Network Working Group T. Clancy
Request for Comments: 4746 LTS
Category: Informational W. Arbaugh
UMD
November 2006
Extensible Authentication Protocol (EAP)
Password Authenticated Exchange
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2006).
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines an Extensible Authentication Protocol (EAP)
method called EAP-PAX (Password Authenticated eXchange). This method
is a lightweight shared-key authentication protocol with optional
support for key provisioning, key management, identity protection,
and authenticated data exchange.
Table of Contents
1. Introduction ....................................................2
1.1. Language Requirements ......................................3
1.2. Terminology ................................................3
2. Overview ........................................................5
2.1. PAX_STD Protocol ...........................................6
2.2. PAX_SEC Protocol ...........................................7
2.3. Authenticated Data Exchange ................................9
2.4. Key Derivation ............................................10
2.5. Verification Requirements .................................11
2.6. PAX Key Derivation Function ...............................12
3. Protocol Specification .........................................13
3.1. Header Specification ......................................13
3.1.1. Op-Code ............................................13
3.1.2. Flags ..............................................14
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3.1.3. MAC ID .............................................14
3.1.4. DH Group ID ........................................14
3.1.5. Public Key ID ......................................15
3.1.6. Mandatory to Implement .............................15
3.2. Payload Formatting ........................................16
3.3. Authenticated Data Exchange (ADE) .........................18
3.4. Integrity Check Value (ICV) ...............................19
4. Security Considerations ........................................19
4.1. Server Certificates .......................................20
4.2. Server Security ...........................................20
4.3. EAP Security Claims .......................................21
4.3.1. Protected Ciphersuite Negotiation ..................21
4.3.2. Mutual Authentication ..............................21
4.3.3. Integrity Protection ...............................21
4.3.4. Replay Protection ..................................21
4.3.5. Confidentiality ....................................21
4.3.6. Key Derivation .....................................21
4.3.7. Key Strength .......................................22
4.3.8. Dictionary Attack Resistance .......................22
4.3.9. Fast Reconnect .....................................22
4.3.10. Session Independence ..............................22
4.3.11. Fragmentation .....................................23
4.3.12. Channel Binding ...................................23
4.3.13. Cryptographic Binding .............................23
4.3.14. Negotiation Attack Prevention .....................23
5. IANA Considerations ............................................23
6. Acknowledgments ................................................24
7. References .....................................................24
7.1. Normative References ......................................24
7.2. Informative References ....................................25
Appendix A. Key Generation from Passwords ........................ 27
Appendix B. Implementation Suggestions ........................... 27
B.1. WiFi Enterprise Network ................................... 27
B.2. Mobile Phone Network ...................................... 28
1. Introduction
EAP-PAX (Password Authenticated eXchange) is an Extensible
Authentication Protocol (EAP) method [RFC3748] designed for
authentication using a shared key. It makes use of two separate
subprotocols, PAX_STD and PAX_SEC. PAX_STD is a simple, lightweight
protocol for mutual authentication using a shared key, supporting
Authenticated Data Exchange (ADE). PAX_SEC complements PAX_STD by
providing support for shared-key provisioning and identity protection
using a server-side public key.
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The idea motivating EAP-PAX is a desire for device authentication
bootstrapped by a simple Personal Identification Number (PIN). If a
weak key is used or a expiration period has elapsed, the
authentication server forces a key update. Rather than using a
symmetric key exchange, the client and server perform a Diffie-
Hellman key exchange, which provides forward secrecy.
Since implementing a Public Key Infrastructure (PKI) can be
cumbersome, PAX_SEC defines multiple client security policies,
selectable based on one's threat model. In the weakest mode, PAX_SEC
allows the use of raw public keys completely eliminating the need for
a PKI. In the strongest mode, PAX_SEC requires that EAP servers use
certificates signed by a trusted Certification Authority (CA). In
the weaker modes, during provisioning PAX_SEC is vulnerable to a
man-in-the-middle dictionary attack. In the strongest mode, EAP-PAX
is provably secure under the Random Oracle model.
EAP-PAX supports the generation of strong key material; mutual
authentication; resistance to desynchronization, dictionary, and
man-in-the-middle attacks; ciphersuite extensibility with protected
negotiation; identity protection; and the authenticated exchange of
data, useful for implementing channel binding. These features
satisfy the EAP method requirements for wireless LANs [RFC4017],
making EAP-PAX ideal for wireless environments such as IEEE 802.11
[IEEE.80211].
1.1. Language 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
[RFC2119].
1.2. Terminology
This section describes the various variables and functions used in
the EAP-PAX protocol. They will be referenced frequently in later
sections.
Variables:
CID
User-supplied client ID, specified as a Network Access Identifier
(NAI) [RFC4282], restricted to 65535 octets
g
public Diffie-Hellman generator, typically the integer 2 [RFC2631]
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M
128-bit random integer generated by the server
N
128-bit random integer generated by the client
X
256-bit random integer generated by the server
Y
256-bit random integer generated by the client
Keys:
AK
authentication key shared between the client and EAP server
AK'
new authentication key generated during a key update
CertPK
EAP server's certificate containing public key PK
CK
Confirmation Key generated from the MK and used during
authentication to prove knowledge of AK
EMSK
Extended Master Session Key also generated from the MK and
containing additional keying material
IV
Initialization Vector used to seed ciphers; exported to the
authenticator
MID
Method ID used to construct the EAP Session ID and consequently
name all the exported keys [IETF.KEY]
MK
Master Key between the client and EAP server from which all other
EAP method session keys are derived
MSK
Master Session Key generated from the MK and exported by the EAP
method to the authenticator
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PK
EAP server's public key
Operations:
enc_X(Y)
encryption of message Y with public key X
MAC_X(Y)
keyed message authentication code computed over message Y with
symmetric key X
PAX-KDF-W(X, Y, Z)
PAX Key Derivation Function computed using secret X, identifier Y,
and seed Z, and producing W octets of output
||
string or binary data concatenation
2. Overview
The EAP framework [RFC3748] defines four basic steps that occur
during the execution of an EAP conversation between client and
server. The first phase, discovery, is handled by the underlying
link-layer protocol. The authentication phase is defined here. The
key distribution and secure association phases are handled
differently depending on the underlying protocol, and are not
discussed in this document.
+--------+ +--------+
| | EAP-Request/Identity | |
| CLIENT |<------------------------------------| SERVER |
| | | |
| | EAP-Response/Identity | |
| |------------------------------------>| |
| | | |
| | EAP-PAX (STD or SEC) | |
| |<----------------------------------->| |
| | ... ... | |
| |<----------------------------------->| |
| | | |
| | EAP-Success or EAP-Failure | |
| |<------------------------------------| |
+--------+ +--------+
Figure 1: EAP-PAX Packet Exchanges
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There are two distinct subprotocols that can be executed. The first,
PAX_STD, is used during typical authentications. The second,
PAX_SEC, provides more secure features such as key provisioning and
identity protection.
PAX_STD and PAX_SEC have two modes of operation. When an AK update
is being performed, the client and server exchange Diffie-Hellman
exponents g^X and g^Y, which are computed modulo prime P or over an
elliptic curve multiplicative group. When no update is being
performed, and only session keys are being derived, X and Y are
exchanged. Using Diffie-Hellman during the key update provides
forward secrecy, and secure key derivation when a weak provisioned
key is used.
The main deployment difference between PAX_STD and PAX_SEC is that
PAX_SEC requires a server-side public key. More specifically, that
means a private key known only to the server with corresponding
public key being transmitted to the client during each PAX_SEC
session. For every authentication, the client is required to compute
computationally intensive public-key operations. PAX_STD, on the
other hand, uses purely symmetric operations, other than a possible
Diffie-Hellman exchange.
Each of the protocols is now defined.
2.1. PAX_STD Protocol
PAX_STD is a simple nonce-based authentication using the strong
long-term key. The client and server each exchange 256 bits of
random data, which is used to seed the PAX-KDF for generation of
session keys. The randomly exchanged data in the protocol differs
depending on whether a key update is being performed. If no key
update is being performed, then let:
o A = X
o B = Y
o E = X || Y
Thus, A and B are 256-bit values and E is their 512-bit
concatenation. To provide forward secrecy and security, let the
following be true when a key update is being performed:
o A = g^X
o B = g^Y
o E = g^(XY)
Here A and B are Diffie-Hellman exponents whose length is determined
by the Diffie-Hellman group size. The value A is data transmitted
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from the server to the client, and B is data transmitted from the
client to the server. The value E is the entropy computed by each
that is used in Section 2.4 to perform key derivation.
The full protocol is as follows:
o PAX_STD-1 : client <- server : A
o PAX_STD-2 : client -> server : B, CID, MAC_CK(A, B, CID),
[optional ADE]
o PAX_STD-3 : client <- server : MAC_CK(B, CID), [optional ADE]
o PAX-ACK : client -> server : [optional ADE]
See Section 2.3 for more information on the ADE component, and
Section 2.4 for the key derivation process, including derivation of
CK.
2.2. PAX_SEC Protocol
PAX_SEC is the high-security protocol designed to provide identity
protection and support for provisioning. PAX_SEC requires a server-
side public key, and public-key operations for every authentication.
PAX_SEC can be performed with and without key update. Let A, B, and
E be defined as in the previous section.
The exchanges for PAX_SEC are as follows:
o PAX_SEC-1 : client <- server : M, PK or CertPK
o PAX_SEC-2 : client -> server : Enc_PK(M, N, CID)
o PAX_SEC-3 : client <- server : A, MAC_N(A, CID)
o PAX_SEC-4 : client -> server : B, MAC_CK(A, B, CID), [optional
ADE]
o PAX_SEC-5 : client <- server : MAC_CK(B, CID), [optional ADE]
o PAX-ACK : client -> server : [optional ADE]
See Section 2.3 for more information on the ADE component, and
Section 2.4 for the key derivation process, including derivation of
CK.
Use of CertPK is optional in PAX_SEC; however, careful consideration
should be given before omitting the CertPK. The following table
describes the risks involved when using PAX_SEC without a
certificate.
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Certificate | Provisioning | Identity
Mode | | Protection
==================+=====================+======================
No Certificate | MiTM offline | ID reveal attack
| dictionary attack |
------------------+---------------------+---------------------
Self-Signed | MiTM offline | ID reveal attack
Certificate | dictionary attack |
------------------+---------------------+---------------------
Certificate/PK | MiTM offline | ID reveal attack
Caching | dictionary attack | during first auth
------------------+---------------------+---------------------
CA-Signed | secure mutual | secure mutual
Certificate | authentication | authentication
Figure 2: Table of Different Security Modes
When using PAX_SEC to support provisioning with a weak key, use of a
CA-signed certificate is RECOMMENDED. When not using a CA-signed
certificate, the initial authentication is vulnerable to an offline
man-in-the-middle (MiTM) dictionary attack.
When using PAX_SEC to support identity protection, use of either a
CA-signed certificate or key caching is RECOMMENDED. Caching
involves a client recording the public key of the EAP server and
verifying its consistency between sessions, similar to Secure SHell
(SSH) Protocol [RFC4252]. Otherwise, an attacker can spoof an EAP
server during a session and gain knowledge of a client's identity.
Whenever certificates are used, clients MUST validate that the
certificate's extended key usage, KeyPurposeID, is either
"eapOverPPP" or "eapOverLAN" [RFC3280][RFC4334]. If the underlying
EAP transport protocol is known, then the client MUST differentiate
between these values. For example, an IEEE 802.11 supplicant SHOULD
require KeyPurposeID == eapOverLAN. By not distinguishing, a client
could accept as valid an unauthorized server certificate.
When using EAP-PAX with Wireless LAN, clients SHOULD validate that
the certificate's wlanSSID extension matches the SSID of the network
to which it is currently authenticating.
In order to facilitate discussion of packet validations, three client
security policies for PAX_SEC are defined.
open
Clients support both use of PK and CertPK. If CertPK is used, the
client MUST validate the KeyPurposeID.
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caching
Clients save PK for each EAP server the first time it encounters
the server, and SHOULD NOT authenticate to EAP servers whose
public key has been changed. If CertPK is used, the client MUST
validate the KeyPurposeID.
strict
In strict mode, clients require servers to present a valid
certificate signed by a trusted CA. As with the other modes, the
KeyPurposeID MUST be validated.
Servers SHOULD support the PAX_SEC mode of operation, and SHOULD
support both the use of PK and CertPK with PAX_SEC. Clients MUST
support PAX_SEC, and MUST be capable of accepting both raw public
keys and certificates from the server. Local security policy will
define which forms of key or certificate authentications are
permissible. Default configurations SHOULD require a minimum of the
caching security policy, and MAY require strict.
The ability to perform key management on the AK is built in to EAP-
PAX through the use of AK'. However, key management of the server
public key is beyond the scope of this document. If self-signed
certificates are used, the deployers should be aware that expired
certificates may be difficult to replace when the caching security
mode is used.
See Section 4 for further discussion on security considerations.
2.3. Authenticated Data Exchange
Messages PAX_STD-2, PAX_STD-3, PAX_SEC-4, PAX_SEC-5, and PAX_ACK
contain optional component ADE. This component is used to convey
authenticated data between the client and server during the
authentication.
The Authenticated Data Exchanged (ADE) can be used in a variety of
ways, including the implementation of channel bindings. Channel
bindings allow link-layer network properties to be securely validated
by the EAP client and server during the authentication session.
It is important to note that ADE is not encrypted, so any data
included will not be confidential. However, since these packets are
all protected by the Integrity Check Value (ICV), authenticity is
guaranteed.
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The ADE element consists of an arbitrary number of subelements, each
with length and type specified. If the number and size of
subelements is too large, packet fragmentation will be necessary.
Vendor-specific options are supported. See Section 3.3.
Note that more than 1.5 round-trips may be necessary to execute a
particular authenticated protocol within EAP-PAX. In this case,
instead of sending an EAP-Success after receiving the PAX_ACK, the
server can continue sending PAX_ACK messages with attached elements.
The client responds to these PAX_ACK messages with PAX_ACK messages
possibly containing more ADE elements. Such an execution could look
something like the following:
+--------+ +--------+
| | PAX_STD-1 | |
| |<------------------------------------| |
| | PAX_STD-2(ADE[1]) | |
| |------------------------------------>| |
| | PAX_STD-3(ADE[2]) | |
| |<------------------------------------| |
| | PAX_ACK(ADE[3]) | |
| |------------------------------------>| |
| | PAX_ACK(ADE[4]) | |
| |<------------------------------------| |
| | | |
| | ... | |
| | | |
| | PAX_ACK(ADE[i]) | |
| |------------------------------------>| |
| | PAX_ACK(ADE[i+1]) | |
| |<------------------------------------| |
| | | |
| | ... | |
| | | |
| | EAP-Success or EAP-Failure | |
| |<------------------------------------| |
+--------+ +--------+
Figure 3: Extended Diagram of EAP-PAX Packet Exchanges
2.4. Key Derivation
Keys are derived independently of which authentication mechanism was
used. The process uses the entropy value E computed as described
above. Session and authentication keys are computed as follows:
o AK' = PAX-KDF-16(AK, "Authentication Key", E)
o MK = PAX-KDF-16(AK, "Master Key", E)
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o CK = PAX-KDF-16(MK, "Confirmation Key", E)
o ICK = PAX-KDF-16(MK, "Integrity Check Key", E)
o MID = PAX-KDF-16(MK, "Method ID", E)
o MSK = PAX-KDF-64(MK, "Master Session Key", E)
o EMSK = PAX-KDF-64(MK, "Extended Master Session Key", E)
o IV = PAX-KDF-64(0x00^16, "Initialization Vector", E)
The IV is computed using a 16-octet NULL key. The value of AK' is
only used to replace AK if a key update is being performed. The EAP
Method ID is represented in ASCII as 32 hexadecimal characters
without any octet delimiters such as colons or dashes.
The EAP Key Management Framework [IETF.KEY] recommends specification
of key names and scope. The EAP-PAX Method-ID is the MID value
computed as described above. The EAP peer name is the CID value
exchanged in PAX_STD-2 and PAX_SEC-2. The EAP server name is an
empty string.
2.5. Verification Requirements
In order for EAP-PAX to be secure, MACs must be properly verified
each step of the way. Any packet with an ICV (see Section 3.4) that
fails validation must be silently discarded. After ICV validation,
the following checks must be performed:
PAX_STD-2
The server MUST validate the included MAC, as it serves to
authenticate the client to the server. If this validation fails,
the server MUST send an EAP-Failure message.
PAX_STD-3
The client MUST validate the included MAC, as it serves to
authenticate the server to the client. If this validation fails,
the client MUST send an EAP-Failure message.
PAX_SEC-1
The client MUST validate PK or CertPK in a manner specified by its
local security policy (see Section 2.2). If this validation
fails, the client MUST send an EAP-Failure message.
PAX_SEC-2
The server MUST verify that the decrypted value of M matches the
value transmitted in PAX_SEC-1. If this validation fails, the
server MUST send an EAP-Failure message.
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PAX_SEC-3
The client MUST validate the included MAC, as it serves to prevent
replay attacks. If this validation fails, the client MUST send an
EAP-Failure message.
PAX_SEC-4
The server MUST validate the included MAC, as it serves to
authenticate the client to the server. If this validation fails,
the server MUST send an EAP-Failure message.
PAX_SEC-5
The client MUST validate the included MAC, as it serves to
authenticate the server to the client. If this validation fails,
the client MUST send an EAP-Failure message.
PAX-ACK
If PAX-ACK is received in response to a message fragment, the
receiver continues the protocol execution. If PAX-ACK is received
in response to PAX_STD-3 or PAX_SEC-5, then the server MUST send
an EAP-Success message. This indicates a successful execution of
PAX.
2.6. PAX Key Derivation Function
The PAX-KDF is a secure key derivation function used to generate
various keys from the provided entropy and shared key.
PAX-KDF-W(X, Y, Z)
W length, in octets, of the desired output
X secret key used to protect the computation
Y public identifier for the key being derived
Z exchanged entropy used to seed the KDF
Let's define some variables and functions:
o M_i = MAC_X(Y || Z || i), where i is an 8-bit unsigned integer
o L = ceiling(W/16)
o F(A, B) = first A octets of binary data B
We define PAX-KDF-W(X, Y, Z) = F(W, M_1 || M_2 || ... || M_L).
Consequently for the two values of W used in this document, we have:
o PAX-KDF-16(X, Y, Z) = MAC_X(Y || Z || 0x01)
o PAX-KDF-64(X, Y, Z) = MAC_X(Y || Z || 0x01) || MAC_X(Y || Z ||
0x02) || MAC_X(Y || Z || 0x03) || MAC_X(Y || Z || 0x04)
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The MAC used in the PRF is extensible and is the same MAC used in the
rest of the protocol. It is specified in the EAP-PAX header.
3. Protocol Specification
In this section, the packet format and content for the EAP-PAX
messages are defined.
EAP-PAX packets have the following structure:
--- bit offset --->
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 | OP-Code | Flags | MAC ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DH Group ID | Public Key ID | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
... Payload ...
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
... ICV ...
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: EAP-PAX Packet Structure
3.1. Header Specification
The Code, Identifier, Length, and Type fields are all part of the EAP
header, and defined in [RFC3748]. IANA has allocated EAP Method Type
46 for EAP-PAX; thus, the Type field in the EAP header MUST be 46.
3.1.1. Op-Code
The OP-Code field is one of the following values:
o 0x01 : PAX_STD-1
o 0x02 : PAX_STD-2
o 0x03 : PAX_STD-3
o 0x11 : PAX_SEC-1
o 0x12 : PAX_SEC-2
o 0x13 : PAX_SEC-3
o 0x14 : PAX_SEC-4
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o 0x15 : PAX_SEC-5
o 0x21 : PAX-ACK
3.1.2. Flags
The flags field is broken up into 8 bits each representing a binary
flag. The field is defined as the Logical OR of the following
values:
o 0x01 : more fragments (MF)
o 0x02 : certificate enabled (CE)
o 0x04 : ADE Included (AI)
o 0x08 - 0x80 : reserved
The MF flag is set if the current packet required fragmentation, and
further fragments need to be transmitted. If a packet does not
require fragmentation, the MF flag is not set.
When a payload requires fragmentation, each fragment is transmitted,
and the receiving party responds with a PAX-ACK packet for each
received fragment.
When using PAX_STD, the CE flag MUST be zero. When using PAX_SEC,
the CE flag MUST be set if PAX_SEC-1 includes CertPK. It MUST NOT be
set if PAX_SEC-1 includes PK. If CE is set in PAX_SEC-1, it MUST be
set in PAX_SEC-2, PAX_SEC-3, PAX_SEC-4, and PAX_SEC-5. If either
party detects an inconsistent value of the CE flag, he MUST send an
EAP-Failure message and discontinue the session.
The AI flag indicates the presence of an ADE element. AI MUST only
be set on packets PAX_STD-2, PAX_STD-3, PAX_SEC-4, PAX_SEC-5, and
PAX_ACK if an ADE element is included. On packets of other types,
ADE elements MUST be silently discarded as they cannot be
authenticated.
3.1.3. MAC ID
The MAC field specifies the cryptographic hash used to generate the
keyed hash value. The following are currently supported:
o 0x01 : HMAC_SHA1_128 [FIPS198] [FIPS180]
o 0x02 : HMAC_SHA256_128 [FIPS180]
3.1.4. DH Group ID
The Diffie-Hellman group field specifies the group used in the
Diffie-Hellman computations. The following are currently supported:
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o 0x00 : NONE (iff not performing a key update)
o 0x01 : 2048-bit MODP Group (IANA DH Group 14) [RFC3526]
o 0x02 : 3072-bit MODP Group (IANA DH Group 15) [RFC3526]
o 0x03 : NIST ECC Group P-256 [FIPS186]
If no key update is being performed, the DH Group ID field MUST be
zero. Otherwise, the DH Group ID field MUST NOT be zero.
3.1.5. Public Key ID
The Public Key ID field specifies the cipher used to encrypt the
client's EAP-Response in PAX_SEC-2.
The following are currently supported:
o 0x00 : NONE (if using PAX_STD)
o 0x01 : RSAES-OAEP [RFC3447]
o 0x02 : RSA-PKCS1-V1_5 [RFC3447]
o 0x03 : El-Gamal Over NIST ECC Group P-256 [FIPS186]
If PAX_STD is being executed, the Public Key ID field MUST be zero.
If PAX_SEC is being executed, the Public Key ID field MUST NOT be
zero.
When using RSAES-OAEP, the hash algorithm and mask generation
algorithm used SHALL be the MAC specified by the MAC ID, keyed using
an all-zero key. The label SHALL be null.
The RSA-based schemes specified here do not dictate the length of the
public keys. DER encoding rules will specify the key size in the key
or certificate [X.690]. Key sizes SHOULD be used that reflect the
desired level of security.
3.1.6. Mandatory to Implement
The following ciphersuite is mandatory to implement and achieves
roughly 112 bits of security:
o HMAC_SHA1_128
o IANA DH Group 14 (2048 bits)
o RSA-PKCS1-V1_5 (RECOMMEND 2048-bit public key)
The following ciphersuite is RECOMMENDED and achieves 128 bits of
security:
o HMAC_SHA256_128
o IANA DH Group 15 (3072 bits)
o RSAES-OAEP (RECOMMEND 3072-bit public key)
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3.2. Payload Formatting
This section describes how to format the payload field. Depending on
the packet type, different values are transmitted. Sections 2.1 and
2.2 define the fields, and in what order they are to be concatenated.
For simplicity and since many field lengths can vary with the
ciphersuite, each value is prepended with a 2-octet length value
encoded as an integer as described below. This length field MUST
equal the length in octets of the subsequent value field.
--- octet offset --->
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---------------------
|len| value ....
+---+--------
Figure 5: Length Encoding for Data Elements
All integer values are stored as octet arrays in network-byte order,
with the most significant octet first. Integers are padded on the
most significant end to reach octet boundaries.
Public keys and certificates SHALL be in X.509 format [RFC3280]
encoded using the Distinguished Encoding Rules (DER) format [X.690].
Strings are not null-terminated and are encoded using UTF-8. Binary
data, such as message authentication codes, are transmitted as-is.
MACs are computed by concatenating the specified values in the
specified order. Note that for MACs, length fields are not included,
though the resulting MAC will itself have a length field. Values are
encoded as described above, except that no length field is specified.
To illustrate this process, an example is presented. What follows is
the encoding of the payload for PAX_STD-2. The three basic steps
will be computing the MAC, forming the payload, and encrypting the
payload.
To create the MAC, we first need to form the buffer that will be
MACed. For this example, assume that no key update is being done and
HMAC_SHA1_128 is used such that the result will be a 16-octet value.
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--- octet offset --->
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 32-octet integer A |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 32-octet integer B |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
... variable length CID ...
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
||
||
CK --> MAC
||
\/
--- octet offset --->
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 16-octet MAC output |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Example Encoding of PAX_STD-2 MAC Data
With this, we can now create the encoded payload:
--- octet offset --->
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|32 | 32-octet integer B
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L | |
+-+-+-+-+ +
| |
... L-octet CID ...
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|16 | MAC computed above |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Example Encoding of PAX_STD-2 Packet
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These 52+L octets are then attached to the packet as the payload.
The ICV is then computed by MACing the packet headers and payload,
and appended after the payload (see Section 3.4).
3.3. Authenticated Data Exchange (ADE)
This section describes the formatting of the ADE elements. ADE
elements can only occur on packets of type PAX_STD-2, PAX_STD-3,
PAX_SEC-4, PAX_SEC-5, and PAX_ACK. Values included in other packets
MUST be silently ignored.
The ADE element is preceded by its 2-octet length L. Each subelement
has first a 2-octet length Li followed by a 2-octet type Ti. The
entire ADE element looks as follows:
--- octet offset --->
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L |L1 |T1 | |
+-+-+-+-+-+-+ +
| |
... subADE-1, type T1, length L1 ...
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |L2 |T2 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
... subADE-2, type T2, length L2 ...
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | more subADE elements... ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Encoding of ADE Components
The following type values have been allocated:
o 0x01 : Vendor Specific
o 0x02 : Client Channel Binding Data
o 0x03 : Server Channel Binding Data
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The first three octets of a subADE utilizing type code 0x01 must be
the vendor's Enterprise Number [RFC3232] as registered with IANA.
The format for such a subADE is as follows:
--- octet offset --->
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Li | 1 | ENi | |
+-+-+-+-+-+-+-+ +
| |
... subADE-i, type Vendor Specific, length Li, vendor ENi ...
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Encoding of Vendor-specific ADE
Channel binding subADEs have yet to be defined. Future IETF
documents will specify the format for these subADE fields.
3.4. Integrity Check Value (ICV)
The ICV is computed as the MAC over the entire EAP packet, including
the EAP header, the EAP-PAX header, and the EAP-PAX payload. The MAC
is keyed using the 16-octet ICK, using the MAC type specified by the
MAC ID in the EAP-PAX header. For packets of type PAX_STD-1,
PAX_SEC-1, PAX_SEC-2, and PAX_SEC-3, where the MK has not yet been
derived, the MAC is keyed using a zero-octet NULL key.
If the ICV field is incorrect, the receiver MUST silently discard the
packet.
4. Security Considerations
Any authentication protocol, especially one geared for wireless
environments, must assume that adversaries have many capabilities.
In general, one must assume that all messages between the client and
server are delivered via the adversary. This allows passive
attackers to eavesdrop on all traffic, while active attackers can
modify data in any way before delivery.
In this section, we discuss the security properties and requirements
of EAP-PAX with respect to this threat model. Also note that the
security of PAX can be proved using under the Random Oracle model.
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4.1. Server Certificates
PAX_SEC can be used in several configurations. It can be used with
or without a server-side certificate. Section 2.2 details the
possible modes and the resulting security risk.
When using PAX_SEC for identity protection and not using a CA-signed
certificate, an attacker can convince a client to reveal his
username. To achieve this, an attacker can simply forge a PAX_SEC-1
message and send it to the client. The client would respond with a
PAX_SEC-2 message containing his encrypted username. The attacker
can then use his associated private key to decrypt the client's
username. Use of key caching can reduce the risk of identity
revelation by allowing clients to detect when the EAP server to which
they are accustom has a different public key.
When provisioning with PAX_SEC and not using a CA-signed certificate,
an attacker could first forge a PAX_SEC-1 message and send it to the
client. The client would respond with a PAX_SEC-2 message. Using
the decrypted value of N, an attacker could forge a PAX_SEC-3
message. Once the client responds with a PAX_SEC-4 message, an
attacker can guess values of the weak AK and compute CK = PAX-KDF(AK,
"Confirmation Key", g^XY). Given enough time, the attacker can
obtain both the old AK and new AK' and forge a responding PAX_SEC-5.
4.2. Server Security
In order to maintain a reasonable security policy, the server should
manage five pieces of information concerning each user, most
obviously, the username and current key. In addition, the server
must keep a bit that indicates whether the current key is weak. Weak
keys must be updated prior to key derivation. Also, the server
should track the date of last key update. To implement the coarse-
grained forward secrecy, the authentication key must be updated on a
regular basis, and this field can be used to expire keys. Last, the
server should track the previous key, to prevent attacks where an
adversary desynchronizes the key state by interfering with PAX-ACK
packets. See Appendix B for more suggested implementation strategies
that prevent key desynchronization attacks.
Since the client keys are stored in plaintext on the server, special
care should be given to the overall security of the authentication
server. An operating system-level attack yielding root access to an
intruder would result in the compromise of all client credentials.
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4.3. EAP Security Claims
This section describes EAP-PAX in terms of specific security
terminology as required by [RFC3748].
4.3.1. Protected Ciphersuite Negotiation
In the initial packet from the server, the server specifies the
ciphersuite in the packet header. The server is in total control of
the ciphersuite; thus, a client not supporting the specified
ciphersuite will not be able to authenticate. In addition, each
client's local security policy should specify secure ciphersuites the
client will accept. The ciphersuite specified in PAX_STD-1 and
PAX_SEC-1 MUST remain the same in successive packets within the same
authentication session. Since later packets are covered by an ICV
keyed with the ICK, the server can verify that the originally
transmitted ciphersuite was not altered by an adversary.
4.3.2. Mutual Authentication
Both PAX_STD and PAX_SEC authenticate the client and the server, and
consequently achieve explicit mutual authentication.
4.3.3. Integrity Protection
The ICV described in Section 3.4 provides integrity protection once
the integrity check key has been derived. The header values in the
unprotected packets can be verified when an ICV is received later in
the session.
4.3.4. Replay Protection
EAP-PAX is inherently designed to avoid replay attacks by
cryptographically binding each packet to the previous one. Also the
EAP sequence number is covered by the ICV to further strengthen
resistance to replay attacks.
4.3.5. Confidentiality
With identity protection enabled, PAX_SEC provides full
confidentiality.
4.3.6. Key Derivation
Session keys are derived using the PAX-KDF and fresh entropy supplied
by both the client and the server. Since the key hierarchy is
derived from the shared password, only someone with knowledge of that
password or the capability of guessing it is capable of deriving the
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session keys. One of the main benefits of PAX_SEC is that it allows
you to bootstrap a strong shared secret using a weak password while
preventing offline dictionary attacks.
4.3.7. Key Strength
Authentication keys are 128 bits. The key generation is protected by
a Diffie-Hellman key exchange. It is believed that a 3000-bit MODP
public-key scheme is roughly equivalent [RFC3766] to a 128-bit
symmetric-key scheme. Consequently, EAP-PAX requires the use of a
Diffie-Hellman group with modulus larger than 3000. Also, the
exponent used as the private DH parameter must be at least twice as
large as the key eventually generated. Consequently, EAP-PAX uses
256-bit DH exponents. Thus, the authentication keys contain the full
128 bits of security.
Future ciphersuites defined for EAP-PAX MUST contain a minimum of 128
bits of security.
4.3.8. Dictionary Attack Resistance
EAP-PAX is resistant to dictionary attacks, except for the case where
a weak password is initially used and the server is not using a
certificate for authentication. See Section 4.1 for more information
on resistance to dictionary attacks.
4.3.9. Fast Reconnect
Although a specific fast reconnection option is not included,
execution of PAX_STD requires very little computation time and is
therefore bound primarily by the latency of the Authentication,
Authorization, and Accounting (AAA) server.
4.3.10. Session Independence
This protocol easily achieves backward secrecy through, among other
things, use of the PAX-KDF. Given a current session key, attackers
can discover neither the entropy used to generate it nor the key used
to encrypt that entropy as it was transmitted across the network.
This protocol has coarse-grained forward secrecy. Compromised
session keys are only useful on data for that session, and one cannot
derive AK from them. If an attacker can discover AK, that value can
only be used to compromise session keys derived using that AK.
Reasonably frequent password updates will help mitigate such attacks.
Session keys are independently generated using fresh nonces for each
session, and therefore the sessions are independent.
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4.3.11. Fragmentation
Fragmentation and reassembly is supported through the fragmentation
flag in the header.
4.3.12. Channel Binding
EAP-PAX can be extended to support channel bindings through the use
of its subADE fields.
4.3.13. Cryptographic Binding
EAP-PAX does not include any cryptographic binding. This is relevant
only for tunneled methods.
4.3.14. Negotiation Attack Prevention
EAP is susceptible to an attack where an attacker uses NAKs to
convince an EAP client and server to use a less secure method, and
can be prevented using method-specific integrity protection on NAK
messages. Since EAP-PAX does not have suitable keys derived for this
integrity protection at the beginning of a PAX conversation, this is
not included.
5. IANA Considerations
This document requires IANA to maintain the namespace for the
following header fields: MAC ID, DH Group ID, Public Key ID, and ADE
type. The initial namespace populations are as follows.
MAC ID Namespace:
o 0x01 : HMAC_SHA1_128
o 0x02 : HMAC_SHA256_128
DH Group ID Namespace:
o 0x00 : NONE
o 0x01 : IANA DH Group 14
o 0x02 : IANA DH Group 15
o 0x03 : NIST ECC Group P-256
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Public Key ID Namespace:
o 0x00 : NONE
o 0x01 : RSAES-OAEP
o 0x02 : RSA-PKCS1-V1_5
o 0x03 : El-Gamal Over NIST ECC Group P-256
ADE Type Namespace:
o 0x01 : Vendor Specific
o 0x02 : Client Channel Binding Data
o 0x03 : Server Channel Binding Data
Allocation of values for these namespaces shall be reviewed by a
Designated Expert appointed by the IESG. The Designated Expert will
post a request to the EAP WG mailing list (or a successor designated
by the Designated Expert) for comment and review, including an
Internet-Draft. Before a period of 30 days has passed, the
Designated Expert will either approve or deny the registration
request and publish a notice of the decision to the EAP WG mailing
list or its successor, as well as informing IANA. A denial notice
must be justified by an explanation and, in the cases where it is
possible, concrete suggestions on how the request can be modified so
as to become acceptable.
6. Acknowledgments
The authors would like to thank Jonathan Katz for discussion with
respect to provable security, Bernard Aboba for technical guidance,
Jari Arkko for his expert review, and Florent Bersani for feedback
and suggestions. Finally, the authors would like to thank the
Defense Information Systems Agency for initially funding this work.
7. References
7.1. Normative References
[FIPS180] National Institute for Standards and Technology, "Secure
Hash Standard", Federal Information Processing Standard
180-2, August 2002.
[FIPS186] National Institute for Standards and Technology,
"Digital Signature Standard (DSS)", Federal Information
Processing Standard 186, May 1994.
[FIPS198] National Institute for Standards and Technology, "The
Keyed-Hash Message Authentication Code (HMAC)", Federal
Information Processing Standard 198, March 2002.
Clancy & Arbaugh Informational [Page 24]
RFC 4746 EAP-PAX November 2006
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
an On-line Database", RFC 3232, January 2002.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential
(MODP) Diffie-Hellman groups for Internet Key Exchange
(IKE)", RFC 3526, May 2003.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
H. Levkowetz, "Extensible Authentication Protocol
(EAP)", RFC 3748, June 2004.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 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)",
RFC 4334, February 2006.
[X.690] International Telecommunications Union, "Information
technology - ASN.1 encoding rules: Specification of
Basic Encoding Rules (BER), Canonical Encoding Rules
(CER) and Distinguished Encoding Rules (DER)", Data
Networks and Open System Communication Recommendation
X.690, July 2002.
7.2. Informative References
[IETF.KEY] Aboba, B., Simon, D., Arkko, J., Eronen, P., and H.
Levkowetz, "Extensible Authentication Protocol (EAP) Key
Management Framework", Work in Progress.
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RFC 4746 EAP-PAX November 2006
[IEEE.80211] 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
802.11-1997, 1997.
[RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC
2631, June 1999.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, April 2004.
[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs", RFC 4017, March 2005.
[RFC4252] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, January 2006.
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Appendix A. Key Generation from Passwords
If a 128-bit key is not available to bootstrap the authentication
process, then one must be generated from some sort of weak preshared
key. Note that the security of the hashing process is unimportant,
as long as it does not significantly decrease the password's entropy.
Resistance to dictionary attacks is provided by PAX_SEC.
Consequently, computing the SHA-1 of the password and truncating the
output to 128 bits is RECOMMENDED as a means of converting a weak
password to a key for provisioning.
When using other preshared credentials, such as a Kerberos Data
Encryption Standard (DES) key, or an MD4-hashed Microsoft Challenge
Handshake Authentication Protocol (MSCHAP) password, to provision
clients, these keys SHOULD still be put through SHA-1 before being
used. This serves to protect the credentials from possible
compromise, and also keeps things uniform. As an example, consider
provisioning using an existing Kerberos credential. The initial key
computation could be SHA1_128(string2key(password)). The KDC,
storing string2key(password), would also be able to compute this
initial key value.
Appendix B. Implementation Suggestions
In this section, two implementation strategies are discussed. The
first describes how best to implement and deploy EAP-PAX in an
enterprise network for IEEE 802.11i authentication. The second
describes how to use EAP-PAX for device authentication in a 3G-style
mobile phone network.
B.1. WiFi Enterprise Network
For the purposes of this section, a wireless enterprise network is
defined to have the following characteristics:
o Users wish to obtain network access through IEEE 802.11 access
points.
o Users can possibly have multiple devices (laptops, PDAs, etc.)
they wish to authenticate.
o A preexisting authentication framework already exists, for
example, a Microsoft Active Directory domain or a Kerberos realm.
Two of the biggest challenges in an enterprise WiFi network is key
provisioning and support for multiple devices. Consequently, it is
recommended that the client's Network Access Identifier (NAI) have
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the format username/KID@realm, where KID is a key ID that can be used
to distinguish between different devices.
The client's supplicant can use a variety of sources to automatically
generate the KID. Two of the better choices would likely be the
computer's NETBIOS name, or local Ethernet adapter's MAC address.
The wireless adapter's address may be a suboptimal choice, as the
user may only have one PCCARD adapter for multiple systems.
With an authentication system already in place, there is a natural
choice for the provisioned key. Clients can authenticate using their
preexisting password. When the server is presented with a new KID,
it can create a new key record on the server and use the user's
current password as the provisioned key. For example, for Active
Directory, the supplicant could use Microsoft's NtPasswordHash
function to generate a key verifiable by the server. It is suggested
that this key then be fed through SHA1_128 before being used in a
non-Microsoft authentication protocol.
After a key update, the server should keep track of both the old and
new authentication keys. When two keys exist, the server should
attempt to use both to validate the MACs on transmitted packets.
Once a client successfully authenticates using the new key, the
server should discard the old key. This prevents desynchronization
attacks.
B.2. Mobile Phone Network
In a mobile phone system, we no longer need to worry about supporting
multiple keys per identity. Presumably, each mobile device has a
unique identity. However, if multiple devices per identity are
desired, a method similar to that presented in Section B.1 could be
used.
Provisioning could easily be accomplished by issuing customers a 6-
digit PIN they could type into their phone's keypad.
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Authors' Addresses
T. Charles Clancy
DoD Laboratory for Telecommunications Sciences
8080 Greenmeade Drive
College Park, MD 20740
USA
EMail: clancy@ltsnet.net
William A. Arbaugh
University of Maryland
Department of Computer Science
College Park, MD 20742
USA
EMail: waa@cs.umd.edu
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RFC 4746 EAP-PAX November 2006
Full Copyright Statement
Copyright (C) The IETF Trust (2006).
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contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
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Acknowledgement
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Clancy & Arbaugh Informational [Page 30]
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