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Internet Engineering Task Force (IETF)                   D. Harkins, Ed.Request for Comments: 8110                                 HP EnterpriseCategory: Informational                                   W. Kumari, Ed.ISSN: 2070-1721                                                   Google                                                              March 2017Opportunistic Wireless EncryptionAbstract   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; seeSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc8110.Copyright Notice   Copyright (c) 2017 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Harkins & Kumari              Informational                     [Page 1]

RFC 8110            Opportunistic Wireless Encryption         March 2017Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .31.1.  Requirements Language . . . . . . . . . . . . . . . . . .31.2.  Notation  . . . . . . . . . . . . . . . . . . . . . . . .32.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .33.  802.11 Network Access . . . . . . . . . . . . . . . . . . . .44.  Opportunistic Wireless Encryption . . . . . . . . . . . . . .54.1.  Cryptography  . . . . . . . . . . . . . . . . . . . . . .54.2.  OWE Discovery . . . . . . . . . . . . . . . . . . . . . .64.3.  OWE Association . . . . . . . . . . . . . . . . . . . . .74.4.  OWE Post-Association  . . . . . . . . . . . . . . . . . .84.5.  OWE PMK Caching . . . . . . . . . . . . . . . . . . . . .105.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .106.  Implementation Considerations . . . . . . . . . . . . . . . .107.  Security Considerations . . . . . . . . . . . . . . . . . . .118.  References  . . . . . . . . . . . . . . . . . . . . . . . . .118.1.  Normative References  . . . . . . . . . . . . . . . . . .118.2.  Informative References  . . . . . . . . . . . . . . . . .12   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .12Harkins & Kumari              Informational                     [Page 2]

RFC 8110            Opportunistic Wireless Encryption         March 20171.  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 inRFC 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 theseHarkins & Kumari              Informational                     [Page 3]

RFC 8110            Opportunistic Wireless Encryption         March 2017   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.Harkins & Kumari              Informational                     [Page 4]

RFC 8110            Opportunistic Wireless Encryption         March 2017   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 Encryption4.1.  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)Harkins & Kumari              Informational                     [Page 5]

RFC 8110            Opportunistic Wireless Encryption         March 2017   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.Harkins & Kumari              Informational                     [Page 6]

RFC 8110            Opportunistic Wireless Encryption         March 20174.3.  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" (seeSection 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 OWEHarkins & Kumari              Informational                     [Page 7]

RFC 8110            Opportunistic Wireless Encryption         March 2017   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 isHarkins & Kumari              Informational                     [Page 8]

RFC 8110            Opportunistic Wireless Encryption         March 2017   passed as the salt to the HMAC-based Extract-and-Expand Key   Derivation Function (HKDF) using the hash algorithm defined inSection 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   inSection 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 inSection 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.Harkins & Kumari              Informational                     [Page 9]

RFC 8110            Opportunistic Wireless Encryption         March 20174.5.  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 SetHarkins & Kumari              Informational                    [Page 10]

RFC 8110            Opportunistic Wireless Encryption         March 2017   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.  References8.1.  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>.Harkins & Kumari              Informational                    [Page 11]

RFC 8110            Opportunistic Wireless Encryption         March 2017   [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.netHarkins & Kumari              Informational                    [Page 12]