RFC 4764
The EAP-PSK Protocol: A Pre-Shared Key Extensible Authentication Protocol (EAP) Method
Pages: 64
Experimental
Experimental
Part 2 of 3 – Pages 15 to 41
3. Cryptographic Design of EAP-PSK
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 [7]. AES-128 has been chosen because: o It is standardized and implementations are widely available.
o It has been carefully reviewed by the cryptographic community and
is believed to be secure.
Other block ciphers could easily be proposed for EAP-PSK, as EAP-PSK
does not intrinsically 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 the case that 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 [3], 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:
o A key setup to derive AK and KDK from the PSK.
o An authenticated key exchange protocol to mutually authenticate
the communicating parties and derive session keys.
o A protected channel protocol for both mutually authenticated
parties to communicate over.
Each part is discussed in more detail in the subsequent paragraphs.
3.1. The Key Setup
EAP-PSK needs two cryptographically separated 16-byte subkeys for
mutual authentication and session key derivation. Indeed, it is a
rule of 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 way, 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] for guidance on secure deletion of the PSK). Derivation of AK and KDK from the PSK is called the key setup: o The input to the key setup is the PSK. o The outputs of the key setup are AK and KDK. 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 3.2). The details of the derivation of AK and KDK from the PSK are shown in Figure 3.
+--------------------------+ | "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.
3.2. The Authenticated Key Exchange
The authentication protocol used by EAP-PSK is inspired by AKEP2, which is described in [14]. AKEP2 consists of a one-and-a-half round-trip exchange, as shown in Figure 4, which is inspired by Figure 5 of [14]. Bob Alice | RA | |<---------------------------------------------------------| | | | [B||A||RA||RB] | |--------------------------------------------------------->| | | | [A||RB] | |<---------------------------------------------------------| Figure 4: Overview of AKEP2 It is also worth noting that [14] focuses on cryptography and not on designing a real-life protocol. Thus, as noted in subsection "Out- Of-Band-Data" of [14], Alice has to send A, its identity, to Bob so that Bob may select the appropriate credential for the sequel to the conversation. This leads to a slightly complemented version of AKEP2 for EAP-PSK as depicted in Figure 5. Bob Alice | A||RA | |<---------------------------------------------------------| | | | [B||A||RA||RB] | |--------------------------------------------------------->| | | | [A||RB] | |<---------------------------------------------------------| Figure 5: Overview of AKEP2 In AKEP2, o RA and RB are random numbers chosen respectively by Alice and Bob. o A and B are Alice's and Bob's respective identities. They allow Alice and Bob to retrieve the key that they have to use to run an authenticated key exchange between each other. They are also included in the protocol for cryptographic reasons.
o The MACs (see Section 1.3 for the notation "[]") are calculated using a dedicated key. EAP-PSK instantiates this protocol with: o The server as Alice and the peer as Bob. o RA and RB as 16-byte random numbers, using Section 4.1 notations; this means RA=RAND_S and RB=RAND_P. o A and B as Alice's and Bob's respective NAIs, using Section 4.1 notations; this means A=ID_S and B=ID_P. o The MAC algorithm as CMAC with AES-128 using AK and producing a tag length of 16 bytes. o The modified counter mode of operation of AES-128 using KDK, to derive session keys as a result of this exchange. CMAC was chosen as the MAC algorithm because it is capable of handling 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 [8]. 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 [5]. This mode was chosen because it is a simple key derivation scheme 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.
+--------------------------+ +-------------------------------+ | 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: o The 16-byte TEK (the first output block). o The 64-byte MSK (the concatenation of the second to fifth output blocks). o 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.
+--------------------------+ | RAND_P | | 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 8.10 for further discussion).
3.3. The Protected Channel
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 [4]. Figure 8 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: o Provides replay protection. o Encrypts and authenticates a Plain Text Payload that becomes an Encrypted Payload. The Plain Text Payload must not exceed 960 bytes; see Sections 5.3, 5.4, and 8.11. o Only authenticates a Header that is thus sent in clear. 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 reusing 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 billion). The Header H consists of 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 [3]) and seems to be existing practice at IETF (see, for instance, [35]). 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:
o It strongly relies on OMAC in its design and OMAC1, a variant of
OMAC, had already been chosen in EAP-PSK for the authentication
part (please remember that OMAC1 and CMAC are analogous).
o Its design is simple.
o It enjoys a security proof.
o It is free of any Intellectual Property Rights claims.
4. EAP-PSK Message Flows
EAP-PSK may consist of two different types of message flows:
o The "standard authentication", which is:
* Mandatory to implement.
* Fully specified in this document.
* The simpler type of message flow, which is expected to be used
most frequently.
o The "extended authentication", which is:
* Optional to implement (i.e., there are no mandatory
extensions).
* Partly specified in this document since it depends on
extensions and none are currently specified, let alone in this
document.
* The type of message flow that should be used when extensions of
EAP-PSK are needed by more sophisticated usage scenarios and
are available.
EAP-PSK introduces the concept of a 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 occurred 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.4.1. EAP-PSK Standard Authentication
EAP-PSK standard authentication is comprised of four messages, i.e., two round-trips; see Figure 9. 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 o The first message is sent by the server to the peer to: * Send a 16-byte random challenge (RAND_S). RAND_S was called RA in Section 3.2 * State its identity (ID_S). ID_S was denoted by A in Section 3.2. o The second message is sent by the peer to the server to: * Send another 16-byte random challenge (RAND_P). RAND_P was called RB in Section 3.2 * State its identity (ID_P). ID_P was denoted by B in Section 3.2. * Authenticate to the server by proving that it is able to compute a particular MAC (MAC_P), which is a function of the two challenges and AK: MAC_P = CMAC-AES-128(AK, ID_P||ID_S||RAND_S||RAND_P)
o The third message is sent by the server to the peer to:
* Authenticate to the peer by proving that it is able to compute
another MAC (MAC_S), which is a function of the peer's
challenge and AK:
MAC_S = CMAC-AES-128(AK, ID_S||RAND_P)
* Set up the protected channel (P_CHANNEL_S_0) to:
+ Confirm that it has derived session keys (at least the TEK).
+ Give a protected result indication of the authentication.
o The fourth message is sent by the peer to the server to finish the
setup of the protected channel (P_CHANNEL_P_1) to:
* Confirm that it has derived session keys (at least the TEK).
* Give a protected result indication of the 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 3.3.
All EAP-PSK messages include a sort of header, which is comprised of
two fields:
o Flags, a 1-byte field that is currently only used to number EAP-
PSK messages.
o RAND_S, a 16-byte challenge sent by the server that is used as a
session identifier.
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 [3], 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 syntactically valid in accordance with the message formats defined in Section 5. If the message is syntactically incorrect, then it is silently discarded. Then it checks the cryptographic validity of this message, i.e., it checks the MAC(s) as follows: o If the received message is the first EAP-PSK message, there is no MAC to check as none is included in message 1. o If the received message is the second EAP-PSK message, the validity of MAC_P is checked. o If the received message is the third EAP-PSK message, the validity of MAC_S is checked and then the validity of the Tag included in P_CHANNEL_S_0 is checked. The validity checks must be done in this order to avoid unnecessarily deriving TEK, MSK, and EMSK in case MAC_S is invalid, meaning that mutual authentication has failed. Indeed, TEK is used to verify the validity of the Tag included in P_CHANNEL_S_0. o If the received message is the fourth EAP-PSK message, the validity of the Tag included in P_CHANNEL_P_1 is checked. If a validity check fails, the message is silently discarded. There can be a counter to track the number of silently discarded messages Section 8.8. If 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 syntactically incorrect, the message is silently discarded.4.2. EAP-PSK Extended Authentication
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 presents the message sequence for EAP-PSK extended authentication. Extended authentication MUST be supported, i.e., any EAP-PSK implementation MUST support sending and reception of an EXT attribute according to rules of operation described in Section 6. Yet, although support of the EXT field is mandatory, there is no mandatory extension type to support. This means that if a server engages in EAP-PSK extended authentication, as only the server can start extended authentication per Section 6, a peer will recognize the attempt to start extended authentication through its EXT support. If
the peer does not support the particular extension type used by the
server, the peer will still be able to conclude the EAP-PSK dialog.
The mandatory support of the EXT field is dictated:
o To guarantee a robust behavior in the future where some peers
might support some extensions and others not. All peers will thus
be able to understand that an extended authentication is being
attempted and indicate whether or not they support the extension
that is tried.
o To ensure that all implementations will indeed be extensible.
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 6 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 3.3. When a party receives an EAP-PSK message, it checks that the message is syntactically valid in accordance with the message formats defined in Section 5. If the message is syntactically incorrect, then it is silently discarded. Then it checks the cryptographic validity of this message, i.e., it checks the MAC(s) as follows: o If the received message is the first EAP-PSK message, there is no MAC to check as none is included in message 1. o If the received message is the second EAP-PSK message, the validity of MAC_P is checked. o If the received message is the third EAP-PSK message, the validity of MAC_S is checked and then the validity of the Tag included in P_CHANNEL_S_0 is checked. The validity checks must be done in this order to avoid unnecessarily deriving TEK, MSK, and EMSK in case MAC_S is invalid, meaning that mutual authentication has failed. Indeed, TEK is used to verify the validity of the Tag included in P_CHANNEL_S_0. o If the received message is the fourth EAP-PSK message, the validity of the Tag included in P_CHANNEL_P_1 is checked. o If the received message is an EAP-PSK message different from the first four ones, then validity of the Tag included in P_CHANNEL is checked. If a validity check fails, the message is silently discarded. There can be a counter to track the number of silently discarded messages Section 8.8. If 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 syntactically incorrect, the message is silently discarded.
5. EAP-PSK Message Format
For the sake of simplicity, EAP-PSK uses a fixed message format. There are four different types of EAP-PSK messages: o The first EAP-PSK message, which is sent by the server to the peer. o The second EAP-PSK message, which is sent by the peer to the server. o The third EAP-PSK message, which is sent by the server to the peer. o The fourth EAP-PSK message, which is sent by the peer to the server. This is also the type of message that the peer further sends to the server in case of an extended authentication. This is also essentially the type of message that the server further sends to the peer in case of an extended authentication: the only slight modification that occurs in this last case is the setting of the EAP Code to 1 instead of 2 in the other cases. 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) is depicted in Figures 11, 13, 14, and 18.
5.1. EAP-PSK First Message
The first EAP-PSK message is sent by the server to the peer. It has the format presented in Figure 11. 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 Since IANA has allocated EAP method type 47 for EAP-PSK, Type EAP-PSK for the first EAP-PSK message as well as any other EAP-PSK message MUST be 47. The first EAP-PSK message consists of: o A 1-byte Flags field o A 16-byte random number: RAND_S o A variable length field that conveys the server's NAI: ID_S. The length of this field is deduced from the EAP length field. The length of this NAI must not exceed 966 bytes. This restriction aims at avoiding fragmentation issues (see Section 8.11). The Flags field has the format presented in Figure 12.
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: o A 2-bit T subfield, which indicates the type of EAP-PSK message: * T=0 for the first EAP-PSK message presented in Section 5.1. * T=1 for the second EAP-PSK message presented in Section 5.2. * T=2 for the third EAP-PSK message presented in Section 5.3. * T=3 for the fourth EAP-PSK message presented in Section 5.4 and the subsequent EAP-PSK messages that may be exchanged during extended authentication. o A 6-bit Reserved subfield that is set to zero on transmission and ignored on reception. The PCHANNEL Nonce field N (see Section 5.3) is used to distinguish between the different EAP-PSK messages that may be exchanged during extended authentication that all have T set to 3, i.e., the fourth EAP-PSK message and possibly the next ones.
5.2. EAP-PSK Second Message
The second EAP-PSK message is sent by the peer to the server. It has the format presented in Figure 13. 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:
o A 1-byte Flags field
o The 16-byte random number sent by the server in the first EAP-PSK
message (RAND_S) that serves as a session identifier
o A 16-byte random number: RAND_P
o A 16-byte MAC: MAC_P
o A variable length field that conveys the peer's NAI: ID_P. The
length of this field is deduced from the EAP length field. The
length of this NAI must not exceed 966 bytes. This restriction
aims at avoiding fragmentation issues (see Section 8.11).
The Flags field format is presented in Figure 12.
5.3. EAP-PSK Third Message
The third EAP-PSK message is sent by the server to the peer. It has the format presented in Figure 14. 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: o A 1-byte Flags field o The 16-byte random number sent by the server in the first EAP-PSK message (RAND_S) that is used as a session identifier o A 16-byte MAC: MAC_S o A variable length field that constitutes the protected channel: PCHANNEL
The Flags field format is presented in Figure 12. If there is no extension, i.e., if the authentication is standard, the PCHANNEL field consists of: o A 4-byte Nonce N (see Section 3.3). o A 16-byte Tag (see Section 3.3). o A 2-bit result indication flag R. o A 1-bit extension flag E, which is set to 0. o A 5-bit Reserved field, which is set to zero on emission and ignored on reception. R, E, and Reserved are sent encrypted by the protected channel (see Section 3.3). If there is no extension, PCHANNEL has the format presented in Figure 15 (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: o A 4-byte Nonce N (see Section 3.3). o A 16-byte Tag (see Section 3.3).
o A 2-bit result indication flag R.
o A 1-bit extension flag E, which is set to 1.
o A 5-bit Reserved field, which is set to zero on emission and
ignored on reception.
o A variable length EXT field.
R, E, Reserved, and EXT are sent encrypted by the protected channel
(see Section 3.3).
If there is an extension, PCHANNEL has the format presented in
Figure 16 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:
o The EXT_Type subfield, which indicates the type of the extension
o The EXT_Payload subfield, which consists of the payload of the
extension. The EXT_Payload length is derived from the EAP Length
field. EXT_Payload must have a bit length that is a multiple of 8
bits and must not exceed 960 bytes. The latter restriction aims
at avoiding fragmentation issues (see Section 8.11), whereas the
former comes from the EAP length being specified in bytes.
The format of the EXT field is presented in Figure 17.
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
5.4. EAP-PSK Fourth Message
The fourth EAP-PSK message is sent by the peer to the server. It has
the format presented in Figure 18.
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:
o A 1-byte Flags field
o The 16-byte random number sent by the server in the first EAP-PSK
message (RAND_S) that is used as a session identifier
o A variable length field that constitutes the protected channel:
PCHANNEL
The Flags field format is presented in Figure 12.
The PCHANNEL field has the following structure, which was already
described in Section 5.3.
If there is no extension, i.e., if the authentication is standard,
the PCHANNEL field consists of:
o A 4-byte Nonce N (see Section 3.3).
o A 16-byte Tag (see Section 3.3).
o A 2-bit result indication flag R.
o A 1-bit extension flag E, which is set to 0.
o A 5-bit Reserved field, which is set to zero on emission and
ignored on reception.
R, E, and Reserved are sent encrypted by the protected channel (see
Section 3.3).
If there is no extension, PCHANNEL has the format presented in
Figure 15.
If there is an extension, i.e., if the authentication is extended,
the PCHANNEL field consists of:
o A 4-byte Nonce N (see Section 3.3).
o A 16-byte Tag (see Section 3.3).
o A 2-bit result indication flag R.
o A 1-bit extension flag E, which is set to 1.
o A 5-bit Reserved field, which is set to zero on emission and
ignored on reception.
o A variable length EXT field. R, E, Reserved, and EXT are sent encrypted by the protected channel (see Section 3.3). If there is an extension, PCHANNEL has the format presented in Figure 16. This EXT field is split in two subfields: o The EXT_Type subfield, which indicates the type of the extension o The EXT_Payload subfield, which consists of the payload of the extension. The EXT_Payload length is derived from the EAP Length field. EXT_Payload must have a bit length that is a multiple of 8 bits and must not exceed 960 bytes. The latter restriction aims at avoiding fragmentation issues (see Section 8.11). The format of the EXT field is presented in Figure 17.
(page 41 continued on part 3)