Network Working Group S. Hanna
Internet Draft Juniper Networks
Intended status: Proposed Standard P. Sangster
Expires: May 2010 Symantec Corporation
January 4, 2010
PT-EAP: Posture Transport (PT) Protocol For EAP Tunnel Methods
draft-hanna-nea-pt-eap-00.txt
Abstract
This document specifies PT-EAP, a Posture Broker Protocol identical
to the Trusted Computing Group's IF-T Protocol Bindings for Tunneled
EAP Methods (also known as EAP-TNC). The document then evaluates PT-
EAP against the requirements defined in the NEA Requirements and PB-
TNC specifications.
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document authors. All rights reserved.
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Table of Contents
1. Introduction......................................... 3
1.1. Prerequisites.................................... 4
1.2. Message Diagram Conventions........................ 4
1.3. Terminology..................................... 5
1.4. Conventions used in this document................... 5
2. Use of EAP-TNC....................................... 5
3. Definition of EAP-TNC................................. 5
3.1. Protocol Overview ................................ 6
3.2. Version Negotiation............................... 7
3.3. Fragmentation.................................... 7
3.4. EAP-TNC Message Format............................ 8
3.5. Diffie-Hellman (D-H) Pre-Negotiation................ 11
3.5.1. Use of D Flag............................... 11
3.5.2. D-H Pre-Negotiation Message Syntax............. 12
3.5.2.1. D-H PN Hello Request Format............... 12
3.5.2.2. D-H PN Hello Response Format.............. 12
3.5.2.3. D-H PN Parameters Request Format........... 13
3.5.2.4. D-H PN Parameters Response Format.......... 14
3.5.3. Diffie-Hellman Pre-Negotiation Protocol......... 15
3.5.4. Diffie-Hellman Pre-Negotiation Hash Algorithm Values18
3.5.5. Diffie-Hellman Group Values................... 19
3.5.5.1. Diffie-Hellman Group 1 Definitions......... 19
3.5.5.2. Diffie-Hellman Group 2 Definitions......... 20
3.5.5.3. Diffie-Hellman Group 3 Definitions......... 20
4. Security Considerations............................... 21
4.1. Trust Relationships.............................. 21
4.1.1. Posture Transport Client...................... 21
4.1.2. Posture Transport Server...................... 22
4.2. Security Threats and Countermeasures................ 23
4.2.1. Message Theft............................... 24
4.2.2. Message Fabrication.......................... 24
4.2.3. Message Modification......................... 25
4.2.4. Denial of Service........................... 25
4.2.5. Nested Tunnel Attacks........................ 26
4.3. Requirements for EAP Tunnel Methods................. 28
4.4. Candidate EAP Tunnel Method Protections............. 29
4.5. Security Claims for EAP-TNC as per RFC3748........... 30
5. Privacy Considerations ............................... 30
6. IANA Considerations.................................. 31
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6.1. Registry for EAP-TNC Versions...................... 31
6.2. Registry for PT-EAP D-H PN Hash Algorithm IDs ........ 31
6.3. Registry for PT-EAP D-H PN Group IDs................ 32
7. References......................................... 32
7.1. Normative References............................. 32
7.2. Informative References........................... 33
8. Acknowledgments..................................... 34
Appendix A. Evaluation Against NEA Requirements............. 35
A.1. Evaluation Against Requirement C-1 ................. 35
A.2. Evaluation Against Requirements C-2................. 35
A.3. Evaluation Against Requirements C-3................. 35
A.4. Evaluation Against Requirements C-4................. 36
A.5. Evaluation Against Requirements C-5................. 36
A.6. Evaluation Against Requirements C-6................. 36
A.7. Evaluation Against Requirements C-7................. 37
A.8. Evaluation Against Requirements C-8................. 37
A.9. Evaluation Against Requirements C-9................. 37
A.10. Evaluation Against Requirements C-10............... 38
A.11. Evaluation Against Requirements C-11............... 38
A.12. Evaluation Against Requirements PT-1............... 38
A.13. Evaluation Against Requirements PT-2............... 39
A.14. Evaluation Against Requirements PT-3............... 39
A.15. Evaluation Against Requirements PT-4............... 39
A.16. Evaluation Against Requirements PT-5............... 39
A.17. Evaluation Against Requirements PT-6 (from PB-TNC
specification)...................................... 40
A.18. Evaluation Against Requirements PT-7 (from PB-TNC
specification)...................................... 40
A.19. Evaluation Against Requirements PT-8 (from PB-TNC
specification)...................................... 40
A.20. Evaluation Against Requirements PT-9 (from PB-TNC
specification)...................................... 40
1. Introduction
This document specifies PT-EAP, a Posture Transport Protocol (PT)
identical to the Trusted Computing Group's IF-T Protocol Bindings for
Tunneled EAP Methods (also known as EAP-TNC) [12]. The document then
evaluates PT-EAP against the requirements defined in the NEA
Requirements [9] and PB-TNC specifications [4].
The PT protocol in the NEA architecture is responsible for
transporting PB-TNC batches (often containing PA-TNC [3] attributes)
across the network between the NEA Client and NEA Server. The PT
protocol also offers strong security protections to ensure the
exchanged messages are protected from a variety of threats from
hostile intermediaries.
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NEA protocols are intended to be used both for pre-admission
assessment of endpoints joining the network and to assess endpoints
already present on the network. In order to support both usage
models, two types of PT protocols are needed. One type of PT
operates after the endpoint has an assigned IP address, layering on
top of the IP protocol to carry a NEA exchange. The other type of PT
operates before the endpoint gains any access to the IP network. This
specification defines PT-EAP, the PT protocol used to assess
endpoints before they gain access to the network.
PT-EAP is comprised of two related protocols, an outer EAP tunnel
method (not defined in this specification) and an inner EAP method
that carries the NEA assessment inside the protections of the outer
EAP tunnel method. This specification uses the term PT-EAP to refer
to both collectively. The inner EAP method is based upon a method
submitted by the Trusted Computing Group's TNC architecture and
standards so the inner EAP method is named EAP-TNC. This
specification defines the EAP-TNC inner EAP method, while allowing
the EAP tunnel method to be specified in another specification
(possibly defined by another IETF WG). The reason to define PT-EAP as
including both the outer EAP tunnel method and the inner EAP method
is because both are required to meet the PT requirements.
EAP-TNC is designed to operate as an inner EAP [10] method over an
EAP tunnel method that meets the Requirements for a Tunnel Based EAP
Method [17]. PT-EAP therefore can operate over a number of existing
access protocols that support EAP for authentication. Some examples
of such access protocols include 802.1X [7] for wired and wireless
networks and IKEv2 [15] for establishing VPNs over IP networks.
This document defines a standard EAP inner method called EAP-TNC. It
also shows how EAP-TNC may be carried over two existing EAP tunnel
EAP methods: EAP-FAST [14] and EAP-TTLS [16].
1.1. Prerequisites
This document does not define an architecture or reference model.
Instead, it defines a protocol that works within the reference model
described in the NEA Requirements specification [9]. The reader is
assumed to be thoroughly familiar with that document. No familiarity
with Trusted Computing Group (TCG) specifications is assumed.
1.2. Message Diagram Conventions
This specification defines the syntax of EAP-TNC messages using
diagrams. Each diagram depicts the format and size of each field in
bits. Implementations MUST send the bits in each diagram as they are
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shown, traversing the diagram from top to bottom and then from left
to right within each line (which represents a 32-bit quantity).
Multi-byte fields representing numeric values must be sent in network
(big endian) byte order.
Descriptions of bit field (e.g. flag) values are described referring
to the position of the bit within the field. These bit positions are
numbered from the most significant bit through the least significant
bit so a one octet field with only bit 0 set has the value 0x80.
1.3. Terminology
This document reuses many terms defined in the NEA Requirements
document [9], such as Posture Transport Client and Posture Transport
Server. The reader is assumed to have read that document and
understood it.
When defining the EAP-TNC method, this specification does not use the
terms "EAP peer" and "EAP authenticator". Instead, it uses the terms
"NEA Client" and "NEA Server" since those are considered to be more
familiar to NEA WG participants. However, these terms are equivalent
for the purposes of these specifications. The part of the NEA Client
that terminates EAP-TNC (generally in the Posture Transport Client)
is the EAP peer for EAP-TNC. The part of the NEA Server that
terminates EAP-TNC (generally in the Posture Transport Server) is the
EAP authenticator for EAP-TNC.
1.4. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
2. Use of EAP-TNC
EAP-TNC is designed to encapsulate PB-TNC batches in a simple EAP
method that can be carried within EAP tunnel methods. The EAP tunnel
methods provide confidentiality and message integrity, so EAP-TNC
does not have to do so. Therefore, EAP-TNC MUST only be used inside
an EAP tunnel method that provides strong cryptographic
authentication (possibly server only), message integrity and
confidentiality services.
3. Definition of EAP-TNC
The EAP-TNC protocol operates between a Posture Transport Client and
a Posture Transport Server, allowing them to send PB-TNC batches to
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each other over an EAP tunnel method. When EAP-TNC is used, the
Posture Transport Client in the NEA reference model acts as an EAP
peer (terminating the EAP-TNC method on the endpoint) and the Posture
Transport Server acts as an EAP authenticator (terminating the EAP-
TNC method on the NEA Server).
This section describes and defines the EAP-TNC method. First, it
provides a protocol overview and a flow diagram. Second, it describes
specific features like version negotiation and fragmentation. Third,
it gives a detailed packet description. Finally, it describes the
Diffie-Hellman Pre-Negotiation (DH-PN) feature, which allows the EAP-
TNC implementations on the NEA Client and NEA Server to derive a key
from the EAP-TNC exchange. This key may be used to cryptographically
bind the EAP-TNC exchange to the EAP tunnel method, defeating MITM
attacks.
3.1. Protocol Overview
EAP-TNC has two phases that follow each other in strict sequence:
negotiation and data transport.
The EAP-TNC method begins with the negotiation phase. The NEA Server
starts this phase by sending an EAP-TNC Start message: an EAP Request
message of type EAP-TNC with the S (Start) flag set. The NEA Server
may set the D flag in the Start message if it wants to engage in
Diffie-Hellman Pre-Negotiation (D-H PN for short). If the D flag is
set, the NEA Client MAY respond by starting D-H PN. If the NEA
Client does not support D-H PN or wishes to skip it, the NEA Client
ignores the D flag and the Start message is the last step in the
negotiation phase. If the NEA Client and NEA Server do engage in D-H
PN, that is the last step in the negotiation phase. In either case,
the negotiation phase ends with a message from the NEA Server to the
NEA Client.
The data transport phase is the only phase of EAP-TNC where PB-TNC
batches are allowed to be exchanged. This phase always starts with
the NEA Client sending a PB-TNC batch to the NEA Server. The NEA
Client and NEA Server then engage in a round-robin exchange with one
PB-TNC batch in flight at a time. The data transport phase always
ends with an EAP Response message from the NEA Client to the NEA
Server. This message may be empty (not contain any data) if the NEA
Server has just sent the last PB-TNC batch in the PB-TNC exchange.
At the end of the EAP-TNC method, the NEA Server will indicate
success or failure to the EAP tunnel method. Some EAP tunnel methods
may provide explicit confirmation of inner method success; others may
not. This is out of scope for the EAP-TNC method. Successful
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completion of EAP-TNC does not imply successful completion of the
overall authentication nor does EAP-TNC failure imply overall
failure. This depends on the administrative policy in place.
The NEA Server and NEA Client may engage in an abnormal termination
of the EAP-TNC exchange at any time by simply stopping the exchange.
This may also require terminating the EAP tunnel method, depending on
the capabilities of the EAP tunnel method.
The NEA Server and NEA Client MUST follow the protocol sequence
described in this section.
3.2. Version Negotiation
EAP-TNC version negotiation takes place in the first EAP-TNC message
sent by the NEA Server (the Start message) and the first EAP-TNC sent
by the NEA Client (the response to the Start message). The NEA Server
MUST set the Version field in the Start message to the maximum EAP-
TNC version that the NEA Server supports and is willing to accept.
The NEA Client chooses the EAP-TNC version to be used for the
exchange and places this value in the Version field in its response
to the Start message. The NEA Client SHOULD choose the value sent by
the NEA Server if the NEA Client supports it. However, the NEA Client
MAY set the Version field to a value less than the value sent by the
NEA Server (for example, if the NEA Client only supports lesser EAP-
TNC versions). If the NEA Client only supports EAP-TNC versions
greater than the value sent by the NEA Server, the EAP client MUST
abnormally terminate the EAP negotiation.
If the version sent by the NEA Client is not acceptable to the NEA
Server, the NEA Server MUST terminate the EAP-TNC session
immediately. Otherwise, the version sent by the NEA Client is the
version of EAP-TNC that MUST be used. Both the NEA Client and the NEA
Server MUST set the Version field to the chosen version number in all
subsequent EAP-TNC messages in this exchange.
This specification defines version 1 of EAP-TNC. Version 0 is
reserved and MUST never be sent. New versions of EAP-TNC (values 2-7)
may be defined by Standards Action, as defined in RFC 5226 [8].
3.3. Fragmentation
In most cases, EAP-TNC fragmentation will not be required. But PB-TNC
batches can be very long and EAP message length is sometimes tightly
constrained so EAP-TNC includes a fragmentation mechanism to be used
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when a particular PB-TNC batch is too long to fit into a single EAP-
TNC message.
The fragmentation mechanism used in EAP-TNC is quite similar to the
mechanism used by EAP-TLS [18], EAP-TTLS, and EAP-FAST [14]. It uses
the L flag (length included) and the M flag (more fragments) as well
as the Data Length field.
A party (NEA Client or NEA Server) that needs to fragment a long PB-
TNC batch SHOULD break the batch into pieces (called "fragments")
that will fit into EAP-TNC messages. Then this party sends the
fragments in proper sequence, one fragment per EAP-TNC message. The
receiving party recognizes the fragments and holds them for
reassembly, sending an acknowledgment for each fragment so that the
next fragment can be sent (since EAP only allows one message in
flight and is half duplex).
The EAP-TNC message that contains the first fragment MUST have the L
flag set to indicate that fragmentation is being initiated. This
packet also MUST contain the Data Length field, indicating the total
octet length of the unfragmented batch and allowing the party
receiving the fragments to know how much data will eventually be
coming. The L flag MUST NOT be set and the Data Length field MUST NOT
be present in any EAP-TNC message unless that message contains the
first fragment of a fragmented PB-TNC batch. The M flag MUST be set
on all but the last fragment and MUST NOT be set on the last
fragment.
A party that receives an EAP-TNC message with the M flag set MUST
respond with an EAP-TNC Acknowledgement message: an EAP-TNC message
with no Data and with the L, M, and S flags set to 0. The party that
sent an EAP-TNC message with the M flag set MUST wait for the EAP-TNC
Acknowledgement packet before sending the next fragment.
EAP-TNC authenticators and NEA Clients MUST include support for EAP-
TNC fragmentation with Data Lengths up to 100,000 octets. However, a
NEA Server or peer still MAY decide to terminate an EAP-TNC exchange
at any time for a variety of reasons.
3.4. EAP-TNC Message Format
This section provides a detailed description of the fields in an EAP-
TNC message. For a description of the diagram conventions used here,
see section 1.2. Since EAP-TNC is an EAP method, the first four
fields in each message are mandated by and defined in EAP.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Flags | Ver | Data Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Length | Data ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
The Code field is one octet and identifies the type of the EAP
message. The only values used for EAP-TNC are:
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 in octets
of this EAP-TNC message, starting from the Code field. If an EAP-
TNC message has been fragmented, the Length field will cover only
this fragment and thus doesn't reflect the overall length of the
entire unfragmented EAP-TNC message.
Type
38
[IANA Note: This value was previously reserved for another purpose
but has been used for EAP-TNC for some time and never used for the
other purpose so please assign this value to EAP-TNC.]
Flags
+-+-+-+-+-+
|L M S D R|
+-+-+-+-+-+
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L: Length included
Indicates the presence of the Data Length field in the EAP-TNC
message. This flag MUST be set for an EAP-TNC message that
contains the first fragment of a fragmented EAP-TNC message and
only for such a message. This flag MUST NOT be set for non-
fragmented messages.
M: More fragments
Indicates that more fragments are to follow. This flag MUST be set
for all EAP-TNC messages that contain a fragmented EAP-TNC message
except that this bit MUST NOT be set for EAP-TNC messages that
contain the last fragment of a fragmented message. This flag MUST
NOT be set for EAP-TNC messages that contain unfragmented Data.
S: Start
Indicates the beginning of an EAP-TNC exchange. This flag MUST be
set only for the first message from the NEA Server. If the S flag
is set, the EAP message MUST NOT contain Data or have the L or M
flags set.
D: Diffie-Hellman Pre-Negotiation
Indicates the use of a Diffie-Hellman (D-H) based exchange to
provide key derivation. See section 3.6 for specifics of when to
set this flag and how to handle it.
R: Reserved
This flag MUST be set to 0 and ignored upon receipt.
Version
This field is used for version negotiation, as described in
section 3.2.
Data Length
Data Length is an optional field four octets in length. It MUST be
present if and only if the L flag is set. When present, it
indicates the total length, before fragmentation, of a fragmented
PB-TNC batch. The Data Length field MUST be set in the EAP-TNC
message that contains the first in a series of fragments and MUST
NOT be set in subsequent fragments.
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Data
Variable length data. The length of the Data field in a particular
EAP-TNC message may be determined by subtracting the length of the
EAP-TNC header fields from the value of the two octet Length
field. Note, however, that this data may only be part of a longer
fragmented PB-TNC batch conveyed in multiple EAP-TNC messages.
3.5. Diffie-Hellman (D-H) Pre-Negotiation
This section describes the optional Diffie-Hellman Pre-Negotiation
feature of EAP-TNC (known as D-H PN). The D-H PN feature allows the
EAP-TNC implementations on the NEA Client and NEA Server to derive a
key from the EAP-TNC exchange. This key may be used to
cryptographically bind the EAP-TNC exchange to the EAP tunnel method,
defeating MITM attacks such as those described in section 4.2.5.
All EAP-TNC implementations on NEA Servers MUST support D-H PN. EAP-
TNC implementations on NEA Clients MAY support D-H PN. However,
administrative configuration and policy SHOULD determine whether this
feature is disabled, permitted, or required.
D-H PN was designed to enable it to be added without causing backward
compatibility issues. Legacy clients only supporting IF-T Protocol
Bindings for Tunneled EAP Methods 1.0 are required to ignore the use
of the D flag. D-H PN was added in version 1.1 of that protocol. As
a result the NEA Server (which initiates a D-H PN request) can not
assume the Posture Transport Client will support DH-PN. Therefore
the Posture Transport Server MUST wait until the Posture Transport
Client has sent a message indicating it supports D-H PN (D flag set)
before sending messages with the D-H PN described below. This
decision causes a full roundtrip to occur prior to exchanging D-H PN
messages.
3.5.1. Use of D Flag
The use of the "D" flag in the EAP-TNC header MUST follow very strict
rules described in Section 3.4. If the D flag is one (1), this
indicates the data field of the EAP-TNC message MUST only contain the
pre-negotiation information or be empty (as in the initial exchange
messages) and not contain PB-TNC batches. PB-TNC batches MUST NOT be
included in messages with the D flag set to one (1). Either entity
MAY set the D flag to 0 at any time indicating it does not wish to
(or is incapable of) perform the D-H PN exchange. The other party
MAY then determine whether to proceed with the dialog without a D-H
PN exchange.
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3.5.2. D-H Pre-Negotiation Message Syntax
This section describes the format of the data field within each of
the four D-H PN messages exchanged. These messages are only present
in the data field when the D flag is set to one. The D flag MUST be
set for all D-H PN messages. If the NEA Client or NEA Server
receives a message with the D flag set to zero in the middle of a D-H
PN exchange, it SHOULD interpret this as meaning that the sender has
decided to terminate the D-H PN exchange but would like to proceed
with the NEA exchange. The recipient MAY proceed or terminate the
entire NEA exchange. The messages are presented in the order they
would appear in a D-H PN message exchange.
3.5.2.1. D-H PN Hello Request Format
The data field of the initial D-H PN message from the NEA Server MUST
be empty for backward compatibility. This message occurs at the
start of a session with the start flag set to one and the D flag set
to one (indicating a desire to initiate D-H PN without sending a data
field that would be confusing to legacy NEA Clients that might ignore
the D flag.)
3.5.2.2. D-H PN Hello Response Format
The data field of the initial response from the Posture Transport
Client allows a NEA Client to notify the Posture Transport Server
that it is able and willing to perform a D-H PN (by replying with the
D flag set to one.) The defined protocol expects the NEA Server to
lead the negotiation except for the D-H group which needs to be
negotiated before the public value exchange can occur (since it
affects the size.) In order to reduce the number of messages
required, this message includes the set of supported/preferred D-H
groups and any minimum nonce size.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| D-H Group | Min. Nonce Len| Reserved for future use |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
D-H Group
Flag field indicating the supported D-H groups. See section
3.5.5. for description of the D-H groups and their representation
in this field. The NEA Client policy MAY dictate what groups are
allowable for a particular NEA Server.
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Min Nonce Len
NEA Client can send a minimum acceptable length for the nonce in
bytes. This value should be set to zero if there is no minimum
required.
3.5.2.3. D-H PN Parameters Request Format
This is the data field of the NEA Server's request message trying to
finalize the negotiation of the parameters of the D-H PN exchange.
This message proposes the NEA Server's set of supported hash
algorithms. The D-H group MUST be selected from the set offered in
the D-H PN Hello Response message. If the NEA Server's policy does
not allow the use of any of the D-H groups offered by the NEA Client,
this MUST result in the unsuccessful termination of the D-H PN. The
NEA Server MAY decide to continue with an EAP-TNC exchange without
the D-H PN protections by sending a message with the S or D flags set
to zero and an empty data field. If the NEA Server decides to select
an offered D-H group, the Posture Transport Server can offer its D-H
public value (using the size from the selected group) and include a
nonce for freshness of the exchange in the following D-H PN
Parameters Request message.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | D-H Group | Hash Alg. | Nonce Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NEA Server Nonce (S-Nonce) ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| D-H Public Value (S-Pub) ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved
This field MUST be set to zero and MUST be ignored by compliant
implementations.
D-H Group
Selected D-H Group (single flag) from set offered by the Posture
Transport Client in the D-H PN Hello Response message. See
section 3.5.5. for description of the D-H groups and their
representation in this field.
Hash Alg(orithm)
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Flag field indicating the set of supported hash algorithms for the
NEA Server. See section 3.5.4. for a description of the defined
hash algorithms and their representation in this field.
Nonce Length
Length of the nonce field in bytes. This value MUST be greater
than 16 and MUST be greater then or equal to the Min Nonce Len
specified by the NEA Client's D-H PN Hello Response message.
NEA Server Nonce (S-Nonce)
High entropy random data used to assure the freshness of the
session. Nonces MUST NOT be repeated or be predictable by other
parties.
D-H Public Value
NEA Server's public value for this D-H exchange. The size of this
field is determined by the Posture Transport Server selected D-H
group to use. See section 3.5.5. for the lengths used for each D-
H group.
3.5.2.4. D-H PN Parameters Response Format
This section describes the data field of the NEA Client's parameter
response message that completes the D-H PN exchange. This message
establishes the particular hash algorithm for the derivation.
Because the D-H group has been established, the Posture Transport
Client can offer its D-H public value and include a nonce for
freshness of the exchange. When the Posture Transport Server
receives this message, both parties will have everything they need to
perform the remaining transforms to derive the shared secrets.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nonce Length | Hash Alg. | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| D-H Public Value (C-Pub) ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NEA Client Nonce (C-Nonce) ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Nonce Length
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Length of the nonce field in bytes. This value MUST be greater
than 16 and SHOULD match the length used by the NEA Server's
nonce.
Hash Alg(orithm)
Selected hash algorithm (single flag) from offered set for use
later in D-H PN. See section 3.5.4. for a description of the
defined hash algorithms.
Reserved
This field MUST be set to zero and MUST be ignored by compliant
implementations.
D-H Public Value (C-Pub)
NEA Client's public value for this D-H exchange. The size of this
field is indicated by the selected D-H group.
NEA Client Nonce (C-Nonce)
High entropy random data used to assure the freshness of the
session (nonces MUST NOT be repeated or be predictable.)
3.5.3. Diffie-Hellman Pre-Negotiation Protocol
This section describes the message exchange protocol which occurs
during the D-H PN. At any point during the exchange if a party is
unwilling to accept the options offered by the other party, it SHOULD
set the D flag to zero indicating it no longer wishes to continue the
D-H PN. This MAY result in a fallback to a standard request/response
protocol if acceptable by both parties. All D-H PN protocol messages
MUST have the D flag set to one.
The following sequence explains the details of the processing of each
D-H PN message carried by EAP and some background on how it provides
security against MiTM attacks:
1. Initially, the NEA Server sends an EAP-Request with the S
(Start) flag set to one to indicate the beginning of the
session. The NEA Server SHOULD check policy to determine if
the NEA Client should be asked to use the D-H PN and if so
set the D (D-H PN) flag. If the D-H PN flag is set, the
message MUST NOT contain data in the data section and is
known as a D-H PN Hello Request message.
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2. The NEA Client receives the D-H PN Hello Request message. If
the NEA Client only supports version 1.0 of IF-T Protocol
Binding for Tunneled EAP Methods and therefore does not
recognize the D flag, it would ignore the D flag and try to
process the data section as a PB-TNC request message so it
MUST find no data field for backward compatibility. The NEA
Client supports the D flag so it will perform the following:
a. If the D flag is zero, the NEA Client MUST NOT respond with
a message with the D flag set to one. Instead, it MAY
terminate the exchange if it requires a D-H PN but will
usually proceed with EAP-TNC without D-H PN.
b. If the D flag is one, the NEA Client MAY consult policy to
decide whether to respond with the D-H PN Hello Response
message indicating a willingness to perform a D-H PN. If
willing to use D-H PN, the NEA Client includes a set of
acceptable D-H groups and any minimum nonce lengths it
requires. The NEA Client MAY decline to perform D-H PN by
sending an EAP-TNC message with the D flag set to zero. In
this case, the NEA Server MAY proceed with a NEA exchange
unprotected by D-H PN or terminate the entire NEA exchange.
3. If the NEA Server receives the D-H PN Hello Response message,
this indicates an ability and willingness to perform a D-H PN
by the NEA Client. The NEA Server sends a D-H PN Parameters
Request message selecting a D-H group from those offered by
the NEA Client. This message also indicates its set of
supported hash algorithms, and the NEA Server's public value
and freshness nonce.
4. The NEA Client responds with a D-H PN Parameters Response
Message. This message MUST select a hash algorithm from the
offered set. If no acceptable options were offered the NEA
Client SHOULD respond with a message with the D flag set to
zero and proceed with an EAP-TNC response (to the Start
message) without D-H PN protection. The NEA Client also
sends its D-H public value corresponding to the selected D-H
group and a freshness nonce.
5. The NEA Server receives the D-H PN Parameters Response
message and assures the response is consistent with its
request message and meets its policy.
At this point the NEA Client and NEA Server compute the shared secret
key using the Diffie-Hellman algorithm. Passive MiTM listeners can
not determine the key value, although an active MiTM that
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participates in the D-H PN exchange and acts as a proxy between the
true NEA Client and NEA Server could share keys with each party. In
the proxy case, the true NEA Client and NEA Server do not share a
common secret key (they each only share a secret with the MiTM
proxy.) To detect this style of attack, the NEA Client uses a
byproduct (Unique-Value-1) of the secret key and both nonces later in
a computation of a quiz answer sent in an PA-TNC attribute request
during the assessment. Because the true NEA Client and NEA Server
know different D-H values, the true NEA Client computes a different
quiz result than what is expected by the NEA Server so the assessment
fails.
Both parties compute the following:
6.Compute Unique-Value-1 = HASH ("1" | C-Nonce | S-Nonce | D-H
Shared Secret Key) The NEA Client saves Unique-Value-1 for
later use with the PA-TNC quiz requests. If the value length
is >20 bytes (e.g. when the selected HASH is SHA-256), the
value MUST be truncated to the 20 most significant bytes.
Later when the NEA Client is asked to produce a quiz result
during the PA-TNC assessment, this value is used in the
computation (e.g. hashed with other posture information).
The NEA Server can also compute Unique-Value-1 and the quiz
result so can recognize a correct response. Note that
Unique-Value-1 isn't the actual secret key used to protect
traffic.
7. Next, the NEA Client and NEA Server compute the following
value that will be used later by EAP-TNC:
Unique-Value-2 = HASH ("2" | C-Nonce | S-Nonce | D-H Shared
Secret Key)
8. After the completion of the D-H PN protocol, both entities
MUST set the D flag to zero and then use the data field to
exchange PB-TNC batches. The NEA Server will start by
sending an EAP-TNC message with no Data and the D and S flags
set to zero. The NEA Client will respond with an EAP-TNC
message containing its first PB-TNC batch.
9. When a D-H PN has successfully completed, the NEA Client and
NEA Server MUST compute a running hash (using the selected
algorithm) including the complete contents (from the Code
field through the Data field, inclusive) of each EAP-TNC
message sent/received in sequence during the assessment.
This running hash is performed by repeated use of the
following after receiving or sending an EAP-TNC Message:
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Unique-Value-2 = HASH (Unique-Value-2 | HASH (EAP-TNC
Message))
10. The result (final Unique-Value-2) is a value that is
cryptographically computed from the D-H PN secret key, nonce
pair and the contents of all of the messages exchanged (thus
all the posture information responses.)
11. At the completion of the EAP-TNC exchanges when the D-H PN
has been used, the final Unique-Value-2 MUST be exported and
mixed into the EAP tunnel method's session keys. An
additional EAP tunnel method round trip is required to assure
that NEA Client and NEA Server both computed the same value.
If this occurred, then both parties knew the secret D-H PN
key, nonce pair and observed the same set of EAP-TNC message
(thus allowing for detection of MiTM message tampering.) If
both do not compute the same final Unique-Value-2, then the
final EAP tunnel method message exchange (cryptographic
binding check) will not properly decrypt, so the session MUST
be considered compromised.
12. Finally after the outer EAP tunnel method completes, it's
critical that the subsequent communications continue to be
protected from active attacks by a MiTM. This SHOULD be
achieved by leveraging keys derived from the Unique-Value-2
known by both parties to encrypt and integrity protect future
traffic. Wireless 802.1X has provisions for provisioning a
key for this purpose, but wired 802.1X requires an equivalent
mechanism (possibly part of 802.1AE.)
3.5.4. Diffie-Hellman Pre-Negotiation Hash Algorithm Values
This section defines the values for the Hash Alg(orithm) field for
the various hashing algorithms supported by D-H PN. The values are
as follows:
+-+-+-+-+-+-+-+-+
|R R R R R R 2 1|
+-+-+-+-+-+-+-+-+
1 - SHA-1 [5]
2 - SHA-256 [5]
R - Reserved for future use
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Implementations compliant with this specification MUST ignore flags
set that they are unable to support. Compliant implementations MUST
NOT set hash algorithm values that they are unable to support.
3.5.5. Diffie-Hellman Group Values
This section defines the flag values for the D-H Group field used in
several D-H PN messages. The values are as follows:
+-+-+-+-+-+-+-+-+
|R R R R R 3 2 1|
+-+-+-+-+-+-+-+-+
1 - Use of values from group 2 from IKE.
2 - Use of values from group 5 from IKE.
3 - Use of values from group 14 from IKE.
R - Reserved for future use
Implementations compliant with this specification MUST ignore flags
set that they are unable to support. Compliant implementations MUST
NOT set D-H Group values that they are unable to support.
3.5.5.1. Diffie-Hellman Group 1 Definitions
This section defines the Diffie-Hellman algorithm values that MUST be
used when using group 1 (flag 1 above) of the D-H PN. This group is
taken from group 2 of IKE.
The public values exchanged when using this group MUST be 128 bytes
in length.
The Diffie-Hellman generator (g) MUST be 2.
The prime modulus is the 128 byte value:
2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }
which has a hexadecimal value of:
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08
8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B
302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9
A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6
49286651 ECE65381 FFFFFFFF FFFFFFFF
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3.5.5.2. Diffie-Hellman Group 2 Definitions
This section defines the Diffie-Hellman algorithm values that MUST be
used when using group 2 (flag 2 above) of the D-H PN. This group is
based on group 5 from IKE MODP Groups [6].
The public values exchanged when using this group MUST be 192 bytes
in length.
The Diffie-Hellman generator (g) MUST be 2.
The prime modulus is the 192 byte value:
2^1536 - 2^1472 - 1 + 2^64 * { [2^1406 pi] + 741804 }
which has a hexadecimal value of:
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
670C354E 4ABC9804 F1746C08 CA237327 FFFFFFFF FFFFFFFF
3.5.5.3. Diffie-Hellman Group 3 Definitions
This section defines the Diffie-Hellman algorithm values that MUST be
used when using group 3 (flag 3 above) of the D-H PN. This group is
based on group 14 from IKE MODP Groups [6].
The public values exchanged when using this group MUST be 256 bytes
in length.
The Diffie-Hellman generator (g) MUST be 2.
The prime modulus is the 256 byte value:
2^2048 - 2^1984 - 1 + 2^64 * { [2^1918 pi] + 124476 }
which has a hexadecimal value of:
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
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EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
670C354E 4ABC9804 F1746C08 CA18217C 32905E46 2E36CE3B
E39E772C 180E8603 9B2783A2 EC07A28F B5C55DF0 6F4C52C9
DE2BCBF6 95581718 3995497C EA956AE5 15D22618 98FA0510
15728E5A 8AACAA68 FFFFFFFF FFFFFFFF
4. Security Considerations
This section discusses the major threats and countermeasures provided
by the EAP-TNC inner EAP method. As discussed throughout the
document, the EAP-TNC method is designed to run inside an EAP tunnel
method which is capable of protecting the EAP-TNC protocol from many
threats.
4.1. Trust Relationships
In order to understand where security countermeasures are necessary,
this section starts with a discussion of where the NEA architecture
envisions some trust relationships between the processing elements of
the PT-EAP protocol. The following sub-sections discuss the trust
properties associated with each portion of the NEA reference model
directly involved with the processing of the PT-TNC protocol.
4.1.1. Posture Transport Client
The Posture Transport Client is trusted by the Posture Broker Client
to:
o Not to observe, fabricate or alter the contents of the PB-TNC
batches received from the network
o Not to observe, fabricate or alter the PB-TNC batches passed down
from the Posture Broker Client for transmission on the network
o Transmit on the network any PB-TNC batches passed down from the
Posture Broker Client
o Deliver properly security protected messages received from the
network that are destined for the Posture Broker Client
o Provide configured security protections (e.g. authentication,
integrity and confidentiality) for the Posture Broker Client's PB-
TNC batches sent on the network
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o Expose the authenticated identity of the Posture Transport Server
o Verify the security protections placed upon messages received from
the network to ensure the messages are authentic and protected
from attacks on the network
o Provide a secure, reliable, in order delivery, full duplex
transport for the Posture Broker Client's messages
The Posture Transport Client is trusted by the Posture Transport
Server to:
o Not send malicious traffic intending to harm (e.g. denial of
service) the Posture Transport Server
o Not to intentionally send malformed messages to cause processing
problems for the Posture Transport Server
o Not to send invalid or incorrect responses to messages (e.g.
errors when no error is warranted)
o Not to ignore or drop messages causing issues for the protocol
processing
o Verify the security protections placed upon messages received from
the network to ensure the messages are authentic and protected
from attacks on the network
4.1.2. Posture Transport Server
The Posture Transport Server is trusted by the Posture Broker Server
to:
o Not to observe, fabricate or alter the contents of the PB-TNC
batches received from the network
o Not to observe, fabricate or alter the PB-TNC batches passed down
from the Posture Broker Server for transmission on the network
o Transmit on the network any PB-TNC batches passed down from the
Posture Broker Server
o Deliver properly security protected messages received from the
network that are destined for the Posture Broker Server
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o Provide configured security protections (e.g. authentication,
integrity and confidentiality) for the Posture Broker Server's
messages sent on the network
o Expose the authenticated identity of the Posture Transport Client
o Verify the security protections placed upon messages received from
the network to ensure the messages are authentic and protected
from attacks on the network
The Posture Transport Server is trusted by the Posture Transport
Client to:
o Not send malicious traffic intending to harm (e.g. denial of
service) the Posture Transport Server
o Not to send malformed messages
o Not to send invalid or incorrect responses to messages (e.g.
errors when no error is warranted)
o Not to ignore or drop messages causing issues for the protocol
processing
o Verify the security protections placed upon messages received from
the network to ensure the messages are authentic and protected
from attacks on the network
4.2. Security Threats and Countermeasures
Beyond the trusted relationships assumed in section 4.1. the PT-EAP
EAP method faces a number of potential security attacks that could
require security countermeasures.
Generally, the PT protocol is responsible for providing strong
security protections for all of the NEA protocols so any threats to
PT's ability to protect NEA protocol messages could be very damaging
to deployments. For the PT-EAP method, most of the cryptographic
security in provided by the outer EAP tunnel method and EAP-TNC is
encapsulated within the protected tunnel. Therefore, this section
highlights the cryptographic requirements that need to be met by the
EAP tunnel method carrying EAP-TNC in order to meet the NEA PT
requirements.
Once the message is delivered to the Posture Broker Client or Posture
Broker Server, the posture brokers are trusted to properly safely
process the messages.
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4.2.1. Message Theft
When EAP-TNC messages are sent over unprotected network links or
spanning local software stacks that are not trusted, the contents of
the messages may be subject to information theft by an intermediary
party. This theft could result in information being recorded for
future use or analysis by the adversary. Messages observed by
eavesdroppers could contain information that exposes potential
weaknesses in the security of the endpoint, or system fingerprinting
information easing the ability of the attacker to employ attacks more
likely to be successful against the endpoint. The eavesdropper might
also learn information about the endpoint or network policies that
either singularly or collectively is considered sensitive
information. For example, if EAP-TNC is housed in an EAP tunnel
method that does not provide confidentiality protection, an adversary
could observe the PA-TNC attributes included in the PB-TNC batch and
determine that the endpoint is lacking patches, or particular sub-
networks have more lenient policies.
In order to protect again NEA assessment message theft, the EAP
tunnel method carrying EAP-TNC MUST provide strong cryptographic
authentication, integrity and confidentiality protection. The use of
bi-directional authentication in the EAP tunnel method carrying EAP-
TNC ensures that only properly authenticated and authorized parties
may be involved in an assessment message exchange. When EAP-TNC is
carried with a cryptographically protected EAP tunnel method like
EAP-TTLS, all of the PB-TNC and PA-TNC protocol messages contents are
hidden from potential theft by intermediaries lurking on the network.
4.2.2. Message Fabrication
Attackers on the network or present within the NEA system could
introduce fabricated PT-EAP messages intending to trick or create a
denial of service against aspects of an assessment. For example, an
adversary could attempt to insert into the message exchange fake PT-
EAP error codes in order to disrupt communications.
The EAP tunnel method carrying an EAP-TNC method needs to provide
strong security protections for the complete message exchange over
the network. These security protections prevent an intermediary from
being able to insert fake messages into the assessment. For example,
the EAP-TTLS method's use of hashing algorithms provides strong
integrity protections that allow for detection of any changes in the
content of the message exchange. Additionally, adversaries are
unable to observe the EAP-TNC method housed inside of an encrypting
EAP tunnel method (e.g. EAP-TTLS) because the messages are encrypted
by the TLS [2] ciphers, so an attacker would have difficulty in
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determining where to insert the falsified message, since the attacker
is unable to determine where the message boundaries exist.
4.2.3. Message Modification
This attack could allow an active attacker capable of intercepting a
message to modify a PT-EAP message or transported PA-TNC attribute to
a desired value to ease the compromise of an endpoint. Without the
ability for message recipients to detect whether a received message
contains the same content as what was originally sent, active
attackers can stealthily modify the attribute exchange.
The EAP-TNC method leverages the EAP tunnel method (e.g. EAP-TTLS) to
provide strong authentication and integrity protections as a
countermeasure to this threat. The bi-directional authentication
prevents the attacker from acting as an active man-in-the-middle to
the protocol that could be used to modify the message exchange. The
strong integrity protections (hashing) offered by EAP-TTLS allows the
EAP-TNC message recipients to detect message alterations by other
types of network based adversaries. Because EAP-TNC does not itself
provide explicit integrity protection for the EAP-TNC payload, an EAP
tunnel method that offers strong integrity protection is required to
mitigate this threat.
4.2.4. Denial of Service
A variety of types of denial of service attacks are possible against
the PT-EAP if the message exchange are left unprotected while
traveling over the network. The Posture Transport Client and
Posture Transport Server are trusted not to participate in the denial
of service of the assessment session, leaving the threats to come
from the network.
The EAP-TNC method primarily relies on the outer EAP tunnel method to
provide strong authentication (at least of one party) and deployers
are expected to leverage other EAP methods to authenticate the other
party (typically the client) within the protected tunnel. The use of
a protected bi-directional authentication will prevent unauthorized
parties from participating in a PT-EAP exchange.
After the cryptographic authentication by the EAP tunnel method, the
session can be encrypted and hashed to prevent undetected
modification that could create a denial of service situation.
However it is possible for an adversary to alter the message flows
causing each message to be rejected by the recipient because it fails
the integrity checking.
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4.2.5. Nested Tunnel Attacks
The PT-EAP protocol works on the premise that the EAP tunnel method
is capable of carrying (and protecting) various inner methods that
perform additional security exchanges to establish the authenticity
and integrity of the endpoint. While this model is very flexible
since it allows for the variety of existing EAP methods to be
leveraged within the tunnel, it may introduce vulnerabilities. One
such vulnerability is an attack described in "Man-in-the-Middle
Attack against Tunneled Authentication Protocols" described in the
2003 Security Protocols Workshop paper by Asokan, Niemi, and Nyberg
[13]. This document will refer to the attack discussed by Asokan and
others as the "nested tunnel attack" for brevity.
The nested tunnel attack takes advantage of the fact that there is no
strong linkage between the outer EAP tunnel method and the inner EAP
method so a MiTM could be forwarding traffic learned from an earlier
unprotected observed authentication (or assessment) or actively
proxying an ongoing unprotected assessment.
For example, if a normally compliant and authorized enterprise laptop
(referred to as "laptop1") became infected with malware and wished to
access the enterprise network despite now being non-compliant the
following might occur:
1. Attacker sets up laptop2 with same software as on compliant
laptop1 (minus malware) and configures to provide posture to
laptop1 using NEA protocols.
2. When legitimate user attempts to join enterprise network with
laptop1, malware delays join and notifies the attacker who
triggers laptop2 to attempt to join network by sending
authentication and posture information to laptop1 (even
securely in an EAP tunnel).
3. Now armed with a NEA session to laptop2, laptop1 attempts to
join enterprise network with secure tunneled PT-EAP exchange
to enterprise's NEA Server
4. After EAP tunnel method establishes tunnel, NEA Server uses
EAP-TNC method to request laptop1 provide posture to join
network
5. Laptop1 relays posture requests over other EAP tunnel to
laptop2 (using EAP-TNC) who responds with compliant posture
information.
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6. Laptop1 relays the laptop2 responses to the enterprise NEA
Server
7. Steps 4-6 repeat as necessary
8. Enterprise NEA Server believes laptop1 is compliant with
policy despite it containing significant malware
This attack exploits the fact that the inner and outer EAP methods
are independent from each other since laptop1 is able to obtain
compliant posture information from laptop2 over a different tunnel
and re-use it. In order to bind the inner and out methods together,
this specification includes a Diffie-Hellman Pre-Negotiation (D-H PN)
which creates a per-assessment freshness value.
When the D-H PN value is combined with a hash of the assessment
messages and the resulting value is exported and mixed into the outer
EAP tunnel method's keys, both parties can perform a simple roundtrip
confirmation message to ensure both know the D-H PN secret, the hash
of the assessment and the original outer tunnel methods encryption
key. This cryptographically binds the one (or more) inner EAP
methods exporting keys with the outer tunnel method and provides more
freshness to the assessment session. For details of D-H PN, see
section 3.5.
In addition, a special pair of PA-TNC attributes can be exchanged
after the D-H PN has completed that include a simple proof of
knowledge quiz that the intermediary is not able to easily solve with
information seen on the network. For example, the NEA Server could
send a "quiz" question to the NEA Client where the response would
include a hash(D-H PN Secret, "quiz_answer"). The intermediary won't
know the D-H PN Secret if it wasn't involved in the D-H PN. If it
was involved, it shouldn't be able to figure out the quiz answer
which involves a question about what a clean system should look like.
The countermeasure provide mitigation because if an active MiTM
(laptop1) takes part in the D-H PN it establishes shared values with
the enterprise NEA Server that aren't actually known by laptop2, so
can't be included in laptop2's exported keys or in the quiz answer.
If a MiTM (laptop1) just forwarded the D-H PN protocol over a tunnel
to laptop2 so that clean laptop2 and the enterprise NEA Server were
selecting the D-H values and nonces, laptop1 would be unable to
determine the established secret since it lacks knowledge of any D-H
private values and would be unable to complete the outer EAP tunnel
exchange once the secret was mixed into the session keys.
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In order for the MiTM protection to continue during the subsequent
communications on the network, the communications SHOULD protect the
data exchanges using keys based on the final EAP tunnel method keys
that were mixed with the D-H secret keys. At present, 802.1X for
wireless use has provisions for such a key to be used. However wired
802.1X lacks the use of keys to protect the communications. These
unprotected flows are again vulnerable to a variety of attacks
including alteration or replay by a MiTM. It is believed that the
use of 802.1AE will address this issue so deployers should consider
this if their threat model (especially with respect to wired 802.1X)
warrants ongoing protections.
Note that this process does not prevent the malware on laptop1 from
lying about its posture; this approach merely addresses the network
based MiTM attack. Detection of local malware lying about its
posture is outside the scope of NEA but is being researched and
standardized in the Trusted Computing Group.
4.3. Requirements for EAP Tunnel Methods
Because the PT-EAP inner method described in this specification
relies on the outer EAP tunnel method for a majority of its security
protections, this section reiterates the PT requirements that MUST be
met by the IETF standard EAP tunnel method for use with PT-EAP.
The security requirements described in this specification MUST be
implemented in any product claiming to be PT-EAP compliant. The
decision of whether a particular deployment chooses to use these
protections is a deployment issue. A customer may choose to avoid
potential deployment issues or performance penalties associated with
the use of cryptography when the required protection has been
achieved through other mechanisms (e.g. physical isolation). If
security mechanisms may be deactivated by policy, an implementation
should offer an interface to query how a message will be (or was)
protected by PT so higher layer NEA protocols can factor this into
their decisions.
RFC 5209 includes the following requirement that is to be applied
during the selection of the EAP tunnel method(s) used in conjunction
with EAP-TNC:
PT-2 The PT protocol MUST be capable of supporting mutual
authentication, integrity, confidentiality, and replay
protection of the PB messages between the Posture Transport
Client and the Posture Transport Server.
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Note that mutual authentication could be achieved by a combination of
a strong authentication of one party (e.g. TLS server when EAP-TTLS
is used) by the EAP tunnel method in conjunction with a second
authentication of the other party (e.g. client authentication inside
the protected tunnel) by another EAP method running prior to EAP-TNC.
Having the Posture Transport Client always authenticate the Posture
Transport Server provides assurance to the NEA Client that the NEA
Server is authentic (not a rogue or MiTM) prior to disclosing secret
or potentially privacy sensitive information about what is running or
configured on the endpoint. However the NEA Server's policy may
allow for the delay of the authentication of the NEA Client until a
suitable protected channel has been established allowing for non-
cryptographic NEA Client credentials (e.g. username/password) to be
used. Whether the communication channel is established with both or
one party performing a cryptographic authentication, the resulting
channel needs to provide strong integrity and confidentiality
protection to its contents. These protections are to be bound to at
least the authentication of the NEA Client, so the session is
cryptographically bound to a particular authentication event.
4.4. Candidate EAP Tunnel Method Protections
This section discusses how EAP-TNC is used within various EAP tunnel
methods meet the PT requirements from section 4.3.
EAP-FAST and EAP-TTLS make use of TLS [2] to protect the transport of
information between the NEA Client and NEA Server. Each of these EAP
tunnel methods has two phases. In the first phase, a TLS tunnel is
established between NEA Client and NEA Server. In the second phase,
the tunnel is used to pass other information. PT-EAP requires that
establishing this tunnel include at least an authentication of the
NEA Server by the NEA Client.
The phase two dialog may include authentication of the user by doing
other EAP methods or in the case of TTLS by using non-EAP
authentication dialogs. EAP-TNC is also carried by the phase two
tunnel allowing the NEA assessment to be within an encrypted and
integrity protected transport.
With all these methods, a cryptographic key is derived from the
authentication that may be used to secure later transmissions. Each
of these methods employs at least a NEA Server authentication using
an X.509 certificates. Within each EAP tunnel method will exist a
set of inner EAP method (or an equivalent using TLVs if inner methods
aren't directly supported.) These inner methods may perform
additional security handshakes including more granular
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authentications or exchanges of integrity information (such as EAP-
TNC.) At some point after the conclusion of each inner EAP method,
some of the methods will export the established secret keys to the
outer tunnel method. It's expected that the outer method will
cryptographically mix these keys into any keys it is currently using
to protect the session and perform a final operation to determine
whether both parties have arrived at the same mixed key. This
cryptographic binding of the inner method results to the outer
methods keys is essential for detection of nested method attacks, see
section 4.2.5.
4.5. Security Claims for EAP-TNC as per RFC3748
This section summarizes the security claims as required by RFC3748
Section 7.2:
Auth. mechanism: None
Ciphersuite negotiation: No
Mutual authentication: No
Integrity protection: No
Replay protection: No
Confidentiality: No
Key derivation: Yes
Key strength: Depends on D-H Group and Hash used
Dictionary attack resistant: N/A
Fast reconnect: No
Crypt. binding: N/A
Session independence: N/A
Fragmentation: Yes
Channel binding: No
5. Privacy Considerations
The role of PT-EAP is to act as a secure transport for PB-TNC over a
network before the endpoint has been admitted to the network. As a
transport protocol, PT-EAP does not directly utilize or require
direct knowledge of any personally identifiable information (PII).
PT-EAP will typically be used in conjunction with other EAP methods
that provide for the user authentication (if bi-directional
authentication is used), so the user's credentials are not directly
seen by the EAP-TNC inner method. Therefore, the Posture Transport
Client and Posture Transport Server's implementation of EAP-TNC MUST
NOT observe the contents of the carried PB-TNC batches that could
contain PII carried by PA-TNC or PB-TNC.
While EAP-TNC does not provide cryptographic protection for the PB-
TNC batches, it is designed to operate within an EAP tunnel method
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that provides strong authentication, integrity and confidentiality
services. Therefore, it is important for deployers to leverage these
protections in order to prevent disclosure of PII potentially
contained within PA-TNC or PB-TNC within the EAP-TNC payload.
6. IANA Considerations
This document defines an EAP method type named EAP-TNC with the
value 38.
[IANA Note: This value was previously reserved for another
purpose but has been used for EAP-TNC for some time and never
used for another purpose so please assign this value to EAP-
TNC.]
This document also defines three new IANA registries: EAP-TNC
Versions, PT-EAP D-H PN Hash Algorithm IDs, and PT-EAP D-H PN
Group IDs. This section explains how these registries work.
Because only eight (8) values are available in each of these
registries, a high bar is set for new assignments. The only way
to register new values in these registries is through Standards
Action (via an approved Standards Track RFC).
6.1. Registry for EAP-TNC Versions
The name for this registry is "EAP-TNC Versions". Each entry
in this registry should include a decimal integer value between
1 and 7 identifying the version, and a reference to the RFC
where the version is defined.
The following entries for this registry are defined in this
document. Once this document becomes an RFC, they should
become the initial entries in the registry for EAP-TNC
Versions. Additional entries to this registry are added by
Standards Action, as defined in RFC 5226 [8].
Value Defining Specification
----- ----------------------
1 RFC # Assigned to this I-D
6.2. Registry for PT-EAP D-H PN Hash Algorithm IDs
The name for this registry is "PT-EAP D-H PN Hash Algorithm
IDs". Each entry in this registry should include a human-
readable name, a decimal integer value between 1 and 8
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representing its location in the bit map, and a reference to
the RFC where the contents of this message type are defined.
The following entries for this registry are defined in this
document. Once this document becomes an RFC, they should
become the initial entries in the registry for PT-EAP D-H PN
Hash Algorithm IDs. Additional entries to this registry are
added by Standards Action, as defined in RFC 5226 [8].
Bit Position Name Defining Specification
------------ ---- ----------------------
1 SHA-1 RFC # Assigned to this I-D
2 SHA-256 RFC # Assigned to this I-D
6.3. Registry for PT-EAP D-H PN Group IDs
The name for this registry is "PT-EAP D-H PN Group IDs". Each
entry in this registry should include a human-readable name, a
decimal integer value between 1 and 8 representing its location
in the bit map, and a reference to the specification where the
contents of this message type are defined.
The following entries for this registry are defined in this
document. Once this document becomes an RFC, they should
become the initial entries in the registry for PT-EAP D-H PN
Group IDs. Additional entries to this registry are added by
Standards Action, as defined in RFC 5226.
Bit Position Name Defining Specification
------------ ---- ----------------------
1 Group 2 (IKE) RFC # Assigned to this I-D
2 Group 5 (IKE) RFC # Assigned to this I-D
3 Group 14 (IKE) RFC # Assigned to this I-D
7. References
7.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Dierks T., Rescorla E., "The Transport Layer Security (TLS)
Protocol Version 1.2", RFC 5246, August 2008.
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[3] Sangster P., Narayan K., "PA-TNC: A Posture Attribute Protocol
(PA) Compatible with TNC", RFC XXXX, January 2010.
[4] Sahita, R., Hanna, S., and R. Hurst, "PB-TNC: A Posture Broker
Protocol (PB) Compatible with TNC", RFC YYYY, January 2010.
[5] NIST, "Secure Hash Standard", FIPS 180-1, National Institute of
Standards and Technology, U.S. Department of Commerce, May
1994, http://csrc.nist.gov/CryptoToolkit/shs/dfips-180-2.pdf
[6] T. Kivinen, M. Kojo, "More Modular Exponential (MODP) Diffie-
Hellman groups for Internet Key Exchange (IKE)", RFC 3526, May
2003.
[7] LAN/MAN Standards Committee of the IEEE Computer Society,
Standard for Local and Metropolitan Area Networks - Port Based
Network Access Control, IEEE Std. 802.1X-2004, December 2004.
[8] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 5226, May 2008.
7.2. Informative References
[9] Sangster, P., Khosravi, H., Mani, M., Narayan, K., and J.
Tardo, "Network Endpoint Assessment (NEA): Overview and
Requirements", RFC 5209, June 2008.
[10] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
[11] Sangster, P., "PT-TLS: A Posture Transport Protocol (PT)
Compatible with TNC Using Transport Layer Security (TLS)",
draft-sangster-nea-pt-tnc-00.txt, work in progress, January
2010.
[12] Trusted Computing Group, "TNC IF-T: Binding to TLS",
http://www.trustedcomputinggroup.org/files/resource_files/51F07
57E-1D09-3519-AD63B6FD099658A6/TNC_IFT_TLS_v1_0_r16.pdf, May
2009.
[13] N. Asokan, Valtteri Niemi, Kaisa Nyberg, "Man in the Middle
Attacks in Tunneled Authentication Protocols", Nokia Research
Center, Finland, Nov. 11, 2002,
http://eprint.iacr.org/2002/163.pdf
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[14] N. Cam-Winget, D. McGrew, J. Salowey, H. Zhou, "The Flexible
Authentication via Secure Tunneling Extensible Authentication
Protocol Method (EAP-FAST)", RFC 4851, May 2007.
[15] C. Kaufman, "Internet Key Exchange (IKEv2) Protocol", RFC 4306,
December 2005.
[16] P. Funk, S. Blake-Wilson, "Extensible Authentication Protocol
Tunneled Transport Layer Security Authenticated Protocol
Version 0 (EAP-TTLSv0)", RFC 5281, August 2008.
[17] K. Hoeper, S. Hanna, H. Zhou, J. Salowey, "Requirements for a
Tunnel Based EAP Method", draft-ietf-emu-eaptunnel-req-04.txt
(work in progress), October 2009.
[18] D. Simon, B. Aboba, R. Hurst, "The EAP-TLS Authentication
Protocol", RFC 5216, March 2008.
8. Acknowledgments
Thanks to the Trusted Computing Group for contributing the initial
text upon which this document was based.
The authors of this draft would like to acknowledge the following
people who have contributed to or provided substantial input on the
preparation of this document or predecessors to it: Amit Agarwal,
Morteza Ansari, Diana Arroyo, Stuart Bailey, Boris Balacheff, Uri
Blumenthal, Gene Chang, Scott Cochrane, Pasi Eronen, Aman Garg,
Sandilya Garimella, David Grawrock, Thomas Hardjono, Chris Hessing,
Ryan Hurst, Hidenobu Ito, John Jerrim, Meenakshi Kaushik, Greg
Kazmierczak, Scott Kelly, Bryan Kingsford, PJ Kirner, Sung Lee, Lisa
Lorenzin, Mahalingam Mani, Bipin Mistry, Seiji Munetoh, Rod
Murchison, Barbara Nelson, Kazuaki Nimura, Ron Pon, Ivan Pulleyn,
Alex Romanyuk, Ravi Sahita, Chris Salter, Mauricio Sanchez, Paul
Sangster, Dean Sheffield, Curtis Simonson, Jeff Six, Ned Smith,
Michelle Sommerstad, Joseph Tardo, Lee Terrell, Chris Trytten, and
John Vollbrecht.
This document was prepared using 2-Word-v2.0.template.dot.
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Appendix A. Evaluation Against NEA Requirements
This section evaluates the PT-EAP protocol against the PT
requirements defined in the NEA Overview and Requirements and
PB-TNC specifications. Each subsection considers a separate
requirement and highlights how PT-EAP meets the requirement.
A.1. Evaluation Against Requirement C-1
Requirement C-1 says:
C-1 NEA protocols MUST support multiple round trips between
the NEA Client and NEA Server in a single assessment.
PT-EAP meets this requirement. Use of the EAP protocol along
with EAP-TNC and suitable EAP tunnel methods will allow for
multiple roundtrips.
A.2. Evaluation Against Requirements C-2
Requirement C-2 says:
C-2 NEA protocols SHOULD provide a way for both the NEA
Client and the NEA Server to initiate a posture assessment or
reassessment as needed.
PT-EAP does NOT meet this requirement. Generally EAP is used
by the endpoint during the joining of the network. At that
time, the endpoint lacks an IP address so is unable to accept
inbound posture assessment requests from the NEA Server.
Subsequent reassessments of the endpoint after it has been
given access to a portion of the IP network can use the PT-TLS
protocol that supports the NEA Client and NEA Server to
initiate an assessment.
A.3. Evaluation Against Requirements C-3
Requirement C-3 says:
C-3 NEA protocols including security capabilities MUST be
capable of protecting against active and passive attacks by
intermediaries and endpoints including prevention from replay
based attacks.
PT-EAP meets this requirement by leveraging the security
capabilities of the underlying EAP tunnel method. EAP-TNC
itself does not provide protection against a variety of
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potential attacks (beside the Diffie-Hellman Pre-Negotiation
support) so must rely on cryptographic support by the EAP
tunnel method.
A.4. Evaluation Against Requirements C-4
Requirement C-4 says:
C-4 The PA and PB protocols MUST be capable of operating over
any PT protocol. For example, the PB protocol must provide a
transport independent interface allowing the PA protocol to
operate without change across a variety of network protocol
environments (e.g. EAP/802.1X, PANA, TLS and IKE/IPsec).
Not applicable to PT, but the PT-EAP is independent of PA and
PB allowing those protocols to operate over other PT protocols.
A.5. Evaluation Against Requirements C-5
Requirement C-5 says:
C-5 The selection process for NEA protocols MUST evaluate and
prefer the reuse of existing open standards that meet the
requirements before defining new ones. The goal of NEA is not
to create additional alternative protocols where acceptable
solutions already exist.
Based on this requirement, PT-EAP should receive a strong
preference. PT-EAP is equivalent with IF-T Binding to Tunneled
EAP Methods 1.1, an open TCG specification that has been widely
implemented.
A.6. Evaluation Against Requirements C-6
Requirement C-6 says:
C-6 NEA protocols MUST be highly scalable; the protocols MUST
support many Posture Collectors on a large number of NEA
Clients to be assessed by numerous Posture Validators residing
on multiple NEA Servers.
PT-EAP meets this requirement. The PT-EAP protocol is
independent of the number of Posture Collectors and Posture
Validators.
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A.7. Evaluation Against Requirements C-7
Requirement C-7 says:
C-7 The protocols MUST support efficient transport of a large
number of attribute messages between the NEA Client and the NEA
Server.
PT-EAP meets this requirement, subject to the limitations of
the underlying EAP protocol. PT-EAP allows for the transport
of a very large number of attributes, up to 2^32 - 1 octets per
PB-TNC batch. Furthermore, the PT-EAP protocol transports data
efficiently, only adding 10 octets of overhead per PT-EAP
message, which is small considering that a single PT-EAP
message may carry multiple PA-TNC attributes.
However, it is important to note that the EAP protocol that
underlies PT-EAP is not a good choice for transporting large
amounts of data. EAP only supports one packet in flight at a
time, which severely limits throughput. Further, some network
equipment imposes timeout restrictions on EAP exchanges.
Therefore, PT-EAP should not be used to transport large amounts
of attributes.
A.8. Evaluation Against Requirements C-8
Requirement C-8 says:
C-8 NEA protocols MUST operate efficiently over low bandwidth
or high latency links.
PT-EAP protocols meet this requirement. PT-EAP was designed to
minimize the amount of overhead included in the protocol to
allow for efficient use over bandwidth or latency constrained
network links.
A.9. Evaluation Against Requirements C-9
Requirement C-9 says:
C-9 For any strings intended for display to a user, the
protocols MUST support adapting these strings to the user's
language preferences.
PT-EAP meets this requirement. PT-EAP does not include
messages intended for display to the user.
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A.10. Evaluation Against Requirements C-10
Requirement C-10 says:
C-10 NEA protocols MUST support encoding of strings in UTF-8
format.
PA-EAP meets this requirement. The PT-EAP protocol does not
include any strings in its fields but it allows higher-layer
protocols to encode their strings in UTF-8 format. This allows
the protocol to support a wide range of languages efficiently.
A.11. Evaluation Against Requirements C-11
Requirement C-11 says:
C-11 Due to the potentially different transport
characteristics provided by the underlying candidate PT
protocols, the NEA Client and NEA Server MUST be capable of
becoming aware of and adapting to the limitations of the
available PT protocol. For example, some PT protocol
characteristics that might impact the operation of PA and PB
include restrictions on: which end can initiate a NEA
connection, maximum data size in a message or full assessment,
upper bound on number of roundtrips, and ordering (duplex) of
messages exchanged. The selection process for the PT protocols
MUST consider the limitations the candidate PT protocol would
impose upon the PA and PB protocols.
PT-EAP meets this requirement. The PT-EAP implementations may
be limited in number of roundtrips, assessment overall time, or
data transmission. These constraints will be exposed up the
protocol stack so the Posture Broker Client and Posture Broker
Server can optimize and make most efficient use of the
available resources during the assessment.
A.12. Evaluation Against Requirements PT-1
Requirement PT-1 says:
PT-1 The PT protocol MUST NOT interpret the contents of PB
messages being transported, i.e., the data it is carrying must
be opaque to it.
PT-EAP meets this requirement. The PT-EAP encapsulates PB-TNC
batches without interpreting their contents.
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A.13. Evaluation Against Requirements PT-2
Requirement PT-2 says:
PT-2 The PT protocol MUST be capable of supporting mutual
authentication, integrity, confidentiality, and replay
protection of the PB messages between the Posture Transport
Client and the Posture Transport Server.
PT-EAP meets this requirement. The PT-EAP leverages an EAP
tunnel method to provide mutual authentication, integrity
protection and confidentiality as well as replay protection.
For more information see the Security Considerations section 4.
A.14. Evaluation Against Requirements PT-3
Requirement PT-3 says:
PT-3 The PT protocol MUST provide reliable delivery for the PB
protocol. This includes the ability to perform fragmentation
and reassembly, detect duplicates, and reorder to provide in-
sequence delivery, as required.
EAP-TNC includes support for fragmentation and the underlying
EAP tunnel methods include support for duplicate detection and
reordering to provide in-sequence delivery.
A.15. Evaluation Against Requirements PT-4
Requirement PT-4 says:
PT-4 The PT protocol SHOULD be able to run over existing
network access protocols such as 802.1X and IKEv2.
PT-EAP meets this requirement. The PT-EAP operates on top of
the 802.1X and IKEv2 protocols.
A.16. Evaluation Against Requirements PT-5
Requirement PT-5 says:
PT-5 The PT protocol SHOULD be able to run between a NEA Client
and NEA Server over TCP or UDP (similar to Lightweight
Directory Access Protocol (LDAP)).
PT-EAP does NOT meet this requirement. PT-EAP is intended for
a different usage. PT-EAP is intended to be used for pre-
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network admission before the endpoint has been given an IP
address and routes on the network. This means that network
layer protocols such as IP are not yet able to communicate with
the system. The PT-TLS (PT Binding to TLS) [11] meets this
requirement.
A.17. Evaluation Against Requirements PT-6 (from PB-TNC specification)
Requirement PT-6 says:
PT-6 The PT protocol MUST be connection oriented; it MUST
support confirmed initiation and close down.
PT-EAP meets this requirement. The PT-EAP fits into the EAP
framework which provides for orderly initiation and shutdown.
A.18. Evaluation Against Requirements PT-7 (from PB-TNC specification)
Requirement PT-7 says:
PT-7 The PT protocol MUST be able to carry binary data.
PT-EAP meets this requirement. The PT-EAP is capable of
carrying binary data.
A.19. Evaluation Against Requirements PT-8 (from PB-TNC specification)
Requirement PT-8 says:
PT-8 The PT protocol MUST provide mechanisms for flow control
and congestion control.
PT-EAP meets this requirement. The PT-EAP utilizes EAP's half
duplex, round robin message exchange to provide flow and
congestion control.
A.20. Evaluation Against Requirements PT-9 (from PB-TNC specification)
Requirement PT-9 says:
PT-9 PT protocol specifications MUST describe the capabilities
that they provide for and limitations that they impose on the
PB protocol (e.g. half/full duplex, maximum message size).
PT-EAP specification meets this requirement. This
specification discusses the level of transport service provided
to the Posture Broker Client and Posture Broker Server.
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Generally, the PT-EAP method supports the pre-network admission
usages discussed in RFC 5209. The maximum message size for PT-
EAP is 2^16-10 octets. EAP by its very nature is half duplex
and very simple which allows it to be used in a wide variety of
settings including over link layer protocols during the
entrance to the network.
Authors' Addresses
Steve Hanna
Juniper Networks, Inc.
79 Parsons Street
Brighton, MA 02135 USA
Email: shanna@juniper.net
Paul Sangster
Symantec Corporation
6825 Citrine Drive
Carlsbad, CA 92009 USA
Email: paul_sangster@symantec.com
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