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Network Working Group M. Vanderveen
Request for Comments: 4763 H. Soliman
Category: Informational Qualcomm Flarion Technologies
November 2006
Extensible Authentication Protocol Method for
Shared-secret Authentication and Key Establishment (EAP-SAKE)
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).
IESG Note
This RFC is not a candidate for any level of Internet Standard. The
IETF disclaims any knowledge of the fitness of this RFC for any
purpose and in particular notes that the decision to publish is not
based on IETF review for such things as security, congestion control,
or inappropriate interaction with deployed protocols. The RFC Editor
has chosen to publish this document at its discretion. Readers of
this document should exercise caution in evaluating its value for
implementation and deployment. See RFC 3932 for more information.
Abstract
This document specifies an Extensible Authentication Protocol (EAP)
mechanism for Shared-secret Authentication and Key Establishment
(SAKE). This RFC is published as documentation for the IANA
assignment of an EAP Type for a vendor's EAP method per RFC 3748.
The specification has passed Designated Expert review for this IANA
assignment.
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RFC 4763 EAP-SAKE November 2006
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
3. Protocol Description ............................................4
3.1. Overview and Motivation of EAP-SAKE ........................4
3.2. Protocol Operation .........................................5
3.2.1. Successful Exchange .................................5
3.2.2. Authentication Failure ..............................7
3.2.3. Identity Management ................................11
3.2.4. Obtaining Peer Identity ............................11
3.2.5. Key Hierarchy ......................................13
3.2.6. Key Derivation .....................................15
3.2.7. Ciphersuite Negotiation ............................17
3.2.8. Message Integrity and Encryption ...................17
3.2.9. Fragmentation ......................................21
3.2.10. Error Cases .......................................21
3.3. Message Formats ...........................................22
3.3.1. Message Format Summary .............................22
3.3.2. Attribute Format ...................................23
3.3.3. Use of AT_ENCR_DATA Attribute ......................25
3.3.4. EAP.Request/SAKE/Challenge Format ..................26
3.3.5. EAP.Response/SAKE/Challenge Format .................28
3.3.6. EAP.Request/SAKE/Confirm Format ....................30
3.3.7. EAP.Response/SAKE/Confirm Format ...................32
3.3.8. EAP.Response/SAKE/Auth-Reject Format ...............33
3.3.9. EAP.Request/SAKE/Identity Format ...................34
3.3.10. EAP.Response/SAKE/Identity Format .................36
3.3.11. Other EAP Messages Formats ........................37
4. IANA Considerations ............................................37
5. Security Considerations ........................................38
5.1. Denial-of-Service Attacks .................................38
5.2. Root Secret Considerations ................................38
5.3. Mutual Authentication .....................................39
5.4. Integrity Protection ......................................39
5.5. Replay Protection .........................................39
5.6. Confidentiality ...........................................40
5.7. Key Derivation, Strength ..................................40
5.8. Dictionary Attacks ........................................41
5.9. Man-in-the-Middle Attacks .................................41
5.10. Result Indication Protection .............................41
5.11. Cryptographic Separation of Keys .........................41
5.12. Session Independence .....................................41
5.13. Identity Protection ......................................42
5.14. Channel Binding ..........................................42
5.15. Ciphersuite Negotiation ..................................42
5.16. Random Number Generation .................................43
6. Security Claims ................................................43
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7. Acknowledgements ...............................................44
8. References .....................................................44
8.1. Normative References ......................................44
8.2. Informative References ....................................45
1. Introduction
The Extensible Authentication Protocol (EAP), described in [EAP],
provides a standard mechanism for support of multiple authentication
methods. EAP is also used within IEEE 802 networks through the IEEE
802.11i [IEEE802.11i] framework.
EAP supports several authentication schemes, including smart cards,
Kerberos, Public Key, One Time Passwords, TLS, and others. This
document defines an authentication scheme, called the EAP-SAKE.
EAP-SAKE supports mutual authentication and session key derivation,
based on a static pre-shared secret data. This RFC is published as
documentation for the IANA assignment of an EAP Type for a vendor's
EAP method per RFC 3748. The specification has passed Designated
Expert review for this IANA assignment.
2. Terminology
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. The key
words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document
are to be interpreted as described in BCP 14 [KEYWORDS].
In addition to the terms used in [EAP], this document frequently uses
the following terms and abbreviations:
MIC Message Integrity Check
SMS SAKE Master Secret
NAI Network Access Identifier
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3. Protocol Description
3.1. Overview and Motivation of EAP-SAKE
The EAP-SAKE authentication protocol is a native EAP authentication
method. That is, a stand-alone version of EAP-SAKE outside of EAP is
not defined. EAP-SAKE is designed to enable mutual authentication,
based on pre-shared keys and well-known public cryptographic
algorithms. This method is ideal for subscription-based public
access networks, e.g., cellular networks.
EAP-SAKE assumes a long-term or pre-shared secret known only to the
Peer and the Server. This pre-shared secret is called the Root
Secret. The Root Secret consists of a 16-byte part Root-Secret-A,
and 16-byte part Root-Secret-B. The Root-Secret-A secret is reserved
for use local to the EAP-SAKE method, i.e., to mutually authenticate
the EAP Peer and EAP Server. The Root-Secret-B secret is used to
derive other quantities such as the Master Session Key (MSK) and
Extended Master Session Key (EMSK). Root-Secret-B and Root-Secret-A
MUST be cryptographically separate.
When a Backend Authentication Server is used, the Server typically
communicates with the Authenticator using an AAA protocol. The AAA
communications are outside the scope of this document.
Some of the advantages of EAP-SAKE are as follows:
o It is based on well-established Bellare-Rogaway mutual
authentication protocol.
o It supports key derivation for local EAP method use and for export
to other third parties to use them independently of EAP.
o It employs only two request/response exchanges.
o It relies on the (corrected) IEEE 802.11i function for key
derivation and message integrity. This way a device implementing a
lower-layer secure association protocol compliant with IEEE 802.11i
standard will not need additional cryptographic code.
o Its encryption algorithm is securely negotiated (with no extra
messages), yet encryption use is optional.
o Keys used for authentication and those used for encryption are
cryptographically separate.
o User identity anonymity can be optionally supported.
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o No synchronization (e.g., of counters) needed between server and
peer.
o There is no one-time key pre-processing step.
3.2. Protocol Operation
EAP-SAKE uses four messages consisting of two EAP request/response
exchanges. The EAP.Success and EAP.Failure messages shown in the
figures are not part of the EAP-SAKE method. As a convention, method
attributes are named AT_XX, where XX is the name of the parameter the
attribute value is set to.
3.2.1. Successful Exchange
A successful EAP-SAKE authentication exchange is depicted in Figure
1. The following steps take place:
Peer Server
| |
| EAP.Request/SAKE/Challenge |
| (AT_RAND_S, AT_SERVERID) |
1 |<---------------------------------------------------------|
| |
| EAP.Response/SAKE/Challenge |
| (AT_RAND_P, AT_PEERID, AT_SPI_P, AT_MIC_P) |
2 |--------------------------------------------------------->|
| |
| EAP.Request/SAKE/Confirm |
| (AT_SPI_S, AT_ENCR_DATA, AT_MIC_S)|
3 |<---------------------------------------------------------|
| |
| EAP.Response/SAKE/Confirm |
| (AT_MIC_P) |
4 |--------------------------------------------------------->|
| |
| |
| EAP-Success |
5 |<---------------------------------------------------------|
| |
Figure 1. EAP-SAKE Authentication Procedure (with ciphersuite
negotiation)
1. The EAP server sends the first message of the EAP-SAKE exchange.
This message is an EAP.Request message of type SAKE and subtype
Challenge. The EAP.Request/SAKE/Challenge message contains the
attributes: AT_RAND_S, whose value is a nonce freshly generated
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by the Server; and AT_SERVERID, which carries an identifier of
the Server (a fully qualified domain name, such as the realm of
the user NAI [NAI]). The AT_SERVERID attribute is OPTIONAL but
SHOULD be included in this message. The purpose of the
AT_SERVERID is to aid the Peer in selecting the correct security
association (i.e., Root-Secret, PEERID, and SERVERID) to use
during this EAP phase.
2. The Peer responds by sending an EAP.Response message of type SAKE
and subtype Challenge. The EAP.Response/SAKE/Challenge message
contains the attributes: AT_RAND_P, which carries a nonce freshly
generated by the Peer; AT_PEERID, which carries a Peer
identifier; AT_SPI_P, which carries a list of one or more
ciphersuites supported by the Peer; and AT_MIC_P, containing a
message integrity check. The AT_SPI_P and AT_PEERID attributes
are OPTIONAL. The AT_PEERID attribute SHOULD be included, in
order to cover the case of multi-homed hosts. A supported
ciphersuite is represented by a value local to the EAP-SAKE
method, or "Security Parameter Index", see section 3.2.8.3. The
Peer uses both nonces, along with the Root-Secret-A key, to
derive a SAKE Master Secret (SMS) and Temporary EAP Keys (TEKs),
which also include the TEK-Auth and TEK-Cipher keys. The MIC_P
value is computed based on both nonces RAND_S and RAND_P, and the
entire EAP packet, using the key TEK-Auth, see Section 3.2.6.
3. Upon receipt of the EAP.Response/SAKE/Challenge message, the
Server uses both nonces RAND_S and RAND_P, along with the Root-
Secret-A key, to compute the SMS and TEK in exactly the same way
the Peer did. Following that, the Server computes the Peer's
MIC_P in exactly the same way the Peer did. The Server then
compares the computed MIC_P with the MIC_P it received from the
Peer. If they match, the Server considers the Peer
authenticated. If encryption is needed, the Server selects the
strongest ciphersuite from the Peer's ciphersuite list SPI_P, or
a suitable ciphersuite if the Peer did not include the AT_SPI_P
attribute. The Server sends an EAP.Request message of type SAKE
and subtype Confirm, containing the attributes: AT_SPI_S,
carrying the ciphersuite chosen by the Server; AT_ENCR_DATA,
containing encrypted data; and AT_MIC_S, carrying a message
integrity check. The AT_SPI_S and AT_ENCR_DATA are OPTIONAL
attributes, included if confidentiality and/or user identity
anonymity is desired. Other OPTIONAL attributes that MAY be
included are AT_NEXT_TMPID, and AT_MSK_LIFE. As before, the
MIC_S value is computed using both nonces RAND_S and RAND_P, and
the entire EAP packet, using the key TEK-Auth.
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4. If the Peer receives the EAP.Request/SAKE/Confirm message
indicating successful authentication by the Server, the Peer
computes the MIC_S in the same manner as the Server did. The
Peer then compares the received MIC_S with the MIC_S it computed.
If they match, the Peer considers the Server authenticated, and
it sends an EAP.Response message of type SAKE and subtype
Confirm, with the attribute AT_MIC_P containing a message
integrity check, computed in the same manner as before.
5. After a successful EAP-SAKE exchange, the Server concludes the
EAP conversation by sending an EAP.Success message to the Peer.
All EAP-SAKE messages contain a Session ID, which is chosen by the
Server, sent in the first message, and replicated in subsequent
messages until an EAP.Success or EAP.Failure is sent. Upon receipt
of an EAP-SAKE message, both Peer and Server MUST check the format of
the message to ensure that it is well-formed and contains a valid
Session ID.
Note that the Session ID is introduced mainly for replay protection
purposes, as it helps uniquely identify a session between a Peer and
a Server. In most cases, there is a one-to-one relationship between
the Session ID and the Peer/Server pair.
The parameters used by the EAP-SAKE method are summarized in the
table below:
Name Length (bytes) Description
---------+---------------+-------------
RAND_P 16 Peer nonce
RAND_S 16 Server nonce
MIC_P 16 Peer MIC
MIC_S 16 Server MIC
SPI_P variable Peer ciphersuite preference(s)
SPI_S variable Server chosen ciphersuite
PEERID variable Peer identifier
SERVERID variable Server identifier (FQDN)
3.2.2. Authentication Failure
If the Authenticator receives an EAP.Failure message from the Server,
the Authenticator MUST terminate the connection with the Peer
immediately.
The Server considers the Peer to have failed authentication if either
of the two received MIC_P values does not match the computed MIC_P.
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The Server SHOULD deny authorization for a Peer that does not
advertise any of the ciphersuites that are considered acceptable
(e.g., by local system policy) and send an EAP.Failure message.
In case of authentication failure, the Server MUST send an
EAP.Failure message to the Peer as in Figure 2. The following steps
take place:
Peer Server
| |
| EAP.Request/SAKE/Challenge |
| (AT_RAND_S, AT_SERVERID) |
1 |<---------------------------------------------------------|
| |
| EAP.Response/SAKE/Challenge |
| (AT_RAND_P, AT_PEERID, AT_SPI_P, AT_MIC_P) |
2 |--------------------------------------------------------->|
| |
| +-------------------------------------------+
| | Server finds MIC_P invalid. |
| +-------------------------------------------+
| |
| EAP-Failure |
3 |<---------------------------------------------------------|
Figure 2. EAP-SAKE Authentication Procedure, Peer Fails
Authentication
1. As in step 1 of Figure 1.
2. As in step 2 of Figure 1.
3. Upon receipt of the EAP.Response/SAKE/Challenge message, the
Server uses both nonces RAND_S and RAND_P, along with the Root-
Secret-A key, to compute the SMS and TEK in exactly the same way
the Peer did. Following that, the Server computes the Peer's MIC
in exactly the same way the Peer did. The Server then compares
the computed MIC_P with the MIC_P it received from the Peer.
Since they do not match, the Server considers the Peer to have
failed authentication and sends an EAP.Failure message back to
the Peer.
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If the AT_SPI_S attribute does not contain one of the SPI values that
the Peer listed in the previous message, or if the Peer did not
include an AT_SPI_P attribute yet does not accept the ciphersuite the
Server has chosen, then the Peer SHOULD silently discard this
message. Alternatively, the Peer MAY send a SAKE/Auth-Reject message
back to the Server; in response to this message, the Server MUST send
an EAP.Failure message to the Peer.
The AT_SPI_S attribute MUST be included if encryption is to be used.
The Server knows whether or not encryption is to be used, as it is
usually the Server that needs to protect some data intended for the
Peer (e.g., temporary ID, group keys, etc). If the Peer receives an
AT_SPI_S attribute yet there is no AT_ENCR_DATA attribute, the Peer
SHOULD process the message and skip the AT_SPI_S attribute.
The Peer considers the Server to have failed authentication if the
received and the computed MIC_S values do not match. In this case,
the Peer MUST send to the Server an EAP.Response message of type SAKE
and subtype Auth-Reject, indicating an authentication failure. In
this case, the Server MUST send an EAP.Failure message to the Peer as
in Figure 3. The following steps take place:
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Peer Server
| |
| EAP.Request/SAKE/Challenge |
| (AT_RAND_S, AT_SERVERID) |
1 |<---------------------------------------------------------|
| |
| EAP.Response/SAKE/Challenge |
| (AT_RAND_P, AT_PEERID, AT_SPI_P, AT_MIC_P) |
2 |--------------------------------------------------------->|
| |
| EAP.Request/SAKE/Confirm |
| (AT_SPI_S, AT_ENCR_DATA, AT_MIC_S)|
3 |<---------------------------------------------------------|
| |
+-----------------------------------------------+ |
| Peer finds MIC_S invalid. | |
+-----------------------------------------------+ |
| |
| EAP.Response/SAKE/Auth-Reject |
4 |--------------------------------------------------------->|
| |
| EAP-Failure |
5 |<---------------------------------------------------------|
| |
Figure 3. EAP-SAKE Authentication Procedure, Server Fails
Authentication
1. As in step 1 of Figure 1.
2. As in step 2 of Figure 1.
3. As in step 3 of Figure 1.
4. The Peer receives a EAP.Request/SAKE/Confirm message indicating
successful authentication by the Server. The Peer computes the
MIC_S in the same manner as the Server did. The Peer then
compares the received MIC_S with the MIC_S it computed. Since
they do not match, the Peer considers the Server to have failed
authentication. In this case, the Peer responds with an
EAP.Response message of type SAKE and subtype Auth-Reject,
indicating authentication failure.
5. Upon receipt of a EAP.Response/SAKE/Auth-Reject message, the
Server sends an EAP.Failure message back to the Peer.
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3.2.3. Identity Management
It may be advisable to assign a temporary identifier (TempID) to a
Peer when user anonymity is desired. The TempID is delivered to the
Peer in the EAP.Request/SAKE/Confirm message. The TempID follows the
format of any NAI [NAI] and is generated by the EAP Server that
engages in the EAP-SAKE authentication task with the Peer. EAP
servers SHOULD be configurable with TempID spaces that can be
distinguished from the permanent identity space. For instance, a
specific realm could be assigned for TempIDs (e.g., tmp.example.biz).
A TempID is not assigned an explicit lifetime. The TempID is valid
until the Server requests the permanent identifier, or the Peer
triggers the start of a new EAP session by sending in its permanent
identifier. A Peer MUST be able to trigger an EAP session at any
time using its permanent identifier. A new TempID assigned during a
successful EAP session MUST replace the existing TempID for future
transactions between the Peer and the Server.
Note that the delivery of a TempID does not imply that the Server
considers the Peer authenticated; the Server still has to check the
MIC in the EAP.Response/SAKE/Confirm message. In case the EAP phase
ends with an EAP.Failure message, then the Server and the Peer MUST
consider the TempID that was just delivered as invalid. Hence, the
Peer MUST NOT use this TempID the next time it tries to authenticate
with the Server.
3.2.4. Obtaining Peer Identity
The types of identities that a Peer may possess are permanent and
temporary identities, referred to as PermID and TempID, respectively.
A PermID can be an NAI associated with the Root Secret, and is long-
lived. A TempID is an identifier generated through the EAP method
and that the Peer can use to identify itself during subsequent EAP
sessions with the Server. The purpose of the TempID is to allow for
user anonymity support. The use of TempIDs is optional in the EAP-
SAKE method.
The Server MAY request the Peer ID via the EAP.Request/SAKE/Identity
message, as shown in Figure 4. This case may happen if, for example,
the Server wishes to initiate an EAP-SAKE session for a Peer it does
not have a subscriber identifier for. The following steps take
place:
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Peer Server
| |
| +---------------------------------+
| | Server wishes to initiate |
| | an EAP-SAKE session |
| | |
| +---------------------------------+
| |
| EAP.Request/SAKE/Identity |
| (AT_ANY_ID_REQ, AT_SERVERID) |
1 |<---------------------------------------------------------|
| |
| EAP.Response/SAKE/Identity |
| (AT_PEERID) |
2 |--------------------------------------------------------->|
| |
+--------------------------------------------------------------+
| If identity found, normal EAP-SAKE authentication follows. |
+--------------------------------------------------------------+
Figure 4. Server Requests Peer Identity
1. The Server sends an EAP.Request message of type SAKE and subtype
Identity, with the attribute AT_ANY_ID_REQ, indicating a request
for any Peer identifier.
2. The Peer constructs an EAP.Response message of type SAKE and
subtype Identity, with the attribute AT_PEER_ID containing any
Peer identifier (PermID or TempID).
If the Server cannot find the Peer identity reported in the
EAP.Response/SAKE/Identity message, or if it does not recognize the
format of the Peer identifier, the Server MAY send an EAP.Failure
message to the Peer.
If the Server is unable to look up a Peer by its TempID, or if policy
dictates that the Peer should now use its permanent id, then the
Server may specifically ask for the permanent identifier, as in
Figure 5. The following steps occur:
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Peer Server
| |
| +---------------------------------+
1 | | Server obtains TempID but |
| | requires PermID |
| +---------------------------------+
| |
| EAP.Request/SAKE/Identity |
| (AT_PERM_ID_REQ, AT_SERVERID) |
2 |<---------------------------------------------------------|
| |
| EAP.Response/SAKE/Identity |
| (AT_PEERID) |
3 |--------------------------------------------------------->|
| |
| +---------------------------------+
| | Server finds and uses |
| | Peer PermID to start a |
| | EAP-SAKE authentication phase |
| +---------------------------------+
|
+---------------------------------------------------------------+
| Normal EAP-SAKE authentication follows. |
+---------------------------------------------------------------+
Figure 5. Server Is Given a TempID as Peer Identity; Server
Requires a PermID
1. The Peer (or the Authenticator on behalf of the Peer) sends an
EAP.Request message of type Identity and includes the TempID.
2. The Server requires a PermID instead, so it sends an EAP.Request
message of type SAKE and subtype Identity with attributes
AT_PERM_ID_REQ and AT_SERVERID.
3. The Peer sends an EAP.Response message of type SAKE and subtype
Identity containing the attribute AT_PEERID carrying the Peer
PermID.
3.2.5. Key Hierarchy
EAP-SAKE uses a three-level key hierarchy.
Level 1 contains the pre-shared secret called Root Secret. This is a
32-byte high-entropy string partitioned into the Root-Secret-A part
(16 bytes) and the Root-Secret-B part (16 bytes).
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Level 2 contains the key derivation key called the SAKE Master Secret
(SMS). SMS-A is derived from the Root-Secret-A key and the Peer and
Server nonces using the EAP-SAKE Key-Derivation Function (KDF), and
similarly for SMS-B. The SMS is known only to the Peer and Server
and is not made known to other parties.
Level 3 contains session keys, such as Transient EAP Keys (TEK),
Master Session Key (MSK), and Extended MSK (EMSK). TEKs are keys for
use local to the EAP method only. They are derived from the SMS-A
and the nonces using the EAP-SAKE KDF. They are partitioned into a
16-byte TEK-Auth, used to compute the MICs, and a 16-byte TEK-Cipher,
used to encrypt selected attributes. Since the SMS is fresh, so are
the derived TEKs.
+--------------------+ +--------------------+
| Root-Secret-A | | Root-Secret-B |
| (pre-shared secret)| | (pre-shared secret)|
+--------------------+ +--------------------+
| |
V V
+-------------------+ +--------------------+
| SAKE Master Secret|<---RAND_S------------->| SAKE Master Secret |
| (SMS-A) | | | (SMS-B) |
| |<-------]---RAND_P----->| |
+-------------------+ | | +--------------------+
| | | |
V | | V
+--------------------+ | | +--------------------+
| Transient EAP Keys |<------+-----+-------->| Session Keys: |
| TEK-Auth,TEK-Cipher|<------------+-------->| MSK, EMSK |
+--------------------+ +--------------------+
Figure 6. Key Hierarchy for the EAP-SAKE Method
On another branch of level 3 of the key hierarchy are the MSK and
EMSK, each mandated to be 64 bytes long. They are derived from the
SMS-B and the nonces using the EAP-SAKE KDF. Again, since the SMS is
fresh, so are the derived MSK/EMSK. The MSK is meant to be exported
and relayed to other parties. The EMSK is reserved for future use,
such as derivation of application-specific keys, and is not shared
with other parties.
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The EAP-SAKE key material is summarized in the table below.
===================================================================
Key Size Scope Lifetime Use
(bytes)
===================================================================
Root-Secret-A 16 Peer, Server Device Derive TEK
--------------------------------------------------------------------
Root-Secret-B 16 Peer, Server Device Derive MSK, EMSK
--------------------------------------------------------------------
TEK-Auth 16 Peer, Server MSK Life Compute MICs
--------------------------------------------------------------------
TEK-Cipher 16 Peer, Server MSK Life Encrypt attribute
--------------------------------------------------------------------
MSK 64 Peer, Server, MSK Life Derive keys for
Authenticator lower-layer use
--------------------------------------------------------------------
EMSK 64 Peer, Server MSK Life Reserved
--------------------------------------------------------------------
A key name format is not provided in this version.
Since this version of EAP-SAKE does not support fast re-
authentication, the lifetime of the TEKs is to extend only until the
next EAP mutual authentication. The MSK lifetime dictates when the
next mutual authentication is to take place. The Server MAY convey
the MSK lifetime to the Peer in the AT_MSK_LIFE attribute. Note that
EAP-SAKE does not support key lifetime negotiation.
The EAP-SAKE Method-Id is the contents of the RAND_S field from the
AT_RAND_S attribute, followed by the contents of the RAND_P field in
the AT_RAND_P attribute.
3.2.6. Key Derivation
3.2.6.1. Key-Derivation Function
For the rest of this document, KDF-X denotes the EAP-SAKE Key-
Derivation Function whose output is X bytes. This function is the
pseudo-random function of [IEEE802.11i].
The function takes three strings of bytes of arbitrary lengths: a
Key, a Label, and a Msg, and outputs a string Out of length X bytes
as follows:
Out = KDF-X (Key, Label, Msg)
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The KDF is a keyed hash function with key "Key" and operating on
input "Label | Msg". The convention followed herein is that
concatenation is denoted by |, FLOOR denotes the floor function, and
[x...y] denotes bytes x through y.
The label is an ASCII character string. It is included in the exact
form it is given without a length byte or trailing null character.
Below, "Length" denotes a single unsigned octet with values between 0
and 255 (bytes). The following functions are defined (see [HMAC],
[SHA1]):
H-SHA1(Key, Label, Msg, Length) := HMAC-SHA1(Key, Label|Y|Msg|Length)
where Y := 0x00
KDF-16(Key, Label, Msg) := KDF(Key, Label, Msg, 16)
KDF-32(Key, Label, Msg) := KDF(Key, Label, Msg, 32)
KDF-128(Key, Label, Msg) := KDF(Key, Label, Msg, 128)
KDF(Key, Label, Msg, Length)
R := [] ;; null string
for i := 0 to FLOOR(Length/20)-1 do
R := R | H-SHA1(Key, Label, Msg, i)
return R[0...(Length-1)]
3.2.6.2. Transient EAP Keys Derivation
The Peer and Server derive the SMS and then the TEK as follows:
SMS-A = KDF-16 (Root-Secret-A, "SAKE Master Secret A", RAND_P|RAND_S)
TEK = KDF-32 (SMS-A, "Transient EAP Key", RAND_S | RAND_P)
TEK-Auth = TEK[0...15]
TEK-Cipher = TEK[16...31]
3.2.6.3. Extended/Master Session Key Derivation
The Peer and the Server derive the MSK/EMSK, as follows:
SMS-B = KDF-16 (Root-Secret-B, "SAKE Master Secret B", RAND_P|RAND_S)
Session-Key-Block=KDF-128(SMS-B, "Master Session Key", RAND_S|RAND_P)
MSK = Session-Key-Block[0...63]
EMSK = Session-Key-Block[64...127]
The derivation above affords the required cryptographic separation
between the MSK and EMSK. That is, knowledge of the EMSK does not
immediately lead to knowledge of the MSK, nor does knowledge of the
MSK immediately lead to knowledge of the EMSK.
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3.2.7. Ciphersuite Negotiation
A ciphersuite is identified by a numeric value called the Security
Parameter Index (SPI). The SPI is used here in the EAP-SAKE method
in order to negotiate a ciphersuite between the Peer and the Server
for EAP data protection only. Obviously, this ciphersuite can only
be used late in the EAP conversation, after it has been agreed upon
by both the Peer and the Server.
The EAP method may or may not need to use this ciphersuite, since
attribute encryption is optional. For example, if the temporary
identifier needs to be delivered to the Peer and needs to be
encrypted, then the negotiated ciphersuite will be used for this
task. If there are no attributes that need encryption as they are
passed to the Peer, then this ciphersuite is never used.
As with most other methods employing ciphersuite negotiation, the
following exchange is employed: the Peer sends an ordered list of one
or more supported ciphersuites, starting with the most preferred one,
in a field SPI_P. The Server then sends back the one ciphersuite
chosen in a field SPI_S. The Server SHOULD choose the strongest
ciphersuite supported by both of them.
Ciphersuite negotiation for data protection is achieved via SAKE
Signaling as follows. In the EAP.Response/SAKE/Challenge, the Peer
sends a list of supported ciphersuites, SPI_P, and protects that with
a MIC. In the EAP.Request/SAKE/Confirm, the Server sends one
selected ciphersuite, SPI_S, and signs that with a MIC. Finally, the
Peer confirms the selected ciphersuite and readiness to use it in a
signed EAP.Response/SAKE/Confirm message. The negotiation is secure
because of the Message Integrity Checks that cover the entire EAP
message.
3.2.8. Message Integrity and Encryption
This section specifies the EAP/SAKE attributes used for message
integrity and attribute encryption: AT_MIC_S, AT_MIC_P, AT_IV,
AT_ENCR_DATA, and AT_PADDING. Only the AT_MIC_S and AT_MIC_P are
mandatory to use during the EAP-SAKE exchange.
Because the TEKs necessary for protection of the attributes and for
message authentication are derived using the nonces RAND_S and
RAND_P, the AT_MIC_S and AT_MIC_P attributes can only be used in the
EAP.Response/SAKE/Challenge message and any messages sent thereafter.
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3.2.8.1. The AT_MIC_S and AT_MIC_P Attributes
The AT_MIC_S and AT_MIC_P attributes are used for EAP-SAKE message
integrity. The AT_MIC_S attribute MUST be included in all EAP-SAKE
packets that the Server sends whenever key material (TEK) has been
derived. That is, the AT_MIC_S attribute MUST be included in the
EAP.Request/SAKE/Confirm message. The AT_MIC_S MUST NOT be included
in EAP.Request/SAKE/Challenge messages, or EAP.Request/SAKE/Identity
messages.
The AT_MIC_P attribute MUST be included in all EAP-SAKE packets the
Peer sends whenever key material (TEK) has been derived. That is,
the AT_MIC_P attribute MUST be included in the
EAP.Response/SAKE/Challenge and EAP.Response/SAKE/Confirm messages.
The AT_MIC_P attribute MUST NOT be included in the
EAP.Response/SAKE/Auth-Reject message since the Peer has not
confirmed that it has the same TEK as the Server.
Messages that do not meet the conditions specified above MUST be
silently discarded upon reception.
The value field of the AT_MIC_S and AT_MIC_P attributes contain a
message integrity check (MIC). The MIC is calculated over the entire
EAP packet, prepended with the Server nonce and identifier and the
Peer nonce and identifier. The value field of the MIC attribute is
set to zero when calculating the MIC. Including the Server and Peer
nonces and identifiers aids in detecting replay and man-in-the-middle
attacks.
The Peer computes its MIC as follows:
MIC_P = KDF-16 (TEK-Auth, "Peer MIC", RAND_S | RAND_P |
PEERID | 0x00 | SERVERID | 0x00 | <EAP-packet>),
while the Server computes its MIC as
MIC_S = KDF-16 (TEK-Auth, "Server MIC", RAND_P |RAND_S |
SERVERID | 0x00 | PEERID | 0x00 | <EAP-packet>).
Here, <EAP-packet> denotes the entire EAP packet, used as a string of
bytes, the MIC value field set to zero. 0x00 denotes a single octet
value used to delimit SERVERID and PEERID. The PEERID and SERVERID,
respectively, are the ones carried in the corresponding attributes in
the SAKE/Challenge messages.
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In case the SAKE/Challenge exchange was preceded by an
EAP.Request/SAKE/Identity message containing the AT_SERVERID
Attribute, then the SERVERID value in the MIC_P and MIC_S computation
MUST be set to the value of this attribute.
If the AT_SERVERID was not included in either the SAKE/Challenge
message or the SAKE/Identity message, then the value of the SERVERID
used in the computation of MIC_P and MIC_S MUST be empty. If the
AT_PEERID was not included in the SAKE/Challenge message, and there
was no EAP.Response/SAKE/Identity message preceding the
SAKE/Challenge messages, then the value of the PEERID used in the
computation of MIC_P and MIC_S MUST be empty.
The Server and Peer identity are included in the computation of the
signed responses so that the Peer and Server can verify each other's
identities, and the possession of a common secret, the TEK-Auth.
However, since the AT_SERVERID is not explicitly signed with a MIC by
the Server, EAP-SAKE does not claim channel binding.
3.2.8.2. The AT_IV, AT_ENCR_DATA, and AT_PADDING Attributes
The AT_IV and AT_ENCR_DATA attributes can be used to transmit
encrypted information between the Server and the Peer. The value
field of AT_IV contains an initialization vector (IV) if one is
required by the encryption algorithm used. It is not mandatory that
the AT_IV attribute be included whenever the AT_ENCR_DATA attribute
is.
However, the AT_IV attribute MUST NOT be included unless the
AT_ENCR_DATA is included. Messages that do not meet this condition
MUST be silently discarded.
Attributes can be encrypted only after a ciphersuite has been agreed
on, i.e., in any message starting with the Server's
EAP.Request/SAKE/Confirm message. The attributes MUST be encrypted
using algorithms corresponding to the SPI value specified by the
AT_SPI_S attribute. The attributes MUST be encrypted using the TEK-
Cipher key, whose derivation is specified in Section 3.2.6.2.
If an IV is required by the encryption algorithm, then the sender of
the AT_IV attribute MUST NOT reuse the IV value from previous EAP-
SAKE packets. The sender MUST choose a new value for each AT_IV
attribute. The sender SHOULD use a good random number generator to
generate the initialization vector (see [RFC4086] for guidelines).
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The value field of the AT_ENCR_DATA attribute consists of bytes
encrypted using the ciphersuite specified in the AT_SPI_S attribute.
The encryption key is the TEK-Cipher, and the plaintext consists of
one or more concatenated EAP-SAKE attributes.
The default encryption algorithm, as described in Section 3.2.8.3,
requires the length of the plaintext to be a multiple of 16 bytes.
The sender MAY need to include the AT_PADDING attribute as the last
attribute within the value field of the AT_ENCR_DATA attribute. The
length of the padding is chosen so that the length of the value field
of the AT_ENCR_DATA attribute becomes a multiple of 16 bytes. The
AT_PADDING attribute SHOULD NOT be included if the total length of
other attributes present within the AT_ENCR_DATA attribute is a
multiple of 16 bytes. The length of the AT_PADDING attribute
includes the Attribute Type and Attribute Length fields. The actual
pad bytes in the value field are set to zero (0x00) on sending. The
recipient of the message MUST verify that the pad bytes are zero
after decryption and MUST silently discard the message otherwise.
The MIC computed on the entire EAP message can be used to obviate the
need for special integrity protection or message authentication of
the encrypted attributes.
An example of the AT_ENCR_DATA attribute is shown in Section 3.3.3.
3.2.8.3. Security Parameter Index (SPI) Considerations
An SPI value is a variable-length string identifying at least an
encryption algorithm and possibly a "security association". EAP-SAKE
does not mandate the format of the SPI; it only mandates that the
default encryption algorithm be supported if encryption is supported.
That is, if an implementation compliant with this document supports
encryption, then it MUST support the AES-CBC cipher.
The default encryption algorithm AES-CBC involves the AES block
cipher [AES] in the Cipher-Block-Chaining (CBC) mode of operation
[CBC].
The Peer in the EAP-SAKE method can send up to four SPI values in its
SPI_P field. Because the length of the AT_SPI_P and AT_SPI_S
attributes must each be a multiple of 2 bytes, the sender pads the
value field with zero bytes when necessary (the AT_PADDING attribute
is not used here). For example, the value field of the AT_SPI_S
contains one byte with the chosen SPI, followed by one byte of zeros.
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3.2.9. Fragmentation
The EAP-SAKE method does not require fragmentation, as the messages
do not get excessively long. That is, EAP packets are well within
the limit of the maximum transmission unit of other layers (link,
network). The only variable fields are those carrying NAIs, which
are limited by their length field to 256 bytes.
3.2.10. Error Cases
Malformed messages: As a general rule, if a Peer or Server receives
an EAP-SAKE packet that contains an error, the implementation SHOULD
silently discard this packet, not change state, and not send any EAP
messages to the other party. Examples of such errors include a
missing mandatory attribute, an attribute that is not allowed in this
type of message, and unrecognized subtypes or attributes.
Non-matching Session Id: If a Peer or Server receives an EAP-SAKE
packet with a Session Id field not matching the Session Id from the
previous packet in this session, that entity SHOULD silently discard
this packet (not applicable for the first message of an EAP-SAKE
session).
Peer Authorization Failure: It may be possible that a Peer is not
authorized for services, such as when the terminal device is reported
stolen. In that case, the Server SHOULD send an EAP.Failure message.
Unexpected EAP.Success: A Server MUST NOT send an EAP-Success message
before the SAKE/Challenge and SAKE/Confirm rounds. The Peer MUST
silently discard any EAP.Success packets before the Peer has
successfully authenticated the Server via the
EAP.Response/SAKE/Confirm packet.
The Peer and Server SHOULD log all error cases.
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3.3. Message Formats
3.3.1. Message Format Summary
A summary of the EAP packet format [EAP] is shown below for
convenience. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Type=EAP-SAKE | Version=2 | Session ID | Subtype |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code
1 - Request
2 - Response
Identifier
The identifier field is one octet and aids in matching responses
with requests.
Length
The Length field is two octets and indicates the number of octets
in an EAP message including the Code, Identifier, Length, Type,
and Data fields.
Type
To be assigned.
Version
The EAP-SAKE method as described herein is version 2.
Session ID
The Session ID is a 1-byte random number that MUST be freshly
generated by the Server that must match all EAP messages in a
particular EAP conversation.
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Subtype
EAP-SAKE subtype: SAKE/Challenge, SAKE/Confirm, SAKE/Auth-Reject,
and SAKE/Identity. All messages of type "EAP-SAKE" that are not
of these subtypes MUST silently discarded.
Message Name Subtype Value (decimal)
=============================================
SAKE/Challenge 1
SAKE/Confirm 2
SAKE/Auth-Reject 3
SAKE/Identity 4
3.3.2. Attribute Format
The EAP-SAKE attributes that are part of the EAP-SAKE packet follow
the Type-Length-Value format with 1-byte Type, 1-byte Length, and
variable-length Value (up to 255 bytes). The Length field is in
octets and includes the length of the Type and Length fields. The
EAP-SAKE attribute format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
1-byte unsigned integer; see Table below.
Length
The total number of octets in the attribute, including Type and
Length.
Value
Attribute-specific.
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The following attribute types are allocated.
-----------------------------------------------------------------
Attr. Name Length
(bytes) Skippable Description
-----------------------------------------------------------------
AT_RAND_S 18 No Server Nonce RAND_S
AT_RAND_P 18 No Peer Nonce RAND_P
AT_MIC_S 10 No Server MIC
AT_MIC_P 10 No Peer MIC
AT_SERVERID variable No Server FQDN
AT_PEERID variable No Peer NAI (tmp, perm)
AT_SPI_S variable No Server chosen SPI SPI_S
AT_SPI_P variable No Peer SPI list SPI_P
AT_ANY_ID_REQ 4 No Requires any Peer Id (tmp, perm)
AT_PERM_ID_REQ 4 No Requires Peer's permanent Id/NAI
AT_ENCR_DATA Variable Yes Contains encrypted attributes
AT_IV Variable Yes IV for encrypted attributes
AT_PADDING 2 to 18 Yes Padding for encrypted attributes
AT_NEXT_TMPID variable Yes TempID for next EAP-SAKE phase
AT_MSK_LIFE 6 Yes MSK Lifetime in seconds
-----------------------------------------------------------------
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3.3.3. Use of AT_ENCR_DATA Attribute
An example of the AT_ENCR_DATA attribute, as used in the
EAP.Request/SAKE/Confirm message, is shown below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_IV | Length = 18 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| Initialization Vector |
| |
| |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |AT_ENCR_DATA | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+}e
| AT_NEXT_TMPID | Length | |}n
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |}c
| |}r
. Peer TempID |}y
. |}p
. |}t
| |}e
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+}d
| AT_MIC_S | Length = 10 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| MIC_S |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |AT_PADDING | Length=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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3.3.4. EAP.Request/SAKE/Challenge Format
The format of the EAP.Request/SAKE/Challenge packet is shown below.
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=EAP-SAKE | Version=2 | Session ID | Subtype=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_RAND_S | Length = 18 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| RAND_S |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| | AT_SERVERID | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: :
| Server ID |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The semantics of the fields is described below:
Code
1 for Request
Identifier
A random number. See [EAP].
Length
The length of the entire EAP packet in octets.
Type
EAP-SAKE
Version
2
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Session ID
A random number chosen by the server to identify this EAP-Session.
Subtype
1 for SAKE/Challenge
AT_RAND_S
The value field of this attribute contains the Server nonce RAND_S
parameter. The RAND_S attribute MUST be present in
EAP.Request/SAKE/Challenge.
AT_SERVERID
The value field of this attribute contains the Server identifier
(e.g., a non-null terminated string). The AT_SERVERID attribute
SHOULD be present in EAP.Request/SAKE Challenge.
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3.3.5. EAP.Response/SAKE/Challenge Format
The format of the EAP.Response/SAKE/Challenge packet is shown below.
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=EAP-SAKE | Version=2 | Session ID | Subtype=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_RAND_P | Length = 18 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| RAND_P |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| | AT_PEERID | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Peer NAI :
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| | AT_SPI_P | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPIP | AT_MIC_P | Length = 18 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MIC_P |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The semantics of the fields is described below:
Code
2 for Response
Identifier
A number that MUST match the Identifier field from the
corresponding Request.
Length
The length of the entire EAP packet in octets.
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Type
EAP-SAKE
Version
2
Session ID
A number matching all other EAP messages in this EAP session.
Subtype
1 for SAKE/Challenge
AT_RAND_P
The value field of this attribute contains the Peer nonce RAND_P
parameter. The AT_RAND_P attribute MUST be present in the
EAP.Response/SAKE/Challenge.
AT_PEERID
The value field of this attribute contains the NAI of the Peer.
The Peer identity follows the same Network Access Identifier
format that is used in EAP.Response/Identity, i.e., including the
NAI realm portion. The identity is the permanent identity, or a
temporary identity. The identity does not include any terminating
null characters. The AT_PEERID attribute is optional in the
EAP.Response/SAKE/Challenge.
AT_SPI_P
The value field of this attribute contains the Peer's ciphersuite
list SPI_P parameter. The AT_SPI_P attribute is optional in the
EAP.Response/SAKE/Challenge.
AT_MIC_P
The value field of this attribute contains the Peer MIC_P
parameter. The AT_MIC_P attribute MUST be present in the
EAP.Response/SAKE/Challenge.
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3.3.6. EAP.Request/SAKE/Confirm Format
The format of the EAP.Request/SAKE/Confirm packet is shown below.
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=EAP-SAKE | Version=2 | Session ID | Subtype=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_SPI_S | Length | SPI_S |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_IV | Length | Initialization Vector ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ :
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | AT_ENCR_DATA | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encrypted Data... |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_MSK_LIFE | Length=6 | MSK Lifetime... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | AT_MIC_S | Length=18 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MIC_S |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The semantics of the fields is described below:
Code
1 for Request
Identifier
A random number. See [EAP].
Length
The length of the entire EAP packet in octets.
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Type
EAP-SAKE
Version
2
Session ID
A number matching all other EAP messages in this EAP session.
Subtype
2 for SAKE Confirm
AT_SPI_S
The value field of this attribute contains the Server chosen
ciphersuite SPI_S parameter. The AT_SPI_S attribute is optional
in the EAP.Request/SAKE/Confirm.
AT_IV
This attribute is optional to use in this message. The value
field of this attribute contains the Initialization Vector that is
used with the encrypted data following.
AT_ENCR_DATA
This attribute is optional to use in this message. The encrypted
data, if present, may contain an attribute AT_NEXT_TMPID,
containing the NAI the Peer should use in the next EAP
authentication.
AT_MSK_LIFE
This attribute is optional to use in this message. The value
field of this attribute contains the MSK Lifetime in seconds.
AT_MIC_S
The value field of this attribute contains the Server MIC_S
parameter. The AT_MIC_S attribute MUST be present in the
EAP.Request/SAKE/Confirm.
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3.3.7. EAP.Response/SAKE/Confirm Format
The format of the EAP.Response/SAKE/Confirm packet is shown below.
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=EAP-SAKE | Version=2 | Session ID | Subtype=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_MIC_P | Length = 18 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| MIC_P |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | AT_PADDING | Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The semantics of the fields is described below:
Code
2 for Response
Identifier
A number that MUST match the Identifier field from the
corresponding Request.
Length
The length of the entire EAP packet in octets.
Type
EAP-SAKE
Version
2
Session ID
A number matching all other EAP messages in this EAP session.
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Subtype
2 for SAKE Confirm
AT_MIC_P
The value field of this attribute contains the Peer's MIC_P
parameter. The AT_MIC_P attribute MUST be present in the
EAP.Response/SAKE/Confirm.
AT_PADDING
The value field is set to zero. Added to achieve 32-bit alignment
of the EAP-SAKE packet.
3.3.8. EAP.Response/SAKE/Auth-Reject Format
The format of the EAP.Response/SAKE/Auth-Reject packet is shown
below.
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=EAP-SAKE | Version=2 | Session ID | Subtype=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The semantics of the fields is described below:
Code
2 for Response
Identifier
A number that MUST match the Identifier field from the
corresponding Request.
Length
The length of the entire EAP packet in octets.
Type
EAP-SAKE
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Version
2
Session ID
A number matching all other EAP messages in this EAP session.
Subtype
3 for SAKE/Auth-Reject
3.3.9. EAP.Request/SAKE/Identity Format
The format of the EAP.Request/SAKE/Identity is shown below.
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=EAP-SAKE | Version=2 | Session ID | Subtype=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AT_PERM_ID_REQ | Length = 4 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AT_ANY_ID_REQ | Length = 4 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AT_SERVERID | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ :
| Server ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The semantics of the fields is described below:
Code
1 for Request
Identifier
A random number. See [EAP].
Length
The length of the entire EAP packet in octets.
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Type
EAP-SAKE
Version
2
Session ID
A number matching all other EAP messages in this EAP session.
Subtype
4 for SAKE/Identity
AT_PERM_ID_REQ
The AT_PERM_ID_REQ attribute is optional, to be included in cases
where the Server requires the Peer to give its permanent
identifier (i.e., PermID). The AT_PERM_ID_REQ MUST NOT be
included if the AT_ANY_ID_REQ attribute is included. The value
field only contains two reserved bytes, which are set to zero on
sending and ignored on reception.
AT_ANY_ID_REQ
The AT_ANY_ID_REQ attribute is optional, to be included in cases
where the Server requires the Peer to send any identifier (e.g.,
PermID, TempID). The AT_ANY_ID_REQ MUST NOT be included if
AT_PERM_ID_REQ is included. The value field only contains two
reserved bytes, which are set to zero on sending and ignored on
reception. One of the AT_PERM_ID_REQ and AT_ANY_ID_REQ MUST be
included.
AT_SERVERID
The value field of this attribute contains the identifier/realm of
the Server. The AT_SERVERID attribute is optional but RECOMMENDED
to include in the EAP.Request/SAKE/Identity.
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3.3.10. EAP.Response/SAKE/Identity Format
The format of the EAP.Response/SAKE/Identity is shown below:
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=EAP-SAKE | Version=2 | Session ID | Subtype=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT_PEERID | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ :
| Peer NAI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The semantics of the fields is described below:
Code
2 for Response
Identifier
A number that MUST match the Identifier field from the
corresponding Request.
Length
The length of the entire EAP packet.
Type
EAP-SAKE
Version
2
Session ID
A number matching all other EAP messages in this EAP session.
Subtype
4 for SAKE/Identity
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AT_PEERID
The value field of this attribute contains the NAI of the Peer.
The AT_PEERID attribute MUST be present in
EAP.Response/SAKE/Identity.
3.3.11. Other EAP Messages Formats
The format of the EAP.Request/Identity and EAP.Response/Identity
packets is described in [EAP]. The user ID (e.g., NAI) SHOULD be
present in this packet.
The format of the EAP-Success and EAP-Failure packet is also shown in
[EAP].
4. IANA Considerations
IANA allocated a new EAP Type for EAP-SAKE.
EAP-SAKE messages include an 8-bit Subtype field. The Subtype is a
new numbering space for which IANA administration is required. The
following subtypes are specified in this memo:
SAKE/Challenge.................1
SAKE/Confirm...................2
SAKE/Auth-Reject...............3
SAKE/Identity..................4
The Subtype-specific data is composed of attributes, which have an
8-bit type number. Attributes numbered within the range 0 through
127 are called non-skippable attributes, and attributes within the
range of 128 through 255 are called skippable attributes. The EAP-
SAKE attribute type number is a new numbering space for which IANA
administration is required. The following attribute types are
specified:
AT_RAND_S.......................................1
AT_RAND_P.......................................2
AT_MIC_S........................................3
AT_MIC_P........................................4
AT_SERVERID.....................................5
AT_PEERID.......................................6
AT_SPI_S........................................7
AT_SPI_P........................................8
AT_ANY_ID_REQ...................................9
AT_PERM_ID_REQ.................................10
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AT_ENCR_DATA..................................128
AT_IV.........................................129
AT_PADDING....................................130
AT_NEXT_TMPID.................................131
AT_MSK_LIFE...................................132
All requests for value assignment from the two number spaces
described in this memo require proper documentation, according to the
"Specification Required" policy described in [IANA].
All assignments of values from the two number spaces described in
this memo require IETF consensus.
5. Security Considerations
The EAP specification [EAP] describes the security vulnerabilities of
EAP, which does not include its method-specific security mechanisms.
This section discusses the claimed security properties of the EAP-
SAKE method, along with vulnerabilities and security recommendations.
5.1. Denial-of-Service Attacks
Since EAP-SAKE is not a tunneling method, the
EAP.Response/SAKE/Auth-Reject, EAP.Success, and EAP.Failure packets
are not integrity or replay protected. This makes it possible for an
attacker to spoof such messages. Note that EAP.Response/SAKE/Auth-
Reject cannot be protected with a MIC since an authentication failure
indicates that the Server and Peer do not agree on a common key.
Most importantly, an attacker cannot cause a Peer to accept an
EAP.Success packet as indication that the Server considers the mutual
authentication to have been achieved. This is because a Peer does
not accept EAP.Success packets before it has authenticated the Server
or after it has considered the Server to have failed authentication.
5.2. Root Secret Considerations
If the Root Secret is known to any party other than the Server and
Peer, then the mutual authentication and key establishment using
EAP-SAKE is compromised.
EAP-SAKE does not address how the Root Secret is generated or
distributed to the Server and Peer. It is RECOMMENDED that the
entropy of the Root Secret be maximized. The Root Secret SHOULD be
machine-generated.
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If the Root Secret is derived from a low-entropy, guessable quantity
such as a human-selected password, then the EAP-SAKE key derivation
is subject to on-line and off-line dictionary attacks. To help
identify whether such a password has been compromised,
implementations SHOULD keep a log of the number of EAP-SAKE messages
received with invalid MIC fields. In these cases, a procedure for
updating the Root Secret securely SHOULD be in place.
5.3. Mutual Authentication
Mutual authentication is accomplished via the SAKE/Challenge and
SAKE/Confirm messages. The EAP.Request/SAKE/Challenge contains the
Server nonce RAND_S; the EAP.Response/SAKE/Challenge contains the
Peer nonce RAND_P, along with the Peer MIC (MIC_P); and the
EAP.Request/SAKE/Confirm contains the Server MIC (MIC_S). Both MICs
(MIC_S and MIC_P) are computed using both nonces RAND_S and RAND_P
and are keyed by the TEK, a shared secret derived from the Root
Secret. The Server considers the Peer authenticated if the MIC_P it
computes matches the one that the Peer sends. Similarly, the Peer
considers the Server authenticated if the MIC_S it computes matches
the one that the Server sends. The way the MICs are computed
involves a keyed one-way hash function, which makes it
computationally hard for an attacker to produce the correct MIC
without knowledge of the shared secret.
5.4. Integrity Protection
Integrity protection of EAP-SAKE messages is accomplished through the
use of the Message Integrity Checks (MIC), which are present in every
message as soon as a common shared secret (TEK) is available, i.e.,
any message after the EAP.Request/SAKE/Challenge. An adversary
cannot modify any of the MIC-protected messages without causing the
recipient to encounter a MIC failure. The extent of the integrity
protection is commensurate with the security of the KDF used to
derive the MIC, the length and entropy of the shared secret used by
the KDF, and the length of the MIC.
5.5. Replay Protection
The first message of most session establishment protocols, such as
EAP-SAKE, is subject to replay. A replayed
EAP.Request/SAKE/Challenge message results in the Peer sending an
EAP.Response/SAKE/Challenge message back, which contains a MIC that
was computed using the attacker's chosen nonce. This poses a minimal
risk to the compromise of the TEK-Auth key, and this EAP Session
cannot proceed successfully as the Peer will find the Server's MIC
invalid.
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Replay protection is achieved via the RAND_S and RAND_P values,
together with the Session ID field, which are included in the
calculation of the MIC present in each packet subsequent to the EAP-
SAKE/Challenge request packet. The Session ID MUST be generated anew
by the Server for each EAP session. Session Ids also aid in
identification of possible multiple EAP sessions between a Peer and a
Server. Within the same session, messages can be replayed by an
attacker, but the state machine SHOULD be able to handle these cases.
Note that a replay within a session is indistinguishable to a
recipient from a network malfunction (e.g., message was first lost
and then re-transmitted, so the recipient thinks it is a duplicate
message).
Replay protection between EAP sessions and within an EAP session is
also accomplished via the MIC, which covers not only the entire EAP
packet (including the Session ID) but also the nonces RAND_S and
RAND_P. Thus, the recipient of an EAP message can be assured that
the message it just received is the one just sent by the other Peer
and not a replay, since it contains a valid MIC of the recipient's
nonce and the other Peer nonce. As before, the extent of replay
protection is commensurate with the security of the KDF used to
derive the MIC, the length and entropy of the shared secret used by
the KDF, and the length of the MIC.
5.6. Confidentiality
Confidentiality of EAP-SAKE attributes is supported through the use
of the AT_ENCR_DATA and AT_IV attributes. A ciphersuite is
negotiated securely (see Section 3.2.7) and can be used to encrypt
any attributes as needed. The default ciphersuite contains a strong
cipher based on AES.
5.7. Key Derivation, Strength
EAP-SAKE derives a Master Key (for EAP use) and Master Session Key,
as well as other lower-level keys, such as TEKs. Some of the lower-
level keys may or may not be used. The strength (entropy) of all
these keys is at most the strength of the Root Secret.
The entropy of the MSK and of the EMSK, assuming that the Server and
Peer 128-bit nonces are generated using good random number
generators, is at most 256-bits.
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5.8. Dictionary Attacks
Dictionary attacks are not feasible to mount on the EAP-SAKE method
because passwords are not used. Instead, the Root Secret is
machine-generated. This does not necessarily pose provisioning
problems.
5.9. Man-in-the-Middle Attacks
Resistance to man-in-the-middle attacks is provided through the
integrity protection that each EAP message carries (i.e., Message
Integrity Check field) as soon as a common key for this EAP session
has been derived through mutual authentication. As before, the
extent of this resistance is commensurate with the strength of the
MIC itself. Man-in-the-middle attacks associated with the use of any
EAP method within a tunneling or sequencing protocol are beyond the
scope of this document.
5.10. Result Indication Protection
EAP-SAKE provides result indication protection in that it provides
result indications, integrity protection, and replay protection. The
Server indicates that it has successfully authenticated the Peer by
sending the EAP.Request/SAKE/Confirm message, which is integrity and
replay protected. The Peer indicates that it has successfully
authenticated the Server by sending the EAP.Response/SAKE/Confirm
message, which is also integrity and replay protected.
5.11. Cryptographic Separation of Keys
The TEKs used to protect EAP-SAKE packets (TEK-Auth, TEK-Cipher), the
Master Session Key, and the Extended Master Session Key are
cryptographically separate. Information about any of these keys does
not lead to information about any other keys. We also note that it
is infeasible to calculate the Root Secret from any or all of the
TEKs, the MSK, or the EMSK.
5.12. Session Independence
Within each EAP-SAKE session, fresh keying material is generated.
The keying material exported by this method from two independent
EAP-SAKE sessions is cryptographically separate, as explained below.
Both the Server and the Peer SHOULD generate fresh random numbers
(i.e., nonces) for the EAP-SAKE exchange. If either entity re-uses a
random number from a previous session, then the fact that the other
does use a freshly generated random number implies that the TEKs,
MSK, and EMSK derived within this session are cryptographically
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separate from the corresponding keys derived in the previous
exchange.
Therefore, compromise of MSK or EMSK on one exchange does not
compromise the MSK and EMSK of previous or subsequent exchanges
between a Peer and a Server.
5.13. Identity Protection
As seen from Section 3.2.3., the Server may assign a temporary NAI to
a Peer in order to achieve user anonymity. This identifier may be
used by the Peer the next time it engages in an EAP-SAKE
authentication phase with the Server. The TempID is protected by
sending it encrypted, within an AT_ENCR_DATA attribute, and signed by
the Server with a MIC. Thus, an eavesdropper cannot link the
original PermID that the Peer first sends (e.g., on power-up) to any
subsequent TempID values sent in the clear to the Server.
The Server and Peer MAY be configured such that only TempID
identities are exchanged after one initial EAP-SAKE phase that uses
the Peer permanent identity. In this case, in order to achieve
maximum identity protection, the TempID SHOULD be stored in non-
volatile memory in the Peer and Server. Thus, compliance with this
document does not preclude or mandate Peer identity protection across
the lifetime of the Peer.
5.14. Channel Binding
The Server identifier and Peer identifier MAY be sent in the
SAKE/Challenge messages. However, since there is no established
authentication key at the time of the first message, the Server
identifier is not integrity-protected here.
All subsequent EAP-SAKE messages exchanged during a successful EAP-
SAKE phase are integrity-protected, as they contain a Message
Integrity Check (MIC). The MIC is computed over the EAP message and
also over the Server and Peer identities. In that, both EAP
endpoints can verify the identity of the other party.
5.15. Ciphersuite Negotiation
EAP-SAKE does not support negotiation of the ciphersuite used to
integrity-protect the EAP conversation. However, negotiation of a
ciphersuite for data protection is supported. This ciphersuite
negotiation is protected in order to minimize the risk of down-
negotiation or man-in-the-middle attacks.
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This negotiation is secure because of the Message Integrity Checks
(MICs) that cover the entire EAP messages used for ciphersuite
negotiation (see Section 3.2.7.). The extent of the security of the
negotiation is commensurate with the security of the KDF used to
derive the MICs, the length and entropy of the shared secret used by
the KDF, and the length of the MICs.
5.16. Random Number Generation
EAP-SAKE supports key derivation from a 32-byte Root Secret. The
entropy of all other keys derived from it is reduced somewhat through
the use of keyed hash functions (e.g. KDF). Thus, assuming
optimistically that the effective key strength of the Root Secret is
32 bytes, the effective key strengths of the derived keys is at most
the effective key strength of the Root Secret quantities they are
derived from: EMSK, at most 16 bytes; MSK, at most 16 bytes.
6. Security Claims
This section provides the security claims as required by [EAP].
[a] Mechanism: EAP-SAKE is a challenge/response authentication and
key establishment mechanism based on a symmetric pre-shared
secret.
[b] Security claims. EAP-SAKE provides:
Mutual authentication (Section 5.3)
Integrity protection (Section 5.4)
Replay protection (Section 5.5)
Confidentiality (optional, Section 5.6 and Section 5.13)
Key derivation (Section 5.7)
Dictionary attack protection (Section 5.8)
Protected result indication of successful authentication from
Server and from Peer (Section 5.10)
Session independence (Section 5.12)
[c] Key strength. EAP-SAKE supports key derivation with 256-bit
effective key strength (Section 5.7)
[d] Description of key hierarchy: see Section 3.2.5.
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[e] Indication of vulnerabilities: EAP-Make does not provide:
Fast reconnect
Fragmentation
Channel binding
Cryptographic binding
7. Acknowledgements
Thanks to R. Dynarski for his helpful comments.
8. References
8.1. Normative References
[AES] National Institute of Standards and Technology,
"Federal Information Processing Standards (FIPS)
Publication 197, Advanced Encryption Standard (AES)",
November 2001. http://csrc.nist.gov/publications/
fips/fips197/fips-197.pdf
[CBC] National Institute of Standards and Technology, NIST
Special Publication 800-38A, "Recommendation for Block
Cipher Modes of Operation - Methods and Techniques",
December 2001. http://csrc.nist.gov/publications/
drafts/Draft-NIST_SP800-38D_Public_Comment.pdf
[EAP] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
H. Levkowetz, "Extensible Authentication Protocol
(EAP)", RFC 3748, June 2004.
[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
February 1997.
[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.
[IEEE802.11i] "IEEE Standard for Information Technology-
Telecommunications and Information Exchange between
Systems - LAN/MAN Specific Requirements - Part 11:
Wireless Medium Access Control (MAC) and physical
layer (PHY) specifications: Amendment 6: Medium Access
Control (MAC) Security Enhancements", June 2004.
Vanderveen & Soliman Informational [Page 44]
RFC 4763 EAP-SAKE November 2006
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SHA1] National Institute of Standards and Technology, U.S.
Department of Commerce, Federal Information Processing
Standard (FIPS) Publication 180-1, "Secure Hash
Standard", April 1995.
8.2. Informative References
[NAI] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC
4086, June 2005.
Authors' Addresses
Michaela Vanderveen
Qualcomm Flarion Technologies
135 Rte. 202/206 South
Bedminster, NJ 07921
USA
EMail: mvandervn@yahoo.com
Hesham Soliman
Qualcomm Flarion Technologies
135 Rte. 202/206 South
Bedminster, NJ 07921
USA
EMail: solimanhs@gmail.com
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RFC 4763 EAP-SAKE November 2006
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