Internet Engineering Task Force (IETF)                          A. Yegin
Request for Comments: 6786                                       Samsung
Category: Standards Track                                      R. Cragie
ISSN: 2070-1721                                           Gridmerge Ltd.
                                                           November 2012


        Encrypting the Protocol for Carrying Authentication for
              Network Access (PANA) Attribute-Value Pairs

Abstract

   This document specifies a mechanism for delivering the Protocol for
   Carrying Authentication for Network Access (PANA) Attribute-Value
   Pairs (AVPs) in encrypted form.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6786.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
      1.1. Specification of Requirements ..............................2
   2. Details .........................................................3
   3. Encryption Keys .................................................3
   4. Encryption-Algorithm AVP ........................................4
      4.1. AES128_CTR Encryption Algorithm ............................5
   5. Encryption-Encap AVP ............................................6
   6. Encryption Policy ...............................................6
      6.1. Encryption Policy Specification ............................7
   7. Security Considerations .........................................8
      7.1. AES-CTR Security Considerations ............................9
   8. IANA Considerations .............................................9
      8.1. PANA AVP Codes .............................................9
      8.2. PANA Encryption-Algorithm AVP Values .......................9
      8.3. PANA AVP Codes Encryption Policy ..........................10
   9. Acknowledgments ................................................10
   10. Normative References ..........................................10

1.  Introduction

   PANA [RFC5191] is a UDP-based protocol to perform an Extensible
   Authentication Protocol (EAP) authentication between a PANA Client
   (PaC) and a PANA Authentication Agent (PAA).

   Various types of payload are exchanged as part of the network access
   authentication and authorization.  These payloads are carried in PANA
   Attribute-Value Pairs (AVPs).  AVPs can be integrity protected using
   the AUTH AVP when EAP authentication generates cryptographic keying
   material.  AVPs are transmitted in the clear (i.e., not encrypted).

   Certain payload types need to be delivered privately (e.g., network
   keys, private identifiers, etc.).  This document defines a mechanism
   for applying encryption to selected AVPs.

1.1.  Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.  The key
   words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
   "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document
   are to be interpreted as described in [RFC2119].

2.  Details

   This document extends the AVP set defined in Section 8 of [RFC5191]
   by defining two new AVPs: the Encryption-Algorithm AVP (see Section
   4) and the Encryption-Encap AVP (see Section 5).  Two new encryption
   keys, PANA_PAC_ENCR_KEY and PANA_PAA_ENCR_KEY, are defined to encrypt
   AVPs from the PaC to the PAA and AVPs from the PAA to the PaC,
   respectively (see Section 3).

   When encryption is needed, the required algorithm is negotiated as
   follows: the PAA SHALL send the initial PANA-Auth-Request carrying
   one or more Encryption-Algorithm AVPs supported by it.  The PaC SHALL
   select one of the algorithms from this AVP, and it SHALL respond with
   the initial PANA-Auth-Answer carrying one Encryption-Algorithm AVP
   for the selected algorithm.  Once PANA_PAC_ENCR_KEY and
   PANA_PAA_ENCR_KEY have been generated, a PANA message MAY contain an
   Encryption-Encap AVP.

3.  Encryption Keys

   PANA_PAC_ENCR_KEY is used for encrypting the AVP payload of the
   Encryption-Encap AVP sent in a PANA message from the PaC to the PAA.

   PANA_PAC_ENCR_KEY SHALL be computed according to the following
   formula:

   PANA_PAC_ENCR_KEY = prf+(MSK, "IETF PANA PaC Encr" | I_PAR |
                         I_PAN | PaC_nonce | PAA_nonce | Key_ID)

   PANA_PAA_ENCR_KEY is used for encrypting the AVP payload of the
   Encryption-Encap AVP sent in a PANA message from the PAA to the PaC.
   PANA_PAA_ENCR_KEY SHALL be computed according to the following
   formula:

   PANA_PAA_ENCR_KEY = prf+(MSK, "IETF PANA PAA Encr" | I_PAR |
                         I_PAN | PaC_nonce | PAA_nonce | Key_ID)

   In both cases:

      -  The prf+ function is defined in the Internet Key Exchange
         Protocol version 2 (IKEv2) [RFC5996].

      -  The pseudo-random function (PRF) to be used for the prf+
         function SHALL be negotiated using the PRF-Algorithm AVP in the
         initial PANA-Auth-Request and PANA-Auth-Answer exchange with
         the 'S' (Start) bit set as described in Section 4.1 of
         [RFC5191].

      -  MSK is the master session key (MSK) generated by the EAP method
         [RFC3748].  PANA_PAC_ENCR_KEY and PANA_PAA_ENCR_KEY MUST be
         recalculated whenever a new MSK is generated by the EAP method.

      -  "IETF PANA PaC Encr" and "IETF PANA PAA Encr" are the ASCII
         code representations of the respective non-NULL terminated
         strings (excluding the double quotes around them).

      -  I_PAR and I_PAN are the initial PANA-Auth-Request and
         PANA-Auth-Answer messages (the PANA header and the following
         PANA AVPs) with the 'S' (Start) bit set, respectively.

      -  PaC_nonce and PAA_nonce are values of the Nonce AVP carried in
         the first non-initial PANA-Auth-Answer and PANA-Auth-Request
         messages in the authentication and authorization phase or the
         first PANA-Auth-Answer and PANA-Auth-Request messages in the
         re-authentication phase, respectively.

      -  Key_ID is the value of the Key-Id AVP.

   The length of PANA_PAC_ENCR_KEY and PANA_PAA_ENCR_KEY depends on the
   encryption algorithm in use.

4.  Encryption-Algorithm AVP

   The Encryption-Algorithm AVP (AVP code 13) is used for conveying the
   encryption algorithm to be used with the Encryption-Encap AVP.  The
   AVP value data is of type Unsigned32.

   Only one encryption algorithm identifier AES128_CTR (code 1) is
   identified by this document.  Encryption algorithm identifier values
   other than 1 are reserved for future use.  Future specifications are
   allowed to extend this list.

      AES128_CTR: 1

   In the absence of an application profile specifying otherwise, all
   implementations SHALL support AES128_CTR.

4.1.  AES128_CTR Encryption Algorithm

   The AES128_CTR encryption algorithm uses the AES-CTR (Counter) mode
   of operation as specified in [NIST_SP800_38A] using the AES-128 block
   cipher.  The formatting function and counter generation function, as
   specified in Appendix A of [NIST_SP800_38C], are used with the
   following parameters:

         n = 12,
         q = 3

   The 12-octet nonce consists of a 4-octet Key-Id, a 4-octet Session
   ID, and a 4-octet Sequence Number in that order where each 4-octet
   value is encoded in network byte order.  The Session ID and Sequence
   Number values SHALL be the same as those in the PANA message carrying
   the key Encryption-Encap AVP.  The Key-Id value SHALL be the same as
   the one used for deriving PANA_PAC_ENCR_KEY and PANA_PAA_ENCR_KEY.
   The output blocks of the encryption processing are encoded as
   OctetString data in the Value field of a Encryption-Encap AVP.

   Note that the first counter block used for encryption is Ctr_1, where
   "_1" denotes "subscript 1" as described in Appendix A.3 of
   [NIST_SP800_38C].  For example, given the following:

         Key-Id = 0x55667788,
         Session ID = 0xaabbccdd,
         Sequence Number = 0x11223344

   The first counter block used for encryption will be:

         0x0255667788aabbccdd11223344000001

   where the initial 0x02 represents the Flags field of the counter
   block.

   The nonce meets the requirement of uniqueness-per-key usage provided
   that the sequence number does not wrap.  Therefore, for the purpose
   of generating new keys:

      -  If Encryption-Encap AVPs are being sent from the PaC to the PAA
         and the sequence number is about to wrap, the PaC SHALL
         initiate PANA re-authentication as described in Section 4.3 of
         [RFC5191].

      -  If Encryption-Encap AVPs are being sent from the PAA to the PaC
         and the sequence number is about to wrap, the PAA SHALL
         initiate PANA re-authentication as described in Section 4.3 of
         [RFC5191].

   Re-authentication ensures the generation of a new MSK [RFC3748] and
   thus a new PANA_PAC_ENCR_KEY and PANA_PAA_ENCR_KEY.

5.  Encryption-Encap AVP

   The Encryption-Encap AVP (AVP code 12) is used to encrypt one or more
   PANA AVPs.  The format of the Encryption-Encap AVP depends on the
   negotiated encryption algorithm.

   When the negotiated encryption algorithm identifier is AES128_CTR
   (code 1), AVP data payload is occupied by the encrypted AVPs.

   There SHALL be only one Encryption-Encap AVP in a PANA message.  All
   AVPs that require encryption SHALL be encapsulated within the
   Encryption-Encap AVP.

   The Encryption-Encap AVP uses either PANA_PAC_ENCR_KEY or
   PANA_PAA_ENCR_KEY, and the encryption algorithm negotiated by the
   Encryption-Algorithm AVP.  The Encryption-Encap AVP SHALL only be
   used if the EAP method generates cryptographic keys (specifically,
   the MSK [RFC3748]).

   The Encryption-Encap AVP MAY be used in a PANA message from the PaC
   to the PAA when the encryption algorithm has been successfully
   negotiated and once PANA_PAC_ENCR_KEY has been generated.

   The Encryption-Encap AVP MAY be used in a PANA message from the PAA
   to the PaC when the encryption algorithm has been successfully
   negotiated and once PANA_PAA_ENCR_KEY has been generated.

   The Encryption-Encap AVP MAY be used in the very first PANA message
   carrying the Result-Code AVP set to PANA_Success value and any
   subsequent message within the same PANA session.

6.  Encryption Policy

   The specification of any AVP SHOULD state that the AVP either shall
   or shall not be encrypted using the Encryption-Encap AVP.  The
   specification of an AVP MAY state that the AVP may (or may not) be
   encrypted using the Encryption-Encap AVP.  The specification SHOULD
   use a table in the format specified in Section 6.1.  If the
   specification of an AVP is silent about whether the AVP shall or
   shall not be encrypted using the Encryption-Encap AVP, this implies
   that the AVP MAY be encrypted using the Encryption-Encap AVP.

6.1.  Encryption Policy Specification

   This section defines a table format for the specification of whether
   an AVP shall or shall not be encrypted using the Encryption-Encap
   AVP.

   The table uses the following symbols:

   Y: The AVP SHALL be encrypted using the Encryption-Encap AVP.  If the
      AVP is encountered not encrypted using the Encryption-Encap AVP,
      it SHALL be considered invalid and the message containing the AVP
      SHALL be discarded.

   N: The AVP SHALL NOT be encrypted using the Encryption-Encap AVP.  If
      the AVP is encountered encrypted using the Encryption-Encap AVP,
      it SHALL be considered invalid and the message containing the AVP
      SHALL be discarded.

   X: The AVP MAY be encrypted using the Encryption-Encap AVP.  If the
      AVP is encountered either encrypted or not encrypted using the
      Encryption-Encap AVP, it SHALL be considered valid.

   The legitimate occurrence of unencrypted AVPs and AVPs after
   decryption and unencapsulation SHALL be subject to the AVP Occurrence
   Table (Figure 4 in [RFC5191]).

   The following table shows the encryption requirements for the
   existing AVPs defined in [RFC5191]:

            Attribute Name        |Enc|
            ----------------------+---+
            AUTH                  | N |
            EAP-Payload           | X |
            Integrity-Algorithm   | N |
            Key-Id                | N |
            Nonce                 | N |
            PRF-Algorithm         | N |
            Result-Code           | N |
            Session-Lifetime      | X |
            Termination-Cause     | X |
            ----------------------+---+

   The following table shows the encryption requirements for the AVPs
   defined in [RFC6345]:

            Attribute Name        |Enc|
            ----------------------+---+
            PaC-Information       | N |
            Relayed-Message       | N |
            ----------------------+---+

   The following table shows the encryption requirements for the AVPs
   defined in this document:

            Attribute Name        |Enc|
            ----------------------+---+
            Encryption-Algorithm  | N |
            Encryption-Encap      | N |
            ----------------------+---+

   The following table is an example showing the encryption requirements
   for a newly defined AVP, Example-AVP:

            Attribute Name        |Enc|
            ----------------------+---+
            Example-AVP           | Y |
            ----------------------+---+

   The guidance for specifying the encryption requirements for a newly
   defined AVP is as follows:

   Y: If the payload needs privacy against eavesdroppers (e.g., carrying
      a secret key).

   N: If the payload may need to be observed by on-path network elements
      (i.e., subject to deep packet inspection).

   X: If neither concern applies.

7.  Security Considerations

   PANA_PAC_ENCR_KEY and PANA_PAA_ENCR_KEY are secret keys shared
   between the PaC and the PAA.  They SHALL NOT be used for purposes
   other than those specified in this document.  Compromise of these
   keys would lead to compromise of the secret information protected by
   these keys.

7.1.  AES-CTR Security Considerations

   The use of AES-CTR encryption has its own security considerations,
   which are detailed in the Security Considerations section of
   [RFC3686].  Specifically:

      -  The nonce specified in Section 4.1 meets the requirement of
         uniqueness-per-key usage.

      -  Section 4.1 of [RFC5191] states that if the EAP method
         generates cryptographic keys, an AUTH AVP will always be
         present in every PANA message after key generation.  Therefore,
         an Encryption-Encap AVP will always be sent in conjunction with
         an AUTH AVP.  This meets the requirement of a companion
         authentication function.

8.  IANA Considerations

   As described in Sections 4 and 5, and following the IANA allocation
   policy on PANA messages [RFC5872], two PANA AVP codes and one set of
   AVP values have been registered.  An additional encryption policy for
   AVP codes has also been registered.

8.1.  PANA AVP Codes

   The following AVP codes have been registered in the "AVP Codes" sub-
   registry of the "Protocol for Carrying Authentication for Network
   Access (PANA) Parameters" registry:

   o  A standard AVP code of 12 for the Encryption-Encap AVP.

   o  A standard AVP code of 13 for the Encryption-Algorithm AVP.

8.2.  PANA Encryption-Algorithm AVP Values

   The following AVP values representing the encryption algorithm
   identifier for the Encryption-Algorithm AVP code have been assigned
   in the "Encryption-Algorithm (AVP Code 13) AVP Values" sub-registry
   under the "Protocol for Carrying Authentication for Network Access
   (PANA) Parameters" registry":

   o  An AVP value of 1 for AES128_CTR.

   o  All other AVP values (0, 2-4294967295) are unassigned.

   The registration procedures are IETF Review or IESG Approval in
   accordance with [RFC5872].

8.3.  PANA AVP Codes Encryption Policy

   The additional encryption policy defined in Section 6.1 has been
   added as a column labeled "Enc" in the "AVP Codes" sub-registry and
   has been applied to all existing AVP codes and those defined in this
   specification.

9.  Acknowledgments

   The authors would like to thank Yoshihiro Ohba, Yasuyuki Tanaka,
   Adrian Farrel, Brian Carpenter, Yaron Sheffer, Ralph Droms, Stephen
   Farrell, Barry Leiba, and Sean Turner for their valuable comments.

10.  Normative References

   [NIST_SP800_38A]
              Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: Methods and Techniques", December 2001.

   [NIST_SP800_38C]
              Dworkin, M., "Recommendation for Block Cipher Modes of
              Operation: The CCM Mode for Authentication and
              Confidentiality", May 2004.

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

   [RFC3686]  Housley, R., "Using Advanced Encryption Standard (AES)
              Counter Mode With IPsec Encapsulating Security Payload
              (ESP)", RFC 3686, January 2004.

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

   [RFC5191]  Forsberg, D., Ohba, Y., Ed., Patil, B., Tschofenig, H.,
              and A. Yegin, "Protocol for Carrying Authentication for
              Network Access (PANA)", RFC 5191, May 2008.

   [RFC5872]  Arkko, J. and A. Yegin, "IANA Rules for the Protocol for
              Carrying Authentication for Network Access (PANA)", RFC
              5872, May 2010.

   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
              5996, September 2010.

   [RFC6345]  Duffy, P., Chakrabarti, S., Cragie, R., Ohba, Y., Ed., and
              A. Yegin, "Protocol for Carrying Authentication for
              Network Access (PANA) Relay Element", RFC 6345, August
              2011.

Authors' Addresses

   Alper Yegin
   Samsung
   Istanbul
   Turkey

   EMail: alper.yegin@yegin.org


   Robert Cragie
   Gridmerge Ltd.
   89 Greenfield Crescent
   Wakefield, WF4 4WA
   United Kingdom

   EMail: robert.cragie@gridmerge.com