capport D. Harkins, Ed.
Internet-Draft HP Enterprise
Intended status: Informational W. Kumari, Ed.
Expires: January 19, 2017 Google
July 18, 2016
OWE
draft-harkins-owe-00
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
1.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. 802.11 Network Access . . . . . . . . . . . . . . . . . . . . 3
4. Opportunistic Wireless Encryption . . . . . . . . . . . . . . 4
4.1. Cryptography . . . . . . . . . . . . . . . . . . . . . . 4
4.2. OWE Discovery . . . . . . . . . . . . . . . . . . . . . . 5
4.3. OWE Association . . . . . . . . . . . . . . . . . . . . . 5
4.4. OWE Post-Association . . . . . . . . . . . . . . . . . . 7
4.5. OWE PMK Caching . . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Implementation Considerations . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
This memo describes a mode of opportunistic encryption [RFC7435] for
802.11 -- OWE -- 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 expontiation of element Y to the
power of x (implied modulo a field defining prime, p) resulting
in an element Z.
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a = len(b)
indicates the length in bits of the string b.
2. Background
Many businesses-- bars, coffee shops, etc.-- 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 not willing and/or not
qualified 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 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. If the attacker is too late to
observe this exchange, he can issue a forged "de-authenticate" 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.
Basically, this shared and public PSK mode of access is as bad as an
open and unencrypted network. [TODO: Explain trade offs; shared PSK
means the attacker has to be active and could provide a false sense
of security.] With OWE, the client and AP, would 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 provides more security to the end user, while also being
easier to use (no public keys to maintain, share, etc.).
3. 802.11 Network Access
Wi-Fi Access Points advertise their presence through frames called
"beacons". These frames inform clients within earshot of the SSID
the AP is advertising, the AP's MAC address (known as its "BSSID"),
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security policy governing access, which 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 (SAE being the exception), this is an empty
exchange known as "Open Authentication-- basically the client says,
"authenticate me", and the AP resonds "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, 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
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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 identifer for OWE. This AKM is
illustrated in Table 1 and is added to the RSN Element in all beacons
and probe responses that the AP issues.
OWE AKM
+----------+--------+-------------------+-------------+-------------+
| OUI | Suite | Authentication | Key | Key |
| | Type | Type | Manamgement | derivation |
| | | | Type | type |
+----------+--------+-------------------+-------------+-------------+
| 00-0F-AC | ANA-1 | Opportunistic | This | [RFC5869] |
| | | Wireless | document | |
| | | Encryption | | |
+----------+--------+-------------------+-------------+-------------+
Table 1: OWE AKM
where ANA-1 is assigned by IEEE 802.11 ANA.
Once a client discovers an OWE-compliant AP, it performs "Open
System" 802.11 authentication as defined in [IEEE802.11], it then
proceeds to 802.11 association.
4.3. OWE Association
Information is added to 802.11 association requests and responses by
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 associate requests and responses.
OWE adds the Diffie-Hellman Parameter element (see Figure 1) to
802.11 association requests and responses. The client adds her
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public key in the 802.11 associate request and the AP adds his public
key in the 802.11 associate response.
The Diffie-Hellman Parameter Element
+------------+----------+------------+------------------------+
| Element ID | Length | ID | element-specific |
| | | Extension | data |
+------------+----------+------------+---------+--------------+
| 255 | variable | ANA-2 | group | public key |
+------------+----------+------------+---------+--------------+
Figure 1
where
o ANA-2 is assigned by IEEE 802.11 ANA;
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 encoded according to section 2.3.3 (Elliptic Curve to
Octet String Conversion) or 2.3.5 (Field Element to Octet String
Conversion) of [SEC1] depending on whether the public key is ECC
or FFC, respectively; and,
o Element ID, Length, and ID Extension are all single octet integers
in little-endian format.
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. TODO:
what to do if the AP doesn't like the client's chosen group?
Received Diffie-Hellman Parameter Elements are checked for validity
upon receipt. For ECC, elements are checked by verifying that
equation for the curve is correct for the given x- and y-
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coordinates, excluding the point at infinity. 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 and a
failure of OWE.
4.4. OWE Post-Association
Once the client and AP have finished 802.11 association they finish
the Diffie-Hellman key exchange and create a "pairwise master key"
(PMK), and its associated identifier, PMKID. 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(NULL, z)
PMK = HKDF-expand(prk, "OWE Key Generation", n)
Where HKDF-expand() and HKDF-extract() are defined in [RFC5869], NULL
indicates the "salt-less" invocation of HKDF using the hash algorithm
defined in section Section 4.1, and n is the bitlength of the digest
produced by that hash algorithm. z and prk are 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 left-most 128 bits:
PMKID = Truncate-128(Hash(C | A))
where C is the client's Diffie-Hellman public key from the 802.11
Association request and A is the AP's Diffie-Hellman public key from
the 802.11 Association response, and Hash is the hash algorithm
defined in section Section 4.1.
Upon completion of 802.11 association, the AP initiates the 4-way
Handshake to the client using the PMK generated above. The result of
the 4-way Handshake are encryption keys to protect bulk unicast data
and broadcast data.
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
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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 Parameeter element.
When attempting "PMK caching" a client SHALL ignore any Diffie-
Hellman Parameter element in an 802.11 association response that
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 memo includes no request to IANA.
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 does not include special security symbols
such as a graphic lock. To a user, an OWE SSID is the same as
"Open", it just provides more security behind the scenes.
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.
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OWE is susceptible to an active attack in which an adversary
impersonates an Access Point, 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. Normative References
[IEEE802.11]
IEEE Computer Society, "Telecommunications and information
exchange between systems Local and metropolitan area
networks--", Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications IEEE Std
802.11-2012, 2012.
[IKE-IANA]
IANA, "Internet Key Exchange (version 2) Parameters",
Transform Type 4: Diffie-Hellman Group Transform IDs,
2005, <http://www.iana.org/assignments/ikev2-parameters/
ikev2-parameters.xhtml#ikev2-parameters-8>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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>.
[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>.
[SEC1] Brown, D., "Elliptic Curve Cryptography", Version 2.0,
2009.
Authors' Addresses
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Dan Harkins (editor)
HP Enterprise
1322 Crossman avenue
Sunnyvale, California 94089
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
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