RFC 4851
The Flexible Authentication via Secure Tunneling Extensible Authentication Protocol Method (EAP-FAST)
4. Message Formats
The following sections describe the message formats used in EAP-FAST. The fields are transmitted from left to right in network byte order.4.1. EAP-FAST Message Format
A summary of the EAP-FAST Request/Response packet format 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 | Flags | Ver | Message Length : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : Message Length | Data... + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code The code field is one octet in length defined as follows: 1 Request 2 Response Identifier The Identifier field is one octet and aids in matching responses with requests. The Identifier field MUST be changed on each Request packet. The Identifier field in the Response packet MUST match the Identifier field from the corresponding request. Length The Length field is two octets and indicates the length of the EAP packet including the Code, Identifier, Length, Type, Flags, Ver, Message Length, and Data fields. Octets outside the range of the Length field should be treated as Data Link Layer padding and should be ignored on reception. Type 43 for EAP-FAST
Flags
0 1 2 3 4
+-+-+-+-+-+
|L M S R R|
+-+-+-+-+-+
L Length included; set to indicate the presence of the four-
octet Message Length field
M More fragments; set on all but the last fragment
S EAP-FAST start; set in an EAP-FAST Start message
R Reserved (must be zero)
Ver
This field contains the version of the protocol. This document
describes version 1 (001 in binary) of EAP-FAST.
Message Length
The Message Length field is four octets, and is present only if
the L bit is set. This field provides the total length of the
message that may be fragmented over the data fields of multiple
packets.
Data
In the case of an EAP-FAST Start request (i.e., when the S bit
is set) the Data field consists of the A-ID described in
Section 4.1.1. In other cases, when the Data field is present,
it consists of an encapsulated TLS packet in TLS record format.
An EAP-FAST packet with Flags and Version fields, but with zero
length data field, is used to indicate EAP-FAST acknowledgement
for either a fragmented message, a TLS Alert message or a TLS
Finished message.
4.1.1. Authority ID Data
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 (0x04) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ID +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type The Type field is two octets. It is set to 0x0004 for Authority ID Length The Length filed is two octets, which contains the length of the ID field in octets. ID Hint of the identity of the server. It should be unique across the deployment.4.2. EAP-FAST TLV Format and Support
The TLVs defined here are standard Type-Length-Value (TLV) objects. The TLV objects could be used to carry arbitrary parameters between EAP peer and EAP server within the protected TLS tunnel. The EAP peer may not necessarily implement all the TLVs supported by the EAP server. To allow for interoperability, TLVs are designed to allow an EAP server to discover if a TLV is supported by the EAP peer, using the NAK TLV. The mandatory bit in a TLV indicates whether support of the TLV is required. If the peer or server does not support a TLV marked mandatory, then it MUST send a NAK TLV in the response, and all the other TLVs in the message MUST be ignored. If an EAP peer or server finds an unsupported TLV that is marked as optional, it can ignore the unsupported TLV. It MUST NOT send an NAK TLV for a TLV that is not marked mandatory. Note that a peer or server may support a TLV with the mandatory bit set, but may not understand the contents. The appropriate response to a supported TLV with content that is not understood is defined by the individual TLV specification.
EAP implementations compliant with this specification MUST support
TLV exchanges, as well as the processing of mandatory/optional
settings on the TLV. Implementations conforming to this
specification MUST support the following TLVs:
Result TLV
NAK TLV
Error TLV
EAP-Payload TLV
Intermediate-Result TLV
Crypto-Binding TLV
Request-Action TLV
4.2.1. General TLV Format
TLVs are defined as described below. The fields are transmitted from
left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 Optional TLV
1 Mandatory TLV
R
Reserved, set to zero (0)
TLV Type
A 14-bit field, denoting the TLV type. Allocated Types
include:
0 Reserved
1 Reserved
2 Reserved
3 Result TLV (Section 4.2.2)
4 NAK TLV (Section 4.2.3)
5 Error TLV (Section 4.2.4)
7 Vendor-Specific TLV (Section 4.2.5)
9 EAP-Payload TLV (Section 4.2.6)
10 Intermediate-Result TLV (Section 4.2.7)
11 PAC TLV [EAP-PROV]
12 Crypto-Binding TLV (Section 4.2.8)
18 Server-Trusted-Root TLV [EAP-PROV]
19 Request-Action TLV (Section 4.2.9)
20 PKCS#7 TLV [EAP-PROV]
Length
The length of the Value field in octets.
Value
The value of the TLV.
4.2.2. Result TLV
The Result TLV provides support for acknowledged success and failure
messages for protected termination within EAP-FAST. If the Status
field does not contain one of the known values, then the peer or EAP
server MUST treat this as a fatal error of Unexpected_TLVs_Exchanged.
The behavior of the Result TLV is further discussed in Section 3.3.2
and Section 3.6.2. A Result TLV indicating failure MUST NOT be
accompanied by the following TLVs: NAK, EAP-Payload TLV, or Crypto-
Binding TLV. The Result TLV is defined 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
Mandatory, set to one (1)
R
Reserved, set to zero (0)
TLV Type
3 for Result TLV
Length
2
Status
The Status field is two octets. Values include:
1 Success
2 Failure
4.2.3. NAK TLV
The NAK TLV allows a peer to detect TLVs that are not supported by
the other peer. An EAP-FAST packet can contain 0 or more NAK TLVs.
A NAK TLV should not be accompanied by other TLVs. A NAK TLV MUST
NOT be sent in response to a message containing a Result TLV, instead
a Result TLV of failure should be sent indicating failure and an
Error TLV of Unexpected_TLVs_Exchanged. The NAK TLV is defined 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NAK-Type | TLVs...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
Mandatory, set to one (1)
R
Reserved, set to zero (0)
TLV Type
4 for NAK TLV
Length
>=6
Vendor-Id
The Vendor-Id field is four octets, and contains the Vendor-Id
of the TLV that was not supported. The high-order octet is 0
and the low-order three octets are the Structure of Management
Information (SMI) Network Management Private Enterprise Code of
the Vendor in network byte order. The Vendor-Id field MUST be
zero for TLVs that are not Vendor-Specific TLVs.
NAK-Type
The NAK-Type field is two octets. The field contains the Type
of the TLV that was not supported. A TLV of this Type MUST
have been included in the previous packet.
TLVs
This field contains a list of zero or more TLVs, each of which
MUST NOT have the mandatory bit set. These optional TLVs are
for future extensibility to communicate why the offending TLV
was determined to be unsupported.
4.2.4. Error TLV
The Error TLV allows an EAP peer or server to indicate errors to the
other party. An EAP-FAST packet can contain 0 or more Error TLVs.
The Error-Code field describes the type of error. Error Codes 1-999
represent successful outcomes (informative messages), 1000-1999
represent warnings, and codes 2000-2999 represent fatal errors. A
fatal Error TLV MUST be accompanied by a Result TLV indicating
failure and the conversation must be terminated as described in
Section 3.6.2. The Error TLV is defined 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error-Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
Mandatory, set to one (1)
R
Reserved, set to zero (0)
TLV Type
5 for Error TLV
Length
4
Error-Code
The Error-Code field is four octets. Currently defined values
for Error-Code include:
2001 Tunnel_Compromise_Error
2002 Unexpected_TLVs_Exchanged
4.2.5. Vendor-Specific TLV
The Vendor-Specific TLV is available to allow vendors to support
their own extended attributes not suitable for general usage. A
Vendor-Specific TLV attribute can contain one or more TLVs, referred
to as Vendor TLVs. The TLV-type of a Vendor-TLV is defined by the
vendor. All the Vendor TLVs inside a single Vendor-Specific TLV
belong to the same vendor. There can be multiple Vendor-Specific
TLVs from different vendors in the same message.
Vendor TLVs may be optional or mandatory. Vendor TLVs sent with
Result TLVs MUST be marked as optional.
The Vendor-Specific TLV is defined 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor-Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor TLVs...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
0 or 1
R
Reserved, set to zero (0)
TLV Type
7 for Vendor Specific TLV
Length
4 + cumulative length of all included Vendor TLVs
Vendor-Id
The Vendor-Id field is four octets, and contains the Vendor-Id
of the TLV. The high-order octet is 0 and the low-order 3
octets are the SMI Network Management Private Enterprise Code
of the Vendor in network byte order.
Vendor TLVs
This field is of indefinite length. It contains vendor-
specific TLVs, in a format defined by the vendor.
4.2.6. EAP-Payload TLV
To allow piggybacking an EAP request or response with other TLVs, the
EAP-Payload TLV is defined, which includes an encapsulated EAP packet
and a list of optional TLVs. The optional TLVs are provided for
future extensibility to provide hints about the current EAP
authentication. Only one EAP-Payload TLV is allowed in a message.
The EAP-Payload TLV is defined 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EAP packet...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLVs...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
Mandatory, set to (1)
R
Reserved, set to zero (0)
TLV Type
9 for EAP-Payload TLV
Length
length of embedded EAP packet + cumulative length of additional
TLVs
EAP packet
This field contains a complete EAP packet, including the EAP
header (Code, Identifier, Length, Type) fields. The length of
this field is determined by the Length field of the
encapsulated EAP packet.
TLVs
This field contains a list of zero or more TLVs associated with
the EAP packet field. The TLVs MUST NOT have the mandatory bit
set. The total length of this field is equal to the Length
field of the EAP-Payload TLV, minus the Length field in the EAP
header of the EAP packet field.
4.2.7. Intermediate-Result TLV
The Intermediate-Result TLV provides support for acknowledged intermediate Success and Failure messages between multiple inner EAP methods within EAP. An Intermediate-Result TLV indicating success MUST be accompanied by a Crypto-Binding TLV. The optional TLVs associated with this TLV are provided for future extensibility to provide hints about the current result. The Intermediate-Result TLV is defined 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|R| TLV Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Status | TLVs... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ M Mandatory, set to (1) R Reserved, set to zero (0) TLV Type 10 for Intermediate-Result TLV Length 2 + cumulative length of the embedded associated TLVs Status The Status field is two octets. Values include: 1 Success 2 Failure TLVs This field is of indeterminate length, and contains zero or more of the TLVs associated with the Intermediate Result TLV. The TLVs in this field MUST NOT have the mandatory bit set.
4.2.8. Crypto-Binding TLV
The Crypto-Binding TLV is used to prove that both the peer and server participated in the tunnel establishment and sequence of authentications. It also provides verification of the EAP-FAST version negotiated before TLS tunnel establishment, see Section 3.1. The Crypto-Binding TLV MUST be included with the Intermediate-Result TLV to perform Cryptographic Binding after each successful EAP method in a sequence of EAP methods. The Crypto-Binding TLV can be issued at other times as well. The Crypto-Binding TLV is valid only if the following checks pass: o The Crypto-Binding TLV version is supported o The MAC verifies correctly o The received version in the Crypto-Binding TLV matches the version sent by the receiver during the EAP version negotiation o The subtype is set to the correct value If any of the above checks fail, then the TLV is invalid. An invalid Crypto-Binding TLV is a fatal error and is handled as described in Section 3.6.2. The Crypto-Binding TLV is defined 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|R| TLV Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Version | Received Ver. | Sub-Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Nonce ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Compound MAC ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ M Mandatory, set to (1)
R
Reserved, set to zero (0)
TLV Type
12 for Crypto-Binding TLV
Length
56
Reserved
Reserved, set to zero (0)
Version
The Version field is a single octet, which is set to the
version of Crypto-Binding TLV the EAP method is using. For an
implementation compliant with this version of EAP-FAST, the
version number MUST be set to 1.
Received Version
The Received Version field is a single octet and MUST be set to
the EAP version number received during version negotiation.
Note that this field only provides protection against downgrade
attacks, where a version of EAP requiring support for this TLV
is required on both sides.
Sub-Type
The Sub-Type field is one octet. Defined values include:
0 Binding Request
1 Binding Response
Nonce
The Nonce field is 32 octets. It contains a 256-bit nonce that
is temporally unique, used for compound MAC key derivation at
each end. The nonce in a request MUST have its least
significant bit set to 0 and the nonce in a response MUST have
the same value as the request nonce except the least
significant bit MUST be set to 1.
Compound MAC
The Compound MAC field is 20 octets. This can be the Server
MAC (B1_MAC) or the Client MAC (B2_MAC). The computation of
the MAC is described in Section 5.3.
4.2.9. Request-Action TLV
The Request-Action TLV MAY be sent by the peer along with a Result
TLV in response to a server's successful Result TLV. It allows the
peer to request the EAP server to negotiate additional EAP methods or
process TLVs specified in the response packet. The server MAY ignore
this TLV.
The Request-Action TLV is defined 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|R| TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Action |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M
Mandatory set to one (1)
R
Reserved, set to zero (0)
TLV Type
19 for Request-Action TLV
Length
2
Action
The Action field is two octets. Values include:
Process-TLV
Negotiate-EAP
4.3. Table of TLVs
The following table provides a guide to which TLVs may be found in which kinds of messages, and in what quantity. The messages are as follows: Request is an EAP-FAST Request, Response is an EAP-FAST Response, Success is a message containing a successful Result TLV, and Failure is a message containing a failed Result TLV. Request Response Success Failure TLVs 0-1 0-1 0-1 0-1 Intermediate-Result 0-1 0-1 0 0 EAP-Payload 0-1 0-1 1 1 Result 0-1 0-1 0-1 0-1 Crypto-Binding 0+ 0+ 0+ 0+ Error 0+ 0+ 0 0 NAK 0+ 0+ 0+ 0+ Vendor-Specific [NOTE1] 0 0-1 0-1 0-1 Request-Action [NOTE1] Vendor TLVs (included in Vendor-Specific TLVs) sent with a Result TLV MUST be marked as optional. The following table defines the meaning of the table entries in the sections below: 0 This TLV MUST NOT be present in the message. 0+ Zero or more instances of this TLV MAY be present in the message. 0-1 Zero or one instance of this TLV MAY be present in the message. 1 Exactly one instance of this TLV MUST be present in the message.5. Cryptographic Calculations
5.1. EAP-FAST Authentication Phase 1: Key Derivations
The EAP-FAST Authentication tunnel key is calculated similarly to the TLS key calculation with an additional 40 octets (referred to as the session_key_seed) generated. The additional session_key_seed is used in the Session Key calculation in the EAP-FAST Tunneled Authentication conversation.
To generate the key material required for the EAP-FAST Authentication tunnel, the following construction from [RFC4346] is used: key_block = PRF(master_secret, "key expansion", server_random + client_random) where '+' denotes concatenation. The PRF function used to generate keying material is defined by [RFC4346]. For example, if the EAP-FAST Authentication employs 128-bit RC4 and SHA1, the key_block is 112 octets long and is partitioned as follows: client_write_MAC_secret[20] server_write_MAC_secret[20] client_write_key[16] server_write_key[16] client_write_IV[0] server_write_IV[0] session_key_seed[40] The session_key_seed is used by the EAP-FAST Authentication Phase 2 conversation to both cryptographically bind the inner method(s) to the tunnel as well as generate the resulting EAP-FAST session keys. The other quantities are used as they are defined in [RFC4346]. The master_secret is generated as specified in TLS unless a PAC is used to establish the TLS tunnel. When a PAC is used to establish the TLS tunnel, the master_secret is calculated from the specified client_random, server_random, and PAC-Key as follows: master_secret = T-PRF(PAC-Key, "PAC to master secret label hash", server_random + client_random, 48) where T-PRF is described in Section 5.5.5.2. Intermediate Compound Key Derivations
The session_key_seed derived as part of EAP-FAST Phase 2 is used in EAP-FAST Phase 2 to generate an Intermediate Compound Key (IMCK) used to verify the integrity of the TLS tunnel after each successful inner authentication and in the generation of Master Session Key (MSK) and Extended Master Session Key (EMSK) defined in [RFC3748]. Note that the IMCK must be recalculated after each successful inner EAP method.
The first step in these calculations is the generation of the base
compound key, IMCK[n] from the session_key_seed and any session keys
derived from the successful execution of n inner EAP methods. The
inner EAP method(s) may provide Master Session Keys, MSK1..MSKn,
corresponding to inner methods 1 through n. The MSK is truncated at
32 octets if it is longer than 32 octets or padded to a length of 32
octets with zeros if it is less than 32 octets. If the ith inner
method does not generate an MSK, then MSKi is set to zero (e.g., MSKi
= 32 octets of 0x00s). If an inner method fails, then it is not
included in this calculation. The derivations of S-IMCK is as
follows:
S-IMCK[0] = session_key_seed
For j = 1 to n-1 do
IMCK[j] = T-PRF(S-IMCK[j-1], "Inner Methods Compound Keys",
MSK[j], 60)
S-IMCK[j] = first 40 octets of IMCK[j]
CMK[j] = last 20 octets of IMCK[j]
where T-PRF is described in Section 5.5.
5.3. Computing the Compound MAC
For authentication methods that generate keying material, further
protection against man-in-the-middle attacks is provided through
cryptographically binding keying material established by both EAP-
FAST Phase 1 and EAP-FAST Phase 2 conversations. After each
successful inner EAP authentication, EAP MSKs are cryptographically
combined with key material from EAP-FAST Phase 1 to generate a
compound session key, CMK. The CMK is used to calculate the Compound
MAC as part of the Crypto-Binding TLV described in Section 4.2.8,
which helps provide assurance that the same entities are involved in
all communications in EAP-FAST. During the calculation of the
Compound-MAC the MAC field is filled with zeros.
The Compound MAC computation is as follows:
CMK = CMK[j]
Compound-MAC = HMAC-SHA1( CMK, Crypto-Binding TLV )
where j is the number of the last successfully executed inner EAP
method.
5.4. EAP Master Session Key Generation
EAP-FAST Authentication assures the master session key (MSK) and Extended Master Session Key (EMSK) output from the EAP method are the result of all authentication conversations by generating an Intermediate Compound Key (IMCK). The IMCK is mutually derived by the peer and the server as described in Section 5.2 by combining the MSKs from inner EAP methods with key material from EAP-FAST Phase 1. The resulting MSK and EMSK are generated as part of the IMCKn key hierarchy as follows: MSK = T-PRF(S-IMCK[j], "Session Key Generating Function", 64) EMSK = T-PRF(S-IMCK[j], "Extended Session Key Generating Function", 64) where j is the number of the last successfully executed inner EAP method. The EMSK is typically only known to the EAP-FAST peer and server and is not provided to a third party. The derivation of additional keys and transportation of these keys to a third party is outside the scope of this document. If no EAP methods have been negotiated inside the tunnel or no EAP methods have been successfully completed inside the tunnel, the MSK and EMSK will be generated directly from the session_key_seed meaning S-IMCK = session_key_seed.5.5. T-PRF
EAP-FAST employs the following PRF prototype and definition: T-PRF = F(key, label, seed, outputlength) Where label is intended to be a unique label for each different use of the T-PRF. The outputlength parameter is a two-octet value that is represented in big endian order. Also note that the seed value may be optional and may be omitted as in the case of the MSK derivation described in Section 5.4.
To generate the desired outputlength octets of key material, the
T-PRF is calculated as follows:
S = label + 0x00 + seed
T-PRF output = T1 + T2 + T3 + ... + Tn
T1 = HMAC-SHA1 (key, S + outputlength + 0x01)
T2 = HMAC-SHA1 (key, T1 + S + outputlength + 0x02)
T3 = HMAC-SHA1 (key, T2 + S + outputlength + 0x03)
Tn = HMAC-SHA1 (key, Tn-1 + S + outputlength + 0xnn)
where '+' indicates concatenation. Each Ti generates 20-octets of
keying material. The last Tn may be truncated to accommodate the
desired length specified by outputlength.
6. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the EAP-
FAST protocol, in accordance with BCP 26, [RFC2434].
EAP-FAST has already been assigned the EAP Method Type number 43.
The document defines a registry for EAP-FAST TLV types, which may be
assigned by Specification Required as defined in [RFC2434].
Section 4.2 defines the TLV types that initially populate the
registry. A summary of the EAP-FAST TLV types is given below:
0 Reserved
1 Reserved
2 Reserved
3 Result TLV
4 NAK TLV
5 Error TLV
7 Vendor-Specific TLV
9 EAP-Payload TLV
10 Intermediate-Result TLV
11 PAC TLV [EAP-PROV]
12 Crypto-Binding TLV
18 Server-Trusted-Root TLV [EAP-PROV]
19 Request-Action TLV
20 PKCS#7 TLV [EAP-PROV]
The Error-TLV defined in Section 4.2.4 requires an error-code. EAP-
FAST Error-TLV error-codes are assigned based on specifications
required as defined in [RFC2434]. The initial list of error codes is
as follows:
2001 Tunnel_Compromise_Error
2002 Unexpected_TLVs_Exchanged
The Request-Action TLV defined in Section 4.2.9 contains an action
code which is assigned on a specification required basis as defined
in [RFC2434]. The initial actions defined are:
1 Process-TLV
2 Negotiate-EAP
The various values under Vendor-Specific TLV are assigned by Private
Use and do not need to be assigned by IANA.
7. Security Considerations
EAP-FAST is designed with a focus on wireless media, where the medium
itself is inherent to eavesdropping. Whereas in wired media, an
attacker would have to gain physical access to the wired medium;
wireless media enables anyone to capture information as it is
transmitted over the air, enabling passive attacks. Thus, physical
security can not be assumed and security vulnerabilities are far
greater. The threat model used for the security evaluation of EAP-
FAST is defined in the EAP [RFC3748].
7.1. Mutual Authentication and Integrity Protection
EAP-FAST as a whole, provides message and integrity protection by
establishing a secure tunnel for protecting the authentication
method(s). The confidentiality and integrity protection is defined
by TLS and provides the same security strengths afforded by TLS
employing a strong entropy shared master secret. The integrity of
the key generating authentication methods executed within the EAP-
FAST tunnel is verified through the calculation of the Crypto-Binding
TLV. This ensures that the tunnel endpoints are the same as the
inner method endpoints.
The Result TLV is protected and conveys the true Success or Failure
of EAP-FAST, and should be used as the indicator of its success or
failure respectively. However, as EAP must terminate with a clear
text EAP Success or Failure, a peer will also receive a clear text
EAP Success or Failure. The received clear text EAP success or
failure must match that received in the Result TLV; the peer SHOULD
silently discard those clear text EAP Success or Failure messages
that do not coincide with the status sent in the protected Result
TLV.
7.2. Method Negotiation
As is true for any negotiated EAP protocol, NAK packets used to suggest an alternate authentication method are sent unprotected and as such, are subject to spoofing. During unprotected EAP method negotiation, NAK packets may be interjected as active attacks to negotiate down to a weaker form of authentication, such as EAP-MD5 (which only provides one-way authentication and does not derive a key). Both the peer and server should have a method selection policy that prevents them from negotiating down to weaker methods. Inner method negotiation resists attacks because it is protected by the mutually authenticated TLS tunnel established. Selection of EAP-FAST as an authentication method does not limit the potential inner authentication methods, so EAP-FAST should be selected when available. An attacker cannot readily determine the inner EAP method used, except perhaps by traffic analysis. It is also important that peer implementations limit the use of credentials with an unauthenticated or unauthorized server.7.3. Separation of Phase 1 and Phase 2 Servers
Separation of the EAP-FAST Phase 1 from the Phase 2 conversation is not recommended. Allowing the Phase 1 conversation to be terminated at a different server than the Phase 2 conversation can introduce vulnerabilities if there is not a proper trust relationship and protection for the protocol between the two servers. Some vulnerabilities include: o Loss of identity protection o Offline dictionary attacks o Lack of policy enforcement There may be cases where a trust relationship exists between the Phase 1 and Phase 2 servers, such as on a campus or between two offices within the same company, where there is no danger in revealing the inner identity and credentials of the peer to entities between the two servers. In these cases, using a proxy solution without end-to-end protection of EAP-FAST MAY be used. The EAP-FAST encrypting/decrypting gateway SHOULD, at a minimum, provide support for IPsec or similar protection in order to provide confidentiality for the portion of the conversation between the gateway and the EAP server.
7.4. Mitigation of Known Vulnerabilities and Protocol Deficiencies
EAP-FAST addresses the known deficiencies and weaknesses in the EAP method. By employing a shared secret between the peer and server to establish a secured tunnel, EAP-FAST enables: o Per packet confidentiality and integrity protection o User identity protection o Better support for notification messages o Protected EAP inner method negotiation o Sequencing of EAP methods o Strong mutually derived master session keys o Acknowledged success/failure indication o Faster re-authentications through session resumption o Mitigation of dictionary attacks o Mitigation of man-in-the-middle attacks o Mitigation of some denial-of-service attacks It should be noted that with EAP-FAST, as in many other authentication protocols, a denial-of-service attack can be mounted by adversaries sending erroneous traffic to disrupt the protocol. This is a problem in many authentication or key agreement protocols and is therefore noted for EAP-FAST as well. EAP-FAST was designed with a focus on protected authentication methods that typically rely on weak credentials, such as password- based secrets. To that extent, the EAP-FAST Authentication mitigates several vulnerabilities, such as dictionary attacks, by protecting the weak credential-based authentication method. The protection is based on strong cryptographic algorithms in TLS to provide message confidentiality and integrity. The keys derived for the protection relies on strong random challenges provided by both peer and server as well as an established key with strong entropy. Implementations should follow the recommendation in [RFC4086] when generating random numbers.7.4.1. User Identity Protection and Verification
The initial identity request response exchange is sent in cleartext outside the protection of EAP-FAST. Typically the Network Access Identifier (NAI) [RFC4282] in the identity response is useful only for the realm information that is used to route the authentication requests to the right EAP server. This means that the identity response may contain an anonymous identity and just contain realm information. In other cases, the identity exchange may be eliminated altogether if there are other means for establishing the destination realm of the request. In no case should an intermediary place any trust in the identity information in the identity response since it
is unauthenticated an may not have any relevance to the authenticated identity. EAP-FAST implementations should not attempt to compare any identity disclosed in the initial cleartext EAP Identity response packet with those Identities authenticated in Phase 2 Identity request-response exchanges sent after the EAP-FAST tunnel is established are protected from modification and eavesdropping by attackers. Note that since TLS client certificates are sent in the clear, if identity protection is required, then it is possible for the TLS authentication to be re-negotiated after the first server authentication. To accomplish this, the server will typically not request a certificate in the server_hello, then after the server_finished message is sent, and before EAP-FAST Phase 2, the server MAY send a TLS hello_request. This allows the client to perform client authentication by sending a client_hello if it wants to, or send a no_renegotiation alert to the server indicating that it wants to continue with EAP-FAST Phase 2 instead. Assuming that the client permits renegotiation by sending a client_hello, then the server will respond with server_hello, a certificate and certificate_request messages. The client replies with certificate, client_key_exchange and certificate_verify messages. Since this re- negotiation occurs within the encrypted TLS channel, it does not reveal client certificate details. It is possible to perform certificate authentication using an EAP method (for example: EAP-TLS) within the TLS session in EAP-FAST Phase 2 instead of using TLS handshake renegotiation.7.4.2. Dictionary Attack Resistance
EAP-FAST was designed with a focus on protected authentication methods that typically rely on weak credentials, such as password- based secrets. EAP-FAST mitigates dictionary attacks by allowing the establishment of a mutually authenticated encrypted TLS tunnel providing confidentiality and integrity to protect the weak credential based authentication method.7.4.3. Protection against Man-in-the-Middle Attacks
Allowing methods to be executed both with and without the protection of a secure tunnel opens up a possibility of a man-in-the-middle attack. To avoid man-in-the-middle attacks it is recommended to always deploy authentication methods with protection of EAP-FAST. EAP-FAST provides protection from man-in-the-middle attacks even if a deployment chooses to execute inner EAP methods both with and without EAP-FAST protection, EAP-FAST prevents this attack in two ways:
1. By using the PAC-Key to mutually authenticate the peer and server
during EAP-FAST Authentication Phase 1 establishment of a secure
tunnel.
2. By using the keys generated by the inner authentication method
(if the inner methods are key generating) in the crypto-binding
exchange and in the generation of the key material exported by
the EAP method described in Section 5.
7.4.4. PAC Binding to User Identity
A PAC may be bound to a user identity. A compliant implementation of
EAP-FAST MUST validate that an identity obtained in the PAC-Opaque
field matches at minimum one of the identities provided in the EAP-
FAST Phase 2 authentication method. This validation provides another
binding to ensure that the intended peer (based on identity) has
successfully completed the EAP-FAST Phase 1 and proved identity in
the Phase 2 conversations.
7.5. Protecting against Forged Clear Text EAP Packets
EAP Success and EAP Failure packets are, in general, sent in clear
text and may be forged by an attacker without detection. Forged EAP
Failure packets can be used to attempt to convince an EAP peer to
disconnect. Forged EAP Success packets may be used to attempt to
convince a peer that authentication has succeeded, even though the
authenticator has not authenticated itself to the peer.
By providing message confidentiality and integrity, EAP-FAST provides
protection against these attacks. Once the peer and AS initiate the
EAP-FAST Authentication Phase 2, compliant EAP-FAST implementations
must silently discard all clear text EAP messages, unless both the
EAP-FAST peer and server have indicated success or failure using a
protected mechanism. Protected mechanisms include TLS alert
mechanism and the protected termination mechanism described in
Section 3.3.2.
The success/failure decisions within the EAP-FAST tunnel indicate the
final decision of the EAP-FAST authentication conversation. After a
success/failure result has been indicated by a protected mechanism,
the EAP-FAST peer can process unprotected EAP success and EAP failure
messages; however the peer MUST ignore any unprotected EAP success or
failure messages where the result does not match the result of the
protected mechanism.
To abide by [RFC3748], the server must send a clear text EAP Success
or EAP Failure packet to terminate the EAP conversation. However,
since EAP Success and EAP Failure packets are not retransmitted, the
final packet may be lost. While an EAP-FAST protected EAP Success or EAP Failure packet should not be a final packet in an EAP-FAST conversation, it may occur based on the conditions stated above, so an EAP peer should not rely upon the unprotected EAP success and failure messages.7.6. Server Certificate Validation
As part of the TLS negotiation, the server presents a certificate to the peer. The peer MUST verify the validity of the EAP server certificate, and SHOULD also examine the EAP server name presented in the certificate, in order to determine whether the EAP server can be trusted. Please note that in the case where the EAP authentication is remote, the EAP server will not reside on the same machine as the authenticator, and therefore the name in the EAP server's certificate cannot be expected to match that of the intended destination. In this case, a more appropriate test might be whether the EAP server's certificate is signed by a CA controlling the intended domain and whether the authenticator can be authorized by a server in that domain.7.7. Tunnel PAC Considerations
Since the Tunnel PAC is stored by the peer, special care should be given to the overall security of the peer. The Tunnel PAC must be securely stored by the peer to prevent theft or forgery of any of the Tunnel PAC components. In particular, the peer must securely store the PAC-Key and protect it from disclosure or modification. Disclosure of the PAC-Key enables an attacker to establish the EAP-FAST tunnel; however, disclosure of the PAC-Key does not reveal the peer or server identity or compromise any other peer's PAC credentials. Modification of the PAC-Key or PAC-Opaque components of the Tunnel PAC may also lead to denial of service as the tunnel establishment will fail. The PAC-Opaque component is the effective TLS ticket extension used to establish the tunnel using the techniques of [RFC4507]. Thus, the security considerations defined by [RFC4507] also apply to the PAC- Opaque. The PAC-Info may contain information about the Tunnel PAC such as the identity of the PAC issuer and the Tunnel PAC lifetime for use in the management of the Tunnel PAC. The PAC-Info should be securely stored by the peer to protect it from disclosure and modification.
7.8. Security Claims
This section provides the needed security claim requirement for EAP [RFC3748]. Auth. mechanism: Certificate based, shared secret based and various tunneled authentication mechanisms. Ciphersuite negotiation: Yes Mutual authentication: Yes Integrity protection: Yes, Any method executed within the EAP-FAST tunnel is integrity protected. The cleartext EAP headers outside the tunnel are not integrity protected. Replay protection: Yes Confidentiality: Yes Key derivation: Yes Key strength: See Note 1 below. Dictionary attack prot.: Yes Fast reconnect: Yes Cryptographic binding: Yes Session independence: Yes Fragmentation: Yes Key Hierarchy: Yes Channel binding: No, but TLVs could be defined for this. Notes 1. BCP 86 [RFC3766] offers advice on appropriate key sizes. The National Institute for Standards and Technology (NIST) also offers advice on appropriate key sizes in [NIST.SP800-57]. [RFC3766] Section 5 advises use of the following required RSA or DH module and DSA subgroup size in bits, for a given level of attack resistance in bits. Based on the table below, a 2048-bit RSA key is required to provide 128-bit equivalent key strength: Attack Resistance RSA or DH Modulus DSA subgroup (bits) size (bits) size (bits) ----------------- ----------------- ------------ 70 947 129 80 1228 148 90 1553 167 100 1926 186 150 4575 284 200 8719 383 250 14596 482
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