One document matched: draft-simon-emu-rfc2716bis-01.txt
Differences from draft-simon-emu-rfc2716bis-00.txt
Network Working Group Dan Simon
INTERNET-DRAFT Bernard Aboba
Category: Proposed Standard Microsoft
<draft-simon-emu-rfc2716bis-01.txt>
24 February 2006
The EAP TLS Authentication Protocol
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Copyright Notice
Copyright (C) The Internet Society 2006.
Abstract
The Extensible Authentication Protocol (EAP), defined in RFC 3748,
provides support for multiple authentication methods. Transport
Level Security (TLS) provides for mutual authentication, integrity-
protected ciphersuite negotiation and key exchange between two
endpoints. This document defines EAP-TLS, which includes support for
certificate-based mutual authentication and key derivation.
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Table of Contents
1. Introduction.............................................. 3
1.1 Requirements Language ........................... 3
1.2 Terminology ..................................... 3
2. Protocol Overview ........................................ 4
2.1 Overview of the EAP-TLS Conversation ............ 4
2.2 Retry Behavior .................................. 7
2.3 Fragmentation ................................... 8
2.4 Identity Verification ........................... 9
2.5 Key Hierarchy ................................... 10
2.6 Ciphersuite and Compression Negotiation ......... 12
3. Detailed Description of the EAP-TLS Protocol ............. 13
3.1 EAP TLS Packet Format .......................... 13
3.2 EAP TLS Request Packet ......................... 14
3.3 EAP TLS Response Packet ........................ 15
4. Security Considerations .................................. 17
4.1 Security Claims ................................ 17
4.2 Certificate Revocation ......................... 18
4.3 Certificate Usage Restrictions ................. 18
4.4 Separation of EAP Authenticator and Server ..... 18
4.5 Lower Layer Security Mechanisms ................ 19
5. References ............................................... 19
5.1 Normative references ....... .................... 19
5.2 Informative references .......................... 20
Acknowledgments .............................................. 21
Authors' Addresses ........................................... 22
Appendix A - Examples ........................................ 23
Intellectual Property Statement .............................. 29
Disclaimer of Validity ....................................... 29
Copyright Statement .......................................... 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.
While the EAP methods defined in [RFC3748] did not support mutual
authentication, the use of EAP with wireless technologies such as
[IEEE-802.11i] has resulted in development of a new set of
requirements [RFC4017]. As described in [RFC4017] it is desirable
for EAP methods used for wireless LAN authentication to support
mutual authentication and key derivation. Since PPP encryption
protocols such as [RFC2419] and [RFC2420] assume existence of a
session key, it is useful to have a mechanism for session key
establishment. Since design of secure key management protocols is
non-trivial, it is desirable to avoid creating new mechanisms for
this. The EAP protocol described in this document allows a EAP peer
to take advantage of the protected ciphersuite negotiation, mutual
authentication and key management capabilities of the TLS protocol,
described in [RFC2246bis].
1.1. Requirements
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
"Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].
1.2. 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].
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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.
2. Protocol Overview
2.1. Overview of the EAP-TLS Conversation
As described in [RFC3748], the EAP-TLS 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.
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/Start packet, which is an EAP-Request packet with
EAP-Type=EAP-TLS, the Start (S) bit set, and no data. The EAP-TLS
conversation will then begin, with the peer sending an EAP-Response
packet with EAP-Type=EAP-TLS. 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
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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. 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 certificate, server_key_exchange,
certificate_request, 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. While
this model was developed for use with HTTP authentication, it may
also have application to PPP authentication (e.g. multilink).
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.
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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.
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.
The certificate_request message is included when the server desires
the client to authenticate itself via public key. While the EAP
server SHOULD require client authentication, this is not a
requirement, since it may be possible that the server will require
that the peer authenticate via some other means.
The peer MUST respond to the EAP-Request with an EAP-Response packet
of EAP-Type=EAP-TLS. The data field of this packet will encapsulate
one or more TLS records containing a TLS change_cipher_spec message
and finished handshake message, and possibly certificate,
certificate_verify and/or client_key_exchange handshake messages. 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 EAP server sent a
certificate_request message in the preceding EAP-Request packet, then
the peer MUST send, in addition, certificate and certificate_verify
handshake messages. The former contains a certificate for the peer's
signature public key, while the latter contains the peer's signed
authentication response to the EAP server. After receiving this
packet, the EAP server will verify the peer's certificate and digital
signature, if requested.
If the peer's authentication is unsuccessful, the EAP server SHOULD
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send an EAP-Request packet with EAP-Type=EAP-TLS, encapsulating a TLS
record containing the appropriate TLS alert message. The EAP 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 conversation to be restarted, or it MAY contain an
EAP-Response packet with EAP-Type=EAP-TLS 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, 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
EAP-Response packet of EAP-Type=EAP-TLS 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, 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
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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 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 start packet
would be resent by the EAP server. Once the peer received the EAP-TLS
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 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 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.
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
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
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fragment offset field as is provided in IPv4.
EAP-TLS 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 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 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 peer receives an EAP-Request packet with the M bit
set, it MUST respond with an EAP-Response with EAP-Type=EAP-TLS 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
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 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 TLS negotiation, the server presents a certificate to
the peer, and if mutual authentication is requested, the peer
presents a certificate to the server. EAP-TLS therefore provides a
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mechanism for determining both the peer and server identities. The
peer identity (Peer-ID in [KEYFRAME]) is either directly determined
from the altSubjectName in the peer certificate or by a mapping of
the altSubjectName to the Peer-ID using a directory service. The
server identity (Server-ID in [KEYFRAME]) is 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.
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.
2.5. Key Hierarchy
In EAP-TLS, 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:
MSK = TLS-PRF-64(TMS, "client EAP encryption",
client.random || server.random)
EMSK = second 64 octets of:
TLS-PRF-128(TMS, "client EAP encryption",
client.random || server.random)
IV = TLS-PRF-64("", "client EAP encryption",
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client.random || server.random)
MSK(0,31) = Peer to Authenticator Encryption Key (Enc-RECV-Key)
(MS-MPPE-Recv-Key in [RFC2548]). Also known as the
PMK in [IEEE-802.11i].
MSK(32,63) = Authenticator to Peer Encryption Key (Enc-SEND-Key)
(MS-MPPE-Send-Key in [RFC2548])
EMSK(0,31) = Peer to Authenticator Authentication Key (Auth-RECV-Key)
EMSK(32,63 = Authenticator to Peer Authentication Key (Auth-Send-Key)
IV(0,31) = Peer to Authenticator Initialization Vector (RECV-IV)
IV(32,63) = Authenticator to Peer Initialization vector (SEND-IV)
Where:
IV(W,Z) = Octets W through Z inclusive of the IV.
MSK(W,Z) = Octets W through Z inclusive of the MSK.
EMSK(W,Z) = Octets W through Z inclusive of the EMSK.
TMS = TLS master_secret
TLS-PRF-X = TLS PRF function defined in [RFC2246] computed to X
octets
client.random = Nonce generated by the TLS client.
server.random = Nonce generated by the TLS server.
Figure 1 illustrates the TEK key hierarchy for EAP-TLS which is based
on the TLS key hierarchy described in [RFC2246bis]. 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)
Where:
TLS-PRF-X = TLS pseudo-random function defined in [RFC2246bis],
computed to X octets.
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| | pre_master_secret |
server| | | client
Random| V | Random
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | | |
+---->| master_secret |<------+
| | (TMS) | |
| | | |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | |
V V V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Key Block (TEKs) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | |
| client | server | client | server | client | server
| MAC | MAC | write | write | IV | IV
| | | | | |
V V V V V V
Figure 1 - TLS [RFC2246bis] Key Hierarchy
The use of these encryption and authentication keys is specific to
the lower layer. For example, PPP encryption mechanisms are defined
in [RFC2419] and [RFC2420]; security mechanisms for IEEE 802.11 are
defined in [IEEE-802.11i].
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 applies only to the EAP conversation, TLS ciphersuite
negotiation SHOULD NOT be used to negotiate the ciphersuites used to
secure data.
TLS also supports compression as well as ciphersuite negotiation.
Since compression negotiated within EAP-TLS 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. Detailed description of the EAP-TLS protocol
3.1. EAP TLS 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 | Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 - Request
2 - Response
Identifier
The identifier field is one octet and aids in matching responses
with requests.
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
13 - EAP TLS
Data
The format of the Data field is determined by the Code field.
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3.2. EAP TLS Request Packet
A summary of the EAP TLS Request 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...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1
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 TLS
Response fields.
Type
13 - EAP TLS
Flags
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 start
R = Reserved
The L bit (length included) is set to indicate the presence of the
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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 start) is set in an EAP-TLS Start message. This
differentiates the EAP-TLS 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.
3.3. EAP TLS Response Packet
A summary of the EAP TLS 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...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
2
Identifier
The Identifier field is one octet and 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 TLS data
fields.
Simon & Aboba Proposed Standard [Page 15]
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Type
13 - EAP TLS
Flags
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 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 start) is set in an EAP-TLS Start message. This
differentiates the EAP-TLS 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. Security Considerations
4.1. Security Claims
EAP security claims are defined in [RFC3748] Section 7.2.1. The
security claims for EAP-TLS are as follows:
Auth. mechanism: Certificates
Ciphersuite negotiation: Yes [1]
Mutual authentication: Yes [1]
Integrity protection: Yes [1]
Replay protection: Yes [1]
Confidentiality: Yes [2]
Key derivation: Yes
Key strength: [3]
Dictionary attack prot.: Yes
Fast reconnect: Yes
Crypt. binding: N/A
Session independence: Yes [1]
Fragmentation: Yes
Channel binding: No
Notes
-----
[1] A formal proof of the security of EAP-TLS when used with
[IEEE-802.11i] is provided in [He]. This proof relies on the
assumption that the private key pairs used by the EAP peer and server
are not shared with other parties or applications. For example, a
backend authentication server supporting EAP-TLS should not utilize
the same certificate with https.
[2] Most EAP-TLS implementations do not support privacy, since they
send the client certificate in the clear. However, it is possible
for EAP-TLS implementations to support privacy by bringing up a
protected channel with server-only authentication, then having the
server request the client certificate.
[3] As noted in [RFC3766] Section 5, the effective level of attack
resistance provided by EAP-TLS is related to the RSA or DH module and
DSA subgroup size in bits. Based on the table below, a 2048-bit RSA
key is required to provide 128-bit equivalent key strength.
Simon & Aboba Proposed Standard [Page 17]
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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
4.2. Certificate revocation
Since the EAP server is on the Internet during the EAP conversation,
the server is capable of following a certificate chain or verifying
whether the peer's certificate has been revoked. In contrast, the
peer may or may not have Internet connectivity, and thus while it can
validate the EAP server's certificate based on a pre-configured set
of CAs, it may not be able to follow a certificate chain or verify
whether the EAP server's certificate has been revoked.
In the case where the peer is initiating a voluntary Layer 2 tunnel
using PPTP [RFC2637] or L2TP [RFC2661], the peer will typically
already have a PPP interface and Internet connectivity established at
the time of tunnel initiation. As a result, during the EAP
conversation it is capable of checking for certificate revocation.
However, in the case where the peer is initiating an initial PPP
conversation, it will not have Internet connectivity and is therefore
not capable of checking for certificate revocation until after NCP
negotiation completes and the peer has access to the Internet. In
this case, the peer SHOULD check for certificate revocation after
connecting to the Internet.
4.3. Certificate Usage Restrictions
As discussed in [He], the security of EAP-TLS can be compromised if
the same credentials are used for authentication within multiple
applications. Certificate extensions for use with EAP-TLS are
discussed in [RFC4334]. These extensions enable certificate usage to
be restricted to use with lower layers such as PPP or IEEE 802.11.
4.4. Separation of the EAP Authenticator and Server
As a result of the EAP-TLS conversation, the EAP peer and server
endpoints will mutually authenticate and derive the MSK and EMSK.
Subsequently the EAP peer and authenticator may negotiate a
ciphersuite for protection of data and derive a session key for
Simon & Aboba Proposed Standard [Page 18]
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subsequent use. Since the peer and EAP client reside on the same
machine, it is necessary for the EAP client module to pass the
required keying material to the lower layer, as described in
[KEYFRAME].
The situation may be more complex on the EAP authenticator, which may
or may not reside on the same machine as the EAP server. In the case
where the EAP server and authenticator reside on different machines,
there are several implications for security. Firstly, the mutual
authentication defined in EAP-TLS will occur between the EAP peer and
server, not between the peer and the authenticator. This means that
as a result of the EAP-TLS conversation, it is not possible for the
EAP peer to validate the identity of the NAS or tunnel server that it
is speaking to.
The second issue is that the EAP keying material derived between the
peer and EAP server will need to be transported to the authenticator.
The implications of this are described in more detail in [KEYFRAME];
however the specification of this transport mechanism is outside the
scope of this document.
4.5. Lower Layer Security Mechanisms
EAP-TLS has been deployed for use with a variety of lower layers,
including PPP, Layer 2 tunneling protocols such as PPTP and L2TP,
IEEE 802 wired networks [IEEE-802.1X] and wireless technologies such
as IEEE 802.11 [IEEE-802.11i] and IEEE 802.16 [IEEE-802.16e].
In compulsory layer 2 tunneling, a PPP peer makes a connection to a
NAS or router which tunnels the PPP packets to a tunnel server.
Since with compulsory tunneling a PPP peer cannot tell whether its
packets are being tunneled, let alone whether the network device is
securing the tunnel, if security is required then the client must
make its own arrangements. In the case where all endpoints cannot be
relied upon to implement IPSEC, TLS, or another suitable security
protocol, PPP encryption provides a convenient means to ensure the
privacy of packets transiting between the client and the tunnel
server.
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2246bis] Dierks, T. and E. Rescorla, "The TLS Protocol Version
1.1", Internet Draft (work in progress), draft-ietf-tls-
Simon & Aboba Proposed Standard [Page 19]
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rfc2246bis-13.txt, June 2005.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H.
Lefkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
5.2. Informative References
[FIPS] National Bureau of Standards, "Data Encryption Standard",
FIPS PUB 46 (January 1977).
[IEEE-802.11] Institute of Electrical and Electronics Engineers,
"Information technology - Telecommunications and
information exchange between systems - Local and
metropolitan area networks - Specific Requirements Part
11: Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) Specifications", IEEE Standard
802.11-2003, 2003.
[IEEE-802.1X] Institute of Electrical and Electronics Engineers, "Local
and Metropolitan Area Networks: Port-Based Network Access
Control", IEEE Standard 802.1X-2004, December 2004.
[IEEE-802.11i] Institute of Electrical and Electronics Engineers,
"Supplement to STANDARD FOR Telecommunications and
Information Exchange between Systems - LAN/MAN Specific
Requirements - Part 11: Wireless Medium Access Control
(MAC) and physical layer (PHY) specifications:
Specification for Enhanced Security", IEEE 802.11i,
December 2004.
[IEEE-802.16e] Institute of Electrical and Electronics Engineers, "IEEE
Standard for Local and Metropolitan Area Networks: Part
16: Air Interface for Fixed and Mobile Broadband Wireless
Access Systems: Amendment for Physical and Medium Access
Control Layers for Combined Fixed and Mobile Operations
in Licensed Bands" IEEE 802.16e, August 2005.
[He] He, C., Sundararajan, M., Datta, A., Derek, A. and J.
Mitchell, "A Modular Correctness Proof of IEEE 802.11i
and TLS", CCS '05, November 7-11, 2005, Alexandria,
Virginia, USA
[KEYFRAME] Aboba, B., Simon, D., Arkko, J., Eronen, P. and H.
Levkowetz, "Extensible Authentication Protocol (EAP) Key
Management Framework", Internet Draft (work in progress),
draft-ietf-eap-keying-10.txt, March 2006.
Simon & Aboba Proposed Standard [Page 20]
INTERNET-DRAFT EAP TLS Authentication Protocol 24 February 2006
[RFC1321] Rivest, R. and S. Dusse, "The MD5 Message-Digest
Algorithm", RFC 1321, April 1992.
[RFC1570] Simpson, W., Editor, "PPP LCP Extensions", RFC 1570,
January 1994.
[RFC1661] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.
[RFC1962] D. Rand, "The PPP Compression Control Protocol", RFC
1962, Novell, June 1996.
[RFC1968] Meyer, G., "The PPP Encryption Protocol (ECP)", RFC 1968,
June 1996.
[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] Hummert, K., "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", 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.
[RFC4334] Housley, R. and T. Moore, "Certificate Extensions and
Attributes Supporting Authentication in Point-to-Point
Protocol (PPP) and Wireless Local Area Networks (WLAN)",
Simon & Aboba Proposed Standard [Page 21]
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RFC 4334, February 2006.
Acknowledgments
Thanks to Terence Spies, Glen Zorn and Narendra Gidwani of Microsoft
for useful discussions of this problem space.
Authors' Addresses
Dan Simon
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 706 6711
EMail: dansimon@microsoft.com
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 706 6605
EMail: bernarda@microsoft.com
Simon & Aboba Proposed Standard [Page 22]
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Appendix A - Examples
In the case where the EAP-TLS mutual authentication is successful,
the conversation will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
[TLS certificate_request,]
TLS server_hello_done)
EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
EAP-Response/
EAP-Type=EAP-TLS ->
<- EAP-Success
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In the case where the EAP-TLS mutual authentication is successful,
and fragmentation is required, the conversation will appear as
follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS Start, S bit set)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
[TLS certificate_request,]
TLS server_hello_done)
(Fragment 1: L, M bits set)
EAP-Response/
EAP-Type=EAP-TLS ->
<- EAP-Request/
EAP-Type=EAP-TLS
(Fragment 2: M bit set)
EAP-Response/
EAP-Type=EAP-TLS ->
<- EAP-Request/
EAP-Type=EAP-TLS
(Fragment 3)
EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS inished)(Fragment 1:
L, M bits set)->
<- EAP-Request/
EAP-Type=EAP-TLS
EAP-Response/
EAP-Type=EAP-TLS
(Fragment 2)->
<- EAP-Request/
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EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
EAP-Response/
EAP-Type=EAP-TLS ->
<- EAP-Success
In the case where the server authenticates to the client
successfully, but the client fails to authenticate to the server, the
conversation will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
TLS certificate_request,
TLS server_hello_done)
EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
TLS client_key_exchange,
TLS certificate_verify,
TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
EAP-Response/
EAP-Type=EAP-TLS ->
<- EAP-Request
EAP-Type=EAP-TLS
(TLS Alert message)
EAP-Response/
EAP-Type=EAP-TLS ->
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<- EAP-Failure
(User Disconnected)
In the case where server authentication is unsuccessful, the
conversation will appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
[TLS certificate_request,]
TLS server_hello_done)
EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
EAP-Response/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
<- EAP-Request/
EAP-Type=EAP-TLS
EAP-Response/
EAP-Type=EAP-TLS
(TLS Alert message) ->
<- EAP-Failure
(User Disconnected)
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In the case where a previously established session is being resumed,
and both sides authenticate successfully, the conversation will
appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS change_cipher_spec
TLS finished)
EAP-Response/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished) ->
<- EAP-Success
In the case where a previously established session is being resumed,
and the server authenticates to the client successfully but the
client fails to authenticate to the server, the conversation will
appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello) ->
<- EAP-Request/
EAP-Type=EAP-TLS
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(TLS server_hello,
TLS change_cipher_spec,
TLS finished)
EA-Response/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request
EAP-Type=EAP-TLS
(TLS Alert message)
EAP-Response
EAP-Type=EAP-TLS ->
<- EAP-Failure
(User Disconnected)
In the case where a previously established session is being resumed,
and the server authentication is unsuccessful, the conversation will
appear as follows:
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS change_cipher_spec,
TLS finished)
EAP-Response/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
<- EAP-Request/
EAP-Type=EAP-TLS
EAP-Response/
EAP-Type=EAP-TLS
(TLS Alert message) ->
<- EAP-Failure
(User Disconnected)
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Acknowledgment
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Simon & Aboba Proposed Standard [Page 29]
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