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By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79.
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This Internet-Draft will expire on August 16, 2006.
Copyright © The Internet Society (2006).
This document specifies EAP-PSK, an Extensible Authentication Protocol (EAP) method for
mutual authentication and session key derivation using a Pre-Shared Key (PSK).
EAP-PSK provides a protected communication channel when mutual authentication is
successful for both parties to communicate over.
This document
describes the use of this channel only for protected exchange
of result indications, but future EAP-PSK extensions may use
the channel for other purposes.
EAP-PSK is designed for authentication over insecure networks such as IEEE 802.11.
1.
Introduction
1.1.
Design goals for EAP-PSK
1.1.1.
Simplicity
1.1.2.
Wide Applicability
1.1.3.
Security
1.1.4.
Extensibility
1.2.
Terminology
1.3.
Conventions
1.4.
Related Work
2.
Protocol overview
2.1.
EAP-PSK key hierarchy
2.1.1.
The PSK
2.1.2.
The TEK
2.1.3.
The MSK
2.1.4.
The EMSK
2.1.5.
The IV
2.2.
Cryptographic design of EAP-PSK
2.2.1.
The Key Setup
2.2.2.
The Authenticated Key Exchange
2.2.3.
The Protected Channel
2.3.
EAP-PSK Message Flows
2.3.1.
EAP-PSK Standard Authentication
2.3.2.
EAP-PSK Extended Authentication
3.
EAP-PSK Message format
3.1.
EAP-PSK First Message
3.2.
EAP-PSK Second Message
3.3.
EAP-PSK Third Message
3.4.
EAP-PSK Fourth Message
4.
Rules of Operation for the EAP-PSK Protected Channel
4.1.
Protected Result Indications
4.1.1.
CONT
4.1.2.
DONE_SUCCESS
4.1.3.
DONE_FAILURE
4.2.
Extended Authentication
5.
IANA considerations
5.1.
Allocation of an EAP-Request/Response Type for EAP-PSK
5.2.
Allocation of EXT Type numbers
6.
Security Considerations
6.1.
Mutual Authentication
6.2.
Protected Result Indications
6.3.
Integrity Protection
6.4.
Replay Protection
6.5.
Reflection attacks
6.6.
Dictionary Attacks
6.7.
Key Derivation
6.8.
Denial of Service Resistance
6.9.
Session Independence
6.10.
Exposition of the PSK
6.11.
Fragmentation
6.12.
Channel Binding
6.13.
Fast Reconnect
6.14.
Identity Protection
6.15.
Protected Ciphersuite Negotiation
6.16.
Confidentiality
6.17.
Cryptographic Binding
6.18.
Implementation of EAP-PSK
7.
Security Claims
8.
Acknowledgments
9.
References
9.1.
Normative References
9.2.
Informative References
Appendix A.
Generation of the PSK from a password - Discouraged
§
Authors' Addresses
§
Intellectual Property and Copyright Statements
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The Extensible Authentication Protocol (EAP) [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.) provides an authentication framework which supports multiple authentication methods.
This document specifies an EAP method, called EAP-PSK, that uses a Pre-Shared Key (PSK).
EAP-PSK was developed at France Telecom R&D in 2003-2004. It is published as an RFC for the general information of the Internet community and to allow independent implementations.
Because PSKs are of frequent use in security protocols, other protocols may also refer to a PSK or contain this word in their name. For instance, Wi-Fi Protected Access (WPA) [53] (Wi-Fi Alliance, “Wi-Fi Protected Access, version 2.0,” April 2003.) specifies an authentication mode called "WPA-PSK". EAP-PSK is distinct from these protocols and should not be confused with them.
Design goals for EAP-PSK were:
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For the sake of simplicity, EAP-PSK relies on a single cryptographic primitive, AES-128 [6] (National Institute of Standards and Technology, “Specification for the Advanced Encryption Standard (AES),” November 2001.).
Restriction to such a primitive, and in particular, not using asymmetric cryptography like Diffie-Hellman key exchange, makes EAP-PSK:
However, as further discussed in Section 6 (Security Considerations), this prevents EAP-PSK from offering advanced features such as identity protection, password support, or Perfect Forward Secrecy (PFS). This choice has been deliberately made as a trade-off between simplicity and security.
For the sake of simplicity, EAP-PSK has also chosen a fixed message format and not a Type-Length-Value (TLV) design.
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EAP-PSK has been designed in a threat model where the attacker has full control over the communication channel. This is the EAP threat model that is presented in Section 7.1 of [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.).
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Since the design of authenticated key exchange is notoriously known to be hard and error prone, EAP-PSK tries to avoid inventing any new cryptographic mechanism. It attempts to build instead on existing primitives and protocols that have been reviewed by the cryptographic community.
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EAP-PSK explicitly provides a mechanism to allow future extensions within its protected channel (see Section 2.2.3 (The Protected Channel)). Thanks to this mechanism, EAP-PSK will be able to provide more sophisticated services as the need to do so appears.
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- Authentication, Authorization and Accounting (AAA)
- Please refer to [10] (Aboba, B., Calhoun, P., Glass, S., Hiller, T., McCann, P., Shiino, H., Zorn, G., Dommety, G., Perkins, C., Patil, B., Mitton, D., Manning, S., Beadles, M., Walsh, P., Chen, X., Sivalingham, S., Hameed, A., Munson, M., Jacobs, S., Lim, B., Hirschman, B., Hsu, R., Xu, Y., Campbell, E., Baba, S., and E. Jaques, “Criteria for Evaluating AAA Protocols for Network Access,” November 2000.) for more details.
- AES-128
- A block cipher specified in the Advanced Encryption Standard [6] (National Institute of Standards and Technology, “Specification for the Advanced Encryption Standard (AES),” November 2001.).
- Authentication Key (AK)
- A 16-byte key derived from the PSK that the EAP peer and server use to mutually authenticate.
- AKEP2
- An authenticated key exchange protocol, please refer to [14] (Bellare, M. and P. Rogaway, “Entity Authentication and Key Distribution,” 1994.) for more details.
- Backend Authentication Server
- An entity that provides an authentication service to an Authenticator. When used, this server typically executes EAP Methods for the Authenticator (This terminology is also used in [28] (Institute of Electrical and Electronics Engineers, “Local and Metropolitan Area Networks: Port-Based Network Access Control,” September 2001.), and has the same meaning in this document).
- CMAC
- CMAC stands for cipher-based message authentication code (MAC). It is the authentication mode of operation of AES recommended by NIST in [7] (National Institute of Standards and Technology, “Recommendation for Block Cipher Modes of Operation: The CMAC Mode for Authentication,” May 2005.).
- Extensible Authentication Protocol (EAP)
- Defined in [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.).
- EAP Authenticator (or simply Authenticator)
- The end of the EAP link initiating the EAP authentication methods. (This terminology is also used in [28] (Institute of Electrical and Electronics Engineers, “Local and Metropolitan Area Networks: Port-Based Network Access Control,” September 2001.), and has the same meaning in this document).
- EAP peer (or simply peer)
- The end of the EAP link that responds to the Authenticator. (In [28] (Institute of Electrical and Electronics Engineers, “Local and Metropolitan Area Networks: Port-Based Network Access Control,” September 2001.), this end is known as the Supplicant).
- EAP server (or simply server)
- The entity that terminates the EAP authentication with the peer. When there is no Backend Authentication Server, this term refers to the EAP Authenticator. Where the EAP Authenticator operates in pass-through mode, it refers to the Backend Authentication Server.
- EAX
- An authenticated-encryption with associated data mode of operation for block ciphers, [3] (Bellare, M., Rogaway, P., and D. Wagner, “The EAX mode of operation,” 2004.).
- Extended Master Session Key (EMSK)
- Additional keying material derived between the EAP peer and server that is exported by the EAP method. The EMSK is reserved for future uses that are not defined yet and is not provided to a third party. Please refer to [8] (Aboba, B., “Extensible Authentication Protocol (EAP) Key Management Framework,” January 2006.) for more details. EAP-PSK generates a 64-byte EMSK.
- Initialization Vector (IV)
- A quantity of at least 64 bytes, suitable for use in an initialization vector field, that is derived between the peer and EAP server. Since the IV is a known value in methods such as EAP-TLS [11] (Aboba, B. and D. Simon, “PPP EAP TLS Authentication Protocol,” October 1999.), it cannot be used by itself for computation of any quantity that needs to remain secret. As a result, its use has been deprecated and EAP methods are not required to generate it. Please refer to [8] (Aboba, B., “Extensible Authentication Protocol (EAP) Key Management Framework,” January 2006.) for more details. EAP-PSK does not generate an IV.
- Key-Derivation Key (KDK)
- A 16-byte key derived from the PSK that the EAP peer and server use to derive session keys (namely, the TEK, MSK and EMSK).
- Message Authentication Code (MAC)
- Informally, the purpose of a MAC is to provide assurances regarding both the source of a message and its integrity [43] (Menezes, A., van Oorschot, P., and S. Vanstone, “Handbook of Applied Cryptography,” 1996.). IEEE 802.11i uses the acronym MIC (Message Integrity Check) to avoid confusion with the other meaning of the acronym MAC (Medium Access Control).
- Master Session Key (MSK)
- Keying material that is derived between the EAP peer and server and exported by the EAP method. In existing implementations a AAA server acting as an EAP server transports the MSK to the Authenticator [8] (Aboba, B., “Extensible Authentication Protocol (EAP) Key Management Framework,” January 2006.). EAP-PSK generates a 64-byte MSK.
- Network Access Identifier (NAI)
- Identifier used to identify the communicating parties [1] (Aboba, B. and M. Beadles, “The Network Access Identifier,” January 1999.).
- One Key CBC-MAC 1 (OMAC1)
- A method to generate a Message Authentication Code [31] (Iwata, T. and K. Kurosawa, “OMAC: One-Key CBC MAC,” 2003.). CMAC is the name under which NIST has standardized OMAC1.
- Perfect Forward Secrecy (PFS)
- The confidence that the compromise of a long-term private key does not compromise any earlier session keys. In other words, once an EAP dialog is finished and its corresponding keys are forgotten, even someone who has recorded all of the data from the connection and gets access to all of the long-term keys of the peer and the server cannot reconstruct the keys used to protect the conversation without doing a brute force search of the session key space. EAP-PSK does not have this property.
- Pre-Shared Key (PSK)
- A Pre-Shared Key simply means a key in symmetric cryptography. This key is derived by some prior mechanism and shared between the parties before the protocol using it takes place. It is merely a bit sequence of given length, each bit of which has been chosen at random uniformly and independently. For EAP-PSK, the PSK is the long term 16-byte credential shared by the EAP peer and server.
- Protected Result Indication
- Please refer to Section 7.16 of [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.) for a definition of this term. This feature has been introduced because EAP-Success/Failure packets are unidirectional and are not protected.
- Transient EAP Key (TEK)
- A session key which is used to establish a protected channel between the EAP peer and server during the EAP authentication exchange. The TEK is appropriate for use with the ciphersuite negotiated between the EAP peer and server to protect the EAP conversation. Note that the ciphersuite used to set up the protected channel between the EAP peer and server during EAP authentication is unrelated to the ciphersuite used to subsequently protect data sent between the EAP peer and Authenticator [8] (Aboba, B., “Extensible Authentication Protocol (EAP) Key Management Framework,” January 2006.). EAP-PSK uses a 16-byte TEK for its protected channel, which is the only ciphersuite available between the EAP peer and server to protect the EAP conversation. This ciphersuite uses AES-128 in the EAX mode of operation.
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All numbers presented in this document are considered in network-byte order.
|| denotes concatenation of strings (and not the logical OR).
MAC(K, String) denotes the MAC of String under the key K (the algorithm used in this document to compute the MACs is CMAC with AES-128, see Section 2.2.2 (The Authenticated Key Exchange)).
[String] denotes the concatenation of String with the MAC of String
calculated as specified by the context. Hence, we have, with K specified by the context:
[String]=String||MAC(K,String) .
** denotes integer exponentiation.
"i" denotes the unsigned binary representation on 16 bytes of the integer i in network byte order. Therefore this notation only makes sense when i is between 0 and 2**128-1.
<i> denotes the unsigned binary representation on 4 bytes of the integer i in network byte order. Therefore this notation only makes sense when i is between 0 and 2**32-1.
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At the time this document is written, only three EAP methods are standards track EAP methods per IETF terminology (see [17] (Bradner, S., “The Internet Standards Process -- Revision 3,” October 1996.)), namely:
Unfortunately, all three methods are deprecated for security reasons that are explained in part in [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.).
Myriads of EAP methods have however been otherwise proposed:
However, no secure and mature Pre-Shared Key EAP method is yet easily and widely available, which is all the more regrettable that Pre-Shared Key methods are the most basic ones!
The existing proposals for a future Pre-Shared Key EAP method are briefly reviewed hereafter (please refer to [16] (Bersani, F., “EAP shared key methods: a tentative synthesis of those proposed so far,” April 2004.) for a more thorough synthesis on EAP methods).
Among these proposals, there are some which:
EAP-PSK differs from the aforementioned methods on the following points:
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Figure 1 (EAP-PSK overview) presents an overview of the EAP-PSK protocol.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++ ---+
| | ^
| EAP-PSK Protocol: a Pre-Shared Key EAP Method | |
| | |
| +----------+ | |
| | PSK | | |
| |(16 bytes)| | |
| +----------+ | |
| | | |
| v | |
| *********************** | |
| *Modified Counter Mode* | |
| *********************** | |
| | | | |
| v v | |
| +----------+ +----------+ +----------------+ | |
| | AK | | KDK | | RAND_P | | |
| |(16 bytes)| |(16 bytes)| |(16 bytes) | | |
| +----------+ +----------+ +----------------+ | |
| | | | |
| | | | |
| +-----------+ | | | |
| +--------+|Plain Text | | | | |
|+-------+|Header H||Var.Length | | | | |
||Nonce N||22 bytes|+-----------+ v v Local |
||4 bytes|+--------+ | *********************** to EAP |
|+-------+ | +--------+ +------*Modified Counter Mode* Method |
| | v v v *********************** | |
| | ******* +--------+ |64 |64 | |
| | * EAX *-------|TEK | |bytes |bytes | |
| +-->******* |16 bytes| | | | |
| | +--------+ | | | |
| +-----+----+ | | | |
| v v | | | |
|+--------+ +-------------------+ | | | |
||Tag | |Cipher Text Payload| | | | |
||16 bytes| | Variable length L | | | | |
|+--------+ +-------------------+ | | | V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++ ---+
| | ^
+-+-+-+-+-++ +-+-+-+-+-++ |
|MSK | |EMSK | |
| | | | Exported |
+-+-+-+-+-++ +-+-+-+-+-++ by EAP |
| | Method |
| | |
V V |
************************* V
* AAA Key Derivation * ---+
* Naming & Binding *
*************************
| Figure 1: EAP-PSK overview |
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This section presents the key hierarchy used by EAP-PSK. This hierarchy is inspired by the EAP Key hierarchy described in [8] (Aboba, B., “Extensible Authentication Protocol (EAP) Key Management Framework,” January 2006.).
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EAP-PSK uses a triplet (ID_P, ID_S, PSK) as its initial static credential.
The PSK is shared between the EAP peer and the EAP server.
EAP-PSK assumes that the PSK is known only to the EAP peer and EAP server. The security properties of the protocol is compromised if it has wider distribution. Please note that EAP-PSK shares this property with all other symmetric key methods (including all password based methods).
EAP-PSK also assumes the EAP server and EAP peer identify the correct PSK to use with each other thanks to their respective NAIs. This means that there MUST only be at most one PSK shared between an EAP server using a given server NAI and an EAP peer using a given peer NAI.
This PSK is used, as shown in Figure 2 (Derivation of AK and KDK from the PSK), to derive two 16-byte static long-lived subkeys, respectively called the Authentication Key (AK) and the Key-Derivation Key (KDK). This derivation should only be done once: it is called the key setup. For an explanation of why PSK is not used a static long-lived key but only as the initial keying material from which the static long-lived keys, AK and KDK, that are actually used by the protocol EAP-PSK, see Section 2.2.1 (The Key Setup).
+---------------------------+
| PSK |
| (16 bytes) |
+---------------------------+
| |
v v
+---------------------------+ +---------------------------+
| AK | | KDK |
| (16 bytes) | | (16 bytes) |
+---------------------------+ +---------------------------+
| Figure 2: Derivation of AK and KDK from the PSK |
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EAP-PSK uses AK to mutually authenticate the EAP peer and the EAP server.
AK is a static long-lived key derived from the PSK, see Section 2.2.1 (The Key Setup). AK is not a session key.
The EAP server and EAP peer identify the correct AK to use with each other thanks to their respective NAIs. This means that there MUST only be at most one AK shared between an EAP server using a given server NAI and an EAP peer using a given peer NAI. This is the case when there is at most one PSK shared between an EAP server using a given server NAI and an EAP peer using a given peer NAI, see Section 2.1.1 (The PSK).
The EAP peer chooses the AK to use based on the EAP server NAI that has been sent by the EAP server in the first EAP-PSK message (namely ID_S, see Section 2.3.1 (EAP-PSK Standard Authentication)) and the EAP peer NAI it chooses to include in the second EAP-PSK message (namely ID_P, see Section 2.3.1 (EAP-PSK Standard Authentication)).
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EAP-PSK uses KDK to derive session keys shared by the EAP peer and the EAP server (namely, the TEK, MSK and EMSK).
KDK is a static long-lived key derived from the PSK, see Section 2.2.1 (The Key Setup). KDK is not a session key.
The EAP server and EAP peer identify the correct KDK to use with each other thanks to their respective NAIs. This means that there MUST only be at most one KDK shared between an EAP server using a given server NAI and an EAP peer using a given peer NAI. This is the case when there is at most one PSK shared between an EAP server using a given server NAI and an EAP peer using a given peer NAI, see Section 2.1.1 (The PSK).
The EAP peer chooses the KDK to use based on the EAP server NAI that has been sent by the EAP server in the first EAP-PSK message (namely ID_S, see Section 2.3.1 (EAP-PSK Standard Authentication)) and the EAP peer NAI it chooses to include in the second EAP-PSK message (namely ID_P, see Section 2.3.1 (EAP-PSK Standard Authentication)).
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EAP-PSK derives a 16-byte TEK thanks to a random number exchanged during authentication (RAND_P, see Section 3.1 (EAP-PSK First Message)) and KDK.
This TEK is used to implement a protected channel for both mutually authenticated parties to communicate over securely.
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EAP-PSK derives a MSK thanks to a random number exchanged during authentication (RAND_P, see Section 3.1 (EAP-PSK First Message)) and the KDK.
The MSK is 64 bytes long, which complies with [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.).
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EAP-PSK derives an EMSK thanks to a random number exchanged during authentication (RAND_P, see Section 3.1 (EAP-PSK First Message)) and the KDK.
The EMSK is 64 bytes long, which complies with [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.).
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EAP-PSK does not derive any IV, which complies with [8] (Aboba, B., “Extensible Authentication Protocol (EAP) Key Management Framework,” January 2006.).
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EAP-PSK relies on a single cryptographic primitive, a block cipher, which is instantiated with AES-128. AES-128 takes a 16-byte Pre-Shared Key and a 16-byte Plain Text block as inputs. It outputs a 16-byte Cipher Text block. For a detailed description of AES-128, please refer to [6] (National Institute of Standards and Technology, “Specification for the Advanced Encryption Standard (AES),” November 2001.).
AES-128 has been chosen because:
Other block ciphers could easily be proposed for EAP-PSK, as EAP-PSK does not intricately depend on AES-128. The only parameters of AES-128 that EAP-PSK depends on, are the AES-128 block and key size (16 bytes). For the sake of simplicity, EAP-PSK has however been chosen to restrict to a single mandatory block cipher and not allow the negotiation of other block ciphers. In case AES-128 is deprecated for security reasons, EAP-PSK should also be deprecated and a cut-and-paste EAP-PSK' should be defined with another block cipher. This EAP-PSK' should not be backward compatible with EAP-PSK because of the security issues with AES-128. EAP-PSK' should therefore use a different EAP-Request/Response Type number. With the EAP-Request/Response Type number space structure defined in [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.), this should not be a problem. The use of a different EAP-Request/Response Type number for EAP-PSK' will prevent this new method from being vulnerable to chosen protocol attacks.
EAP-PSK uses three cryptographic parts:
Each part is discussed in more detail in the subsequent paragraphs.
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EAP-PSK needs two cryptographically separated 16-byte subkeys for mutual authentication and session key derivation. Indeed, it is a rule of the thumb in cryptography to use different keys for different applications.
It could have implemented these two subkeys either by specifying a 32-byte PSK that would then be split in two 16-byte subkeys, or by specifying a 16-byte PSK that would then be cryptographically expanded to two 16-byte subkeys.
Because provisioning a 32-byte long term credential is more cumbersome than a 16-byte one, and the strength of the derived session keys is 16 bytes either ways, the latter option was chosen.
Hence, the PSK is only used by EAP-PSK to derive AK and KDK. This derivation should be done only once, immediately after the PSK has been provisioned. As soon as AK and KDK have been derived, the PSK should be deleted. If the PSK is deleted, it should be done so securely (see, for instance, [19] (Department of Defense of the United States, “National Industrial Security Program Operating Manual,” January 1995.) for guidance on secure deletion of the PSK).
Derivation of AK and KDK from the PSK is called the key setup:
AK and KDK are derived from the PSK using the modified counter mode of operation of AES-128. The modified counter mode is a length increasing function, i.e., it expands one AES-128 input block into a longer t-block output, where t>=2. This mode was chosen for the key setup because it had already been chosen for the derivation of the session keys (see Section 2.2.2 (The Authenticated Key Exchange)).
The details of the derivation of AK and KDK from the PSK are shown in Figure 3 (Derivation of AK and KDK from the PSK in Details).
+--------------------------+
| "0" |
| Input Block (16 bytes) |
+--------------------------+
|
v
+----------------+
| |
| AES-128(PSK,.) |
| |
+----------------+
|
|
+----------------------------+
| |
v v
+--------+ +---+ +--------+ +---+
| c1="1" |->|XOR| | c2="2" |->|XOR|
|16 bytes| +---+ |16 bytes| +---+
+--------+ | +--------+ |
| |
+----------------+ +----------------+
| | | |
| AES-128(PSK,.) | | AES-128(PSK,.) |
| | | |
+----------------+ +----------------+
| |
| |
v v
+------------------------+ +------------------------+
| AK | | KDK |
| (16 bytes) | | (16 bytes) |
+------------------------+ +------------------------+
| Figure 3: Derivation of AK and KDK from the PSK in Details |
The input block is "0". For the sake of simplicity, this input block has been chosen constant: it could have been set to a value depending on the peer and the server (for instance, the XOR of their respective NAIs appropriately truncated or zero-padded), but this did not seem to add much security to the scheme, whereas it added complexity. Any 16-byte constant could have been chosen, as the security is not supposed to depend on the particular value taken by the constant. "0" was arbitrarily chosen.
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The authentication protocol used by EAP-PSK is inspired of AKEP2 which is described in [14] (Bellare, M. and P. Rogaway, “Entity Authentication and Key Distribution,” 1994.).
AKEP2 consists of a one and half round trip exchange, as shown in Figure 4 (Overview of AKEP2) which is inspired of Figure 5 of [14] (Bellare, M. and P. Rogaway, “Entity Authentication and Key Distribution,” 1994.).
Bob Alice | RA | |<---------------------------------------------------------| | | | [B||A||RA||RB] | |--------------------------------------------------------->| | | | [A||RB] | |<---------------------------------------------------------|
| Figure 4: Overview of AKEP2 |
It is also worth noting that [14] (Bellare, M. and P. Rogaway, “Entity Authentication and Key Distribution,” 1994.) focuses on cryptography and not on designing a real-life protocol. Thus, as noted in subsection "Out-Of-Band-Data" of [14] (Bellare, M. and P. Rogaway, “Entity Authentication and Key Distribution,” 1994.), Alice has to send A, its identity, to Bob so that Bob may select the appropriate credential for the sequel of the conversation. This leads to a slightly complemented version of AKEP2 for EAP-PSK as depicted in Figure 5 (Overview of AKEP2 with out-of-band data).
Bob Alice | A||RA | |<---------------------------------------------------------| | | | [B||A||RA||RB] | |--------------------------------------------------------->| | | | [A||RB] | |<---------------------------------------------------------|
| Figure 5: Overview of AKEP2 with out-of-band data |
In AKEP2,
EAP-PSK instantiates this protocol with:
CMAC was chosen as the MAC algorithm because it is capable of handling of arbitrary length messages, and its design is simple. It also enjoys up to date review by the cryptographic community, especially using provable security concepts. It has been recommended by the NIST. For a detailed description of CMAC, please refer to [7] (National Institute of Standards and Technology, “Recommendation for Block Cipher Modes of Operation: The CMAC Mode for Authentication,” May 2005.).
In AKEP2 the key exchange is "implicit": the session keys are derived from RB. In EAP-PSK, the session keys are thus derived from RAND_P by using KDK and the modified counter mode of operation of AES-128 described in [4] (Gilbert, H., “The Security of One-Block-to-Many Modes of Operation,” 2003.). This mode was chosen because it is a simple key derivation schemes that relies on a block cipher and has a proof of its security. It is a length increasing function, i.e., it expands one AES-128 input block into a longer t-block output, where t>=2. The derivation of the session keys is shown in Figure 6 (Derivation of the Session Keys).
+--------------------------+ +-------------------------------+
| RAND_P | | KDK |
| Input Block (16 bytes) | | Key Derivation Key (16 bytes) |
+--------------------------+ +-------------------------------+
| |
v v
+-----------------------------------------------------------------+
| |
| Modified Counter Mode |
| |
+-----------------------------------------------------------------+
| | |
v v v
+------------+ +----------------------+ +----------------------+
| TEK | | MSK | | EMSK |
| (16 bytes) | | (64 bytes) | | (64 bytes) |
+------------+ +----------------------+ +----------------------+
| Figure 6: Derivation of the Session Keys |
The input to the derivation of the session keys is RAND_P.
The outputs of the derivation of the session keys are:
- The 16-byte TEK (the first output block).
- The 64-byte MSK (the concatenation of the second to fifth output blocks).
- The 64-byte EMSK (the concatenation of the sixth to ninth output blocks).
The details of the derivation of the session keys are shown in Figure 7 (Derivation of the Session Keys in Details).
+--------------------------+
| RB |
| Input Block (16 bytes) |
+--------------------------+
|
v
+----------------+
| |
| AES-128(KDK,.) |
| |
+----------------+
|
|
+---------------------+-- - - - - - - - - - --+
| | |
v v v
+--------+ +---+ +--------+ +---+ +--------+ +---+
| c1="1" |->|XOR| | c2="2" |->|XOR|.......| c9="9" |->|XOR|
|16 bytes| +---+ |16 bytes| +---+ |16 bytes| +---+
+--------+ | +--------+ | +--------+ |
| | |
+----------------+ +----------------+ +----------------+
| | | | | |
| AES-128(KDK,.) | | AES-128(KDK,.) |......| AES-128(KDK,.) |
| | | | | |
+----------------+ +----------------+ +----------------+
| | |
| | |
v v v
+-----------------+ +-----------------+ +------------------+
| Output Block #1 | | Output Block #2 | | Output Block #9 |
| (16 bytes) | | (16 bytes) |.....| (16 bytes) |
| TEK | | MSK (block 1/4) | | EMSK (block 4/4) |
+-----------------+ +-----------------+ +------------------+
| Figure 7: Derivation of the Session Keys in Details |
The counter values are set respectively to the first t integers (that is ci="i", with i=1 to 9).
Keying material is sensitive information and should be handled accordingly (see Section 6.10 (Exposition of the PSK) for further discussion.
| TOC |
EAP-PSK provides a protected channel for both parties to communicate over, in case of a successful authentication. This protected channel is currently used to exchange protected result indications and may be used in the future to implement extensions.
EAP-PSK uses the EAX mode of operation to provide this protected channel. For a detailed description of EAX, please refer to [3] (Bellare, M., Rogaway, P., and D. Wagner, “The EAX mode of operation,” 2004.). Figure 8 (The Protected Channel) shows how EAX is used to implement EAP-PSK protected channel.
+-----------+ +----------------+ +---------------------+ +----------+
| Nonce N | | Header H | | Plain Text Payload | | TEK |
| 4 bytes | | 22 bytes | | Variable length L | | 16 bytes |
+-----------+ +----------------+ +---------------------+ +----------+
| | | |
v v v v
+-------------------------------------------------------------------+
| |
| EAX |
| |
+-------------------------------------------------------------------+
| |
v v
+---------------------+ +----------+
| Cipher Text Payload | | Tag |
| Variable length L | | 16 bytes |
+---------------------+ +----------+
| Figure 8: The Protected Channel |
This protected channel:
EAX is instantiated with AES-128 as the underlying block cipher.
AES-128 is keyed with the TEK.
The nonce N is used to provide cryptographic security to the
encryption and data origin authentication as well as protection replay.
Indeed, N is a 4-byte sequence number starting from <0> that is
monotonically incremented at each EAP-PSK message within
one EAP-PSK dialog, except retransmissions of course.
N was taken to be 4 bytes to avoid 16-byte
arithmetic. Since EAX uses a 16-byte nonce, N is padded with 96 zero
bits for its high order bits.
For cryptographic reasons, N is not allowed to wrap around. In the unlikely, yet possible, event of the server sending an EAP-PSK message with
N set to <2**32-2>, it must not send any
further message on this protected channel, which would cause to reuse the value 0.
Either the conversation is
finished after the server receives the EAP-PSK answer from the peer
with N set to <2**32-1> and the server
proceeds (typically by sending an EAP-Success or
Failure), or the conversation is not finished and must then be
aborted (a new EAP-PSK dialog may subsequently be started to try
again to authenticate).
Thus, the maximum number of messages that can be exchanged over the
same protected channel is 2**32 (which should not be a limitation in
practice as this is approximately equal to 4 billions).
The Header H consists in the first 22 bytes of the EAP Request or Response packet (i.e. the EAP Code, Identifier, Length and Type fields followed by the EAP-PSK Flags and RAND_S fields). Although it may appear unorthodox that an upper layer (EAP-PSK) protects some information of the lower layer (EAP), this was chosen to comply with EAP recommendation (see Section 7.5. of [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.)) and seems to be existing practice at IETF (see, for instance, [38] (Kent, S. and R. Atkinson, “IP Authentication Header,” November 1998.)).
The Plain Text Payload is the payload that is to be encrypted and integrity protected. The Cipher Text payload is the result of the encryption of the Plain Text.
The Tag is a MAC that protects both the Header and the Plain Text
Payload.
The verification of the Tag must only be done after a successful verification of the Nonce
for replay protection.
If the verification of the Tag succeeds, then the Encrypted Payload is
decrypted to recover the Plain Text Payload. If the verification of the Tag fails,
then no decryption is performed and this MAC failure should be logged.
The tag length is chosen to be 16 bytes for EAX within EAP-PSK.
This length is considered appropriate by the cryptographic community.
EAX was mainly chosen because:
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EAP-PSK may consist of two different types of message flows:
EAP-PSK introduces the concept of session to facilitate its analysis and provide a cleaner interface to other layers. A session is a particular instance of an EAP-PSK dialog between two parties. This session is identified by a session identifier.
In the first EAP-PSK message, the EAP server asserts its identity. Given that the EAP-Request/Identity and EAP-Response/Identity may not be assumed to have occured prior to this sending and that the response included in EAP-Response/Identity (if this EAP Identity exchange takes) place may not contain the actual NAI the peer shall use with EAP-PSK, this means that an EAP server implementing EAP-PSK must use the same EAP server NAI for all EAP-PSK dialogs with any EAP peer implementing EAP-PSK.
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EAP-PSK standard authentication is comprised of four messages, i.e., two round trips; see Figure 9 (EAP-PSK Standard Authentication).
peer server | Flags||RAND_S||ID_S | |<---------------------------------------------------------| | | | Flags||RAND_S||RAND_P||MAC_P||ID_P | |--------------------------------------------------------->| | | | Flags||RAND_S||MAC_S||PCHANNEL_S_0 | |<---------------------------------------------------------| | | | Flags||RAND_S||PCHANNEL_P_1 | |--------------------------------------------------------->| | |
| Figure 9: EAP-PSK Standard Authentication |
The PCHANNEL_S_0 and PCHANNEL_P_1 fields of the third and fourth EAP-PSK messages contain a MAC computed thanks to TEK that protects the integrity of the messages. For a detailed list of the fields of the messages that are integrity protected please refer to Section 2.2.3 (The Protected Channel).
All EAP-PSK messages include a sort of header which is comprised of two fields:
This standard message flow could be comprised of only three messages, like AKEP2, were it not the request/response nature of EAP that prevents the third message to be the last one. Since the fourth message is mandatory, EAP-PSK chose to take advantage of this and set up a protected channel.
The standard message flow also includes a statement by the peer of its identity, in addition to the EAP-Response/Identity it may have sent. This behavior follows Section 5.1 of [2] (Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, “Extensible Authentication Protocol (EAP),” June 2004.) which recommends that the EAP-Response/Identity be used primarily for routing purposes and selecting which EAP method to use, and therefore that EAP methods include a method-specific mechanism for obtaining the identity, so that they do not have to rely on the Identity Response.
When a party receives an EAP-PSK message, it checks that the message is syntaxically valid in accordance with the message formats defined in Section 3 (EAP-PSK Message format). If the message is syntaxically incorrect, then it is silently discarded.Then it checks the cryptographic validity of this message, i.e. it checks the MAC(s), namely
In case a validity check fails, the message is silently discarded. There can be a counter to track the number of silently discarded messages Section 6.8 (Denial of Service Resistance). In case, there is an encrypted payload in the message (namely in the PCHANNEL attribute), then the encrypted payload is decrypted. Then, if the decrypted payload is syntaxically incorrect then the message is silently discarded.
| TOC |
To remain simple and yet be extensible to meet future requirements, EAP-PSK provides an extension mechanism within its protected channel: the payload of the protected channel may contain an optional extension field (EXT).
Figure 10 (EAP-PSK Extended Authentication) presents the message sequence for EAP-PSK extended authentication.
Although support of the EXT field is mandatory, there is no mandatory extension type to support. The mandatory support of the EXT field is dictated:
No extension is currently defined.
At most One extension may be run within a single EAP-PSK dialog: there can neither be sequences of extensions nor interleaved extensions. However, extensions may take a variable number of round trips to complete.
Only the server can start an extension and, if it does so, it must start it in the first payload it sends over the protected channel.
peer server | Flags||RAND_S||ID_S | |<---------------------------------------------------------| | | | Flags||RAND_S||RAND_P||MAC_P||ID_P | |--------------------------------------------------------->| | | | Flags||RAND_S||MAC_S||PCHANNEL_S_0(EXT) | |<---------------------------------------------------------| | | | Flags||RAND_S||PCHANNEL_P_1(EXT) | |--------------------------------------------------------->| | | . . . . . . | Flags||RAND_S||PCHANNEL_S_2i(EXT) | |<---------------------------------------------------------| | | | Flags||RAND_S||PCHANNEL_P_2i+1(EXT) | |--------------------------------------------------------->| | |
| Figure 10: EAP-PSK Extended Authentication |
Please refer to Section 4 (Rules of Operation for the EAP-PSK Protected Channel) for more details on how extended authentication works.
The PCHANNEL_S_2j and PCHANNEL_P_2j+1 fields of the EAP-PSK messages (where j varies from 0 to i) contain a MAC computed thanks to TEK that protects the integrity of the messages. For a detailed list of the fields of the messages that are integrity protected please refer to Section 2.2.3 (The Protected Channel).
When a party receives an EAP-PSK message, it checks that the message is syntaxically valid in accordance with the message formats defined in Section 3 (EAP-PSK Message format). If the message is syntaxically incorrect, then it is silently discarded.Then it checks the cryptographic validity of this message, i.e. it checks the MAC(s), namely
In case a validity check fails, the message is silently discarded. There can be a counter to track the number of silently discarded messages Section 6.8 (Denial of Service Resistance). In case, there is an encrypted payload in the message (namely in the PCHANNEL attribute), then the encrypted payload is decrypted. Then, if the decrypted payload is syntaxically incorrect then the message is silently discarded.
| TOC |
For the sake of simplicity, EAP-PSK uses a fixed message format. There are four different types of EAP-PSK messages:
For the sake of clarity, the whole EAP packet that encapsulates the EAP-PSK message, i.e., the EAP-PSK message plus its EAP headers, are depicted in Figure 11 (EAP-PSK First Message), Figure 13 (EAP-PSK Second Message), Figure 14 (EAP-PSK Third Message) and Figure 18 (EAP-PSK Fourth Message).
| TOC |
The first EAP-PSK message is sent by the server to the peer. It has the format presented in Figure 11 (EAP-PSK First Message).
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=1 | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type EAP-PSK | Flags | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + + | RAND_S | + + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + : : : ID_S : : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Figure 11: EAP-PSK First Message |
The first EAP-PSK message consists of:
The Flags field has the format presented in Figure 12 (EAP-PSK Flags Field).
0 0 1 2 3 4 5 6 7 8 +-+-+-+-+-+-+-+-+ | T | Reserved | +-+-+-+-+-+-+-+-+
| Figure 12: EAP-PSK Flags Field |
The Flags field is comprised of two subfields:
The PCHANNEL Nonce field N (see Section 3.3 (EAP-PSK Third Message)) is used to distinguish between the different EAP-PSK messages that may be exchanged during extended authentication which all have T set to 3, i.e., the fourth EAP-PSK message and possibly the next ones.
| TOC |
The second EAP-PSK message is sent by the peer to the server. It has the format presented in Figure 13 (EAP-PSK Second Message).
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=2 | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type EAP-PSK | Flags | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + + | RAND_S | + + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + + | RAND_P | + + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+ + | | + + | MAC_P | + + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+ + : ID_P : : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Figure 13: EAP-PSK Second Message |
It consists of:
The Flags field format is presented in Figure 12 (EAP-PSK Flags Field).
| TOC |
The third EAP-PSK message is sent by the server to the peer. It has the format presented in Figure 14 (EAP-PSK Third Message).
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=1 | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type EAP-PSK | Flags | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + + | RAND_S | + + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + + | MAC_S | + + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+ + : PCHANNEL : : : : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Figure 14: EAP-PSK Third Message |
It consists of:
The Flags field format is presented in Figure 12 (EAP-PSK Flags Field).
If there is no extension, i.e., if the authentication is standard, the PCHANNEL field consists of:
R, E and Reserved are sent encrypted by the protected channel (see Section 2.2.3 (The Protected Channel)).
If there is no extension, PCHANNEL has the format presented in Figure 15 (The PCHANNEL Field with E=0) (where R, E and Reserved are presented in the clear for the sake of clarity, although in reality they are sent encrypted).
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | Tag | + + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | R |0| Reserved| +-+-+-+-+-+-+-+-+
| Figure 15: The PCHANNEL Field with E=0 |
If there is an extension, i.e., if the authentication is extended, the PCHANNEL field consists of:
R, E, Reserved and EXT are sent encrypted by the protected channel (see Section 2.2.3 (The Protected Channel)).
If there is an extension, PCHANNEL has the format presented in Figure 16 (The PCHANNEL Field with E=1) where R, E, Reserved and EXT are presented in the clear for the sake of clarity, although in reality they are sent encrypted)..
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Nonce | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | Tag | + + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | R |1| Reserved| | +-+-+-+-+-+-+-+-+ + : EXT : : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Figure 16: The PCHANNEL Field with E=1 |
This EXT field is split in two subfields:
The format of the EXT field is presented in Figure 17 (The EXT Field).
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | EXT_Type | | +-+-+-+-+-+-+-+-+ + : EXT_Payload : : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Figure 17: The EXT Field |
| TOC |
The fourth EAP-PSK message is sent by the peer to the server. It has the format presented in Figure 18 (EAP-PSK Fourth Message).
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=2 | Identifier | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type EAP-PSK | Flags | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | + + | RAND_S | + + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + : : : PCHANNEL : : : : : + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Figure 18: EAP-PSK Fourth Message |
It consists of:
The Flags field format is presented in Figure 12 (EAP-PSK Flags Field).
The PCHANNEL field has the following structure, which was already described in Section 3.3 (EAP-PSK Third Message).
If there is no extension, i.e., if the authentication is standard, the PCHANNEL field consists of:
R, E and Reserved are sent encrypted by the protected channel (see Section 2.2.3 (The Protected Channel)).
If there is no extension, PCHANNEL has the format presented in Figure 15 (The PCHANNEL Field with E=0).
If there is an extension, i.e., if the authentication is extended, the PCHANNEL field consists of:
R, E, Reserved and EXT are sent encrypted by the protected channel (see Section 2.2.3 (The Protected Channel)).
If there is an extension, PCHANNEL has the format presented in Figure 16 (The PCHANNEL Field with E=1).
This EXT field is split in two subfields:
The format of the EXT field is presented in Figure 17 (The EXT Field).
| TOC |
In this section, the rules of operation of the EAP-PSK protected channel are presented:
| TOC |
The R flag of the PCHANNEL field in the third and fourth type of EAP-PSK messages is used to provide result indications.
Since this 2-bit flag is communicated over the protected channel, it is:
This 2-bit R flag can take the following values:
The peer and the server each remember some information about both the values of R that they have sent and the values of R they have received. It is the conjunction of both sent and received R values that indicate the success or the failure of the EAP-PSK dialog.
In case of a standard authentication, the following values of R should be exchanged:
In case of an extended authentication, more complex exchanges may occur, which is why the CONT value was introduced.
The rules of operation for each value R may take are presented in details hereafter.
| TOC |
The server and the peer each initialize the values of R they intend to send and receive as CONT.
Here CONT stands for "Continue". It indicates that the EAP-PSK dialog is not yet successful and that the party sending it wants to continue the dialog to try and reach success.
Indeed, although the peer and the server must have successfully authenticated each other, thanks to MAC_P and MAC_S, before they start communicating over the protected channel, the EAP-PSK dialog may not yet be deemed successful after this mutual authentication because of authorization issues. For instance, a prepaid customer of a wireless Hot-Spot might have successfully authenticated but has to refill its account, e.g., with a credit card transaction over the protected channel, before it is authorized.
| TOC |
DONE_SUCCESS indicates that the party that sent it deems the EAP-PSK dialog successful and therefore proposes to end this dialog.
Once the server has sent a DONE_SUCCESS, it must keep sending this value for R.
The peer must first receive a DONE_SUCCESS from the server before it is allowed to send a DONE_SUCCESS.
After the peer has received a DONE_SUCCESS from the server, it may:
| TOC |
DONE_FAILURE indicates that the party that sent it deems the EAP-PSK dialog unsuccessful and proposes to end this dialog because nothing will make it change its mind.
If the server is the first to send a DONE_FAILURE, then, the peer that receives this DONE_FAILURE must reply with a DONE_FAILURE and fail, which ends the EAP-PSK dialog.
If the peer is the first to send a DONE_FAILURE, then, the server that receives this DONE_FAILURE must immediately end this EAP-PSK dialog without sending any further EAP-PSK message, and fail.
| TOC |