EMU Working Group T. Otto
Internet-Draft
Intended status: Standards Track H. Tschofenig
Expires: April 26, 2007 Siemens Networks GmbH & Co KG
October 23, 2006
The EAP-TLS-PSK Authentication Protocol
draft-otto-emu-eap-tls-psk-01.txt
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Abstract
The Extensible Authentication Protocol (EAP), defined in RFC 3748, is
a network access authentication framework which provides support for
multiple authentication methods. One proposal is EAP-TLS, which
relies on the Transport Layer Security (TLS) protocol and allows for
certificate-based authentication. This document specifies EAP-TLS-
PSK, which also relies on TLS, but allows for shared secret-based
authentication. EAP-TLS-PSK supports the pre-shared key ciphersuites
specified in RFC 4279.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Overview about pre-shared key TLS ciphersuites . . . . . . 4
1.2. Requirements notation . . . . . . . . . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Overview of the EAP-TLS-PSK Conversation . . . . . . . . . 7
2.2. Retry Behavior . . . . . . . . . . . . . . . . . . . . . . 11
2.3. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 11
2.4. Identity Verification . . . . . . . . . . . . . . . . . . 13
2.5. Key Hierarchy . . . . . . . . . . . . . . . . . . . . . . 14
2.6. Ciphersuite and Compression Negotiation . . . . . . . . . 15
3. EAP-TLS-PSK Packet Format . . . . . . . . . . . . . . . . . . 16
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 19
5.1. Mutual Authentication . . . . . . . . . . . . . . . . . . 19
5.2. Protected Result Indications . . . . . . . . . . . . . . . 19
5.3. Integrity Protection . . . . . . . . . . . . . . . . . . . 19
5.4. Replay Protection . . . . . . . . . . . . . . . . . . . . 19
5.5. Dictionary Attacks . . . . . . . . . . . . . . . . . . . . 19
5.6. Key Derivation . . . . . . . . . . . . . . . . . . . . . . 20
5.7. Session Independence . . . . . . . . . . . . . . . . . . . 20
5.8. Exposition of the PSK . . . . . . . . . . . . . . . . . . 20
5.9. Fragmentation . . . . . . . . . . . . . . . . . . . . . . 21
5.10. Channel Binding . . . . . . . . . . . . . . . . . . . . . 21
5.11. Fast Reconnect . . . . . . . . . . . . . . . . . . . . . . 21
5.12. Identity Protection . . . . . . . . . . . . . . . . . . . 21
5.13. Protected Ciphersuite Negotiation . . . . . . . . . . . . 21
5.14. Confidentiality . . . . . . . . . . . . . . . . . . . . . 21
5.15. Cryptographic Binding . . . . . . . . . . . . . . . . . . 22
5.16. Security Claims . . . . . . . . . . . . . . . . . . . . . 22
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
7.1. Normative References . . . . . . . . . . . . . . . . . . . 25
7.2. Informative References . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . . . . 29
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1. Introduction
The Extensible Authentication Protocol (EAP), described in [RFC3748],
provides a standard mechanism for support of multiple authentication
methods. Through the use of EAP, support for a number of
authentication schemes may be added, including smart cards, Kerberos,
Public Key, One Time Passwords, and others.
In 1998, EAP-TLS ([RFC2716]) was published. It performs mutual
authentication based on the Transport Layer Security (TLS) protocol.
EAP-TLS allows an EAP peer to take advantage of the protected
ciphersuite negotiation, mutual authentication and key management
capabilities of the TLS protocol, described in [RFC2246bis].
Nonetheless, EAP-TLS restricts on certificate-based authentication.
In December 2005, IETF standardized pre-shared key ciphersuites for
TLS [RFC4279]. At IETF 65 in March 2006, the EMU Working Group
agreed on leaving EAP-TLS in its current form, i.e. not to enhance it
by support of the pre-shared key ciphersuites, but rather to specify
a new EAP method for this purpose.
This is the rationale for the EAP method specified in this document,
called EAP-TLS-PSK.
1.1. Overview about pre-shared key TLS ciphersuites
The goal of this subsection is to survey the pre-shared key TLS
ciphersuites specified in [RFC4279]. These ciphersuites are divided
into three sets, which distinguish in the underlying key exchange
mechanism and in the way the premaster_secret is computed. The three
key exchange mechanisms are henceforth referred to as PSK, DHE_PSK,
and RSA_PSK, in compliance with [RFC4279].
Basically, the pre-shared key extensions are realized by adding
attributes to the TLS client_key_exchange and TLS server_key_exchange
message, or also by changing the semantic of existing attributes.
For instance, all three sets extend the the client_key_exchange
message by a PSK identity, and the server_key_exchange message by an
attribute "PSK identity hint". This attribute is optional, and can
be used by the server to send some hint to the client which identity
to choose.
The three key exchange mechanisms PSK, DHE_PSK and RSA_PSK shall be
contrasted as next.
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PSK
These ciphersuites fully relies on symmetric key algorithms for
authentication, are thus very efficient and therefore well
suited for constrainted environments. where
The premaster_secret results from a concatenation of the pre-
shared key and its length.
DHE_PSK
DHE_PSK uses a PSK to authenticate a Diffie-Hellman key
exchange. The Diffie-Hellman public keys are exchanged within
the server_key_exchange and client_key_exchange messages. The
DHE_PSK ciphersuites provide Perfect Forward Secrecy (PFS).
The premaster_secret results from a concatenation of the pre-
shared key, its length, and the Diffie-Hellman shared secret and
its length.
RSA_PSK
RSA_PSK combines public key authentication of the server (using
RSA and certificates) with pre-shared key authentication of the
client. The server sends a certificate message to the client
which contains his public key. In this sense, RSA_PSK equals
TLS ciphersuites with RSA key exchange. However, [RFC4279] does
not further specify what the certificates contain. The
client_key_exchange message contains next to the PSK identity a
parameter "EncryptedPreMasterSecret" in length of 48 byte. The
first two octets are the TLS version number, the other 46 byte
are random data. For a RSA-based ciphersuite, this value is
exactly the TLS premaster_secret. For the pre-shared key
ciphersuites with RSA_PSK key exchange, however, the
premaster_secret results from a concatenation of its 48-byte
value with the pre-shared key and its length.
For further information on the respective mechanism, however, please
refer to the original specification [RFC4279].
1.2. Requirements notation
In this document, several words are used to signify the requirements
of the specification. 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].
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1.3. Terminology
This document frequently uses the following terms:
authenticator:
The end of the link initiating EAP authentication. The term
authenticator is used in [IEEE-802.1X], and 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.
backend authentication server:
A backend authentication server is an entity that provides an
authentication service to an authenticator. When used, this
server typically executes EAP methods for the authenticator.
This terminology is also used in [IEEE-802.1X].
EAP server:
The entity that terminates the EAP authentication method with the
peer. In the case where no backend authentication server is
used, the EAP server is part of the authenticator. In the case
where the authenticator operates in pass-through mode, the EAP
server is located on the backend authentication server.
Master Session Key (MSK):
Keying material that is derived between the EAP peer and server
and exported by the EAP method. The MSK is at least 64 octets in
length.
Extended Master Session Key (EMSK):
Additional keying material derived between the EAP client and
server that is exported by the EAP method. The EMSK is at least
64 octets in length.
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2. Protocol Overview
2.1. Overview of the EAP-TLS-PSK Conversation
The following figure depicts the EAP-TLS-PSK message flow in the
successful case.
Authenticating Peer Authenticator /
EAP Server
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=EAP-TLS-PSK
(TLS Start)
EAP-Response/
EAP-Type=EAP-TLS-PSK
(TLS client_hello)->
<- EAP-Request/
EAP-Type=EAP-TLS-PSK
(TLS server_hello,
[TLS server_key_exchange,]
TLS server_hello_done)
EAP-Response/
EAP-Type=EAP-TLS-PSK
(TLS client_key_exchange,
TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=EAP-TLS-PSK
(TLS change_cipher_spec,
TLS finished)
EAP-Response/
EAP-Type=EAP-TLS-PSK ->
<- EAP-Success
Figure 1: EAP-TLS-PSK message flow
As described in [RFC3748], the EAP-TLS-PSK conversation will
typically begin with the authenticator and the peer negotiating EAP.
The authenticator will then 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 peer's userId.
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From this point forward, while nominally the EAP conversation occurs
between the EAP authenticator and the peer, the authenticator MAY act
as a passthrough device, with the EAP packets received from the peer
being encapsulated for transmission to a backend security server. In
the discussion that follows, we will use the term "EAP server" to
denote the ultimate endpoint conversing with the peer.
Once having received the peer's Identity, the EAP server MUST respond
with an EAP-TLS-PSK/Start packet, which is an EAP-Request packet with
EAP-Type=EAP-TLS-PSK, the Start (S) bit set, and no data. The EAP-
TLS-PSK conversation will then begin, with the peer sending an EAP-
Response packet with EAP-Type=EAP-TLS-PSK. The data field of that
packet will encapsulate one or more TLS records in TLS record layer
format, containing a TLS client_hello handshake message.
The current cipher spec for the TLS records will be
TLS_NULL_WITH_NULL_NULL and null compression. This current cipher
spec remains the same until the change_cipher_spec message signals
that subsequent records will have the negotiated attributes for the
remainder of the handshake.
The client_hello message contains the client's TLS version number, a
sessionId, a random number, and a set of ciphersuites supported by
the client. The version offered by the client MUST correspond to TLS
v1.0 or later.
The EAP server will then respond with an EAP-Request packet with EAP-
Type=EAP-TLS-PSK. The data field of this packet will encapsulate one
or more TLS records. These will contain a TLS server_hello handshake
message, possibly followed by TLS server_key_exchange,
server_hello_done and/or finished handshake messages, and/or a TLS
change_cipher_spec message. The server_hello handshake message
contains a TLS version number, another random number, a sessionId,
and a ciphersuite. The version offered by the server MUST correspond
to TLS v1.0 or later.
If the client's sessionId is null or unrecognized by the server, the
server MUST choose the sessionId to establish a new session;
otherwise, the sessionId will match that offered by the client,
indicating a resumption of the previously established session with
that sessionID. The server will also choose a ciphersuite from those
offered by the client; if the session matches the client's, then the
ciphersuite MUST match the one negotiated during the handshake
protocol execution that established the session.
The purpose of the sessionId within the TLS protocol is to allow for
improved efficiency in the case where a client repeatedly attempts to
authenticate to an EAP server within a short period of time.
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As a result, it is left up to the peer whether to attempt to continue
a previous session, thus shortening the TLS conversation. Typically
the peer's decision will be made based on the time elapsed since the
previous authentication attempt to that EAP server. Based on the
sessionId chosen by the peer, and the time elapsed since the previous
authentication, the EAP server will decide whether to allow the
continuation, or whether to choose a new session.
In the case where the EAP server and authenticator reside on the same
device, then client will only be able to continue sessions when
connecting to the same NAS or tunnel server. Should these devices be
set up in a rotary or round-robin then it may not be possible for the
peer to know in advance the authenticator it will be connecting to,
and therefore which sessionId to attempt to reuse. As a result, it
is likely that the continuation attempt will fail. In the case where
the EAP authentication is remoted then continuation is much more
likely to be successful, since multiple NAS devices and tunnel
servers will remote their EAP authentications to the same backend
authentication server.
If the EAP server is resuming a previously established session, then
it MUST include only a TLS change_cipher_spec message and a TLS
finished handshake message after the server_hello message. The
finished message contains the EAP server's authentication response to
the peer. If the EAP server is not resuming a previously established
session, then it MUST include a TLS server_certificate handshake
message, and a server_hello_done handshake message MUST be the last
handshake message encapsulated in this EAP-Request packet.
In case of a RSA_PSK ciphersuite, the server sends a certificate
message. This message MUST contain the server's public key. In
accordance to EAP-TLS, the certificate message contains a public key
certificate chain for either a key exchange public key (such as an
RSA or Diffie-Hellman key exchange public key) or a signature public
key (such as an RSA or DSS signature public key). In the latter
case, a TLS server_key_exchange handshake message MUST also be
included to allow the key exchange to take place.
In an EAP-TLS-PSK message exchange, there will never the client will
never send a certificate and certificate_verify message, and the
server will never send a certificate_request message.
The peer MUST respond to the EAP-Request with an EAP-Response packet
of EAP-Type=EAP-TLS-PSK. The data field of this packet will
encapsulate one or more TLS records containing a TLS
client_key_exchange, change_cipher_spec and and finished handshake
message.
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If the preceding server_hello message sent by the EAP server in the
preceding EAP-Request packet indicated the resumption of a previous
session, then the peer MUST send only the change_cipher_spec and
finished handshake messages. The finished message contains the
peer's authentication response to the EAP server.
If the preceding server_hello message sent by the EAP server in the
preceeding EAP-Request packet did not indicate the resumption of a
previous session, then the peer MUST send, in addition to the
change_cipher_spec and finished messages, a client_key_exchange
message, which completes the exchange of a shared master secret
between the peer and the EAP server.
If the peer's authentication is unsuccessful, the EAP server SHOULD
send an EAP-Request packet with EAP-Type=EAP-TLS-PSK, encapsulating a
TLS record containing the appropriate TLS alert message. In
particular, if the server does not recognize the PSK identity, it
MUST respond with either an "unknown_psk_identity" TLS alert pessage
or continue with the protocol and send a TLS "decrypt_error" alert,
which stands for an incorrect key.
The server SHOULD send a TLS alert message rather 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.
The EAP-Response packet sent by the peer MAY encapsulate a TLS
client_hello handshake message, in which case the EAP server MAY
allow the EAP-TLS-PSK conversation to be restarted, or it MAY contain
an EAP-Response packet with EAP-Type=EAP-TLS-PSK and no data, in
which case the EAP server MUST send an EAP-Failure packet, and
terminate the conversation. It is up to the EAP server whether to
allow restarts, and if so, how many times the conversation can be
restarted. An EAP server implementing restart capability SHOULD
impose a limit on the number of restarts, so as to protect against
denial of service attacks.
If the peers authenticates successfully, the EAP server MUST respond
with an EAP-Request packet with EAP-Type=EAP-TLS-PSK, which includes,
in the case of a new TLS session, one or more TLS records containing
TLS change_cipher_spec and finished handshke messages. The latter
contains the EAP server's authentication response to the peer. The
peer will then verify the hash in order to authenticate the EAP
server.
If the EAP server authenticates unsuccessfully, the peer MAY send an
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EAP-Response packet of EAP-Type=EAP-TLS-PSK containing a TLS Alert
message identifying the reason for the failed authentication. The
peer MAY send a TLS alert message rather than immediately terminating
the conversation so as to allow the EAP server to log the cause of
the error for examination by the system administrator.
To ensure that the EAP server receives the TLS alert message, the
peer MUST wait for the EAP server to reply before terminating the
conversation. The EAP server MUST reply with an EAP-Failure packet
since server authentication failure is a terminal condition.
If the EAP server authenticates successfully, the peer MUST send an
EAP-Response packet of EAP-Type=EAP-TLS-PSK, and no data. The EAP
server then MUST respond with an EAP-Success message.
2.2. Retry Behavior
As with other EAP protocols, the EAP server is responsible for retry
behavior. This means that if the EAP server does not receive a reply
from the peer, it MUST resend the EAP-Request for which it has not
yet received an EAP-Response. However, the peer MUST NOT resend EAP-
Response packets without first being prompted by the EAP server.
For example, if the initial EAP-TLS-PSK start packet sent by the EAP
server were to be lost, then the peer would not receive this packet,
and would not respond to it. As a result, the EAP-TLS-PSK start
packet would be resent by the EAP server. Once the peer received the
EAP-TLS-PSK start packet, it would send an EAP-Response encapsulating
the client_hello message. If the EAP-Response were to be lost, then
the EAP server would resend the initial EAP-TLS-PSK start, and the
peer would resend the EAP-Response.
As a result, it is possible that a peer will receive duplicate EAP-
Request messages, and may send duplicate EAP-Responses. Both the
peer and the EAP server should be engineered to handle this
possibility.
2.3. 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-TLS-PSK
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 [RFC1990], 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.
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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 anwhere 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, there may be cases in which multilink or the MRRU
LCP option cannot be negotiated. As a result, an EAP-TLS-PSK
implementation MUST provide its own support for fragmentation and
reassembly.
Since EAP is a simple 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-TLS-PSK fragmentation support is provided through addition of a
flags octet 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-TLS-PSK Start (S)
bits. The 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-TLS-PSK
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-TLS-PSK peer receives an EAP-Request packet with the M
bit set, it MUST respond with an EAP-Response with EAP-Type=EAP-TLS-
PSK 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 server receives an EAP-Response with the M
bit set, it MUST respond with an EAP-Request with EAP-Type=EAP-TLS-
PSK and no data. This serves as a fragment ACK. The EAP peer MUST
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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 use 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.
2.4. Identity Verification
As noted in [RFC3748] Section 5.1: It is RECOMMENDED that the
Identity Response be used primarily for routing purposes and
selecting which EAP method to use. EAP Methods SHOULD include a
method-specific mechanism for obtaining the identity, so that they do
not have to rely on the Identity Response.
As part of the EAP-TLS-PSK authentication, the peer uses the
client_key_exchange message to transmit his identity.
For RSA_PSK ciphersuites, the server presents a certificate to the
peer, which contains his identity.
EAP-TLS-PSK therefore provides a mechanism for determining both the
peer and server identities.
o The peer identity (Peer-ID in is exactly the PSK identity of the
client_key_exchange message.
o The server identity (Server-ID in is for ciphersuites of type
"RSA_PSK" either directly determined from the altSubjectName in
the server certificate or by a mapping of the altSubjectName to
the Server-ID using a directory service.
For ciphersuites of type "PSK" and "DHE_PSK", the server identity
is uniquely defined by means of the pre-shared key, which is
shared exclusively between an EAP peer and EAP server.
For RSA_PSK, the peer MUST verify the validity of the EAP server
certificate, and SHOULD also examine the EAP server name presented in
the certificate, in order to determine whether the EAP server can be
trusted. Please note that in the case where the EAP authentication
is remoted that the EAP server will not reside on the same machine as
the authenticator, and therefore the name in the EAP server's
certificate cannot be expected to match that of the intended
destination. In this case, a more appropriate test might be whether
the EAP server's certificate is signed by a CA controlling the
intended destination and whether the EAP server exists within a
target sub-domain.
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2.5. Key Hierarchy
In EAP-TLS-PSK, the MSK, EMSK and IV are derived from the TLS master
secret via a one-way function. This ensures that the TLS master
secret cannot be derived from the MSK, EMSK or IV unless the one-way
function (TLS PRF) is broken. Since the MSK is derived from the the
TLS master secret, if the TLS master secret is compromised then the
MSK is also compromised.
The MSK is divided into two halves, corresponding to the "Peer to
Authenticator Encryption Key" (Enc- RECV-Key, 32 octets) and
"Authenticator to Peer Encryption Key" (Enc- SEND-Key, 32 octets).
The EMSK is also divided into two halves, corresponding to the "Peer
to Authenticator Authentication Key" (Auth-RECV-Key, 32 octets) and
"Authenticator to Peer Authentication Key" (Auth-SEND-Key, 32
octets). The IV is a 64 octet quantity that is a known value; octets
0-31 are known as the "Peer to Authenticator IV" or RECV-IV, and
Octets 32-63 are known as the "Authenticator to Peer IV", or SEND-IV.
The key derivation scheme is as follows. The notation X[A..B] means
byte A to B of X. The notation TLS-PRF-X means that the TLS-PRF is
iterated as long as possible to generate X byte output.
MSK = TLS-PRF-128 (master_secret, "client EAP encryption",
client.random || server.random)[0..63]
EMSK = TLS-PRF-128 (master_secret, "client EAP encryption",
client.random || server.random)[64..127]
IV = TLS-PRF-64 ("", "client EAP encryption",
client.random || server.random)[0..63]
The TLS-negotiated ciphersuite is used to set up a protected channel
for use in protecting the EAP conversation, keyed by the derived
TEKs. The TEK derivation proceeds as follows:
master_secret = TLS-PRF-48(pre_master_secret, "master secret",
client.random || server.random)
TEK = TLS-PRF-X(master_secret, "key expansion",
server.random || client.random)
To meet the requirements of [I-D.ietf-eap-keying] EAP-TLS-PSK defines
a Method-ID, which is used for computing a session-ID and key names.
In the current version, the Method-ID is set to the concatenation of
the server and client Finished messages. The Method-ID uniquely
identifies an EAP-TLS-PSK session, because the Hashes in the Finished
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message contain the random values exchanged with the ClientHello- and
ServerHello messages as well as the identities of client and EAP
server.
2.6. Ciphersuite and Compression Negotiation
Since TLS supports ciphersuite negotiation, peers completing the TLS
negotiation will also have selected a ciphersuite, which includes
encryption and hashing methods. Since the ciphersuite negotiated
within EAP-TLS-PSK applies only to the EAP conversation, TLS
ciphersuite negotiation SHOULD NOT be used to negotiate the
ciphersuites used to secure data.
EAP-TLS-PSK is intended to be used only with the ciphersuites defined
in [RFC4279]. For convenience, these ciphersuites are summarized
below.
CipherSuite TLS_PSK_WITH_RC4_128_SHA = { 0x00, 0x8A };
CipherSuite TLS_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x8B };
CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0x8C };
CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0x8D };
CipherSuite TLS_DHE_PSK_WITH_RC4_128_SHA = { 0x00, 0x8E };
CipherSuite TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x8F };
CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0x90 };
CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0x91 };
CipherSuite TLS_RSA_PSK_WITH_RC4_128_SHA = { 0x00, 0x92 };
CipherSuite TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0x93 };
CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA = { 0x00, 0x94 };
CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA = { 0x00, 0x95 };
TLS also supports compression as well as ciphersuite negotiation.
Since compression negotiated within EAP-TLS-PSK applies only to the
EAP conversation, TLS compression negotiation MUST NOT be used to
negotiate compression mechanisms to be applied to data.
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3. EAP-TLS-PSK Packet Format
A summary of the EAP TLS 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 | TLS Message Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLS Message Length | TLS Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: EAP-TLS-PSK packet format
Code
1 - EAP-TLS-PSK Request (short: Request)
2 - EAP-TLS-PSK Response (short: Response)
Identifier
The identifier field is one octet and aids in matching responses
with requests. If the message is of type Response, then the
identifier MUST match the Identifier field from the corresponding
request.
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
TBD - EAP-TLS-PSK
Flags
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0 1 2 3 4 5 6 7 8
+-+-+-+-+-+-+-+-+
|L M S R R R R R|
+-+-+-+-+-+-+-+-+
L = Length included
M = More fragments
S = EAP-TLS-PSK start
R = Reserved
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-TLS-PSK start) is set in an EAP-TLS-PSK Start
message. This differentiates the EAP-TLS-PSK Start message from
a fragment acknowledgement. Implementations of this
specification MUST set the reserved bits to zero, and MUST ignore
them on reception.
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 TLS packet in TLS
record format.
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4. IANA Considerations
This document requires IANA to allocate a new EAP Type for EAP-TLS-
PSK.
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5. Security Considerations
[RFC3748] highlights several attacks that are possible against EAP as
EAP does not provide any robust security mechanism. This section
discusses the claimed security properties of EAP-TLS-PSK as well as
vulnerabilities and security recommendations in the threat model of
[RFC3748].
5.1. Mutual Authentication
EAP-TLS-PSK provides mutual authentication. This holds for all three
modes PSK, DHE_PSK and RSA_PSK.
5.2. Protected Result Indications
EAP-TLS-PSK does not provide protected result indications.
5.3. Integrity Protection
EAP-TLS-PSK provides integrity protection thanks to the TLS Finished
message, which contains a Message Authentication Code computed over
the whole previous conversation. That is, the verification of the
Finished message serves as guarantee of the conversation's integrity.
5.4. Replay Protection
EAP-TLS-PSK provides replay protection of its mutual authentication
part thanks to the use of random numbers in the client_hello and
server_hello messages. These random numbers are 16 byte long. One
expects to have to record 2**64 (i.e. approximately 1.84*10**19) EAP-
TLS-PSK successful authentication before an authentication can be
replayed. A good source for randomness is cruicial for the security
of EAP-TLS-PSK.
5.5. Dictionary Attacks
For PSK and DHE_PSK, mutual authentication is based on a shared
secret. While [RFC4279] does not specify the length of this pre-
shared key, EAP-TLS-PSK does so. The pre-shared key MUST be at least
16 byte long and have full entropy. For these two modes, it is
highly discouraged having derived the pre-shared key from low entropy
source, e.g. a password.
For RSA_PSK, the shared secret must also be at least 16 byte long.
In contrast to PSK and DHE_PSK modes, the shared secret may be also
derived from a low entropy source, e.g. a password. This becomes
possible because the server authentications with public-key
techniques first.
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In this draft version, however, the following assertion is made. If
the shared secret is at least 16 byte long and has full entropy, EAP-
TLS-PSK is not susceptible for dictionary attacks.
5.6. Key Derivation
EAP-TLS-PSK supports key derivation according to [RFC3748] and [I.D.-
Keying].
EAP-TLS-PSK exports keys, namely a 64-byte MSK, a 64-byte EMSK and a
64-byte IV.
Some remarks regarding the key strength. In case of RSA_PSK
ciphersuites, the server's private key size should be chosen
accordingly to the length of the pre-shared key. Section 5 in
[RFC3766] contrasts symmetric key sizes with their public key
counterparts, to obtain roughly the same overall key strength. Based
on the table below, a 3072-bit RSA key is required to provide 128-bit
equivalent key strength.
Attack Resistance RSA or DH Modulus DSA subgroup
(bits) size (bits) size (bits)
----------------- ----------------- ------------
70 947 128
80 1228 145
90 1553 153
100 1926 184
150 4575 279
200 8719 373
250 14596 475
5.7. Session Independence
Thanks to its key derivation mechanisms, EAP-PSK provides session
independence: passive attacks (such as capture of the EAP
conversation) or active attacks (including compromise of the MSK or
EMSK) does not enable compromise of subsequent or prior MSKs or
EMSKs. The assumption that the random numbers of the TLS
client_hello and server_hello messages are random is central for the
security of EAP-TLS-PSK in general and session independance in
particular.
5.8. Exposition of the PSK
EAP-TLS-PSK specifies three sets of ciphersuites, which distinguish
in the underlying key derivation mechanism.
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o The PSK and RSA_PSK ciphersuites does not provide perfect forward
secrecy. Compromise of the pre-shared key or the pre-shared key
and the server's private key (in case of RSA_PSK) leads to
compromise of recorded past sessions.
o DHE_PSK ciphersuites provide perfect forward secrecy, if a fresh
Diffie-Hellman private key is generated for each handshake.
5.9. Fragmentation
EAP-TLS-PSK supports fragmentation and reassembly. The mechanism is
inherited from EAP-TLS ([RFC2716]).
5.10. Channel Binding
EAP-TLS-PSK does not provide channel binding.
5.11. Fast Reconnect
EAP-TLS-PSK relies on the Transport Layer Security protocol, which
specifies a fast resumption mode. If peer and server agree on
continuing a previously established session, the session's master
secret can be re-used. That is, related computations can be omitted,
which make the fast resumption mode very efficient.
For PSK ciphersuites, the speedup is believed to be minimal, since
these ciphersuites rely on symmetric key operations only. It is
expected, that DHE_PSK and RSA_PSK may benefit from the fast
resumption mode.
5.12. Identity Protection
EAP-TLS-PSK does not provide user identity protection. The
client_key_exchange message contains the peer's identity. This
message is sent in plaintext.
5.13. Protected Ciphersuite Negotiation
Since EAP-TLS-PSK relies on TLS, it also supports ciphersuite
negotiation. This is done in a securely manner, because the TLS
Finished messages authenticate the whole handshake. Therefore, EAP-
TLS-PSK provides protected ciphersuite negotiation.
5.14. Confidentiality
EAP-TLS-PSK does not support this feature. According to Section
7.2.1 of [RFC3748], this feature would mandate for the feature
'identity protection', which is also not addressed by EAP-TLS-PSK.
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5.15. Cryptographic Binding
This feature is not applicable for EAP-TLS-PSK.
5.16. Security Claims
This section provides the security claims required by [RFC3748].
[a] Mechanism.
* For PSK and DHE_PSK: Pre-shared key.
* For RSA_PSK: Server via Public key, Peer via Pre-shared key.
[b] Security claims. EAP-TLS-PSK provides:
* Mutual authentication (see Section 5.1)
* Integrity protection (see Section 5.3)
* Replay protection (see Section 5.4)
* Key derivation (see Section 5.6)
* Dictionary attack resistance (see Section 5.5)
* Session independence (see section Section 5.5)
* Fast reconnect (see Section 5.11)
* Fragmentation (see Section 5.9)
* Protected cipher suite negotiation (see Section 5.13)
* Perfect Forward Secrecy (at least partially; see Section 5.6)
[c] Key strength. EAP-TLS-PSK provides at least a 16-byte effective
key strength.
[d] Indication of vulnerabilities. EAP-TLS-PSK does not provide:
* Identity protection (see Section 5.12)
* Confidentiality (see Section 5.14)
* Cryptographic binding (see Section 5.15)
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* Key agreement: the session key is chosen by the peer (see
Section 5.6)
* Channel binding (see Section 5.10)
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6. Acknowledgments
The authors would like to thank Bernard Aboba and Dan Simon for
adopting parts of their EAP-TLS specification, and Florent Bersani
for lending parts of the EAP-PSK specification.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC2716] Aboba, B. and D. Simon, "PPP EAP TLS Authentication
Protocol", RFC 2716, October 1999.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279,
December 2005.
7.2. Informative References
[I-D.ietf-eap-keying]
Aboba, B., "Extensible Authentication Protocol (EAP) Key
Management Framework", draft-ietf-eap-keying-14 (work in
progress), June 2006.
[IEEE-802.11]
Institute of Electrical and Electronics Engineers,
"Standard for Telecommunications and Information Exchange
Between Systems - LAN/MAN Specific Requirements - Part 11:
Wireless LAN Medium Access Control (MAC) and Physical
Layer (PHY) Specifications", IEEE Standard 802.11, 1999.
[IEEE-802.11i]
Institute of Electrical and Electronics Engineers,
"Approved Draft Supplement to Standard for
Telecommunications and Information Exchange Between
Systems-LAN/MAN Specific Requirements - Part 11: Wireless
LAN Medium Access Control (MAC) and Physical Layer (PHY)
Specifications: Specification for Enhanced Security",
IEEE 802.11i-2004, June 2004.
[IEEE-802.16e]
Institute of Electrical and Electronics Engineers,
"Standard for Local and Metropolitan Area Networks: Part
16: Air Interface for Fixed and Mobile Broadband Wireless
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Access Systems: Amendment for Physical and Medium Access
Control Layers for Combined Fixed and Mobile Operations in
Licensed Bands" IEEE 802.16e", IEEE 802.16e, August 2005.
[IEEE-802.1X]
Institute of Electrical and Electronics Engineers, "Local
and Metropolitan Area Networks: Port-Based Network Access
Control", IEEE Standard 802.1X, September 2001.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC1570] Simpson, W., "PPP LCP Extensions", RFC 1570, January 1994.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[RFC1662] Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC 1662,
July 1994.
[RFC1990] Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T.
Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990,
August 1996.
[RFC2419] Sklower, K. and G. Meyer, "The PPP DES Encryption
Protocol, Version 2 (DESE-bis)", RFC 2419, September 1998.
[RFC2420] Kummert, H., "The PPP Triple-DES Encryption Protocol
(3DESE)", RFC 2420, September 1998.
[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes",
RFC 2548, March 1999.
[RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little,
W., and G. Zorn, "Point-to-Point Tunneling Protocol",
RFC 2637, July 1999.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
RFC 2661, August 1999.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, April 2004.
[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs", RFC 4017, March 2005.
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[RFC4334] Housley, R. and T. Moore, "Certificate Extensions and
Attributes Supporting Authentication in Point-to-Point
Protocol (PPP) and Wireless Local Area Networks (WLAN)",
RFC 4334, February 2006.
[TLSPSK-Perf]
Fang-Chun Kuo, Hannes Tschofenig, Fabian Meyer, and
Xiaoming Fu, "Comparison Studies between Pre-Shared Key
and Public Key Exchange Mechanisms for Transport Layer
Security (TLS)", IFI-TB-2006-01 URL: http://
user.informatik.uni-goettingen.de/~fkuo/publications/
ptls-ifi-tb-2006-01.pdf, January 2006.
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Authors' Addresses
Thomas Otto
Email: thomas.g.otto@googlemail.com
Hannes Tschofenig
Siemens Networks GmbH & Co KG
Otto-Hahn-Ring 6
Munich 81739
Germany
Email: hannes.tschofenig@siemens.com
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Full Copyright Statement
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