV is enforced to be run after any EAP method that supports
(mutual) authentication ensuring that it was the same peer and AS
that communicated in all transpired methods (including tunnel
establishment).
The Result TLV is protected and conveys the true Success or Failure
of EAP-FAST and should be used as the indicator of its success or
failure respectively. However, as EAP must terminate with a
cleartext EAP Success or Failure, a peer will also receive a
cleartext EAP success or failure. The received cleartext EAP
success or failure must match that received in the Result TLV; the
peer may silently discard those cleartext EAP success or failure
messages that do not coincide with the status sent in the protected
Result TLV.
12.2 Method Negotiation
As is true for any negotiated EAP protocol, NAK packets used to
suggest an alternate authentication method are sent unprotected and
as such, are subject to spoofing. During EAP method negotiation,
NAK packets may be interjected as active attacks to negotiate
down to a weaker form of authentication, such as EAP-MD5 (which
only provides one way authentication and does not derive a key).
Since a subsequent protected EAP conversation can take place within
the TLS session, selection of EAP-FAST as an authentication method
does not limit the potential secondary authentication methods. As a
result, the only legitimate reason for a peer to NAK EAP-FAST as an
authentication method is that it does not support it. Where the
additional security of EAP-FAST is required, the server shall best
determine how to respond to a NAK as this is beyond the scope of
this specification.
Inner method negotiation is protected by the mutually authenticated
TLS tunnel established in EAP-FAST and immune to attacks.
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12.3 Separation of the EAP Server and the Authenticator
When EAP-FAST is successfully invoked to gain network access, the
EAP endpoints will mutually authenticate, and derive a session key
for subsequent use in link layer security. Since it is presumed
that the peer and EAP client reside on the same machine, it is
necessary for the EAP client module to pass the session key to the
link layer encryption module.
As EAP-FAST is defined to achieve mutual authentication between a
peer and AS, it will not achieve direct authentication to the
Authenticator (which is true for most if not all currently
specified EAP methods).
It is implied that there is an established trust between
Authenticator and AS before the AS securely distributes the session
keys to the authenticator. Using the transitive property and the
authenticator to AS trust assumption, if the AS trusts the
authenticator and distributes the session key to the authenticator,
and the peer has successfully gained authorization by mutually
deriving fresh session keys, the peer may then presume trust with
the authenticator who can prove it has those session keys. Note
however, that this presumed trust does not authenticate the
authenticator to the peer, it merely proves that the AS has a trust
relationship with said authenticator. Further, it is presumed that
a secure mechanism is used by the AS to distribute the session key
to the authenticator.
In the case of the AS and the home AAA server logical model,
similar security properties hold as that between the AS and
authenticator. Though in general, it is highly recommended that
the AAA server be reside on the same host as the AS.
In both cases, the presumed trust between authenticator and AS as
well as AS and AAA server as well as the security in the transport
(such as IPsec) and key delivery (such as NIST approved key
wrapping) mechanisms for these links are outside the scope of the
EAP-FAST specification. Without these presumed trusts and secure
transport mechanisms, security vulnerabilities will exist.
12.4 Separation of Phase 1 and Phase 2 Servers
Separation of the EAP-FAST Phase 1 from the Phase 2 conversation is
not recommended. Without a trust relationship and proper
protection (such as IPsec) for RADIUS, by allowing a the Phase 1
conversation to be terminated at a different (proxy) AS (AS1) than
the Phase 2 conversation (terminated at AS2), vulnerabilities are
introduced since cleartext transmission between AS1 and AS2 ensue.
Some vulnerabilities include:
* Loss of identity protection
* Offline dictionary attacks
* Lack of policy enforcement
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In order to find the proper EAP-FAST destination, the peer SHOULD
place a Network Access Identifier (NAI) conforming to [RFC2486] in
the Identity Response.
There may be cases where a natural trust relationship exists
between the (foreign) authentication server and final EAP server,
such as on a campus or between two offices within the same company,
where there is no danger in revealing the identity of the station
to the authentication server. In these cases, using a proxy
solution without end to end protection of EAP-FAST MAY be used. The
EAP-FAST encrypting/decrypting gateway SHOULD provide support for
IPsec protection of RADIUS in order to provide confidentiality for
the portion of the conversation between the gateway and the EAP
server, as described in [RFC3162].
12.5 Mitigation of Known Vulnerabilities and Protocol Deficiencies
EAP-FAST addresses the known deficiencies and weaknesses in the EAP
method. By employing a shared secret between the peer and server
to establish a secured tunnel, EAP-FAST enables:
* Per packet confidentiality and integrity protection
* User identity protection
* Better support for notification messages
* EAP negotiation
* Sequencing of EAP methods
* Strong mutually derived master session keys
* Support for fragmentation and reassembly
* Acknowledged success/failure indication
* Faster re-authentications through session resumption
* Mitigation of dictionary attacks
* Mitigation of man-in-the-middle attacks
* Denial of Service attacks
It should be noted that EAP-FAST as in many other authentication
protocols, a denial of service attack can be easily mounted by
adversaries imposing as either peer or AS and failing to present
the proper credential. This is an inherent problem in most
authentication or key agreement protocols and is so noted for EAP-
FAST as well.
EAP-FAST protection addresses a number of weaknesses present in
LEAP, PEAPv1, EAP-TTLS and the inner EAP methods used in the EAP-
FAST Authentication Phase 2 conversation. These weaknesses have
been described in draft-puthenkulam-eap-binding-03.txt.
EAP-FAST was designed with a focus on protected authentication
methods that typically rely on weak credentials, such as password
based secrets. To that extent, the EAP-FAST Authentication
mitigates several vulnerabilities such as dictionary attacks by
protecting the weak credential based authentication method. The
protection is based on strong cryptographic algorithms such as RC4
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and HMAC-SHA1 to provide message confidentiality and integrity
respectively. The keys derived for the protection relies on strong
random challenges provided by both peer and AS as well as a shared
secret with strong entropy (minimally 32 octets). It is
recommended that peers provide strong random number generators that
can satisfy the criteria as that described by NIST Special
Publication 800-22b (e.g. NIST SP800-22b). The AS acting as the
PAC distributor must generate unique and randomly generated 32
octet keys for each peer.
12.5.1 User Identity Protection and Verification
As EAP-FAST employs TLS to establish a secure tunnel, the initial
Identity request/response may be omitted as it must be transmitted
in the clear and thus subject to snooping and forgery. It may be
omitted also in deployments where it is known that all users are
required to authenticate with EAP-FAST. Alternately, an anonymous
identity may be used in the Identity response.
If the initial Identity request/response has been tampered with,
the AS may be unable to verify the peer's identity. For example,
the peer's userID may not be valid or may not be within a realm
handled by the AS. Rather than attempting to proxy the
authentication to the server within the correct realm, the AS
should terminate the conversation.
The EAP-FAST peer can present the server with multiple identities.
This includes the claim of identity within the initial EAP-
Response/Identity (MyID) packet, which is typically used to route
the EAP conversation to the appropriate home backend AS. There may
also be subsequent EAP-Response/Identity packets sent by the peer
once the secure tunnel has been established.
The PAC-Opaque field conveyed by the peer to the AS contains the
peer's identity that should be validated with at least one identity
presented in the EAP-FAST Authentication Phase 2 conversation.
This ensures that the PAC-Key is employed by the intended peer.
Though EAP-FAST implementations should not attempt to compare the
EAP-FAST Authentication Phase 1 Identity disclosed in the EAP
Identity response packet with those Identities claimed in Phase 2;
the AS should match the identity disclosed in the PAC-Opaque field
with at least one identity disclosed in EAP-FAST Authentication
Phase 2.
12.5.2 Dictionary Attack Resistance
EAP-FAST was designed with a focus on protected authentication
methods that typically rely on weak credentials, such as password
based secrets. To that extent, by establishing a mutually
authenticated protected tunnel, EAP-FAST mitigates dictionary
attacks by protecting the weak credential based authentication
method. The protection is based on strong cryptographic algorithms
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such as RC4 and HMAC-SHA1 to provide message confidentiality and
integrity respectively. The keys derived for the protection relies
on strong random challenges provided by both peer and AS as well as
a strong entropy (minimally 32 octet) shared secret. The AS acting
as the PAC distributor MUST generate unique and randomly generated
32 octet keys for each peer.
12.5.3 Protection against MitM Attacks
The recommended solution is to always deploy authentication methods
with protection of EAP-FAST. If a deployment chooses to allow an
EAP method protected by EAP-FAST without protection of EAP-FAST at
the same time, then this opens up a possibility of a Compound
Authentication Binding man-in-the-middle attack [MITM].
A man-in-the-middle can spoof the client to authenticate to it
instead of the real EAP server; and forward the authentication to
the real server over a protected tunnel. Since the attacker has
access to the keys derived from the tunnel, it can gain access to
the network.
EAP-FAST prevents this attack in two ways:
1. An adversary must have the corresponding peer's PAC-Key to
mutually authenticate during EAP-FAST Authentication Phase 1
establishment of a secure tunnel; and
2. By using the keys generated by the inner authentication method
in the crypto-binding exchange described in above protected
termination section 6.5.
Both compound MAC and compound session key approaches are used to
prevent the aforementioned man-in-the-middle attack. Both the peer
and the EAP server MUST derive compound MAC and compound session
keys using the procedure described in Section 6.7.
As a strong PAC-Key is used to establish mutual authentication in
EAP-FAST Phase 1, this attack is also prevented if the inner
authentication method does not generate keys. Thus, most EAP
authentication methods are protected from these MitM attacks when
protected by EAP-FAST.
To summarize, EAP-FAST Authentication mitigates most MitM attacks
in the following way:
* Identity binding with PAC-Key: in presenting the PAC-Opaque
field to the AS, a peer is presenting an authenticated credential.
With the user identity serving as another validation point for the
inner EAP authentication method, a MitM may not interject and
impersonate itself as the peer unless it has recovered the PAC-Key
as well as the PAC-Opaque field. Thus, the PAC-Key binding to an
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Identity prevents an adversary from interjection regardless of
whether the authentication method generates session keys,
* Cryptographic binding of EAP-FAST Phase 1 and all methods within
Phase 2: by cryptographically binding key material generated in
all methods, peer and AS are assured that they were the sole
participants of all transpired methods.
12.5.4 PAC Validation with User Credentials
The PAC-Opaque field is consumed by the AS during a network access
EAP-FAST invocation to both acquire and validate the authenticity
of the PAC credential. However, during the EAP-FAST Phase 1
conversation it validates the peer based on the secret, PAC-Key and
not on the identity. Further, since the EAP-FAST Phase 1
conversation occurs in cleartext, it is feasible for an adversary
to acquire a PAC-Opaque credential.
While a PAC-Opaque credential can be easily acquired, the shared
secret, PAC-Key is not discernible from the PAC-Opaque field. Thus,
an adversary must resort to a brute force attack to gain the PAC-
Key from PAC-Opaque information.
Another feasible scenario due to the cleartext transmission is the
spoofing of the PAC-Opaque field. While the PAC-Opaque is
authenticated to mitigate forgery, a denial of service and
potential user lockout (based on deployment configurations that may
choose to lock a peer after a configurable number of failed
attempts) is feasible.
The final validation and binding of the PAC credential is the
identity validation in the EAP-FAST Phase 2 conversation. A
compliant implementation of EAP-FAST MUST match the identity
presented to the AS in the PAC-Opaque field with at minimum one of
the identities provided in the EAP-FAST Phase 2 authentication
method. This validation provides another binding to ensure that
the intended peer (based on identity) has successfully completed
the EAP-FAST Phase 1 and proved identity in the Phase 2
conversations. This validation helps mitigate the MitM attack as
described in Section 12.5.3.
12.6 PAC Storage Considerations
The main premise behind EAP-FAST is to protect the authentication
stream over the media link. However, physical security is still an
issue. Some care should be taken to protect the PAC on both the
peer and server. The peer must store securely both the PAC-Key and
PAC-Opaque, while the server must secure storage of its security
association context used to consume the PAC-Opaque. Additionally,
if manual provisioning is employed, the transportation mechanism
used to distribute the PAC must also be secured.
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Most of the attacks described here would require some level of
effort to execute; conceivably greater than their value. The main
focus therefore, should be to ensure that proper protections are
used on both the client and server. There are a number of
potential attacks which can be considered against secure key
storage such as:
* weak passphrases
On the client side, keys are usually protected by a passphrase.
On some environments, this passphrase may be associated with the
user's password. In either case, if an attacker can obtain the
encrypted key for a range of users, he may be able to
successfully attack a weak passphrase. The tools are already in
place today to allow an attacker to easily attack all Outlook or
Outlook Express users in an enterprise environment. Most viruses
or worms of this sort attract attention to themselves by their
action, but that need not be the case. A simple, genuine
appearing email could surreptitiously access keys from known
locations and email them directly to the attacker, attracting
little notice.
* key finding attacks
Key finding attacks are usually mentioned in reference to web
servers, where the private SSL key may be stored securely, but at
some point it must be decrypted and stored in system memory. An
attacker with access to system memory can actually find the key
by identifying their mathematical properties. To date, this
attack appears to be purely theoretical and primarily acts to
argue strongly for secure access controls on the server itself to
prevent such unauthorized code from executing.
* key duplication , key substitution, key modification
Once keys are accessible to an attacker on either the client or
server, they fall under three forms of attack: key duplication,
key substitution and key modification. The first option would be
the most common, allowing the attacker to masquerade as the user
in question. The second option could have some use if an
attacker could implement it on the server.
Another consideration is the use of secure mechanisms afforded by
the particular device. For instance, some laptops enable secure
key storage through a special chip. It would be worthwhile for
implementations to explore the use of such a mechanism.
12.7 Protecting against Forged Clear Text EAP Packets
As described earlier, EAP Success and EAP Failure packets are in
general sent in cleartext and may be forged by an attacker without
fear of detection. Forged EAP Failure packets can be used to
convince an EAP peer to disconnect. Forged EAP Success packets may
be used by any rogue to convince a peer to let itself access the
network, even though the authenticator has not authenticated itself
to the peer.
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By providing message confidentiality and integrity, EAP-FAST
provides protection against these attacks. Once the peer and AS
initiate the EAP-FAST Authentication Phase 2, compliant EAP-FAST
implementations must silently discard all cleartext EAP messages
unless both the EAP-FAST peer and server have indicated success or
failure using a protected mechanism. Protected mechanisms include
TLS alert mechanism and the protected termination mechanism
described in Section 6.5.
The success/failure decisions sent by a protected mechanism
indicate the final decision of the EAP-FAST authentication
conversation. After a success/failure result has been indicated by
a protected mechanism, the EAP-FAST peer can process unprotected
EAP success and EAP failure message; however the peer must ignore
any unprotected EAP success or failure messages where the result
does not match the result of the protected mechanism.
To abide by RFC 2284, the AS must send a cleartext EAP Success or
EAP Failure packet to terminate the EAP conversation, so that no
response is possible. However, since EAP Success and EAP Failure
packets are not retransmitted, if the final packet is lost, then
authentication will fail. As a result, where packet loss is
expected to be non-negligible, unacknowledged success/failure
indications lack robustness.
While an EAP-FAST protected EAP Success or EAP Failure packet
should not be a final packet in an EAP-FAST conversation, it may be
feasible based on the conditions stated above and construed as an
optimization savings of a full round-trip in low packet loss
environments.
12.8 Implementation
Both server and in particular, client implementations must provide
a suitably strong PRNG to ensure good entropy challenges. Suitable
recommendations for PRNGs can be found in PKCS#5, PKCS#11 and
criteria for suitable PRNGS are also defined by NIST Special
Publication 800-22b.
12.9 Security Claims
This section provides needed security claim requirement for
RFC3748 [EAP].
Auth. mechanism: Tunneled authentication as well as
pre-shared key.
Ciphersuite negotiation:Yes. See [RFC2246].
Mutual authentication: Yes. See Section 12.1.
Integrity protection: Yes. See Section 12.1. Only EAP Type
Data field and inner EAP methods
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contained in this field are
protected.
Replay protection: Yes. See [RFC2246].
Confidentiality: Yes. See [RFC2246].
Key derivation: Yes. See Section 6.6.
Key strength: TLS key strength, may be enhanced by
binding keys with inner methods
Dictionary attack prot.:Yes. See Section 12.5.2.
Key hierarchy: Yes. See Section 6.6.
Fast reconnect: Yes. See Section 9.
Cryptographic binding: Yes. See Section 6.7.
Session independence: Yes. See [RFC2716].
Fragmentation: Yes. See Section 10.
Channel binding: No.
13. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the
EAP protocol, in accordance with BCP 26, [RFC2434].
There is a namespace in EAP-FAST that requires registration: TLV
Types. These numbers may be assigned by First Come First Served
[RFC2434].
14. References
14.1 Normative
[RFC2246]
Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[EAP]
Blunk, L., et. al., "Extensible Authentication Protocol
(EAP)", RFC3748, June 2004.
[RFC 3268]
Chown, P., "Advanced Encryption Standard (AES) Ciphersuites
for Transport Layer Security (TLS)", RFC 3268, June 2002.
[RFC2119]
Bradner, S., "Key words for use in RFCs to indicate
Requirement Levels", RFC 2119, March 1997.
[RFC 3546]
Blake-Wilson, S., et al, "Transport Layer Security (TLS)
Extensions", RFC 3546, June 2003.
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14.2 Informative
[RFC2434]
Narten, T., and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434, October
1998.
[RFC2279]
Yergeau, F., "UTF-8, a transformation format of ISO 10646",
RFC 2279, January 1998.
[RFC2631]
Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC
2631, January 1999.
[RFC 3268]
Chown, P., "Advanced Encryption Standard (AES) Ciphersuites
for Transport Layer Security (TLS)", RFC 3268, June 2002.
[RFC 2716]
Aboba, B. and Simon, D, "PPP EAP TLS Authentication
Protocol", RFC 2716, October 1999.
[SHARED KEYS IN TLS]
Gutmann, P., "Use of Shared Keys in the TLS Protocol",
draft-ietf-tls-sharedkeys-02.txt (expired), October 2003.
[IKEv2]
Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
draft-ietf-ipsec-ikev2-17 (work in progress), September
2004.
[PEAP]
Palekar, et. al., "Protected EAP Protocol (PEAP) Version 2",
draft-josefsson-pppext-eap-tls-eap-10 (work in progress),
October 2004
[RFC 3526]
Kivinen, T., "More Modular Exponential (MODP) DIffie-Hellman
groups for Internet Key Exchange (IKE)", RFC 3526, May 2003
[MITM] Puthenkulam, J., "The Compound Authentication Binding
Problem", draft-puthenkulam-eap-binding-04 (expired),
October 2003.
[RFC2486BIS]
Aboba, et. al., "The Network Access Identifier", draft-
arkko-roamops-rfc2486bis-02.txt (work in progress), July,
2004.
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[RFC2548]
Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", RFC
2548, March 1999.
[EAP-TTLS]
Funk & Blake-Wilson, "EAP Tunneled TLS Authentication
Protocol", draft-ietf-pppext-eap-ttls-05.txt (work in
progress), July 2004
[TLS-PSK]
Eronen & Tschofenig, "Pre-Shared Key Ciphersuites for
Transport Layer Security (TLS)", draft-ietf-tls-psk-02,
Septempber 2004
15. Acknowledgments
The EAP-FAST design and protocol specification is based on the
ideas and hard efforts of Pad Jakkahalli, Mark Krischer, Doug
Smith, Ilan Frenkel and Jeremy Steiglitz of Cisco Systems, Inc.
16. Author's Addresses
Nancy Cam-Winget
Cisco Systems
3625 Cisco Way
San Jose, CA 95134
US
Phone: +1 408 853 0532
Email: ncamwing@cisco.com
David McGrew
Cisco Systems
San Jose, CA 95134
US
Email: mcgrew@cisco.com
Joseph Salowey
Cisco Systems
2901 3rd Ave
Seattle, WA 98121
US
Phone: +1 206 256 3380
Email: jsalowey@cisco.com
Hao Zhou
Cisco Systems
4125 Highlander Parkway
Richfield, OH 44286
US
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Phone : +1 330 523 2132
Email: hzhou@cisco.com
17. Appendix A: Examples
17.1 Successful Authentication
The following exchanges show a successful EAP-FAST authentication
with PAC refreshment, the conversation will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID1) ->
<- EAP-Request/
EAP-Type=EAP-FAST, V=1
(EAP-FAST Start, S bit set, A-ID)
EAP-Response/
EAP-Type=EAP-FAST, V=1
(TLS client_hello with
PAC-Opaque)->
<- EAP-Request/
EAP-Type=EAP-FAST, V=1
(TLS server_hello,
(TLS change_cipher_spec,
TLS finished)
EAP-Response/
EAP-Type=EAP-FAST, V=1 ->
(TLS change_cipher_spec,
TLS finished)
TLS channel established
(messages sent within the TLS channel)
<- EAP Payload TLV, EAP-Request,
EAP-GTC, Challenge
EAP Payload TLV, EAP-Response,
EAP-GTC, Response with both
user name and password) ->
optional additional exchanges (new pin mode,
password change etc.) ...
<- Intermediate Result TLV (Success)
Binding-TLV(Version=0,SNonce,
CompoundMAC)
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Intermediate Result TLV (Success)
Binding-TLV(Version=0,
CNonce, CompoundMAC) ->
<- Result TLV (Success)
(Optional PAC TLV)
Result TLV (Success)
(PAC TLV Acknowledgment) ->
TLS channel torn down
(messages sent in cleartext)
<- EAP-Success
17.2 Failed Authentication
The following exchanges show a failed EAP-FAST authentication due
to wrong user credentials, the conversation will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID1) ->
<- EAP-Request/
EAP-Type=EAP-FAST, V=1
(EAP-FAST Start, S bit set, A-ID)
EAP-Response/
EAP-Type=EAP-FAST, V=1
(TLS client_hello with
PAC-Opaque)->
<- EAP-Request/
EAP-Type=EAP-FAST, V=1
(TLS server_hello,
(TLS change_cipher_spec,
TLS finished)
EAP-Response/
EAP-Type=EAP-FAST, V=1 ->
(TLS change_cipher_spec,
TLS finished)
TLS channel established
(messages sent within the TLS channel)
<- EAP Payload TLV, EAP-Request,
EAP-GTC, Challenge
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EAP Payload TLV, EAP-Response,
EAP-GTC, Response with both
user name and password) ->
<- EAP Payload TLV, EAP-Request,
EAP-GTC, error
EAP Payload TLV, EAP-Response,
EAP-GTC, empty data packet to
acknowledge unrecoverable error) ->
<- Result TLV (Failure)
Result TLV (Failure) ->
TLS channel torn down
(messages sent in cleartext)
<- EAP-Failure
18. Appendix B: EAP-FAST PRF (T-PRF)
EAP-FAST employs a simpler PRF than the TLS PRF where possible.
For instance, when generating the master_secret, master session
keys and cryptographic binding keys and computations, EAP-FAST
employs the following PRF construction:
PRF(key, label, seed) = HMAC-SHA1(key, label + "\0" + seed)
Where '+' indicates concatenation and "\0" is a NULL character.
Label is intended to be a unique label for each different use of
the T-PRF.
To generate the desired OutputLength octet length of key material,
the T-PRF is iterated as follows:
T-PRF (Key, S, OutputLength) = T1 + T2 + T3 + T4 + ...
Where S = label + 0x00 + seed; and
T1 = HMAC-SHA1 (Key, S + OutputLength + 0x01)
T2 = HMAC-SHA1 (Key, T1 + S + OutputLength + 0x02)
T3 = HMAC-SHA1 (Key, T2 + S + OutputLength + 0x03)
T4 = HMAC-SHA1 (Key, T3 + S + OutputLength + 0x04)
OutputLength is a two octet value that is represented in big endian
order. The NULL character, 0x00 shall be present when a label
string is provided. Also note that the seed value may be optional
and may be omitted as in the case of the MSK derivation described
in Section 6.8.
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Where each Ti generates 20-octets of keying material, the last Tn
may be truncated to accommodate the desired length specified by
OutputLength.
19. Appendix C: Test Vectors
19.1 Key derivation
PAC KEY:
0B 97 39 0F 37 51 78 09 81 1E FD 9C 6E 65 94 2B
63 2C E9 53 89 38 08 BA 36 0B 03 7C D1 85 E4 14
Server_hello Random
3F FB 11 C4 6C BF A5 7A 54 40 DA E8 22 D3 11 D3
F7 6D E4 1D D9 33 E5 93 70 97 EB A9 B3 66 F4 2A
Client_hello Random
00 00 00 02 6A 66 43 2A 8D 14 43 2C EC 58 2D 2F
C7 9C 33 64 BA 04 AD 3A 52 54 D6 A5 79 AD 1E 00
Master_secret = T-PRF(PAC-Key,
"PAC to master secret label hash",
server_random + Client_random,
48)
4A 1A 51 2C 01 60 BC 02 3C CF BC 83 3F 03 BC 64
88 C1 31 2F 0B A9 A2 77 16 A8 D8 E8 BD C9 D2 29
38 4B 7A 85 BE 16 4D 27 33 D5 24 79 87 B1 C5 A2
Key_block = PRF(Master_secret,
"key expansion",
server_random + Client_random)
59 59 BE 8E 41 3A 77 74 8B B2 E5 D3 60 AC 4D 35
DF FB C8 1E 9C 24 9C 8B 0E C3 1D 72 C8 84 9D 57
48 51 2E 45 97 6C 88 70 BE 5F 01 D3 64 E7 4C BB
11 24 E3 49 E2 3B CD EF 7A B3 05 39 5D 64 8A 44
11 B6 69 88 34 2E 8E 29 D6 4B 7D 72 17 59 28 05
AF F9 B7 FF 66 6D A1 96 8F 0B 5E 06 46 7A 44 84
64 C1 C8 0C 96 44 09 98 FF 92 A8 B4 C6 42 28 71
Session Key Seed
D6 4B 7D 72 17 59 28 05 AF F9 B7 FF 66 6D A1 96
8F 0B 5E 06 46 7A 44 84 64 C1 C8 0C 96 44 09 98
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FF 92 A8 B4 C6 42 28 71
IMCK = T-PRF(SKS,
"Inner Methods Compound Keys",
ISK,
60)
Note: ISK is 32 bytes 0's.
16 15 3C 3F 21 55 EF D9 7F 34 AE C8 1A 4E 66 80
4C C3 76 F2 8A A9 6F 96 C2 54 5F 8C AB 65 02 E1
18 40 7B 56 BE EA A7 C5 76 5D 8F 0B C5 07 C6 B9
04 D0 69 56 72 8B 6B B8 15 EC 57 7B
[SIMCK 1]
16 15 3C 3F 21 55 EF D9 7F 34 AE C8 1A 4E 66 80
4C C3 76 F2 8A A9 6F 96 C2 54 5F 8C AB 65 02 E1
18 40 7B 56 BE EA A7 C5
MSK = T-PRF(S-IMCKn,
"Session Key Generating Function",
64);
4D 83 A9 BE 6F 8A 74 ED 6A 02 66 0A 63 4D 2C 33
C2 DA 60 15 C6 37 04 51 90 38 63 DA 54 3E 14 B9
27 99 18 1E 07 BF 0F 5A 5E 3C 32 93 80 8C 6C 49
67 ED 24 FE 45 40 A0 59 5E 37 C2 E9 D0 5D 0A E3
19.2 Crypto-Bind MIC:
[Compound MAC Key 1]
76 5D 8F 0B C5 07 C6 B9 04 D0 69 56 72 8B 6B B8
15 EC 57 7B
[Crypto-binding TLV]
80 0C 00 38 00 01 01 00 D8 6A 8C 68 3C 32 31 A8 56 63 B6 40 21 FE
21 14 4E E7 54 20 79 2D 42 62 C9 BF 53 7F 54 FD AC 58 43 24 6E 30
92 17 6D CF E6 E0 69 EB 33 61 6A CC 05 C5 5B B7
[Server Nonce]
D8 6A 8C 68 3C 32 31 A8 56 63 B6 40 21 FE 21 14
4E E7 54 20 79 2D 42 62 C9 BF 53 7F 54 FD AC 58
[Compound MAC]
43 24 6E 30 92 17 6D CF E6 E0 69 EB 33 61 6A CC
05 C5 5B B7
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20. Intellectual Property Statement
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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.
21. 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.
22. Copyright Statement
Copyright (C) The Internet Society (2004). 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.
23. Expiration Date
This memo is filed as <draft-cam-winget-eap-fast-01.txt>, and
expires April 25, 2005.
Cam-Winget, et al. Expires - April 25, 2005 [Page 52]
