path: root/doc/rfc/rfc6630.txt

Internet Engineering Task Force (IETF)                            Z. Cao
Request for Comments: 6630                                       H. Deng
Category: Standards Track                                   China Mobile
ISSN: 2070-1721                                                    Q. Wu
                                                                  Huawei
                                                            G. Zorn, Ed.
                                                             Network Zen
                                                               June 2012


               EAP Re-authentication Protocol Extensions
            for Authenticated Anticipatory Keying (ERP/AAK)

Abstract

   The Extensible Authentication Protocol (EAP) is a generic framework
   supporting multiple types of authentication methods.

   The EAP Re-authentication Protocol (ERP) specifies extensions to EAP
   and the EAP keying hierarchy to support an EAP method-independent
   protocol for efficient re-authentication between the peer and an EAP
   re-authentication server through any authenticator.

   Authenticated Anticipatory Keying (AAK) is a method by which
   cryptographic keying material may be established upon one or more
   Candidate Attachment Points (CAPs) prior to handover.  AAK uses the
   AAA infrastructure for key transport.

   This document specifies the extensions necessary to enable AAK
   support in ERP.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6630.







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Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  3
     2.2.  Acronyms . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  ERP/AAK Description  . . . . . . . . . . . . . . . . . . . . .  4
   4.  ERP/AAK Key Hierarchy  . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Derivation of the pRK and pMSK . . . . . . . . . . . . . .  8
   5.  Packet and TLV Extension . . . . . . . . . . . . . . . . . . .  9
     5.1.  EAP-Initiate/Re-auth-Start Packet and TLV Extension  . . .  9
     5.2.  EAP-Initiate/Re-auth Packet and TLV Extension  . . . . . . 10
     5.3.  EAP-Finish/Re-auth Packet and TLV Extension  . . . . . . . 12
     5.4.  TV and TLV Attributes  . . . . . . . . . . . . . . . . . . 14
   6.  Lower-Layer Considerations . . . . . . . . . . . . . . . . . . 15
   7.  AAA Transport Considerations . . . . . . . . . . . . . . . . . 15
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     11.2. Informative References . . . . . . . . . . . . . . . . . . 19














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1.  Introduction

   The Extensible Authentication Protocol (EAP) [RFC3748] is a generic
   framework supporting multiple types of authentication methods.  In
   systems where EAP is used for authentication, it is desirable not to
   repeat the entire EAP exchange with another authenticator.  The EAP
   Re-authentication Protocol (ERP) [RFC5296] specifies extensions to
   EAP and the EAP keying hierarchy to support an EAP method-independent
   protocol for efficient re-authentication between the EAP
   re-authentication peer and an EAP re-authentication server through
   any authenticator.  The re-authentication server may be in the home
   network or in the local network to which the mobile host (i.e., the
   EAP re-authentication peer) is connecting.

   Authenticated Anticipatory Keying (AAK) [RFC5836] is a method by
   which cryptographic keying material may be established upon one or
   more Candidate Attachment Points (CAPs) prior to handover.  AAK
   utilizes the AAA infrastructure for key transport.

   This document specifies the extensions necessary to enable AAK
   support in ERP.

2.  Terminology

2.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.2.  Acronyms

   The following acronyms are used in this document; see the references
   for more details.

   AAA
       Authentication, Authorization, and Accounting [RFC3588]

   CAP
       Candidate Attachment Point [RFC5836]

   DSRK
         Domain-Specific Root Key [RFC5295]

   EA
       Abbreviation for "ERP/AAK"





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   EA Peer
            An EAP peer that supports the ERP/AAK.  Note that all
            references to "peer" in this document imply an EA peer,
            unless specifically noted otherwise.

   NAI
       Network Access Identifier [RFC4282]

   pMSK
         pre-established Master Session Key

   pRK
       pre-established Root Key

   rIK
       re-authentication Integrity Key [RFC5296]

   rRK
       re-authentication Root Key [RFC5296]

   SAP
       Serving Attachment Point [RFC5836]

3.  ERP/AAK Description

   ERP/AAK is intended to allow (upon request by the peer) the
   establishment of cryptographic keying materials on a single Candidate
   Attachment Point prior to the arrival of the peer at the Candidate
   Access Network (CAN).

   In this document, ERP/AAK support by the peer is assumed.  Also, it
   is assumed that the peer has previously completed full EAP
   authentication and that either the peer or the SAP knows the
   identities of neighboring attachment points.  Note that the behavior
   of a peer that does not support the ERP-AAK scheme defined in this
   specification is out of the scope of this document.  Figure 1 shows
   the general protocol exchange by which the keying material is
   established on the CAP.













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     +------+         +-----+        +-----+          +-----------+
     | Peer |         | SAP |        | CAP |          | EA Server |
     +--+---+         +--+--+        +--+--+          +-----+-----+
        |                |              |                   |
     a. | [EAP-Initiate/ |              |                   |
        | Re-auth-start  |              |                   |
        | (E flag)]      |              |                   |
        |<---------------|              |                   |
        |                |              |                   |
     b. | EAP-Initiate/  |              |                   |
        | Re-auth        |              |                   |
        | (E flag)       |              |                   |
        |--------------->|              |                   |
     c. |                | AAA(EAP-Initiate/Re-auth(E flag))|
        |                |--------------------------------->|
        |                |              |         +---------+---------+
        |                |              |         | CA authorized &   |
     d. |                |              |         |  and EA Keying    |
        |                |              |         |   Distribution    |
        |                |              |         +---------+---------+
        |                |              |                   |
        |                |              |                   |
     f. |                | AAA (EAP-Finish/Re-auth(E flag)) |
        |                |<---------------------------------|
     g. | EAP-Finish/    |              |                   |
        | Re-auth(E flag)|              |                   |
        |<---------------|              |                   |
        |                |              |                   |

                        Figure 1: ERP/AAK Exchange





















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                  +-----------+               +---------+
                  |           |               |         |
                  | EA Server |               |   CAP   |
                  |           |               |         |
                  +-----|-----+               +----|----+
                        |                          |
                        |                          |
                        |    AAA Request (pMSK)    |
                     e.1|------------------------->|
                        |                          |
                        |                          |
                        |                          |
                        |  AAA Response (Success)  |
                     e.2|<-------------------------|
                        |                          |
                        |                          |
                        |                          |

                  Figure 2: Key Distribution for ERP/AAK

   ERP/AAK reuses the packet format defined by ERP, but specifies a new
   flag to differentiate EAP early authentication from EAP
   re-authentication.  The peer initiates ERP/AAK without an external
   trigger, or initiates ERP/AAK in response to an EAP-Initiate/
   Re-Auth-Start message from the SAP.

   In the latter case, the SAP MAY send the identity of one or more
   Candidate Attachment Points to which the SAP is adjacent to the peer
   in the EAP-Initiate/Re-auth-Start message (see step a in Figure 1).
   The peer SHOULD override the identity of CAP(s) carried in the
   EAP-Initiate/Re-auth-Start message by sending EAP-Initiate/Re-auth
   with the E flag set if it knows to which CAP it will move.  If the
   EAP-Initiate/Re-auth-Start packet is not supported by the peer, it
   MUST be silently discarded.

   If the peer initiates ERP/AAK, the peer MAY send an early-
   authentication request message (EAP-Initiate/Re-auth with the E flag
   set) containing the keyName-NAI, the CAP-Identifier, rIK, and
   sequence number (see step b in Figure 1).  The realm in the keyName-
   NAI field is used to locate the peer's ERP/AAK server.  The CAP-
   Identifier is used to identify the CAP.  The re-authentication
   Integrity Key (rIK) is defined by Narayanan & Dondeti in [RFC5296]
   and is used to protect the integrity of the message.  The sequence
   number is used for replay protection.

   The SAP SHOULD verify the integrity of this message at step b.  If
   this verification fails, the SAP MUST send an EAP-Finish/Re-auth
   message with the Result flag set to '1' (Failure).  If the



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   verification succeeds, the SAP SHOULD encapsulate the early-
   authentication message into a AAA message and send it to the peer's
   ERP/AAK server in the realm indicated in the keyName-NAI field (see
   step c in Figure 1).

   Upon receiving the message, the ERP/AAK server MUST first use the
   keyName indicated in the keyName-NAI to look up the rIK and check the
   integrity and freshness of the message.  Then, the ERP/AAK server
   MUST verify the identity of the peer by checking the username portion
   of the KeyName-NAI.  If any of the checks fail, the server MUST send
   an early-authentication finish message (EAP-Finish/Re-auth with E
   flag set) with the Result flag set to '1'.  Next, the server MUST
   authorize the CAP specified in the CAP-Identifier TLV.  In the
   success case, the server MUST derive a pMSK from the pRK for the CAP
   carried in the CAP-Identifier field using the sequence number
   associated with CAP-Identifier as an input to the key derivation.
   (see step d in Figure 1).

   Then, the ERP/AAK server MUST transport the pMSK to the authorized
   CAP via AAA (see Section 7) as illustrated above (see steps e.1 and
   e.2 in Figure 2).  Note that key distribution in Figure 2 is one part
   of step d in Figure 1.

   Finally, in response to the EAP-Initiate/Re-auth message, the ERP/AAK
   server SHOULD send the early-authentication finish message (EAP--
   -Finish/Re-auth with E flag set) containing the identity of the
   authorized CAP to the peer via the SAP along with the lifetime of the
   pMSK.  If the peer also requests the rRK Lifetime, the ERP/AAK server
   SHOULD send the rRK Lifetime in the EAP-Finish/Re-auth message (see
   steps f and g in Figure 1).

4.  ERP/AAK Key Hierarchy

   ERP/AAK uses a key hierarchy similar to that of ERP.  The ERP/AAK
   pre-established Root Key (pRK) is derived from either the EMSK or the
   DSRK as specified below (see Section 4.1).  In general, the pRK is
   derived from the EMSK if the peer is located in the home AAA realm
   and derived from the DSRK if the peer is in a visited realm.  The
   DSRK is delivered from the EAP server to the ERP/AAK server as
   specified in [KEYTRAN].  If the peer has previously been
   authenticated by means of ERP or ERP/AAK, the DSRK SHOULD be directly
   reused.









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                                 DSRK    EMSK
                                  |       |
                              +---+---+---+---+
                              |
                             pRK            ...

                   Figure 3: ERP/AAK Root Key Derivation

   Similarly, the pre-established Master Session Key (pMSK) is derived
   from the pRK.  The pMSK is established for the CAP when the peer
   early authenticates to the network.  The hierarchy relationship is
   illustrated Figure 4, below.

                                    pRK
                                     |
                            +--------+--------+
                            |
                            pMSK             ...

                      Figure 4: ERP/AAK Key Hierarchy

4.1.  Derivation of the pRK and pMSK

   The rRK is derived as specified in [RFC5295].

   pRK = KDF (K, S), where

      K = EMSK or K = DSRK and

      S = pRK Label | "\0" | length

   The pRK Label is an IANA-assigned 8-bit ASCII string:

      EAP Early-Authentication Root Key@ietf.org

   assigned from the "User Specific Root Keys (USRK) Key Labels" name
   space in accordance with Salowey, et al. [RFC5295].  The KDF and
   algorithm agility for the KDF are also defined in RFC 5295.  The KDF
   algorithm is indicated in the cryptosuite field or list of
   cryptosuites TLV payload as specified in Sections 5.2 and 5.3.

   The pMSK uses the same KDF as pRK and is derived as follows:

   pMSK = KDF (K, S), where

      K = pRK and

      S = pMSK label | "\0" | SEQ | length



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   The pMSK label is the 8-bit ASCII string:

      EAP Early-Authentication Master Session Key@ietf.org

   The length field refers to the length of the pMSK in octets encoded
   as specified in RFC 5295.  SEQ is sent by either the peer or the
   server in the ERP/AAK message using the SEQ field or the Sequence
   number TLV.  It is encoded as a 16-bit number as specified in
   Sections 5.2 and 5.3.

5.  Packet and TLV Extension

   This section describes the packet and TLV extensions for the ERP/AAK
   exchange.

5.1.  EAP-Initiate/Re-auth-Start Packet and TLV Extension

   Figure 5 shows the new parameters contained in the EAP-Initiate/
   Re-auth-Start packet defined in [RFC5296].

      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      |E| Reserved    |     1 or more TVs or TLVs     ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 5: EAP-Initiate/Re-auth-Start Extension

   Flags

   'E' - The E flag is used to indicate early authentication.  This
   field MUST be set to '1' if early authentication is in use, and it
   MUST be set to '0' otherwise.

   The rest of the 7 bits (Reserved) MUST be set to 0 and ignored on
   reception.

   Type/Values (TVs) and TLVs

   CAP-Identifier: Carried in a TLV payload.  The format is identical to
   that of a DiameterIdentity [RFC3588].  It is used by the SAP to
   advertise the identity of the CAP to the peer.  Exactly one
   CAP-Identifier TLV MAY be included in the EAP-Initiate/Re-auth-Start
   packet if the SAP has performed CAP discovery.





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   If the EAP-Initiate/Re-auth-Start packet is not supported by the
   peer, it SHOULD be discarded silently.

5.2.  EAP-Initiate/Re-auth Packet and TLV Extension

   Figure 6 illustrates the new parameters contained in the
   EAP-Initiate/Re-auth packet defined in [RFC5296].

      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      |R|x|L|E|Resved |             SEQ               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 1 or more TVs or TLVs                         ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Cryptosuite  |         Authentication Tag                     ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 6: EAP-Initiate/Re-auth Extension

   Flags

   'x' - The x flag is reserved.  It MUST be ignored on receipt.

   'L' - As defined in Section 5.3.2 of [RFC5296], this bit is used to
   request the key lifetimes from the server.

   'E' - The E flag is used to indicate early authentication.

   The first bit(R) and final 4 bits (Resved) MUST be set to 0 and
   ignored on reception.

   SEQ

   As defined in Section 5.3.2 of [RFC5296], this field is 16-bit
   sequence number and used for replay protection.

   TVs and TLVs

   keyName-NAI: As defined in [RFC5296], this is carried in a TLV
   payload.  The Type is 1.  The NAI is variable in length, not
   exceeding 253 octets.  The username part of the NAI is the EMSKname
   used to identify the peer.  The realm part of the NAI is the peer's
   home domain name if the peer communicates with the home EA server or
   the domain to which the peer is currently attached (i.e., local
   domain name) if the peer communicates with a local EA server.  The



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   SAP knows whether the KeyName-NAI carries the local domain name by
   comparing the domain name carried in the KeyName-NAI with the local
   domain name that is associated with the SAP.  Exactly one keyName-NAI
   attribute SHALL be present in an EAP-Initiate/Re-auth packet and the
   realm part of it SHOULD follow the use of internationalized domain
   names defined in [RFC5890].

   CAP-Identifier: Carried in a TLV payload.  The Type is 11.  This
   field is used to indicate the Fully Qualified Domain Name (FQDN) of a
   CAP.  The value field MUST be encoded as specified in Section 8 of
   [RFC3315].  Exactly one instance of the CAP-Identifier TLV MUST be
   present in the ERP/AAK-Key TLV.

   Sequence number: The Type is 7.  The value field is a 16-bit field
   and used in the derivation of the pMSK for a CAP.

   Cryptosuite

   This field indicates the integrity algorithm used for ERP/AAK.  Key
   lengths and output lengths are either indicated or obvious from the
   cryptosuite name, e.g., HMAC-SHA256-128 denotes Hashed Message
   Authentication Code (HMAC) computed using the SHA-256 function
   [RFC4868] with 256-bit key length and the output truncated to 128
   bits [RFC2104].  We specify some cryptosuites below:

   0-1  RESERVED

   2    HMAC-SHA256-128

   3    HMAC-SHA256-256

   HMAC-SHA256-128 is REQUIRED to implement, and it SHOULD be enabled in
   the default configuration.

   Authentication Tag

   This field contains an integrity checksum over the ERP/AAK packet
   from the first bit of the Code field to the last bit of the
   Cryptosuite field, excluding the Authentication Tag field itself.
   The value field is calculated using the integrity algorithm indicated
   in the Cryptosuite field and rIK specified in [RFC5296] as the secret
   key.  The length of the field is indicated by the Cryptosuite.

   The peer uses the Authentication Tag to determine the validity of the
   EAP-Finish/Re-auth message from the server.






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   If the message doesn't pass verification or the Authentication Tag is
   not included in the message, the message SHOULD be discarded
   silently.

   If the EAP-Initiate/Re-auth packet is not supported by the SAP, it
   SHOULD be discarded silently.  The peer MUST maintain retransmission
   timers for reliable transport of the EAP-Initiate/Re-auth message.
   If there is no response to the EAP-Initiate/Re-auth message from the
   server after the necessary number of retransmissions (see Section 6),
   the peer MUST assume that ERP/AAK is not supported by the SAP.

5.3.  EAP-Finish/Re-auth Packet and TLV Extension

   Figure 7 shows the new parameters contained in the EAP-Finish/Re-auth
   packet defined in [RFC5296].

      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      |R|x|L|E|Resved |             SEQ               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 1 or more TVs or TLVs                         ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Cryptosuite  |         Authentication Tag                     ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 7: EAP-Finish/Re-auth Extension

   Flags

   'R' - As defined in Section 5.3.3 of [RFC5296], this bit is used as
   the Result flag.  This field MUST be set to '1' to indicate success,
   and it MUST be set to '0' otherwise.

   'x' - The x flag is reserved.  It MUST be ignored on receipt.

   'L' - As defined in Section 5.3.3 of [RFC5296], this bit is used to
   request the key lifetimes from the server.

   'E' - The E flag is used to indicate early authentication.

   The final 4 bits (Resved) MUST be set to 0 and ignored on reception.







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   SEQ

   As defined in Section 5.3.3 of [RFC5296], this field is a 16-bit
   sequence number and is used for replay protection.

   TVs and TLVs

   keyName-NAI: As defined in [RFC5296], this is carried in a TLV
   payload.  The Type is 1.  The NAI is variable in length, not
   exceeding 253 octets.  Exactly one keyName-NAI attribute SHALL be
   present in an EAP-Finish/Re-auth packet.

   ERP/AAK-Key: Carried in a TLV payload for the key container.  The
   Type is 8.  Exactly one ERP/AAK-key SHALL be present in an
   EAP-Finish/Re-auth packet.

   ERP/AAK-Key ::=
        { sub-TLV: CAP-Identifier }
        { sub-TLV: pMSK Lifetime }
        { sub-TLV: pRK Lifetime }
        { sub-TLV: Cryptosuites }

   CAP-Identifier
      Carried in a sub-TLV payload.  The Type is 11 (less than 128).
      This field is used to indicate the identifier of the candidate
      authenticator.  The value field MUST be encoded as specified in
      Section 8 of [RFC3315].  At least one instance of the CAP-
      Identifier TLV MUST be present in the ERP/AAK-Key TLV.

   pMSK Lifetime
      Carried in a sub-TLV payload of the EAP-Finish/Re-auth message.
      The Type is 10.  The value field is an unsigned 32-bit field and
      contains the lifetime of the pMSK in seconds.  This value is
      calculated by the server after performing the pRK Lifetime
      computation upon receiving the EAP-Initiate/Re-auth message.  The
      rIK SHOULD share the same lifetime as the pMSK.  If the 'L' flag
      is set, the pMSK Lifetime attribute MUST be present.

   pRK Lifetime
      Carried in a sub-TLV payload of EAP-Finish/Re-auth message.  The
      Type is 9.  The value field is an unsigned 32-bit field and
      contains the lifetime of the pRK in seconds.  This value is
      calculated by the server before performing the pMSK Lifetime
      computation upon receiving a EAP-Initiate/Re-auth message.  If the
      'L' flag is set, the pRK Lifetime attribute MUST be present.






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   List of Cryptosuites
      Carried in a sub-TLV payload.  The Type is 5 [RFC5296].  The value
      field contains a list of cryptosuites (at least one cryptosuite
      SHOULD be included), each 1 octet in length.  The allowed
      cryptosuite values are as specified in Section 5.2.  The server
      SHOULD include this attribute if the cryptosuite used in the
      EAP-Initiate/Re-auth message was not acceptable and the message is
      being rejected.  The server MAY include this attribute in other
      cases.  The server MAY use this attribute to signal its
      cryptographic algorithm capabilities to the peer.

   Cryptosuite

   This field indicates the integrity algorithm and PRF used for ERP/
   AAK.  HMAC-SHA256-128 is REQUIRED to implement, and it SHOULD be
   enabled in the default configuration.  Key lengths and output lengths
   are either indicated or obvious from the cryptosuite name.

   Authentication Tag

   This field contains the integrity checksum over the ERP/AAK packet
   from the first bit of the Code field to the last bit of the
   Cryptosuite field, excluding the Authentication Tag field itself.
   The value field is calculated using the integrity algorithm indicated
   in the Cryptosuite field and the rIK [RFC5296] as the integrity key.
   The length of the field is indicated by the corresponding
   Cryptosuite.

   The peer uses the authentication tag to determine the validity of the
   EAP-Finish/Re-auth message from a server.

   If the message doesn't pass verification or the authentication tag is
   not included in the message, the message SHOULD be discarded
   silently.

   If the EAP-Initiate/Re-auth packet is not supported by the SAP, it is
   discarded silently.  The peer MUST maintain retransmission timers for
   reliable transport of the EAP-Initiate/Re-auth message.  If there is
   no response to the EAP-Initiate/Re-auth message from the server after
   the necessary number of retransmissions (see Section 6), the peer
   MUST assume that ERP/AAK is not supported by the SAP.

5.4.  TV and TLV Attributes

   With the exception of the rRK Lifetime and rMSK Lifetime TV payloads,
   the attributes specified in Section 5.3.4 of [RFC5296] also apply to
   this document.  In this document, new attributes that may be present
   in the EAP-Initiate and EAP-Finish messages are defined as below:



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   o  Sequence number: This is a TV payload.  The Type is 7.

   o  ERP/AAK-Key: This is a TLV payload.  The Type is 8.

   o  pRK Lifetime: This is a TV payload.  The Type is 9.

   o  pMSK Lifetime: This is a TV payload.  The Type is 10.

   o  CAP-Identifier: This is a TLV payload.  The Type is 11.

6.  Lower-Layer Considerations

   Similar to ERP, some lower-layer specifications may need to be
   revised to support ERP/AAK; refer to Section 6 of [RFC5296] for
   additional guidance.

7.  AAA Transport Considerations

   The AAA transport of ERP/AAK messages is the same as that of the ERP
   message [RFC5296].  In addition, this document requires AAA transport
   of the ERP/AAK keying materials delivered by the ERP/AAK server to
   the CAP.  Hence, a new AAA message for the ERP/AAK application should
   be specified to transport the keying materials.

8.  Security Considerations

   This section provides an analysis of the protocol in accordance with
   the AAA key management requirements specified in [RFC4962].

   o  Cryptographic algorithm independence: ERP-AAK satisfies this
      requirement.  The algorithm chosen by the peer for calculating the
      authentication tag is indicated in the EAP-Initiate/Re-auth
      message.  If the chosen algorithm is unacceptable, the EAP server
      returns an EAP-Finish/Re-auth message with a Failure indication.

   o  Strong, fresh session keys: ERP-AAK results in the derivation of
      strong, fresh keys that are unique for the given CAP.  A pMSK is
      always derived on demand when the peer requires a key with a new
      CAP.  The derivation ensures that the compromise of one pMSK does
      not result in the compromise of a different pMSK at any time.

   o  Limit key scope: The scope of all the keys derived by ERP-AAK is
      well defined.  The pRK is used to derive the pMSK for the CAP.
      Different sequence numbers for each CAP MUST be used to derive a
      unique pMSK.






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   o  Replay detection mechanism: For replay protection, a sequence
      number associated with the pMSK is used.  The peer increments the
      sequence number by one after it sends an ERP/AAK message.  The
      server sets the expected sequence number to the received sequence
      number plus one after verifying the validity of the received
      message, and it responds to the message.

   o  Authenticate all parties: The EAP Re-authentication Protocol
      provides mutual authentication of the peer and the server.  The
      peer and SAP are authenticated via ERP.  The CAP is authenticated
      and trusted by the SAP.

   o  Peer and authenticator authorization: The peer and authenticator
      demonstrate possession of the same keying material without
      disclosing it, as part of the lower-layer secure authentication
      protocol.

   o  Keying material confidentiality: The peer and the server derive
      the keys independently using parameters known to each entity.

   o  Uniquely named keys: All keys produced within the ERP context can
      be referred to uniquely as specified in this document.

   o  Prevent the domino effect: Different sequence numbers for each CAP
      MUST be used to derive the unique pMSK so that the compromise of
      one pMSK does not hurt any other CAP.

   o  Bind key to its context: The pMSKs are bound to the context in
      which the sequence numbers are transmitted.

   o  Confidentiality of identity: This is the same as with ERP
      [RFC5296].

   o  Authorization restriction: All the keys derived are limited in
      lifetime by that of the parent key or by server policy.  Any
      domain-specific keys are further restricted to be used only in the
      domain for which the keys are derived.  Any other restrictions of
      session keys may be imposed by the specific lower layer and are
      out of scope for this specification.

9.  IANA Considerations

   IANA has assigned five TLVs from the registry of EAP Initiate and
   Finish Attributes maintained at
   http://www.iana.org/assignments/eap-numbers/ with the following
   numbers:





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   o  Sequence number: This is a TV payload.  The Type is 7.

   o  ERP/AAK-Key: This is a TLV payload.  The Type is 8.

   o  pRK Lifetime: This is a TLV payload.  The Type is 9.

   o  pMSK Lifetime: This is a TLV payload.  The Type is 10.

   o  CAP-Identifier: This is a TLV payload.  The Type is 11.

   This document reuses the cryptosuites that were created for
   "Re-authentication Cryptosuites" in [RFC5296].

   Further, IANA has added a new label in the "User Specific Root Keys
   (USRK) Key Labels" sub-registry of the "Extended Master Session Key
   (EMSK) Parameters" registry, as follows:

      EAP Early-Authentication Root Key@ietf.org

   A new registry for the flags in the EAP Initiate/Re-auth-Start
   message called the "EAP Initiate/Re-auth-Start Flags" has been
   created and a new flag (E) has been assigned as follows:

      (E) 0x80

   The rest of the values in the 8-bit field are reserved.  New values
   can be assigned by Standards Action or IESG Approval [RFC5226].

   A new registry for the flags in the EAP Initiate/Re-auth message
   called the "EAP Initiate/Re-auth Flags" has also been created.  The
   following flags are reserved:

      (R) 0x80 [RFC5296]

      (B) 0x40 [RFC5296]

      (L) 0x20 [RFC5296]

   This document assigns a new flag (E) as follows:

      (E) 0x10

   The rest of the values in the 8-bit field are reserved.  New values
   can be assigned by Standards Action or IESG Approval.

   Further, this document creates a new registry for the flags in the
   EAP Finish/Re-auth message called the "EAP Finish/Re-auth Flags".
   The following values are assigned.



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      (R) 0x80 [RFC5296]

      (B) 0x40 [RFC5296]

      (L) 0x20 [RFC5296]

   This document assigns a new flag (E) as follows:

      (E) 0x10

   The rest of the values in the 8-bit field are reserved.  New values
   can be assigned by Standards Action or IESG approval.

10.  Acknowledgements

   In writing this document, Yungui Wang contributed to early versions
   of this document and we have received reviews from many experts in
   the IETF, including Tom Taylor, Tena Zou, Tim Polk, Tan Zhang, Semyon
   Mizikovsky, Stephen Farrell, Radia Perlman, Miguel A. Garcia, and
   Sujing Zhou.  We apologize if we miss some of those who have helped
   us.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5295]  Salowey, J., Dondeti, L., Narayanan, V., and M. Nakhjiri,
              "Specification for the Derivation of Root Keys from an
              Extended Master Session Key (EMSK)", RFC 5295,
              August 2008.

   [RFC5296]  Narayanan, V. and L. Dondeti, "EAP Extensions for EAP
              Re-authentication Protocol (ERP)", RFC 5296, August 2008.




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11.2.  Informative References

   [KEYTRAN]  Zorn, G., Wu, W., and V. Cakulev, "Diameter Attribute-
              Value Pairs for Cryptographic Key Transport", Work
              in Progress, August 2011.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

   [RFC4868]  Kelly, S. and S. Frankel, "Using HMAC-SHA-256, HMAC-SHA-
              384, and HMAC-SHA-512 with IPsec", RFC 4868, May 2007.

   [RFC4962]  Housley, R. and B. Aboba, "Guidance for Authentication,
              Authorization, and Accounting (AAA) Key Management",
              BCP 132, RFC 4962, July 2007.

   [RFC5836]  Ohba, Y., Wu, Q., and G. Zorn, "Extensible Authentication
              Protocol (EAP) Early Authentication Problem Statement",
              RFC 5836, April 2010.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.




















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Authors' Addresses

   Zhen Cao
   China Mobile
   53A Xibianmennei Ave., Xuanwu District
   Beijing, Beijing  100053
   P.R. China

   EMail: zehn.cao@gmail.com


   Hui Deng
   China Mobile
   53A Xibianmennei Ave., Xuanwu District
   Beijing, Beijing  100053
   P.R. China

   EMail: denghui02@gmail.com


   Qin Wu
   Huawei
   Floor 12, HuiHong Mansion, No. 91 BaiXia Rd.
   Nanjing, Jiangsu  210001
   P.R. China

   Phone: +86 25 56623633
   EMail: sunseawq@huawei.com


   Glen Zorn (editor)
   Network Zen
   227/358 Thanon Sanphawut
   Bang Na, Bangkok  10260
   Thailand

   Phone: +66 (0) 87-040-4617
   EMail: glenzorn@gmail.com













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