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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 5448

Network Working Group                                           J. Arkko
Internet-Draft                                             V. Lehtovirta
Updates: 4187 (if approved)                                     Ericsson
Intended status: Informational                                 P. Eronen
Expires: May 22, 2009                                              Nokia
                                                       November 18, 2008


 Improved Extensible Authentication Protocol Method for 3rd Generation
              Authentication and Key Agreement (EAP-AKA')
                       draft-arkko-eap-aka-kdf-10

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on May 22, 2009.

Abstract

   This specification defines a new EAP method, EAP-AKA', a small
   revision of the EAP-AKA method.  The change is a new key derivation
   function that binds the keys derived within the method to the name of
   the access network.  The new key derivation mechanism has been
   defined in the 3rd Generation Partnership Project (3GPP).  This
   specification allows its use in EAP in an interoperable manner.  In
   addition, EAP-AKA' employs SHA-256 instead of SHA-1.

   This specification also updates RFC 4187 EAP-AKA to prevent bidding



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   down attacks from EAP-AKA'.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements language  . . . . . . . . . . . . . . . . . . . .  4
   3.  EAP-AKA' . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  AT_KDF_INPUT . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  AT_KDF . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Key Generation . . . . . . . . . . . . . . . . . . . . . . 10
     3.4.  Hash Functions . . . . . . . . . . . . . . . . . . . . . . 12
       3.4.1.  PRF' . . . . . . . . . . . . . . . . . . . . . . . . . 12
       3.4.2.  AT_MAC . . . . . . . . . . . . . . . . . . . . . . . . 12
       3.4.3.  AT_CHECKCODE . . . . . . . . . . . . . . . . . . . . . 12
   4.  Bidding Down Prevention for EAP-AKA  . . . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
     5.1.  Security Properties of Binding Network Names . . . . . . . 17
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
     6.1.  Type Value . . . . . . . . . . . . . . . . . . . . . . . . 18
     6.2.  Attribute Type Values  . . . . . . . . . . . . . . . . . . 18
     6.3.  Key Derivation Function Namespace  . . . . . . . . . . . . 19
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Appendix A.  Changes from RFC 4187 . . . . . . . . . . . . . . . . 21
   Appendix B.  Importance of Explicit Negotiation  . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
   Intellectual Property and Copyright Statements . . . . . . . . . . 23





















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1.  Introduction

   This specification defines a new Extensible Authentication Protocol
   (EAP)[RFC3748] method, EAP-AKA', a small revision of the EAP-AKA
   method originally defined in [RFC4187].  What is new in EAP-AKA' is
   that it has a new key derivation function specified in [3GPP.33.402].
   This function binds the keys derived within the method to the name of
   the access network.  This limits the effects of compromised access
   network nodes and keys.  This specification defines the EAP
   encapsulation for AKA when the new key derivation mechanism is in
   use.

   3GPP has defined a number of applications for the revised AKA
   mechanism, some based on native encapsulation of AKA over 3GPP radio
   access networks and others based on the use of EAP.

   For making the new key derivation mechanisms usable in EAP-AKA
   additional protocol mechanisms are necessary.  Given that RFC 4187
   calls for the use of CK (the encryption key) and IK (the integrity
   key) from AKA, existing implementations continue to use these.  Any
   change of the key derivation must be unambiguous to both sides in the
   protocol.  That is, it must not be possible to accidentally connect
   old equipment to new equipment and get the key derivation wrong or
   attempt to use wrong keys without getting a proper error message.
   The change must also be secure against bidding down attacks that
   attempt to force the participants to use the least secure mechanism.

   This specification therefore introduces a variant of the EAP-AKA
   method, called EAP-AKA'.  This method can employ the derived keys CK'
   and IK' from the 3GPP specification and updates the used hash
   function to SHA-256 [FIPS.180-2.2002].  But it is otherwise
   equivalent to RFC 4187.  Given that a different EAP method Type value
   is used for EAP-AKA and EAP-AKA', a mutually supported method may be
   negotiated using the standard mechanisms in EAP [RFC3748].

      Note: Appendix B explains why it is important to be explicit about
      the change of semantics for the keys, and why other approaches
      would lead to severe interoperability problems.

   The rest of this specification is structured as follows.  Section 3
   defines the EAP-AKA' method.  Section 4 adds support to EAP-AKA to
   prevent bidding down attacks from EAP-AKA'.  Section 5 explains the
   security differences between EAP-AKA and EAP-AKA'.  Section 6
   describes the IANA considerations and Appendix A explains what
   updates to RFC 4187 EAP-AKA have been made in this specification.
   Finally, Appendix B explains some of the design rationale for
   creating EAP-AKA'.




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      Editor's Note: The publication of this RFC depends on its
      normative references [3GPP.24.302], [3GPP.33.102], and
      [3GPP.33.402] from 3GPP reaching their final Release 8 status at
      3GPP.  This is expected to happen shortly.  The RFC Editor should
      check with the 3GPP liaisons that this has happened.  RFC Editor:
      Please delete this note upon publication of this specification as
      an RFC.


2.  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].


3.  EAP-AKA'

   EAP-AKA' is a new EAP method that follows the EAP-AKA specification
   [RFC4187] in all respects except the following:

   o  It uses the Type code TBA1 BY IANA, not 23 which is used by EAP-
      AKA.

   o  It carries the AT_KDF_INPUT attribute, as defined in Section 3.1
      to ensure that both the peer and server know the name of the
      access network.

   o  It supports key derivation function negotiation via the AT_KDF
      attribute (Section 3.2), to allow for future extensions.

   o  It calculates keys as defined in Section 3.3, not as defined in
      EAP-AKA.

   o  It employs SHA-256 [FIPS.180-2.2002], not SHA-1 [FIPS.180-1.1995]
      (Section 3.4).

   Figure 1 shows an example of the authentication process.  Each
   message AKA'-Challenge and so on represents the corresponding message
   from EAP-AKA, but with EAP-AKA' Type code.  The definition of these
   messages, along with the definition of attributes AT_RAND, AT_AUTN,
   AT_MAC, and AT_RES can be found in [RFC4187].









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    Peer                                                    Server
       |                      EAP-Request/Identity             |
       |<------------------------------------------------------|
       |                                                       |
       | EAP-Response/Identity                                 |
       | (Includes user's Network Access Identifier, NAI)      |
       |------------------------------------------------------>|
       |         +-------------------------------------------------+
       |         | Server determines the network name and ensures  |
       |         | that the given access network is authorized to  |
       |         | use the claimed name. The server then runs the  |
       |         | AKA' algorithms generating RAND and AUTN,       |
       |         | derives session keys from CK' and IK'. RAND and |
       |         | AUTN are sent as AT_RAND and AT_AUTN attributes,|
       |         | whereas the network name is transported in the  |
       |         | AT_KDF_INPUT attribute. AT_KDF signals the used |
       |         | key derivation function. The session keys are   |
       |         | used in creating the AT_MAC attribute.          |
       |         +-------------------------------------------------+
       |                        EAP-Request/AKA'-Challenge     |
       |       (AT_RAND, AT_AUTN, AT_KDF, AT_KDF_INPUT, AT_MAC)|
       |<------------------------------------------------------|
   +-----------------------------------------------------+     |
   | The peer determines what the network name should be,|     |
   | based on, e.g.,  what access technology it is using.|     |
   | The peer also retrieves the network name sent by    |     |
   | the network from the AT_KDF_INPUT attribute. The    |     |
   | two names are compared for discrepancies, and if    |     |
   | necessary, the authentication is aborted. Otherwise,|     |
   | the network name from AT_KDF_INPUT attribute is     |     |
   | used in running the AKA' algorithms, verifying AUTN |     |
   | from AT_AUTN and MAC from AT_MAC attributes. The    |     |
   | peer then generates RES. The peer also derives      |     |
   | session keys from CK'/IK'. The AT_RES and AT_MAC    |     |
   | attributes are constructed.                         |     |
   +-----------------------------------------------------+     |
       | EAP-Response/AKA'-Challenge                           |
       | (AT_RES, AT_MAC)                                      |
       |------------------------------------------------------>|
       |         +-------------------------------------------------+
       |         | Server checks the RES and MAC values received   |
       |         | in AT_RES and AT_MAC, respectively. Success     |
       |         | requires both to be found correct.              |
       |         +-------------------------------------------------+
       |                                          EAP-Success  |
       |<------------------------------------------------------|

              Figure 1: EAP-AKA' Authentication Process



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3.1.  AT_KDF_INPUT

   The format of the AT_KDF_INPUT attribute is shown below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | AT_KDF_INPUT  | Length        | Actual Network Name Length    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      .                        Network Name                           .
      .                                                               .
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The fields are as follows:

   AT_KDF_INPUT

      This is set to TBA2 BY IANA.

   Length

      The length of the attribute, calculated as defined in [RFC4187]
      Section 8.1.

   Actual Network Name Length

      This a 2-byte actual length field, needed due to the requirement
      that the previous field is expressed in multiples of 4 bytes per
      the usual EAP-AKA rules.  The Actual Network Name Length field
      provides the length of the Network Name in bytes.

   Network Name

      This field contains the network name of the access network for
      which the authentication is being performed.  The name does not
      include any terminating null characters.  Because the length of
      the entire attribute must be a multiple of 4 bytes, the sender
      pads the name with one, two, or three bytes of all zero bits when
      necessary.

   Only the server sends the AT_KDF_INPUT attribute.  Per [3GPP.33.402],
   the server always verifies the authorization of a given access
   network to use a particular name before sending it to the peer over
   EAP-AKA'.  The value of the AT_KDF_INPUT attribute from the server
   MUST be non-empty.  If it is empty, the peer behaves as if AUTN had



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   been incorrect and authentication fails.  See Section 3 and Figure 3
   of [RFC4187] for an overview of how authentication failures are
   handled.

   In addition, the peer MAY check the received value against its own
   understanding of the network name.  Upon detecting a discrepancy, the
   peer either warns the user and continues, or fails the authentication
   process.  More specifically, the peer SHOULD have a configurable
   policy which it can follow under these circumstances.  If the policy
   indicates that it can continue, the peer SHOULD log a warning message
   or display it to the user.  If the peer chooses to proceed, it MUST
   use the network name as received in the AT_KDF_INPUT attribute.  If
   the policy indicates that the authentication should fail, the peer
   behaves as if AUTN had been incorrect and authentication fails.

   The Network Name field contains an UTF-8 string.  This string MUST be
   constructed as specified in [3GPP.24.302] for "Access Network
   Identity".  The string is structured as fields separated by colons
   (:).  The algorithms and mechanisms to construct the identity string
   depend on the used access technology.

   On the network side, the network name construction is a configuration
   issue in an access network and an authorization check in the
   authentication server.  On the peer, the network name is constructed
   based on the local observations.  For instance, the peer knows which
   access technology it is using on the link, it can see information in
   a link layer beacon, and so on.  The construction rules specify how
   this information maps to an access network name.  Typically, the
   network name consists of the name of the access technology, or name
   of the access technology followed by some operator identifier that
   was advertised in a link layer beacon.  In all cases [3GPP.24.302] is
   the normative specification for the construction in both the network
   and peer side.  If the peer policy allows running EAP-AKA' over an
   access technology for which that specification does not provide
   network name construction rules, the peer SHOULD rely only on the
   information from the AT_KDF_INPUT attribute and not perform a
   comparison.

   If a comparison of the locally determined network name and the one
   received over EAP-AKA' is performed on the peer, it MUST be done as
   follows.  First, each name is broken down to the fields separated by
   colon.  If one of the names has more colons and fields than the other
   one, the additional fields are ignored.  The remaining sequences of
   fields are compared, and they match only if they are equal character
   by character.  This algorithm allows a prefix match where the peer
   would be able to match "", "FOO", and "FOO:BAR" against the value
   "FOO:BAR" received from the server.  This capability is important in
   order to allow possible updates to the specifications that dictate



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   how the network names are constructed.  For instance, if a peer knows
   that it is running on access technology "FOO" it can use the string
   "FOO" even if the server uses an additional, more accurate
   description "FOO:BAR" that contains more information.

   The allocation procedures in [3GPP.24.302] ensure that conflicts
   potentially arising from using the same name in different types of
   networks are avoided.  The specification also has detailed rules
   about how a client can determine these based on information available
   to the client, such as the type of protocol used to attach to the
   network, beacons sent out by the network, and so on.  Information
   that the client cannot directly observe (such as the type or version
   of the home network) is not used by this algorithm.

   The AT_KDF_INPUT attribute MUST be sent and processed as explained
   above when AT_KDF attribute has the value 1.  Future definitions of
   new AT_KDF values MUST define how this attribute is sent and
   processed.

3.2.  AT_KDF

   AT_KDF is an attribute that the server uses to reference a specific
   key derivation function.  It offers a negotiation capability that can
   be useful for future evolution of the key derivation functions.

   The format of the AT_KDF attribute is shown below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | AT_KDF        | Length        |    Key Derivation Function    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The fields are as follows:

   AT_KDF

      This is set to TBA3 BY IANA.

   Length

      The length of the attribute, MUST be set to 1.

   Key Derivation Function

      An enumerated value representing the key derivation function that
      the server (or peer) wishes to use.  Value 1 represents the



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      default key derivation function for EAP-AKA', i.e., employing CK'
      and IK' as defined in Section 3.3.

   Servers MUST send one or more AT_KDF attributes in the EAP-Request/
   AKA'-Challenge message.  These attributes represent the desired
   functions ordered by preference, the most preferred function being
   the first attribute.

   Upon receiving a set of these attributes, if the peer supports and is
   willing to use the key derivation function indicated by the first
   attribute, the function is taken into use without any further
   negotiation.  However, if the peer does not support this function or
   is unwilling to use it, it responds with the EAP-Response/
   AKA'-Challenge message that contains only one attribute, AT_KDF with
   the value set to the selected alternative.  If there is no suitable
   alternative, the peer behaves as if AUTN had been incorrect and
   authentication fails (see Figure 3 of [RFC4187]).  The peer fails the
   authentication also if there are any duplicate values within the list
   of AT_KDF attributes (except where the duplication is due to a
   request to change the key derivation function; see below for further
   information).

   Upon receiving an EAP-Response/AKA'-Challenge with AT_KDF from the
   peer, the server checks that the suggested AT_KDF value was one of
   the alternatives in its offer.  The first AT_KDF value in the message
   from the server is not a valid alternative.  If the peer has replied
   with the first AT_KDF value, the server behaves as if AT_MAC of the
   response had been incorrect and fails the authentication.  For an
   overview of the failed authentication process in the server side, see
   Section 3 and Figure 2 in [RFC4187].  Otherwise, the server re-sends
   the EAP-Response/AKA'-Challenge message, but adds the selected
   alternative to the beginning of the list of AT_KDF attributes, and
   retains the entire list following it.  Note that this means that the
   selected alternative appears twice in the set of AT_KDF values.
   Responding to the peer's request to change the key derivation
   function is the only legal situation where such duplication may
   occur.

   When the peer receives the new EAP-Request/AKA'-Challenge message, it
   MUST check that the requested change, and only the requested change
   occurred in the list of AT_KDF attributes.  If yes, it continues.  If
   not, it behaves as if AT_MAC had been incorrect and fails the
   authentication.  If the peer receives multiple EAP-Request/
   AKA'-Challenge messages with differing AT_KDF attributes without
   having requested negotiation, the peer MUST behave as if AT_MAC had
   been incorrect and fail the authentication.





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3.3.  Key Generation

   Both the peer and server MUST derive the keys as follows.

   AT_KDF set to 1

      In this case MK is derived and used as follows:


       MK = PRF'(IK'|CK',"EAP-AKA'"|Identity)
       K_encr = MK[0..127]
       K_aut  = MK[128..383]
       K_re   = MK[384..639]
       MSK    = MK[640..1151]
       EMSK   = MK[1152..1663]

      Here [n..m] denotes the substring from bit n to m.  PRF' is a new
      pseudo random function specified in Section 3.4.  The 1664 first
      bits from its output are used for K_encr (encryption key, 128
      bits), K_aut (authentication key, 256 bits), K_re (re-
      authentication key, 256 bits), MSK (Master Session Key, 512 bits)
      and EMSK (Extended Master Session Key, 512 bits).  These keys are
      used by the subsequent EAP-AKA' process.  K_encr is used by the
      AT_ENCR_DATA attribute, and K_aut by the AT_MAC attribute.  K_re
      is used later in this section.  MSK and EMSK are outputs from a
      successful EAP method run [RFC3748].

      IK' and CK' are derived as specified in [3GPP.33.402].  The
      functions that derive IK' and CK' take the following parameters:
      CK and IK produced by the AKA algorithm, and value of the Network
      Name field (without length or padding) from AT_KDF_INPUT.

      The value "EAP-AKA'" is an eight characters long ASCII string.  It
      is used as is, without any trailing NUL characters.

      Identity is the peer identity as specified in Section 7 of
      [RFC4187].

      When the server creates an AKA challenge and corresponding AUTN,
      CK, CK', IK, and IK' values, it MUST set the AMF separation bit to
      1 in the AKA algorithm [3GPP.33.102].  Similarly, the peer MUST
      check that the AMF separation bit set is to 1.  If the bit is not
      set to 1, the peer behaves as if the AUTN had been incorrect and
      fails the authentication.

      On fast re-authentication, the following keys are calculated:





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       MK = PRF'(K_re,"EAP-AKA' re-auth"|Identity|counter|NONCE_S)
       MSK  = MK[0..511]
       EMSK = MK[512..1023]


      MSK and EMSK are the resulting 512 bit keys, taking the first 1024
      bits from the result of PRF'.  Note that K_encr and K_aut are not
      re-derived on fast re-authentication.  K_re is the re-
      authentication key from the preceding full authentication and
      stays unchanged over any fast re-authentication(s) that may happen
      based on it.  The value "EAP-AKA' re-auth" is a sixteen characters
      long ASCII string, again represented without any trailing NUL
      characters.  Identity is the fast re-authentication identity,
      counter is the value from the AT_COUNTER attribute, NONCE_S is the
      nonce value from the AT_NONCE_S attribute, all as specified in
      Section 7 of [RFC4187].  To prevent the use of compromised keys on
      other places, it is forbidden to change the network name when
      going from the full to the fast re-authentication process.  The
      peer SHOULD NOT attempt fast re-authentication when it knows that
      the network name in the current access network is different from
      the one in the initial, full authentication.  Upon seeing a re-
      authentication request with a changed network name, the server
      SHOULD behave as if the re-authentication identifier had been
      unrecognized and fall back to full authentication.  The server
      observes the change in the name by comparing where the fast re-
      authentication and full authentication EAP transactions were
      received from at the Authentication, Authorization, and Accounting
      (AAA) protocol level.

   AT_KDF has any other value

      Future variations of key derivation functions may be defined, and
      they will be represented by new values of AT_KDF.  If the peer
      does not recognize the value it cannot calculate the keys and
      behaves as explained in Section 3.2.

   AT_KDF is missing

      The peer behaves as if the AUTN had been incorrect and MUST fail
      the authentication.

   If the peer supports a given key derivation function but is unwilling
   to perform it for policy reasons, it refuses to calculate the keys
   and behaves as explained in Section 3.2.







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3.4.  Hash Functions

   EAP-AKA' uses SHA-256 [FIPS.180-2.2002], not SHA-1 [FIPS.180-1.1995]
   as in EAP-AKA.  This requires a change to the pseudo random function
   (PRF) as well as the AT_MAC and AT_CHECKCODE attributes.

3.4.1.  PRF'

   The PRF' construction is the same one as IKEv2 uses (see Section 2.13
   in [RFC4306]).  The function takes two arguments.  K is a 256 bit
   value and S is an octet string of arbitrary length.  PRF' is defined
   as follows:

   PRF'(K,S) = T1 | T2 | T3 | T4 | ...

      where:
      T1 = HMAC-SHA-256 (K, S | 0x01)
      T2 = HMAC-SHA-256 (K, T1 | S | 0x02)
      T3 = HMAC-SHA-256 (K, T2 | S | 0x03)
      T4 = HMAC-SHA-256 (K, T3 | S | 0x04)
      ...

   PRF' produces as many bits of output as is needed.  HMAC-SHA-256 is
   the application of HMAC [RFC2104] to SHA-256.

3.4.2.  AT_MAC

   When used within EAP-AKA', the AT_MAC attribute is changed as
   follows.  The MAC algorithm is HMAC-SHA-256-128, a keyed hash value.
   The HMAC-SHA-256-128 value is obtained from the 32-byte HMAC-SHA-256
   value by truncating the output to the first 16 bytes.  Hence, the
   length of the MAC is 16 bytes.

   Otherwise the use of AT_MAC in EAP-AKA' follows Section 10.15 of
   [RFC4187].

3.4.3.  AT_CHECKCODE

   When used within EAP-AKA', the AT_CHECKCODE attribute is changed as
   follows.  First, a 32 byte value is needed to accommodate a 256 bit
   hash output:










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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AT_CHECKCODE  | Length        |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Checkcode (0 or 32 bytes)                 |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Second, the checkcode is a hash value, calculated with SHA-256
   [FIPS.180-2.2002], over the data specified in Section 10.13 of
   [RFC4187].


4.  Bidding Down Prevention for EAP-AKA

   As discussed in [RFC3748], negotiation of methods within EAP is
   insecure.  That is, a man-in-the-middle attacker may force the
   endpoints to use a method that is not the strongest one they both
   support.  This is a problem, as we expect EAP-AKA and EAP-AKA' to be
   negotiated via EAP.

   In order to prevent such attacks, this RFC specifies a new mechanism
   for EAP-AKA that allows the endpoints to securely discover the
   capabilities of each other.  This mechanism comes in the form of the
   AT_BIDDING attribute.  This allows both endpoints to communicate
   their desire and support for EAP-AKA' when exchanging EAP-AKA
   messages.  This attribute is not included in EAP-AKA' messages as
   defined in this RFC.  It is only included in EAP-AKA messages.  This
   is based on the assumption that EAP-AKA' is always preferable (see
   Section 5).  If during the EAP-AKA authentication process it is
   discovered that both endpoints would have been able to use EAP-AKA',
   the authentication process SHOULD be aborted, as a bidding down
   attack may have happened.

   The format of the AT_BIDDING attribute is shown below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | AT_BIDDING    | Length        |D|          Reserved           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The fields are as follows:



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   AT_BIDDING

      This is set to TBA4 BY IANA.

   Length

      The length of the attribute, MUST be set to 1.

   D

      This bit is set to 1 if the sender does support EAP-AKA', is
      willing to use it, and prefers it over EAP-AKA.  Otherwise it
      should be set to 0.

   Reserved

      This field MUST be set to zero when sent and ignored on receipt.

   The server sends this attribute in the EAP-Request/AKA-Challenge
   message.  If the peer supports EAP-AKA', it compares the received
   value to its own capabilities.  If it turns out that both the server
   and peer would have been able to use EAP-AKA' and preferred it over
   EAP-AKA, the peer behaves as if AUTN had been incorrect, and fails
   the authentication (see Figure 3 of [RFC4187]).  A peer not
   supporting EAP-AKA' will simply ignore this attribute.  In all cases,
   the attribute is protected by the integrity mechanisms of EAP-AKA, so
   it cannot be removed by a man-in-the-middle attacker.

   Note that we assume (Section 5) that EAP-AKA' is always stronger than
   EAP-AKA.  As a result, there is no need to prevent bidding "down"
   attacks in the other direction, i.e., attackers forcing the endpoints
   to use EAP-AKA'.


5.  Security Considerations

   A summary of the security properties of EAP-AKA' follows.  These
   properties are very similar to those in EAP-AKA.  We assume that SHA-
   256 is at least as secure as SHA-1.  This is called the SHA-256
   assumption in the remainder of this section.  Under this assumption
   EAP-AKA' is at least as secure as EAP-AKA.

   If the AT_KDF attribute has value 1, then the security properties of
   EAP-AKA' are as follows:







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   Protected ciphersuite negotiation

      EAP-AKA' has no ciphersuite negotiation mechanisms.  It does have
      a negotiation mechanism for selecting the key derivation
      functions.  This mechanism is secure against bidding down attacks.
      The negotiation mechanism allows changing the offered key
      derivation function, but the change is visible in the final EAP-
      Request/AKA'-Challenge message that the server sends to the peer.
      This message is authenticated via the AT_MAC attribute, and
      carries both the chosen alternative and the initially offered
      list.  The peer refuses to accept a change it did not initiate.
      As a result, both parties are aware that a change is being made
      and what the original offer was.

   Mutual authentication

      Under the SHA-256 assumption, the properties of EAP-AKA' are at
      least as good as those of EAP-AKA in this respect.  Refer to
      [RFC4187] Section 12 for further details.

   Integrity protection

      Under the SHA-256 assumption, the properties of EAP-AKA' are at
      least as good (most likely better) as those of EAP-AKA in this
      respect.  Refer to [RFC4187] Section 12 for further details.  The
      only difference is that a stronger hash algorithm, SHA-256 is used
      instead of SHA-1.

   Replay protection

      Under the SHA-256 assumption, the properties of EAP-AKA' are at
      least as good as those of EAP-AKA in this respect.  Refer to
      [RFC4187] Section 12 for further details.

   Confidentiality

      The properties of EAP-AKA' are exactly the same as those of EAP-
      AKA in this respect.  Refer to [RFC4187] Section 12 for further
      details.

   Key derivation

      EAP-AKA' supports key derivation with an effective key strength
      against brute force attacks equal to the minimum of the length of
      the derived keys and the length of the AKA base key, i.e. 128-bits
      or more.  The key hierarchy is specified in Section 3.3.

      The Transient EAP Keys used to protect EAP-AKA packets (K_encr,



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      K_aut, K_re), the MSK, and the EMSK are cryptographically
      separate.  If we make the assumption that SHA-256 behaves as a
      pseudo-random function, an attacker is incapable of deriving any
      non-trivial information about any of these keys based on the other
      keys.  An attacker also cannot calculate the pre-shared secret
      from IK, CK, IK', CK', K_encr, K_aut, K_re, MSK, or the EMSK by
      any practically feasible means.

      EAP-AKA' adds an additional layer of key derivation functions
      within itself to protect against the use of compromised keys.
      This is discussed further in Section 5.1.

      EAP-AKA' uses a pseudo random function modeled after the one used
      in IKEv2 [RFC4306] together with SHA-256.

   Key strength

      See above.

   Dictionary attack resistance

      Under the SHA-256 assumption, the properties of EAP-AKA' are at
      least as good as those of EAP-AKA in this respect.  Refer to
      [RFC4187] Section 12 for further details.

   Fast reconnect

      Under the SHA-256 assumption, the properties of EAP-AKA' are at
      least as good as those of EAP-AKA in this respect.  Refer to
      [RFC4187] Section 12 for further details.  Note that
      implementations MUST prevent performing a fast reconnect across
      method types.

   Cryptographic binding

      Note that this term refers to a very specific form of binding,
      something that is performed between two layers of authentication.
      It is not the same as the binding to a particular network name.
      The properties of EAP-AKA' are exactly as those of EAP-AKA in this
      respect, i.e., as it is not a tunnel method this property is not
      applicable to it.  Refer to [RFC4187] Section 12 for further
      details.

   Session independence

      The properties of EAP-AKA' are exactly the same as those of EAP-
      AKA in this respect.  Refer to [RFC4187] Section 12 for further
      details.



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   Fragmentation

      The properties of EAP-AKA' are exactly the same as those of EAP-
      AKA in this respect.  Refer to [RFC4187] Section 12 for further
      details.

   Channel binding

      EAP-AKA', like EAP-AKA, does not provide channel bindings as
      they're defined in [RFC3748] and [RFC5247].  New skippable
      attributes can be used to add channel binding support in the
      future, if required.

      However, including the network name field in the AKA' algorithms
      (which are also used for other purposes than EAP-AKA') does
      provide a form of cryptographic separation between different
      network names, which resembles channel bindings.  However, the
      network name does not typically identify the EAP (pass-through)
      authenticator.  See the following section for more discussion.

5.1.  Security Properties of Binding Network Names

   The ability of EAP-AKA' to bind the network name into the used keys
   provides some additional protection against key leakage to
   inappropriate parties.  The keys used in the protocol are specific to
   a particular network name.  If key leakage occurs due to an accident,
   access node compromise, or another attack, the leaked keys are only
   useful when providing access with that name.  For instance, a
   malicious access point cannot claim to be network Y if has stolen
   keys from network X. Obviously, if an access point is compromised,
   the malicious node can still represent the compromised node.  As a
   result, neither EAP-AKA' or any other extension can prevent such
   attacks, but the binding to a particular name limits the attacker's
   choices, allows better tracking of attacks, makes it possible to
   identify compromised networks, and applies good cryptographic
   hygiene.

   The server receives the EAP transaction from a given access network,
   and verifies that the claim from the access network corresponds to
   the name that this access network should be using.  It becomes
   impossible for an access network to claim over AAA that it is another
   access network.  In addition, if the peer checks that the information
   it has received locally over the network access link layer matches
   with the information the server has given it via EAP-AKA', it becomes
   impossible for the access network to tell one story to the AAA
   network and another one to the peer.  These checks prevent some
   "lying NAS" (Network Access Server) attacks.  For instance, a roaming
   partner, R, might claim that it is the home network H in an effort to



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   lure peers to connect to itself.  Such an attack would be beneficial
   for the roaming partner if it can attract more users, and damaging
   for the users if their access costs in R are higher than those in
   other alternative networks, such as H.

   Any attacker who gets hold of the keys CK, IK produced by the AKA
   algorithm can compute the keys CK', IK' and hence the master key MK
   according to the rules in Section 3.3.  The attacker could then act
   as a lying NAS.  In 3GPP systems in general, the keys CK and IK have
   been distributed to, for instance, nodes in a visited access network
   where they may be vulnerable.  In order to reduce this risk the AKA
   algorithm MUST be computed with the AMF separation bit set to 1, and
   the peer MUST check that this is indeed the case whenever it runs
   EAP-AKA'.  Furthermore, [3GPP.33.402] requires that no CK, IK keys
   computed in this way ever leave the home subscriber system.

   The additional security benefits obtained from the binding depend
   obviously on the way names are assigned to different access networks.
   This is specified in [3GPP.24.302].  See also [3GPP.23.003].
   Ideally, the names allow separating each different access technology,
   each different access network, and each different NAS within a
   domain.  If this is not possible, the full benefits may not be
   achieved.  For instance, if the names identify just an access
   technology, use of compromised keys in a different technology can be
   prevented, but it is not possible to prevent their use by other
   domains or devices using the same technology.


6.  IANA Considerations

6.1.  Type Value

   EAP-AKA' has the EAP Type value TBA1 BY IANA in the Extensible
   Authentication Protocol (EAP) Registry under Method Types.  Per
   [RFC3748] Section 6.2, this allocation can be made with Designated
   Expert and Specification Required.

6.2.  Attribute Type Values

   EAP-AKA' shares its attribute space and subtypes with EAP-SIM
   [RFC4186] and EAP-AKA [RFC4186].  No new registries are needed.

   However, a new Attribute Type value (TBA2) in the non-skippable range
   needs to be assigned for AT_KDF_INPUT (Section 3.1) in the EAP-AKA
   and EAP-SIM Parameters registry under Attribute Types.

   Also, a new Attribute Type value (TBA3) in the non-skippable range
   needs to be assigned for AT_KDF (Section 3.2).



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   Finally, a new Attribute Type value (TBA4) in the skippable range
   needs to be assigned for AT_BIDDING (Section 4).

6.3.  Key Derivation Function Namespace

   IANA also needs to create a new namespace for EAP-AKA' AT_KDF Key
   Derivation Function values.  This namespace can exist under the EAP-
   AKA and EAP-SIM Parameters registry.  The initial contents of this
   namespace are given below; new values can be created through
   Specification Required policy [RFC5226].

   Value      Description              Reference
   ---------  ----------------------   ---------------
   0          Reserved
   1          EAP-AKA' with CK'/IK'    [this document]
   2-65535    Unassigned


7.  Acknowledgments

   The authors would like to thank Guenther Horn, Joe Salowey, Mats
   Naslund, Adrian Escott, Brian Rosenberg, Laksminath Dondeti, Ahmad
   Muhanna, Stefan Rommer, Miguel Garcia, Jan Kall, Ankur Agarwal, Jouni
   Malinen, Brian Weis, Russ Housley, and Alfred Hoenes for their in-
   depth reviews and interesting discussions in this problem space.


8.  References

8.1.  Normative References

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

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

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

   [RFC4187]  Arkko, J. and H. Haverinen, "Extensible Authentication
              Protocol Method for 3rd Generation Authentication and Key
              Agreement (EAP-AKA)", RFC 4187, January 2006.

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



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              May 2008.

   [3GPP.24.302]
              3GPP, "3rd Generation Partnership Project; Technical
              Specification Group Core Network and Terminals; Access to
              the 3GPP Evolved Packet Core (EPC) via non-3GPP access
              networks; Stage 3; (Release 8)", 3GPP Draft Technical
              Specification 24.302 v 1.0.0, September 2008.

   [3GPP.33.102]
              3GPP, "3rd Generation Partnership Project; Technical
              Specification Group Services and System Aspects; 3G
              Security; Security architecture (Release 8)", 3GPP Draft
              Technical Specification 33.102 v 8.0.0, June 2008.

   [3GPP.33.402]
              3GPP, "3GPP System Architecture Evolution (SAE); Security
              aspects of non-3GPP accesses; Release 8", 3GPP Draft
              Technical Specification 33.402 v 8.0.0, June 2008.

   [FIPS.180-2.2002]
              National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS PUB 180-2, August 2002, <http://
              csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf>.

8.2.  Informative References

   [RFC4186]  Haverinen, H. and J. Salowey, "Extensible Authentication
              Protocol Method for Global System for Mobile
              Communications (GSM) Subscriber Identity Modules (EAP-
              SIM)", RFC 4186, January 2006.

   [RFC4284]  Adrangi, F., Lortz, V., Bari, F., and P. Eronen, "Identity
              Selection Hints for the Extensible Authentication Protocol
              (EAP)", RFC 4284, January 2006.

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

   [RFC5113]  Arkko, J., Aboba, B., Korhonen, J., and F. Bari, "Network
              Discovery and Selection Problem", RFC 5113, January 2008.

   [RFC5247]  Aboba, B., Simon, D., and P. Eronen, "Extensible
              Authentication Protocol (EAP) Key Management Framework",
              RFC 5247, August 2008.

   [3GPP.23.003]
              3GPP, "3rd Generation Partnership Project; Technical



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              Specification Group Core Network and Terminals; Numbering,
              addressing and identification (Release 8)", 3GPP Draft
              Technical Specification 23.003 v 8.0.0, June 2008.

   [FIPS.180-1.1995]
              National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS PUB 180-1, April 1995,
              <http://www.itl.nist.gov/fipspubs/fip180-1.htm>.


Appendix A.  Changes from RFC 4187

   The changes to RFC 4187 relate only to the bidding down prevention
   support defined in Section 4.  In particular, this document does not
   change how the Master Key (MK) is calculated in RFC 4187 (it uses CK
   and IK, not CK' and IK'); neither is any processing of the AMF bit
   added to RFC 4187.


Appendix B.  Importance of Explicit Negotiation

   Choosing between the traditional and revised AKA key derivation
   functions is easy when their use is unambiguously tied to a
   particular radio access network, e.g Long Term Evolution (LTE) as
   defined by 3GPP or evolved High Rate Packet Data (eHRPD) as defined
   by 3GPP2.  There is no possibility for interoperability problems if
   this radio access network is always used in conjunction with new
   protocols that cannot be mixed with the old ones; clients will always
   know whether they are connecting to the old or new system.

   However, using the new key derivation functions over EAP introduces
   several degrees of separation, making the choice of the correct key
   derivation functions much harder.  Many different types of networks
   employ EAP.  Most of these networks have no means to carry any
   information about what is expected from the authentication process.
   EAP itself is severely limited in carrying any additional
   information, as noted in [RFC4284] and [RFC5113].  Even if these
   networks or EAP were extended to carry additional information, it
   would not affect millions of deployed access networks and clients
   attaching to them.

   Simply changing the key derivation functions that EAP-AKA [RFC4187]
   uses would cause interoperability problems with all of the existing
   implementations.  Perhaps it would be possible to employ strict
   separation into domain names that should be used by the new clients
   and networks.  Only these new devices would then employ the new key
   derivation mechanism.  While this can be made to work for specific
   cases, it would be an extremely brittle mechanism, ripe to result in



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   problems whenever client configuration, routing of authentication
   requests, or server configuration does not match expectations.  It
   also does not help to assume that the EAP client and server are
   running a particular release of 3GPP network specifications.  Network
   vendors often provide features from the future releases early or do
   not provide all features of the current release.  And obviously,
   there are many EAP and even some EAP-AKA implementations that are not
   bundled with the 3GPP network offerings.  In general, these
   approaches are expected to lead to hard-to-diagnose problems and
   increased support calls.


Authors' Addresses

   Jari Arkko
   Ericsson
   Jorvas  02420
   Finland

   Email: jari.arkko@piuha.net


   Vesa Lehtovirta
   Ericsson
   Jorvas  02420
   Finland

   Email: vesa.lehtovirta@ericsson.com


   Pasi Eronen
   Nokia Research Center
   P.O. Box 407
   FIN-00045 Nokia Group
   Finland

   Email: pasi.eronen@nokia.com














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Full Copyright Statement

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