OWE: Opportunistic Wireless Encryption

Internet Engineering Task Force (IETF)                   D. Harkins, Ed.
Request for Comments: 8110                                 HP Enterprise
Category: Informational                                   W. Kumari, Ed.
ISSN: 2070-1721                                                   Google
                                                              March 2017


                   Opportunistic Wireless Encryption

Abstract

   This memo specifies an extension to IEEE Std 802.11 to provide for
   opportunistic (unauthenticated) encryption to the wireless media.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 7841.

   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/rfc8110.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Notation  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  802.11 Network Access . . . . . . . . . . . . . . . . . . . .   4
   4.  Opportunistic Wireless Encryption . . . . . . . . . . . . . .   5
     4.1.  Cryptography  . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  OWE Discovery . . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  OWE Association . . . . . . . . . . . . . . . . . . . . .   7
     4.4.  OWE Post-Association  . . . . . . . . . . . . . . . . . .   8
     4.5.  OWE PMK Caching . . . . . . . . . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Implementation Considerations . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   This memo describes Opportunistic Wireless Encryption (OWE) -- a mode
   of opportunistic security [RFC7435] for IEEE Std 802.11 that provides
   encryption of the wireless medium but no authentication.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  Notation

   This memo uses the following notation:

   y = F(X)
       An element-to-scalar mapping function.  For an elliptic curve
       group, it takes a point on the curve and returns the
       x-coordinate; for a finite field element, it is the identity
       function, just returning the element itself.

   Z = DH(x,Y)
       For an elliptic curve, DH(x,Y) is the multiplication of point Y
       by the scalar value x, creating a point on the curve Z; for
       finite field cryptography, DH(x,Y) is an exponentiation of
       element Y to the power of x (implied modulo a field defining
       prime, p) resulting in an element Z.

   a = len(b)
       Indicates the length in bits of the string b.

2.  Background

   Internet access has become an expected service at many locations --
   for example, coffee shops, airports, and hotels.  In many cases, this
   is offered over "Open" (unencrypted) wireless networks, because
   distributing a passphrase (or using other authentication solutions)
   is not convenient or realistic.  Ideally, users would always use a
   VPN when using an untrusted network, but often they don't.  This
   leaves their traffic vulnerable to sniffing attacks, for example,
   from someone in the adjacent hotel room running Wireshark, pervasive
   monitors, etc.

   In addition, many businesses (for example, coffee shops and bars)
   offer free Wi-Fi as an inducement to customers to enter and remain in
   the premises.  Many customers will use the availability of free Wi-Fi
   as a deciding factor in which business to patronize.  Since these
   businesses are not Internet service providers, they are often
   unwilling and/or unqualified to perform complex configuration on
   their network.  In addition, customers are generally unwilling to do
   complicated provisioning on their devices just to obtain free Wi-Fi.
   This leads to a popular deployment technique -- a network protected
   using a shared and public Pre-Shared Key (PSK) that is printed on a
   sandwich board at the entrance, on a chalkboard on the wall, or on a
   menu.  The PSK is used in a cryptographic handshake, defined in
   [IEEE802.11], called the "4-way handshake" to prove knowledge of the
   PSK and derive traffic encryption keys for bulk wireless data.

   The belief is that this protects the wireless medium from passive
   sniffing and simple attacks.  That belief is erroneous.  Since the
   PSK is known by everyone, it is possible for a passive attacker to
   observe the 4-way handshake and compute the traffic encryption keys
   used by a client and access point (AP).  If the attacker is too late
   to observe this exchange, he can issue a forged "deauthenticate"
   frame that will cause the client and/or AP to reset the 802.11 state
   machine and cause them to go through the 4-way handshake again,
   thereby allowing the passive attacker to determine the traffic keys.

   With OWE, the client and AP perform a Diffie-Hellman key exchange
   during the access procedure and use the resulting pairwise secret
   with the 4-way handshake instead of using a shared and public PSK in
   the 4-way handshake.

   OWE requires no special configuration or user interaction but
   provides a higher level of security than a common, shared, and public
   PSK.  OWE not only provides more security to the end user, it is also
   easier to use both for the provider and the end user because there
   are no public keys to maintain, share, or manage.

3.  802.11 Network Access

   Wi-Fi access points (APs) advertise their presence through frames
   called "beacons".  These frames inform clients within earshot of the
   SSID (Service Set Identifier) the AP is advertising, the AP's Media
   Access Control (MAC) address (known as its "BSSID" (Basic Service Set
   Identifier)), security policy governing access, the symmetric ciphers
   it uses for unicast and broadcast frames, QoS information, as well as
   support for other optional features of [IEEE802.11].  Wi-Fi clients
   can actively discover APs by issuing "probe requests", which are
   queries for APs that respond with "probe responses".  A probe
   response carries essentially the same information as a beacon.

   After an AP is discovered by a client, actively through probing or
   passively through beacons, the client initiates a two-step method to
   gain network access.  The first step is "802.11 authentication".  For
   most methods of access, this is an empty exchange known as "Open
   Authentication" -- basically, the client says, "authenticate me", and
   the AP responds, "ok, you're authenticated".  After 802.11
   authentication is 802.11 association, in which the client requests
   network access from an AP (the SSID, a selection of the type of
   subsequent authentication to be made, any pairwise and group ciphers,
   etc.) using an 802.11 association request.  The AP acknowledges the
   request with an 802.11 association response.

   If the network is Open (no authentication and no encryption), the
   client has network access immediately after completion of 802.11
   association.  If the network enforces PSK authentication, the 4-way
   handshake is initiated by the AP using the PSK to authenticate the
   client and derive traffic encryption keys.

   To add an opportunistic encryption mode of access to [IEEE802.11], it
   is necessary to perform a Diffie-Hellman key exchange during 802.11
   authentication and use the resulting pairwise secret with the 4-way
   handshake.

4.  Opportunistic Wireless Encryption

4.1.  Cryptography

   Performing a Diffie-Hellman key exchange requires agreement on a
   domain parameter set in which to perform the exchange.  OWE uses a
   registry (see [IKE-IANA]) to map an integer into a complete domain
   parameter set.  OWE supports both Elliptic Curve Cryptography (ECC)
   and Finite Field Cryptography (FFC).

   OWE uses a hash algorithm for generation of a secret and a secret
   identifier.  The particular hash algorithm depends on the group
   chosen for the Diffie-Hellman.  For ECC, the hash algorithm depends
   on the size of the prime defining the curve p:

   o  SHA-256: when len(p) <= 256

   o  SHA-384: when 256 < len(p) <= 384

   o  SHA-512: when 384 < len(p)
   For FFC, the hash algorithm depends on the prime, p, defining the
   finite field:

   o  SHA-256: when len(p) <= 2048

   o  SHA-384: when 2048 < len(p) <= 3072

   o  SHA-512: when 3072 < len(p)

4.2.  OWE Discovery

   An access point advertises support for OWE using an Authentication
   and Key Management (AKM) suite selector for OWE.  This AKM is
   illustrated in Table 1 and is added to the Robust Security Network
   (RSN) element, defined in [IEEE802.11], in all beacons and probe
   response frames the AP issues.

   +----------+--------+-------------------+-------------+-------------+
   |   OUI    | Suite  |   Authentication  |     Key     |     Key     |
   |          |  Type  |        Type       |  Management |  derivation |
   |          |        |                   |     Type    |     type    |
   +----------+--------+-------------------+-------------+-------------+
   | 00-0F-AC |   18   |   Opportunistic   |     This    |  [RFC5869]  |
   |          |        |      Wireless     |   document  |             |
   |          |        |     Encryption    |             |             |
   +----------+--------+-------------------+-------------+-------------+

                             Table 1: OWE AKM

   Once a client discovers an OWE-compliant AP, it performs "Open
   System" 802.11 authentication as defined in [IEEE802.11], and it then
   proceeds to 802.11 association.

4.3.  OWE Association

   Information is added to 802.11 association requests and responses
   using TLVs that [IEEE802.11] calls "elements".  Each element has an
   "Element ID" (including any Element ID extension), a length, and a
   value field that is element specific.  These elements are appended to
   each other to construct 802.11 association requests and responses.

   OWE adds the Diffie-Hellman Parameter element (see Figure 1) to
   802.11 association requests and responses.  The client adds her
   public key in the 802.11 association request, and the AP adds his
   public key in the 802.11 association response.

      +------------+----------+------------+------------------------+
      | Element ID |  Length  | Element ID |   element-specific     |
      |            |          |  Extension |         data           |
      +------------+----------+------------+---------+--------------+
      |    255     | variable |     32     | group   |  public key  |
      +------------+----------+------------+---------+--------------+

              Figure 1: The Diffie-Hellman Parameter Element

   where:

   o  group is an unsigned two-octet integer defined in [IKE-IANA], in
      little-endian format, that identifies a domain parameter set;

   o  public key is an octet string representing the Diffie-Hellman
      public key; and,

   o  Element ID, Length, and Element ID Extension are all single-octet
      integers.

   The encoding of the public key depends on its type.  FFC elements
   SHALL be encoded per the integer-to-octet-string conversion technique
   of [RFC6090].  For ECC elements, the encoding depends on the
   definition of the curve, either that in [RFC6090] or [RFC7748].  If
   the public key is from a curve defined in [RFC6090], compact
   representation SHALL be used.

   A client wishing to do OWE MUST indicate the OWE AKM in the RSN
   element portion of the 802.11 association request and MUST include a
   Diffie-Hellman Parameter element to its 802.11 association request.
   An AP agreeing to do OWE MUST include the OWE AKM in the RSN element
   portion of the 802.11 association response.  If "PMK caching" (see
   Section 4.5) is not performed, it MUST also include a Diffie-Hellman
   Parameter element.  If "PMK caching" is not being performed, a client
   MUST discard any 802.11 association response that indicates the OWE
   AKM in the RSN element but does not have not a Diffie-Hellman
   Parameter element.

   For interoperability purposes, a compliant implementation MUST
   support group nineteen (19), a 256-bit elliptic curve group.  If the
   AP does not support the group indicated in the received 802.11
   association request, it MUST respond with an 802.11 association
   response with a status code of seventy-seven (77) indicating an
   unsupported finite cyclic group.  A client that receives an 802.11
   association response with a status code of seventy-seven SHOULD retry
   OWE with a different supported group and, due to the unsecured nature
   of 802.11 association, MAY request association again using the group
   that resulted in failure.  This failure SHOULD be logged, and if the
   client abandons association due to the failure to agree on any group,
   notification of this fact SHOULD be provided to the user.

   Received Diffie-Hellman Parameter elements are checked for validity
   upon receipt.  For ECC, a validity check depends on the curve
   definition, either that in [RFC6090] or [RFC7748].  For FFC, elements
   are checked that they are between one (1) and one (1) less than the
   prime, p, exclusive (i.e., 1 < element < p-1).  Invalid received
   Diffie-Hellman keys MUST result in unsuccessful association, a
   failure of OWE, and a reset of the 802.11 state machine.  Due to the
   unsecured nature of 802.11 association, a client SHOULD retry OWE a
   number of times (this memo does not specify the number of times).
   This failure should be logged, and if the client abandons association
   due to the (repeated) receipt of invalid elements, notification of
   this fact should be provided to the user.

4.4.  OWE Post-Association

   Once the client and AP have finished 802.11 association, they then
   complete the Diffie-Hellman key exchange and create a Pairwise Master
   Key (PMK) and its associated identifier, PMKID [IEEE802.11].  Given a
   private key x and the peer's (AP's if client, client's if AP) public
   key Y, the following are generated:

      z = F(DH(x, Y))

      prk = HKDF-extract(C | A | group, z)

      PMK = HKDF-expand(prk, "OWE Key Generation", n)

   where HKDF-expand() and HKDF-extract() are defined in [RFC5869]; "C |
   A | group" is a concatenation of the client's Diffie-Hellman public
   key, the AP's Diffie-Hellman public key (from the 802.11 association
   request and response, respectively), and the two-octet group from the
   Diffie-Hellman Parameter element (in little-endian format) and is
   passed as the salt to the HMAC-based Extract-and-Expand Key
   Derivation Function (HKDF) using the hash algorithm defined in
   Section 4.1; and n is the bit length of the digest produced by that
   hash algorithm. z and prk SHOULD be irretrievably deleted once the
   PMK has been generated.

   The PMKID is generated by hashing the two Diffie-Hellman public keys
   (the data, as sent and received, from the "public key" portion of the
   Diffie-Hellman Parameter element in the 802.11 association request
   and response) and returning the leftmost 128 bits:

      PMKID = Truncate-128(Hash(C | A))

   where C is the client's Diffie-Hellman public key from the 802.11
   association request, A is the AP's Diffie-Hellman public key from the
   802.11 association response, and Hash is the hash algorithm defined
   in Section 4.1.

   +---------+--------------+----------+-------+------------+----------+
   |   Hash  |  Integrity   | KCK_bits |  Size |  Key-wrap  | KEK_bits |
   |         |  Algorithm   |          |   of  | Algorithm  |          |
   |         |              |          |  MIC  |            |          |
   +---------+--------------+----------+-------+------------+----------+
   | SHA-256 | HMAC-SHA-256 |   128    |   16  |  NIST AES  |   128    |
   |         |              |          |       |  Key-wrap  |          |
   | SHA-384 | HMAC-SHA-384 |   192    |   24  |  NIST AES  |   256    |
   |         |              |          |       |  Key-wrap  |          |
   | SHA-512 | HMAC-SHA-521 |   256    |   32  |  NIST AES  |   256    |
   |         |              |          |       |  Key-wrap  |          |
   +---------+--------------+----------+-------+------------+----------+

                Table 2: Integrity and Key Wrap Algorithms

   Upon completion of 802.11 association, the AP initiates the 4-way
   handshake to the client using the PMK generated above.  The 4-way
   handshake generates a Key-Encrypting Key (KEK), a Key-Confirmation
   Key (KCK), and a Message Integrity Code (MIC) to use for protection
   of the frames that define the 4-way handshake.  The algorithms and
   key lengths used in the 4-way handshake depend on the hash algorithm
   selected in Section 4.1 and are listed in Table 2.

   The result of the 4-way handshake is encryption keys to protect bulk
   unicast data and broadcast data.  If the 4-way handshake fails, this
   information SHOULD be presented to the user.

4.5.  OWE PMK Caching

   [IEEE802.11] defines "PMK caching" where a client and access point
   can cache a PMK for a certain period of time and reuse it with the
   4-way handshake after subsequent associations to bypass potentially
   expensive authentication.  A client indicates its desire to do "PMK
   caching" by including the identifying PMKID in its 802.11 association
   request.  If an AP has cached the PMK identified by that PMKID, it
   includes the PMKID in its 802.11 association response; otherwise, it
   ignores the PMKID and proceeds with normal 802.11 association.  OWE
   supports the notion of "PMK caching".

   Since "PMK caching" is indicated in the same frame as the Diffie-
   Hellman Parameter element is passed, a client wishing to do "PMK
   caching" MUST include both in her 802.11 association request.  If the
   AP has the PMK identified by the PMKID and wishes to perform "PMK
   caching", he will include the PMKID in his 802.11 association
   response but does not include a Diffie-Hellman Parameter element.  If
   the AP does not have the PMK identified by the PMKID, it ignores the
   PMKID and proceeds with normal OWE 802.11 association by including a
   Diffie-Hellman Parameter element.

   When attempting "PMK caching", a client SHALL ignore any Diffie-
   Hellman Parameter element in an 802.11 association response whose
   PMKID matches that of the client-issued 802.11 association request.
   If the 802.11 association response does not include a PMKID, or if
   the PMKID does not match that of the client-issued 802.11 association
   request, the client SHALL proceed with normal OWE association.

   The client SHALL ignore a PMKID in any 802.11 association response
   frame for which it did not include a PMKID in the corresponding
   802.11 association request frame.

5.  IANA Considerations

   This document does not require any IANA actions.

6.  Implementation Considerations

   OWE is a replacement for 802.11 "Open" authentication.  Therefore,
   when OWE-compliant access points are discovered, the presentation of
   the available SSID to users should not include special security
   symbols such as a "lock icon".  To a user, an OWE SSID is the same as
   "Open"; it simply provides more security behind the scenes.

   When OWE is initially deployed as a replacement for an existing
   network that uses "Open" authentication or a shared and public PSK,
   it will be necessary to create an additional Basic Service Set
   Identifier (BSSID) or a new Extended Service Set (ESS) with a
   separate Service Set Identifier (SSID) for OWE so two distinct 802.11
   networks can exist on the same access point (see [IEEE802.11]).  This
   arrangement should remain until the majority of users have switched
   over to OWE.

7.  Security Considerations

   Opportunistic encryption does not provide authentication.  The client
   will have no authenticated identity for the access point, and vice
   versa.  They will share pairwise traffic encryption keys and have a
   cryptographic assurance that a frame claimed to be from the peer is
   actually from the peer and was not modified in flight.

   OWE only secures data sent over the wireless medium and does not
   provide security for end-to-end traffic.  Users should still use
   application-level security to achieve security end-to-end.

   OWE is susceptible to an active attack in which an adversary
   impersonates an access point and induces a client to connect to it
   via OWE while it makes a connection to the legitimate access point.
   In this particular attack, the adversary is able to inspect, modify,
   and forge any data between the client and legitimate access point.

   OWE is not a replacement for any authentication protocol specified in
   [IEEE802.11] and is not intended to be used when an alternative that
   provides real authentication is available.

8.  References

8.1.  Normative References

   [IEEE802.11]
              IEEE, "IEEE Standard for Information technology--
              Telecommunications and information exchange between
              systems Local and metropolitan area networks--Specific
              requirements - Part 11: Wireless LAN Medium Access Control
              (MAC) and Physical Layer (PHY) Specifications", IEEE Std
              802.11, DOI 10.1109/IEEESTD.2016.7786995.

   [IKE-IANA] IANA, "Transform Type 4 - Diffie-Hellman Group Transform
              IDs", <http://www.iana.org/assignments/ikev2-parameters/>.

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

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <http://www.rfc-editor.org/info/rfc5869>.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090,
              DOI 10.17487/RFC6090, February 2011,
              <http://www.rfc-editor.org/info/rfc6090>.

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

8.2.  Informative References

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <http://www.rfc-editor.org/info/rfc7435>.

Authors' Addresses

   Dan Harkins (editor)
   HP Enterprise
   3333 Scott Boulevard
   Santa Clara, California  95054
   United States of America

   Phone: +1 415 555 1212
   Email: dharkins@arubanetworks.com


   Warren Kumari (editor)
   Google
   1600 Amphitheatre Parkway
   Mountain View, California  94043
   United States of America

   Phone: +1 408 555 1212
   Email: warren@kumari.net