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Versions: 00 01 02

Network Working Group                                           J. Arkko
Internet-Draft                                                K. Norrman
Updates: 5448 (if approved)                                  V. Torvinen
Intended status: Informational                                  Ericsson
Expires: January 3, 2019                                    July 2, 2018


   Perfect-Forward Secrecy for the Extensible Authentication Protocol
       Method for Authentication and Key Agreement (EAP-AKA' PFS)
                       draft-arkko-eap-aka-pfs-02

Abstract

   Many different attacks have been reported as part of revelations
   associated with pervasive surveillance.  Some of the reported attacks
   involved compromising smart cards, such as attacking SIM card
   manufacturers and operators in an effort to compromise shared secrets
   stored on these cards.  Since the publication of those reports,
   manufacturing and provisioning processes have gained much scrutiny
   and have improved.  However, the danger of resourceful attackers for
   these systems is still a concern.

   This specification is an optional extension to the EAP-AKA'
   authentication method which was defined in RFC 5448.  The extension
   provides Perfect Forward Secrecy for the session key generated as a
   part of the authentication run in EAP-AKA'.  This prevents an
   attacker who has gained access to the long-term pre-shared secret in
   a SIM card from merely passively eavesdropping the EAP-AKA' exchanges
   and deriving associated session keys, forcing attackers to use active
   attacks instead.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 3, 2019.




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Copyright Notice

   Copyright (c) 2018 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.  Protocol Design and Deployment Objectives . . . . . . . . . .   4
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  AKA . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  EAP-AKA' Protocol . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Attacks Against Long-Term Shared Secrets in Smart Cards .   8
   4.  Requirements Language . . . . . . . . . . . . . . . . . . . .   8
   5.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Extensions to EAP-AKA'  . . . . . . . . . . . . . . . . . . .  11
     6.1.  AT_PUB_DH . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.2.  AT_KDF_DH . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.3.  New Key Derivation Function . . . . . . . . . . . . . . .  13
     6.4.  Diffie-Hellman Groups . . . . . . . . . . . . . . . . . .  14
     6.5.  Message Processing  . . . . . . . . . . . . . . . . . . .  14
       6.5.1.  EAP-Request/AKA'-Identity . . . . . . . . . . . . . .  15
       6.5.2.  EAP-Response/AKA'-Identity  . . . . . . . . . . . . .  15
       6.5.3.  EAP-Request/AKA'-Challenge  . . . . . . . . . . . . .  15
       6.5.4.  EAP-Response/AKA'-Challenge . . . . . . . . . . . . .  16
       6.5.5.  EAP-Request/AKA'-Reauthentication . . . . . . . . . .  16
       6.5.6.  EAP-Response/AKA'-Reauthentication  . . . . . . . . .  16
       6.5.7.  EAP-Response/AKA'-Synchronization-Failure . . . . . .  16
       6.5.8.  EAP-Response/AKA'-Authentication-Reject . . . . . . .  16
       6.5.9.  EAP-Response/AKA'-Client-Error  . . . . . . . . . . .  17
       6.5.10. EAP-Request/AKA'-Notification . . . . . . . . . . . .  17
       6.5.11. EAP-Response/AKA'-Notification  . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  22



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   Many different attacks have been reported as part of revelations
   associated with pervasive surveillance.  Some of the reported attacks
   involved compromising smart cards, such as attacking SIM card
   manufacturers and operators in an effort to compromise shared secrets
   stored on these cards.  Such attacks are conceivable, for instance,
   during the manufacturing process of cards, or during the transfer of
   cards and associated information to the operator.  Since the
   publication of reports about such attacks, manufacturing and
   provisioning processes have gained much scrutiny and have improved.

   However, the danger of resourceful attackers attempting to gain
   information about SIM cards is still a concern.  They are a high-
   value target and concern a large number of people.  Note that the
   attacks are largely independent of the used authentication
   technology; the issue is not vulnerabilities in algorithms or
   protocols, but rather the possibility of someone gaining unlawful
   access to key material.  While the better protection of manufacturing
   and other processes is essential in protecting against this, there is
   one question that we as protocol designs can ask.  Is there something
   that we can do to limit the consequences of attacks, should they
   occur?

   The authors want to provide a public specification of an extension
   that helps defend against one aspect of pervasive surveillance.  This
   important, given the large number of users such practices may affect.
   It is also a stated goal of the IETF to ensure that we understand the
   surveillance concerns related to IETF protocols and take appropriate
   countermeasures [RFC7258].  This document does that for EAP-AKA'.

   This specification is an optional extension to the EAP-AKA'
   authentication method [RFC5448].  The extension provides Perfect
   Forward Secrecy for the session key generated as a part of the
   authentication run in EAP-AKA'.  This prevents an attacker who has
   gained access to the long-term pre-shared secret in a SIM card from
   merely passively eavesdropping the EAP-AKA' exchanges and deriving
   associated session keys, forcing attackers to use active attacks
   instead.

   Attacks against AKA authentication via compromising the long-term
   secrets in the SIM cards have been an active discussion topic in many
   contexts.  Perfect forward secrecy is on the list of features for the
   next release of 3GPP (5G Phase 2), and this document provides a basis
   for providing this feature in a particular fashion.




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   It should also be noted that 5G network architecture includes the use
   of the EAP framework for authentication.  While any methods can be
   run, the default authentication method within that context will be
   EAP-AKA.  As a result, improvements in EAP-AKA' security have a
   potential to improve security for large number of users.

2.  Protocol Design and Deployment Objectives

   This extension specified here re-uses large portions of the current
   structure of 3GPP interfaces and functions, with the rationale that
   this will make the construction more easily adopted.  In particular,
   the construction maintains the interface between the Universal
   Subscriber Identification Module (USIM) and the mobile terminal
   intact.  As a consequence, there is no need to roll out new
   credentials to existing subscribers.  The work is based on an earlier
   paper [TrustCom2015], and uses much of the same material, but applied
   to EAP rather than the underlying AKA method.  This specification is
   an initial proposal for ensuring SIM-based authentication in EAP
   makes attacks difficult.  Comments and suggestions are much
   appreciated, including design improvements.

   It has been a goal to implement this change as an extension of the
   widely supported EAP-AKA' method, rather than a completely new
   authentication method.  The extension is implemented as a set of new,
   optional attributes, that are provided alongside the base attributes
   in EAP-AKA'.  Old implementations can ignore these attributes, but
   their presence will nevertheless be verified as part of base EAP-AKA'
   integrity verification process, helping protect against bidding down
   attacks.  This extension does not increase the number of rounds
   necessary to complete the protocol.

   The use of this extension is at the discretion of the authenticating
   parties.  It should be noted that PFS and defenses against passive
   attacks are by no means a panacea, but they can provide a partial
   defense that increases the cost and risk associated with pervasive
   surveillance.

   While adding perfect forward secrecy to the existing mobile network
   infrastructure can be done in multiple different ways, the authors
   believe that the approach chosen here is relatively easily
   deployable.  In particular:

   o  As noted above, no new credentials are needed; there is no change
      to SIM cards.

   o  PFS property can be incorporated into any current or future system
      that supports EAP, without changing any network functions beyond
      the EAP endpoints.



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   o  Key generation happens at the endpoints, enabling highest grade
      key material to be used both by the endpoints and the intermediate
      systems (such as access points that are given access to specific
      keys).

   o  While EAP-AKA' is just one EAP method, for practical purposes
      perfect forward secrecy being available for both EAP-TLS [RFC5216]
      [I-D.mattsson-eap-tls13] and EAP-AKA' ensures that for many
      practical systems perfect forward secrecy can be enabled for
      either all or significant fraction of users.

3.  Background

3.1.  AKA

   AKA is based on challenge-response mechanisms and symmetric
   cryptography.  AKA typically runs in a UMTS Subscriber Identity
   Module (USIM) or a CDMA2000 (Removable) User Identity Module
   ((R)UIM).  In contrast with its earlier GSM counterparts, 3rd
   generation AKA provides long key lengths and mutual authentication.

   AKA works in the following manner:

   o  The identity module and the home environment have agreed on a
      secret key beforehand.

   o  The actual authentication process starts by having the home
      environment produce an authentication vector, based on the secret
      key and a sequence number.  The authentication vector contains a
      random part RAND, an authenticator part AUTN used for
      authenticating the network to the identity module, an expected
      result part XRES, a 128-bit session key for integrity check IK,
      and a 128-bit session key for encryption CK.

   o  The authentication vector is passed to the serving network, which
      uses it to authenticate the device.

   o  The RAND and the AUTN are delivered to the identity module.

   o  The identity module verifies the AUTN, again based on the secret
      key and the sequence number.  If this process is successful (the
      AUTN is valid and the sequence number used to generate AUTN is
      within the correct range), the identity module produces an
      authentication result RES and sends it to the serving network.

   o  The serving network verifies the correct result from the identity
      module.  If the result is correct, IK and CK can be used to




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      protect further communications between the identity module and the
      home environment.

3.2.  EAP-AKA' Protocol

   When AKA (and AKA') are embedded into EAP, the authentication on the
   network side is moved to the home environment; the serving network
   performs the role of a pass-through authenticator.  Figure 1
   describes the basic flow in the EAP-AKA' authentication process.  The
   definition of the full protocol behaviour, along with the definition
   of attributes AT_RAND, AT_AUTN, AT_MAC, and AT_RES can be found in
   [RFC5448] and [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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3.3.  Attacks Against Long-Term Shared Secrets in Smart Cards

   Current 3GPP systems use (U)SIM pre-shared key based protocols to
   authenticate subscribers.  Since the addition of replay protection
   and mutual authentication in the third generation 3GPP systems, there
   have been no published attacks that violate the security properties
   defined for the Authentication and Key Agreement (AKA) in, at least
   not within the assumed trust model.  (However, there have been
   attacks using a different trust model [CB2014] [MT2012]; the protocol
   was not designed to counter those situations.  There have also been
   attacks against systems where AKA is used in a different setting than
   initially intended, e.g.  [BT2013].)

   Recent reports of compromised long term pre-shared keys used in AKA
   [Heist2015] indicate a need to look into solutions that allow a
   weaker trust model, in particular for future 5G systems.  It is also
   noted in [Heist2015] that, even if the current trust model is kept,
   some security can be retained in this situation by providing Perfect
   Forward Security (PFS) [DOW1992] for the session key.  If AKA would
   have provided PFS, compromising the pre-shared key would not be
   sufficient to perform passive attacks; the attacker is, in addition,
   forced to be a Man-In-The-Middle (MITM) during the AKA run.
   Introducing PFS for authentication in 3GPP systems can be achieved by
   adding a Diffie-Hellman (DH) exchange.

4.  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].

5.  Protocol Overview

   The enhancements in the protocol specified here are compatible with
   the signaling flow and other basic structures of both AKA and EAP-
   AKA'.  The intent is to implement the enhancement as optional
   attributes that legacy implementations can ignore.

   The purpose of the protocol is to achieve mutual authentication
   between the EAP server and peer, and to establish keying material for
   secure communication between the two.  The enhancements brought in
   this document change the calculation of key material, providing new
   properties that are not present in key material provided by EAP-AKA'
   in its original form.

   Figure 2 below describes the overall process.  Since our goal has
   been to not require new infrastructure or credentials, the flow
   diagrams also show the conceptual interaction with the USIM card and



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   the 3GPP authentication server (HSS).  The details of those
   interactions are outside the scope of this document, however, and the
   reader is referred to the 3GPP specifications .

     USIM             Peer                     Server          HSS
       |               |                          |             |
       |               |    EAP-Req/Identity      |             |
       |               |<-------------------------|             |
       |               |                          |             |
       |               |    EAP-Resp/Identity     |             |
       |               |------------------------->|             |
       |               |                          |             |
       |         +-------------------------------------------------+
       |         | Server now has an identity for the peer.        |
       |         | The server then asks the help of                |
       |         | HSS to run AKA algorithms, generating RAND,     |
       |         | AUTN, XRES, CK, IK. Typically, the HSS performs |
       |         | the first part of key derivations so that the   |
       |         | authentication server gets the CK' and IK' keys |
       |         | already tied to a particular network name.      |
       |         +-------------------------------------------------+
       |               |                          |             |
       |               |                          | ID,         |
       |               |                          | key deriv.  |
       |               |                          | function,   |
       |               |                          | network name|
       |               |                          |------------>|
       |               |                          |             |
       |               |                          | RAND, AUTN, |
       |               |                          | XRES, CK',  |
       |               |                          | IK'         |
       |               |                          |<------------|
       |               |                          |             |
       |         +-------------------------------------------------+
       |         | Server now has the needed authentication vector.|
       |         | It generates an ephemeral DH-parameter G^x      |
       |         | and sends the first EAP method message. In the  |
       |         | message AT_PUB_DH represents sender's generated |
       |         | parameter and AT_KDF_DH carries other DH-       |
       |         | related parameters. All these are skippable     |
       |         | attributes that can be ignored if the peer does |
       |         | not support this extension.                     |
       |         +-------------------------------------------------+
       |               |                          |             |
       |               | EAP-Req/AKA'-Challenge   |             |
       |               | AT_RAND, AT_AUTN, AT_KDF,|             |
       |               | AT_KDF_DH, AT_KDF_INPUT, |             |
       |               | AT_PUB_DH, AT_MAC        |             |



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       |               |<-------------------------|             |
   +-----------------------------------------------------+      |
   | The peer checks if it wants to do the PFS extension.|      |
   | If yes, it will eventually respond with AT_PUB_DH   |      |
   | and AT_MAC. If not, it will ignore AT_PUB_DH and    |      |
   | AT_KDF_DH and base all calculations on basic        |      |
   | EAP-AKA' attributes, continuing just as in EAP-AKA' |      |
   | per RFC 5448 rules. In any case, the peer needs to  |      |
   | query the auth parameters from the USIM card.       |      |
   +-----------------------------------------------------+      |
       |               |                          |             |
       |  RAND, AUTN   |                          |             |
       |<---------------|                         |             |
       |               |                          |             |
       |  CK, IK, RES  |                          |             |
       |-------------->|                          |             |
       |               |                          |             |
   +-----------------------------------------------------+      |
   | The peer now has everything to respond. If it wants |      |
   | to participate in the PFS extension, it will then   |      |
   | generate G^y, calculate G^xy and derive all keys    |      |
   | and construct a full response.                      |      |
   +-----------------------------------------------------+      |
       |               |                          |             |
       |               | EAP-Resp/AKA'-Challenge  |             |
       |               | AT_RES, AT_PUB_DH, AT_MAC|             |
       |               |------------------------->|             |
       |         +-------------------------------------------------+
       |         | The server now has all the necessary values.    |
       |         | It generates the Diffie-Hellman value G^xy      |
       |         | and checks the RES and MAC values received      |
       |         | in AT_RES and AT_MAC, respectively. Success     |
       |         | requires both to be found correct. Note that    |
       |         | keys in this extension are generated based on   |
       |         | both CK/IK as well as the Diffie-Hellman value. |
       |         | This implies that only an active attacker can   |
       |         | determine the used session keys; in basic       |
       |         | EAP-AKA' the keys are only based on CK and IK.  |
       |         +-------------------------------------------------+
       |               |                          |             |
       |               | EAP-Success              |             |
       |               |<-------------------------|             |

              Figure 2: EAP-AKA' PFS Authentication Process







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6.  Extensions to EAP-AKA'

6.1.  AT_PUB_DH

   The AT_PUB_DH carries a Diffie-Hellman value.

   The format of the AT_PUB_DH 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_PUB_DH     | Length        |    Value ...                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The fields are as follows:

   AT_PUB_DH

      This is set to TBA1 BY IANA.

   Length

      The length of the attribute, set as other attributes in EAP-AKA
      [RFC4187].

   Value

      This value is the sender's Diffie-Hellman public value.  For
      Curve25519, the length of this value is 32 bytes, represented as
      specified in [RFC8031] and [RFC7748].

      To retain the security of the keys, the sender SHALL generate a
      fresh value for each run of the protocol.

6.2.  AT_KDF_DH

   The AT_KDF_DH indicates the used or desired key generation function,
   if the Perfect Forward Secrecy extension is taken into use.  It will
   also at the same time indicate the used or desired Diffie-Hellman
   group.  A new attribute is needed to carry this information, as
   AT_KDF carries the legacy KDF value for those EAP peers that cannot
   or do not want to use this extension.

   The format of the AT_KDF_DH attribute is shown below.






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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_KDF_DH     | Length        |    Key Derivation Function    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The fields are as follows:

   AT_KDF_DH

      This is set to TBA2 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.  See Section 6.3 for the
      functions specified in this document.  Note: This field has a
      different name space than the similar field in the AT_KDF
      attribute Key Derivation Function defined in [RFC5448].

   Servers MUST send one or more AT_KDF_DH 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, and is willing and able to use the extension defined in
   this specification, 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 to the server with an
   indication that a different function is needed.  Similarly with the
   negotiation process defined in [RFC5448] for AT_KDF, the peer sends
   EAP-Response/AKA'-Challenge message that contains only one attribute,
   AT_KDF_DH with the value set to the desired alternative function from
   among the ones suggested by the server earlier.  If there is no
   suitable alternative, the peer has a choice of either falling back to
   EAP-AKA' or behaving as if AUTN had been incorrect and failing
   authentication (see Figure 3 of [RFC4187]).  The peer MUST fail the
   authentication if there are any duplicate values within the list of
   AT_KDF_DH attributes (except where the duplication is due to a
   request to change the key derivation function; see below for further
   information).



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   If the peer does not recognize the extension defined in this
   specification or is unwilling to use it, it ignores the AT_KDF_DH
   attribute.

   Upon receiving an EAP-Response/AKA'-Challenge with AT_KDF_DH from the
   peer, the server checks that the suggested AT_KDF_DH value was one of
   the alternatives in its offer.  The first AT_KDF_DH value in the
   message from the server is not a valid alternative.  If the peer has
   replied with the first AT_KDF_DH 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_DH 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_DH 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_DH attributes without having
   requested negotiation, the peer MUST behave as if AT_MAC had been
   incorrect and fail the authentication.

6.3.  New Key Derivation Function

   A new Key Derivation Function type is defined for "EAP-AKA' with DH
   and Curve25519", represented by value 1.  It represents a particular
   choice of key derivation function and at the same time selects a
   Diffie-Hellman group to be used.

   The Key Derivation Function type value is only used in the AT_KDF_DH
   attribute, and should not be confused with the different range of key
   derivation functions that can be represented in the AT_KDF attribute
   as defined in [RFC5448].

   Key derivation in this extension produces exactly the same keys for
   internal use within one authentication run as RFC 5448 EAP-AKA' did.
   For instance, K_aut that is used in AT_MAC is still exactly as it was
   in EAP-AKA'.  The only change to key derivation is in re-
   authentication keys and keys exported out of the EAP method, MSK and
   EMSK.  As a result, EAP-AKA' attributes such as AT_MAC continue to be
   usable even when this extension is in use.



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   When the Key Derivation Function field in the AT_KDF_DH attribute is
   set to 1 and the Key Derivation Function field in the AT_KDF
   attribute is also set to 1, the Master Key (MK) is derived and as
   follows below.

           MK = PRF'(IK'|CK',"EAP-AKA'"|Identity)
           MK_DH = PRF'(IK'|CK'|G^xy,"EAP-AKA' PFS"|Identity)
           K_encr = MK[0..127]
           K_aut  = MK[128..383]
           K_re   = MK_DH[0..255]
           MSK    = MK_DH[256..767]
           EMSK   = MK_DH[768..1279]

   The rest of computation proceeds as defined in Section 3.3 of
   [RFC5448].

   For readability, an explanation of the notation used above is copied
   here: [n..m] denotes the substring from bit n to m.  PRF' is a new
   pseudo-random function specified in [RFC5448].  K_encr is the
   encryption key, 128 bits, K_aut is the authentication key, 256 bits,
   K_re is the re-authentication key, 256 bits, MSK is the Master
   Session Key, 512 bits, and EMSK is the Extended Master Session Key,
   512 bits.  MSK and EMSK are outputs from a successful EAP method run
   [RFC3748].

   CK and IK are produced by the AKA algorithm.  IK' and CK' are derived
   as specified in [RFC5448] from IK and CK.

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

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

6.4.  Diffie-Hellman Groups

   The selection of suitable groups for the Diffie-Hellman computation
   is necessary.  The choice of a group is made at the same time as
   deciding to use of particular key derivation function in AT_KDF_DH.
   For "EAP-AKA' with DH and Curve25519" the Diffie-Hellman group is the
   Curve25519 group specified in [RFC8031].

6.5.  Message Processing

   This section specifies the changes related to message processing when
   this extension is used in EAP-AKA'.  It specifies when a message may
   be transmitted or accepted, which attributes are allowed in a
   message, which attributes are required in a message, and other



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   message-specific details, where those details are different for this
   extension than the base EAP-AKA' or EAP-AKA protocol.  Unless
   otherwise specified here, the rules from [RFC5448] or [RFC4187]
   apply.

6.5.1.  EAP-Request/AKA'-Identity

   No changes, except that the AT_KDF_DH or AT_PUB_DH attributes MUST
   NOT be added to this message.  The appearance of these messages in a
   received message MUST be ignored.

6.5.2.  EAP-Response/AKA'-Identity

   No changes, except that the AT_KDF_DH or AT_PUB_DH attributes MUST
   NOT be added to this message.  The appearance of these messages in a
   received message MUST be ignored.

6.5.3.  EAP-Request/AKA'-Challenge

   The server sends the EAP-Request/AKA'-Challenge on full
   authentication as specified by [RFC4187] and [RFC5448].  The
   attributes AT_RAND, AT_AUTN, and AT_MAC MUST be included and checked
   on reception as specified in in [RFC4187].  They are also necessary
   for backwards compatibility.

   In EAP-Request/AKA'-Challenge, there is no message-specific data
   covered by the MAC for the AT_MAC attribute.  The AT_KDF_DH and
   AT_PUB_DH attributes MUST be included.  The AT_PUB_DH attribute
   carries the server's public Diffie-Hellman key.  If either AT_KDF_DH
   or AT_PUB_DH is missing on reception, the peer MUST treat them as if
   neither one was sent, and the assume that the extension defined in
   this specification is not in use.

   The AT_RESULT_IND, AT_CHECKCODE, AT_IV, AT_ENCR_DATA, AT_PADDING,
   AT_NEXT_PSEUDONYM, AT_NEXT_REAUTH_ID and other attributes may be
   included as specified in Section 9.3 of [RFC4187].

   When processing this message, the peer MUST process AT_RAND, AT_AUTN,
   AT_KDF_DH, AT_PUB_DH before processing other attributes.  Only if
   these attributes are verified to be valid, the peer derives keys and
   verifies AT_MAC.  If the peer is unable or unwilling to perform the
   extension specified in this document, it proceeds as defined in
   [RFC5448].  Finally, the operation in case an error occurs is
   specified in Section 6.3.1. of [RFC4187].







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6.5.4.  EAP-Response/AKA'-Challenge

   The peer sends EAP-Response/AKA'-Challenge in response to a valid
   EAP-Request/AKA'-Challenge message, as specified by [RFC4187] and
   [RFC5448].  If the peer supports and is willing to perform the
   extension specified in this protocol, and the server had made a valid
   request involving the attributes specified in Section 6.5.3, the peer
   responds per the rules specified below.  Otherwise, the peer responds
   as specified in [RFC4187] and [RFC5448] and ignores the attributes
   related to this extension.

   The AT_MAC attribute MUST be included and checked as specified in
   [RFC5448].  In EAP-Response/AKA'-Challenge, there is no message-
   specific data covered by the MAC.  The AT_PUB_DH attribute MUST be
   included, and carries the peer's public Diffie-Hellman key.

   The AT_RES attribute MUST be included and checked as specified in
   [RFC4187].

   The AT_CHECKCODE, AT_RESULT_IND, AT_IV, AT_ENCR_DATA and other
   attributes may be included as specified in Section 9.4 of [RFC4187].

6.5.5.  EAP-Request/AKA'-Reauthentication

   No changes, but note that the re-authentication process uses the keys
   generated in the original EAP-AKA' authentication, which, if the
   extension specified in this documents is in use, employs key material
   from the Diffie-Hellman procedure.

6.5.6.  EAP-Response/AKA'-Reauthentication

   No changes, but as discussed in Section 6.5.5, re-authentication is
   based on the key material generated by EAP-AKA' and the extension
   defined in this document.

6.5.7.  EAP-Response/AKA'-Synchronization-Failure

   No changes, except that the AT_KDF_DH or AT_PUB_DH attributes MUST
   NOT be added to this message.  The appearance of these messages in a
   received message MUST be ignored.

6.5.8.  EAP-Response/AKA'-Authentication-Reject

   No changes, except that the AT_KDF_DH or AT_PUB_DH attributes MUST
   NOT be added to this message.  The appearance of these messages in a
   received message MUST be ignored.





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6.5.9.  EAP-Response/AKA'-Client-Error

   No changes, except that the AT_KDF_DH or AT_PUB_DH attributes MUST
   NOT be added to this message.  The appearance of these messages in a
   received message MUST be ignored.

6.5.10.  EAP-Request/AKA'-Notification

   No changes.

6.5.11.  EAP-Response/AKA'-Notification

   No changes.

7.  Security Considerations

   This section deals only with the changes to security considerations
   as they differ from EAP-AKA', or as new information has been gathered
   since the publication of [RFC5448].

   The possibility of attacks against key storage offered in SIM or
   other smart cards has been a known threat.  But as the discussion in
   Section 3.3 shows, the likelihood of practically feasible attacks has
   increased.  Many of these attacks can be best dealt with improved
   processes, e.g., limiting the access to the key material within the
   factory or personnel, etc.  But not all attacks can be entirely ruled
   out for well-resourced adversaries, irrespective of what the
   technical algorithms and protection measures are.

   This extension can provide assistance in situations where there is a
   danger of attacks against the key material on SIM cards by
   adversaries that can not or who are unwilling to mount active attacks
   against large number of sessions.  This extension is most useful when
   used in a context where EAP keys are used without further mixing that
   can provide Perfect Forward Secrecy.  For instance, when used with
   IKEv2, the session keys produced by IKEv2 have this property, so
   better characteristics of EAP keys is not that useful.  However,
   typical link layer usage of EAP does not involve running Diffie-
   Hellman, so using EAP to authenticate access to a network is one
   situation where the extension defined in this document can be
   helpful.

   The following security properties of EAP-AKA' are impacted through
   this extension:

   Protected ciphersuite negotiation





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      EAP-AKA' has a negotiation mechanism for selecting the key
      derivation functions, and this mechanism has been extended by the
      extension specified in this document.  The resulting mechanism
      continues to be secure against bidding down attacks.

      There are two specific needs in the negotiation mechanism:

      Negotiating key derivation function within the extension

         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.

      Negotiating the use of this extension

         This extension is offered by the server through presenting the
         AT_KDF_DH and AT_PUB_DH attributes in the EAP-Request/AKA'-
         Challenge message.  These attributes are protected by AT_MAC,
         so attempts to change or omit them by an adversary will be
         detected.  (Except of course, if the adversary holds the long-
         term shared secret and is willing to engage in an active
         attack, but that is a case that cannot be solved by a technical
         change in this protocol.)  However, as discussed in the
         introduction, even an attacker with access to the long-term
         keys is required to be MITM on each AKA run, which makes mass
         survailance slightly more laborous.

   Key derivation

      This extension provides key material that is based on the Diffie-
      Hellman keys, yet bound to the authentication through the (U)SIM
      card.  This means that subsequent payload communications between
      the parties are protected with keys that are not solely based on
      information in the clear (such as the RAND) and information
      derivable from the long-term shared secrets on the (U)SIM card.
      As a result, if anyone successfully recovers shared secret
      information, they are unable to decrypt communications protected
      by the keys generated through this extension.  Note that the
      recovery of shared secret information could occur either before or
      after the time that the protected communications are used.  When
      this extension is used, communications at time t0 can be protected
      if at some later time t1 an adversary learns of long-term shared




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      secret and has access to a recording of the encrypted
      communications.

      Obviously, this extension is still vulnerable to attackers that
      are willing to perform an active attack and who at the time of the
      attack have access to the long-term shared secret.

      This extension does not change the properties of related to re-
      authentication.  No new Diffie-Hellman run is performed during the
      re-authentication allowed by EAP-AKA'.  However, if this extension
      was in use when the original EAP-AKA' authentication was
      performed, the keys used for re-authentication (K_re) are based on
      the Diffie-Hellman keys, and hence continue to be equally safe
      against expose of the long-term secrets as the original
      authentication.

8.  IANA Considerations

   This extension of EAP-AKA' shares its attribute space and subtypes
   with EAP-SIM [RFC4186], EAP-AKA [RFC4186], and EAP-AKA' [RFC5448].

   Two new Attribute Type value (TBA1, TBA2) in the skippable range need
   to be assigned for AT_PUB_DH (Section 6.1) and AT_KDF_DH (Section 6.2
   in the EAP-AKA and EAP-SIM Parameters registry under Attribute Types.

   Also, a new registry should be created to represent Diffie-Hellman
   Key Derivation Function types.  The "EAP-AKA' with DH and Curve25519"
   type (1, see Section 6.3) needs to be assigned, along with one
   reserved value.  The initial contents of this namespace are therefore
   as below; new values can be created through the Specification
   Required policy [RFC8126].

   Value     Description                         Reference
   --------  ---------------------------------   ---------------
   0         Reserved                            [TBD BY IANA: THIS RFC]
   1         EAP-AKA' with DH and Curve25519     [TBD BY IANA: THIS RFC]
   2-65535   Unassigned

9.  References

9.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997, <https://www.rfc-
              editor.org/info/rfc2104>.





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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, Ed., "Extensible Authentication Protocol
              (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
              <https://www.rfc-editor.org/info/rfc3748>.

   [RFC4187]  Arkko, J. and H. Haverinen, "Extensible Authentication
              Protocol Method for 3rd Generation Authentication and Key
              Agreement (EAP-AKA)", RFC 4187, DOI 10.17487/RFC4187,
              January 2006, <https://www.rfc-editor.org/info/rfc4187>.

   [RFC5448]  Arkko, J., Lehtovirta, V., and P. Eronen, "Improved
              Extensible Authentication Protocol Method for 3rd
              Generation Authentication and Key Agreement (EAP-AKA')",
              RFC 5448, DOI 10.17487/RFC5448, May 2009,
              <https://www.rfc-editor.org/info/rfc5448>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.

   [RFC8031]  Nir, Y. and S. Josefsson, "Curve25519 and Curve448 for the
              Internet Key Exchange Protocol Version 2 (IKEv2) Key
              Agreement", RFC 8031, DOI 10.17487/RFC8031, December 2016,
              <https://www.rfc-editor.org/info/rfc8031>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

9.2.  Informative References

   [RFC4186]  Haverinen, H., Ed. and J. Salowey, Ed., "Extensible
              Authentication Protocol Method for Global System for
              Mobile Communications (GSM) Subscriber Identity Modules
              (EAP-SIM)", RFC 4186, DOI 10.17487/RFC4186, January 2006,
              <https://www.rfc-editor.org/info/rfc4186>.




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   [RFC5216]  Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
              Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
              March 2008, <https://www.rfc-editor.org/info/rfc5216>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [I-D.mattsson-eap-tls13]
              Mattsson, J. and M. Sethi, "Using EAP-TLS with TLS 1.3",
              draft-mattsson-eap-tls13-02 (work in progress), March
              2018.

   [TrustCom2015]
              Arkko, J., Norrman, K., Naslund, M., and B. Sahlin, "A
              USIM compatible 5G AKA protocol with perfect forward
              secrecy", August 2015 in Proceedings of the TrustCom 2015,
              IEEE.

   [CB2014]   Choudhary, A. and R. Bhandari, "3GPP AKA Protocol:
              Simplified Authentication Process", December 2014,
              International Journal of Advanced Research in Computer
              Science and Software Engineering, Volume 4, Issue 12.

   [MT2012]   Mjolsnes, S. and J-K. Tsay, "A vulnerability in the UMTS
              and LTE authentication and key agreement protocols",
              October 2012, in Proceedings of the 6th international
              conference on Mathematical Methods, Models and
              Architectures for Computer Network Security: computer
              network security.

   [BT2013]   Beekman, J. and C. Thompson, "Breaking Cell Phone
              Authentication: Vulnerabilities in AKA, IMS and Android",
              August 2013, in 7th USENIX Workshop on Offensive
              Technologies, WOOT '13.

   [Heist2015]
              Scahill, J. and J. Begley, "The great SIM heist", February
              2015, in https://firstlook.org/theintercept/2015/02/19/
              great-sim-heist/ .

   [DOW1992]  Diffie, W., vanOorschot, P., and M. Wiener,
              "Authentication and Authenticated Key Exchanges", June
              1992, in Designs, Codes and Cryptography 2 (2): pp.
              107-125.






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Appendix A.  Acknowledgments

   The authors would like to note that the technical solution in this
   document came out of the TrustCom paper [TrustCom2015], whose authors
   were J.  Arkko, K.  Norrman, M.  Naslund, and B.  Sahlin.  This
   document uses also a lot of material from [RFC4187] by J.  Arkko and
   H.  Haverinen as well as [RFC5448] by J.  Arkko, V.  Lehtovirta, and
   P.  Eronen.

   The authors would also like to thank Tero Kivinen, John Mattson,
   Mohit Sethi, Vesa Lehtovirta, Joseph Salowey, Kathleen Moriarty,
   Zhang Fu, Bengt Sahlin, Ben Campbell, Prajwol Kumar Nakarmi, Goran
   Rune, Tim Evans, Helena Vahidi Mazinani, Anand R.  Prasad, and many
   other people at the GSMA and 3GPP groups for interesting discussions
   in this problem space.

Authors' Addresses

   Jari Arkko
   Ericsson
   Jorvas  02420
   Finland

   Email: jari.arkko@piuha.net


   Karl Norrman
   Ericsson
   Stockholm  16483
   Sweden

   Email: karl.norrman@ericsson.com


   Vesa Torvinen
   Ericsson
   Jorvas  02420
   Finland

   Email: vesa.torvinen@ericsson.com











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