Network Working Group T. Aura
Internet-Draft Aalto University
Intended status: Standards Track M. Sethi
Expires: August 11, 2016 Ericsson
February 8, 2016
Nimble out-of-band authentication for EAP (EAP-NOOB)
draft-aura-eap-noob-00
Abstract
Extensible Authentication Protocol (EAP) [RFC3748] provides support
for multiple authentication methods. This document defines the EAP-
NOOB authentication method for nimble out-of-band (OOB)
authentication and key derivation. This EAP method is intended for
bootstrapping all kinds of Internet-of-Things (IoT) devices that have
a minimal user interface and no pre-configured authentication
credentials. The method makes use of a user-assisted one-directional
OOB channel between the peer device and authentication server.
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
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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 August 11, 2016.
Copyright Notice
Copyright (c) 2016 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
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. EAP-NOOB protocol . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Protocol overview . . . . . . . . . . . . . . . . . . . . 4
3.2. Protocol messages and sequences . . . . . . . . . . . . . 7
3.2.1. Initial Exchange . . . . . . . . . . . . . . . . . . 8
3.2.2. OOB Step . . . . . . . . . . . . . . . . . . . . . . 9
3.2.3. Completion Exchange . . . . . . . . . . . . . . . . . 10
3.2.4. Waiting Exchange . . . . . . . . . . . . . . . . . . 11
3.3. Message data items . . . . . . . . . . . . . . . . . . . 12
3.4. Fast reconnect and rekeying . . . . . . . . . . . . . . . 17
3.4.1. Reconnect Exchange . . . . . . . . . . . . . . . . . 17
3.4.2. User reset . . . . . . . . . . . . . . . . . . . . . 19
3.5. Key derivation . . . . . . . . . . . . . . . . . . . . . 20
3.6. Error handling . . . . . . . . . . . . . . . . . . . . . 21
3.6.1. Invalid messages . . . . . . . . . . . . . . . . . . 22
3.6.2. Unwanted peer . . . . . . . . . . . . . . . . . . . . 22
3.6.3. State mismatch . . . . . . . . . . . . . . . . . . . 23
3.6.4. Negotiation failure . . . . . . . . . . . . . . . . . 23
3.6.5. Cryptographic verification failure . . . . . . . . . 23
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
4.1. Cryptosuites . . . . . . . . . . . . . . . . . . . . . . 24
4.2. Error codes . . . . . . . . . . . . . . . . . . . . . . . 25
4.3. Domain name reservation considerations . . . . . . . . . 25
5. Security considerations . . . . . . . . . . . . . . . . . . . 26
5.1. Authentication principle . . . . . . . . . . . . . . . . 26
5.2. Identifying and naming peer devices . . . . . . . . . . . 27
5.3. Downgrading threats . . . . . . . . . . . . . . . . . . . 29
5.4. EAP security claims . . . . . . . . . . . . . . . . . . . 29
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
6.1. Normative references . . . . . . . . . . . . . . . . . . 31
6.2. Informative references . . . . . . . . . . . . . . . . . 32
Appendix A. Exchanges and events per state . . . . . . . . . . . 33
Appendix B. TODO list . . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
1. Introduction
This document describes a method for registration, authentication and
key derivation for network-connected ubiquitous computing devices,
such as consumer and enterprise appliances that are part of the
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Internet of Things (IoT). These devices may be off-the-shelf
hardware that is sold and distributed without any prior registration
or credential-provisioning process. Thus, the device registration in
a server database, ownership of the device, and the authentication
credentials for both network access and application-level security
must all be established at the time of the device deployment.
Furthermore, many such devices have only limited user interfaces that
could be used for their configuration. Often, the interfaces are
limited to either only input (e.g. camera) or output (e.g. display
screen). The device configuration is made more challenging by the
fact that the devices may exist in large numbers or may have to be
deployed or re-configured nimbly based on user needs.
More specifically, the devices may have the following
characteristics:
o no pre-established relation with a specific server or user,
o no pre-provisioned device identifier or authentication
credentials,
o limited user interface and configuration capabilities.
Many proprietary OOB configuration methods exits for specific IoT
devices. The goal of this specification is to provide an open
standard and a generic protocol for bootstrapping the security of
network-connected appliances, such as displays, printers, speaker,
and cameras. The security bootstrapping in this specification makes
use of a user-assisted out-of-band (OOB) channel. The security is
based on the assumption that attackers are not able to observe or
modify the messages conveyed through the OOB channel. We follow the
common approach of performing a Diffie-Hellman key exchange over the
insecure network and authenticating the established key with the help
of the OOB channel in order to prevent man-in-the-middle (MitM)
attacks.
The solution presented here is intended for devices that have either
an input or output interface, such as a camera or display screen,
which is able to send or receive dynamically generated messages of
tens of bytes in length. Naturally, this solution may not be
appropriate for very small sensors or actuators that have no user
interface at all. We also assume that the OOB channel is at least
partly automated (e.g. camera scanning a bar code) and, thus, there
is no need to absolutely minimize the length of the data transferred
through the OOB channel. This differs, for example, from Bluetooth
simple pairing [SimplePairing], where it is critical to minimize the
length of the manually transferred or compared codes. Since the OOB
messages are dynamically generated, we do not support static printed
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registration codes. This also prevents attacks where a static secret
code would be leaked.
2. Terminology
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].
In addition, this document frequently uses the following terms as
they have been defined in [RFC5216]:
authenticator The entity initiating EAP authentication.
peer The entity that responds to the authenticator. In
[IEEE-802.1X], this entity is known as the Supplicant.
server The entity that terminates the EAP authentication method with
the peer. In the case where no backend authentication server
is used, the EAP server is part of the authenticator. In the
case where the authenticator operates in pass-through mode, the
EAP server is located on the backend authentication server.
3. EAP-NOOB protocol
This section defines the EAP-NOOB protocol. The protocol is a
generalized version of the original idea presented by Sethi et al.
[Sethi14].
3.1. Protocol overview
One EAP-NOOB protocol execution spans multiple EAP exchanges. This
is necessary to leave time for the OOB message to be delivered, as
will be explained below.
The overall protocol starts with the Initial Exchange, in which the
server allocates an identifier to the peer, and the server and peer
negotiate the protocol version and cryptosuite (i.e. cryptographic
algorithm suite), exchange nonces and perform an Elliptic Curve
Diffie-Hellman (ECDH) key exchange. The user-assisted OOB Step then
takes place. This step involves only one out-of-band message either
from the peer to the server or from the server to the peer. While
waiting for the OOB Step action, the peer MAY probe the server by
reconnecting to it with EAP-NOOB. If the OOB Step has already taken
place, the probe leads to the Completion Exchange, which completes
the mutual authentication and key confirmation. On the other hand,
if the OOB Step has not yet taken place, the probe leads to the
Waiting Exchange, and the peer will perform another probe after a
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server-defined minimum waiting time. The Initial Exchange and
Waiting Exchange always end in EAP-Failure, while the Completion
Exchange may result in EAP-Success. Once the peer and server have
performed a successful Completion Exchange, both parties store the
created association in persistent storage, and the OOB Step is not
repeated. Thereafter, creation of new temporal keys, ECDH rekeying,
and updates of cryptographic algorithms can be achieved with the
Reconnect Exchange.
Figure 1 shows the association state machine, which is the same for
the server and for the peer. When the client initiates the EAP-NOOB
method, the server chooses the ensuing message exchange based on the
combination of the server and peer states. The EAP server and peer
are initially in the Unregistered state, in which no state
information needs to be stored. Before a successful Completion
Exchange, the server-peer association state is ephemeral in both the
server and peer (ephemeral states 0..2) , and either party may cause
the protocol to fall back to the Initial Exchange. After the
Completion Exchange has resulted in EAP-Success, the association
state becomes persistent (persistent states 3..4), and only user
reset or accidental failure can cause the return of the server or the
peer to the ephemeral states and to the Initial Exchange.
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OOB Output, Initial Exchange,
or Waiting Exchange
.-----.
| v
.------------------. Initial .------------------.
| | Exchange | |
.->| 0. Unregistered |---------------->|1. Waiting for OOB|
| | | | |
| '------------------' '------------------'
| | | ^
User Reset Completion | | |
| Exchange | OOB Initial
|<-------. .<------------------------' Input Exchange
| | | | |
| | v v |
| .------------------. Completion .------------------.
| | | Exchange | |
| | 4. Registered |<----------------| 2. OOB Received |
| | | | |
| '------------------' '------------------'
| | ^
| | |
| Timeout / Reconnect
| Failure Exchange
| | |
| v |
| .-----------------.
| | |
'--| 3. Reconnecting |
| |
'-----------------'
Figure 1: EAP-NOOB server-peer association state machine
The server MUST NOT repeat the OOB Step with the same peer except if
the association with the peer is explicitly reset by the user or lost
due to failure of the persistent storage. In particular, once the
association has entered the Registered state, the server MUST NOT
delete the association or go back to states 0-2 without explicit user
approval. Similarly, the peer MUST NOT repeat the OOB Step unless
the user explicitly deletes the association with the server or resets
it to the Unregistered state. However, it can happen that the client
accidentally loses its persistent state and reconnects to the server
without a previously allocated peer identifier. In that case, the
server MUST treat the peer as a new peer. The server MAY use
auxiliary information, such as the PeerInfo field received in the
Initial Exchange, to detect such multiple association of the same
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peer. However, it MUST NOT automatically delete associations because
there is no secure way of verifying that the two peers are the same
physical device.
A special feature of the EAP-NOOB method is that the server is not
assumed to have any a-priori knowledge of the peer. Therefore, the
peer initially uses the generic identity string "noob@eap-noob.net"
as the NAI. The server then allocates a server-specific identifier
to the peer. The network access identifier NAI is a concatenation of
the server-allocated peer identifier and the generic suffix "@eap-
noob.net". This suffix serves two purposes: firstly, it tells the
server that the peer supports and expects the EAP-NOOB method and,
secondly, it allows routing of the EAP-NOOB sessions to a specific
authentication server in the AAA architecture.
EAP-NOOB is an unusual EAP method in that the peer has to connect to
the server two or more times before it can receive EAP-Success. The
reason is that, while EAP allows delays between the request-response
pairs, e.g. for repeated password entry, the user delays in OOB
authentication can be much longer than in password trials. In
particular, EAP-NOOB supports also peers or servers with no input
capability in the user interface. Since these output-only parties
cannot be told to perform the protocol at the right moment, they have
to perform the initial exchange opportunistically and hope for the
OOB Step to take place within a timeout period, which is why the
timeout needs to be several minutes rather than seconds. For
example, consider a printer (peer) from which the OOB message is
printed as a bar code on paper and then scanned with a camera phone
and communicated to the server. To support such devices and slow OOB
channels, the peer in EAP-NOOB first contacts the server in the
Initial Exchange, then disconnects for some time, and later continues
with the Waiting and Completion Exchanges.
3.2. Protocol messages and sequences
This section defines the EAP-NOOB exchanges. The protocol messages
communicated and the data members in each message are listed in the
diagrams below.
Each EAP-NOOB exchange begins with the authenticator sending an EAP-
Request/Identity packet to the peer. From this point on, the EAP
conversation occurs between the server and the peer, and the
authenticator acts as a pass-through device. The peer responds to
the authenticator with an EAP-Response/Identity packet, containing
the network access identifier (NAI). The peer MUST compose the NAI
as defined in Section 3.3. Essentially, if the peer has no previous
peer identifier (PeerId), it uses the fixed NAI string "noob@eap-
noob.net", and if it has received a PeerId from the server, it
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creates the NAI by concatenating the PeerId, a state indicator "+sX",
and the fixed suffix string "@eap-noob.net".
After receiving the NAI, the server chooses the EAP-NOOB exchange,
i.e. the ensuing message sequence, based on the combination of the
client and server states. The client recognizes the exchange based
on the message type field (Type) of the EAP-NOOB request received
from the server. The available exchanges are defined in the
following subsections. Each exchange comprises one or two EAP
requests-response pairs and ends in either EAP-Failure, indicating
that authentication is not (yet) successful, or in EAP-Success.
3.2.1. Initial Exchange
Upon receiving the EAP-Response/Identity from the peer, if either the
peer or the server is in the Unregistered (0) state and the other is
in one of the ephemeral states (0..2), the server chooses the Initial
Exchange.
The Initial Exchange comprises two EAP-NOOB request-response pairs,
one for version, algorithm and parameter negotiation and the other
for the ECDH key exchange. The first EAP-NOOB request (Type=1) from
the server contains a newly allocated PeerId for the peer, regardless
of the username part of the received NAI. The server also sends in
the request a list of protocol versions supported (Vers),
cryptosuites (Cryptosuites), an indicator of the OOB channel
directions supported by the server (Dirs), and a ServerInfo object.
The peer chooses one of the versions and cryptosuites. The peer
sends a response (Type=1) with the selected protocol version (Verp),
the received PeerId, the selected cryptosuite (Cryptosuitep), an
indicator of the OOB channel directions supported by the peer (Dirp),
and a PeerInfo object. In the second EAP-NOOB request and response
(Type=2), the server and peer exchange the public components of their
ECDH keys and nonces (PKs,Ns,PKp,Np). The ECDH keys MUST be based on
the negotiated cryptosuite. The Initial Exchange ends with EAP-
Failure from the server because the authentication cannot yet be
completed.
At the conclusion of the Initial Exchange, both the server and the
peer move to the Waiting for OOB (1) state.
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EAP Peer EAP Server
| |
|<----------- EAP-Request/Identity -| |
| |
| |
|------------ EAP-Response/Identity -------------->|
| (NAI=noob|PeerId+sX@eap-noob.net) |
| |
| |
|<----------- EAP-Request/EAP-NOOB ----------------|
| (Type=1,Vers,PeerId,Cryptosuites,Dirs,ServerInfo)|
| |
| |
|------------ EAP-Response/EAP-NOOB -------------->|
| (Type=1,Verp,PeerId,Cryptosuitep,Dirp,PeerInfo) |
| |
| |
|<----------- EAP-Request/EAP-NOOB ----------------|
| (Type=2,PeerId,PKs,Ns) |
| |
| |
|------------ EAP-Response/EAP-NOOB -------------->|
| (Type=2,PeerId,PKp,Np) |
| |
| |
|<----------- EAP-Failure -------------------------|
| |
Figure 2: Initial Exchange
3.2.2. OOB Step
The OOB Step, shown as OOB Output and OOB Input in Figure 1, takes
place after the Initial Exchange. Depending on the direction
negotiated, the peer or the server outputs the OOB message containing
the PeerId, the secret nonce Noob, and the cryptographic fingerprint
Hoob, as defined in Section 3.3. This message is then delivered to
the other party via a user-assisted OOB channel. The details of the
OOB channel are defined by the application. The receiver of the OOB
message MUST compare the received value of the fingerprint Hoob with
a value that it computes locally.
Even though not recommended (see Section 3.3), this specification
allows both directions to be negotiated. In this case, both sides
SHOULD output the OOB message, and it is up to the user to deliver
one of them.
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EAP Peer EAP Server
| |
|=================OOB=============================>|
| (PeerId,Noob,Hoob) |
| |
Figure 3: OOB Step, from peer to EAP server
EAP Peer EAP Server
| |
|<================OOB==============================|
| (PeerId,Noob,Hoob) |
| |
Figure 4: OOB Step, from EAP server to peer
3.2.3. Completion Exchange
After the Initial Exchange, if both the server the peer support the
peer-to-server direction for the OOB channel, the peer SHOULD
initiate the EAP-NOOB method again after an applications-specific
waiting time in order to probe for completion of the OOB Step. Also,
if both sides support the server-to-peer direction of the OOB
exchange and the peer receives the OOB message, it SHOULD initiate
the EAP-NOOB method immediately. Once server receives the EAP-
Response/Identity, if one of the server and peer is in the OOB
Received (2) state and the other is in the Waiting for OOB (1) or OOB
Received (2) state, the OOB Step has taken place and the server
SHOULD continue with the Completion Exchange.
The Completion Exchange comprises one EAP-NOOB request-response pair
(Type=4). In these messages, the server and peer exchange message
authentication codes. Both sides MUST compute the keys Kms and Kmp
as defined in Section 3.5 and the message authentication codes MACs
and MACp as defined in Section 3.3. Both sides MUST compare the
received message authentication code with a locally computed value.
If the EAP server finds that it has received the correct value of
MACp, the Completion Exchange ends in EAP-Success; otherwise, in EAP-
Failure.
While it is not expected to occur in practice, poor user interface
design could lead to two OOB messages delivered simultaneously, one
from the peer to the server and the other from the server to the
peer. The server detects this event by observing that both the
server and peer are in the OOB Received state (2). In that case, the
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server MUST behave as if only the server-to-peer message was
delivered.
After successful Completion Exchange, both the server and the peer
move to the Registered (4) state.
EAP Peer EAP Server
| |
|<----------- EAP-Request/Identity -| |
| |
| |
|------------ EAP-Response/Identity -------------->|
| (NAI=PeerId+sX@eap-noob.net) |
| |
| |
|<----------- EAP-Request/EAP-NOOB ----------------|
| (Type=4,PeerId,MACs) |
| |
| |
|------------ EAP-Response/EAP-NOOB -------------->|
| (Type=4,PeerId,MACp) |
| |
| |
|<----------- EAP-Success -------------------------|
| |
Figure 5: Completion Exchange
3.2.4. Waiting Exchange
As explained in Section 3.2.3, if both the server and the peer
support the peer-to-server direction for the OOB channel, the peer
will probe the server for completion of the OOB Step. If both the
server and client states are Waiting for OOB (1), the server will
continue with the Waiting Exchange (message Type=3). The only
purpose of this exchange is to indicate to the peer that the server
has not yet received a peer-to-server OOB message.
In order to limit the rate at which peers probe the server, the
server sends to the peer a minimum time to wait before probing the
server again. The peer MUST wait at least the server-specified
minimum waiting time in seconds (MinSleep) before initiating EAP
again with the same server. If the server omits the MinSleep field
from the request, the peer SHOULD wait for an application-specified
minimum time.
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EAP Peer EAP Server
| |
|<----------- EAP-Request/Identity -| |
| |
| |
|------------ EAP-Response/Identity -------------->|
| (NAI=PeerId+s1@eap-noob.net) |
| |
| |
|<----------- EAP-Request/EAP-NOOB ----------------|
| (Type=3,PeerId,[MinSleep]) |
| |
| |
|------------ EAP-Response/EAP-NOOB -------------->|
| (Type=3,PeerId) |
| |
| |
|<----------- EAP-Failure -------------------------|
| |
Figure 6: Waiting Exchange
3.3. Message data items
Table 1 defines the data items in the protocol messages. The in-band
messages are formatted as JSON objects [RFC7159] in UTF-8 encoding.
The member names are in the left-hand column of table.
+------------------+------------------------------------------------+
| Data field | Description |
+------------------+------------------------------------------------+
| Vers,Verp | EAP-NOOB protocol versions supported by the |
| | EAP server, and the protocol version chosen by |
| | the peer. Vers is a JSON array of unsigned |
| | integers, and Verp is an unsigned integer. |
| | Currently, the only defined values are "[1]" |
| | and "1", respectively. |
| PeerId | Peer identifier. If the peer does not yet have |
| | a peer identifier or it does not remember one, |
| | it uses the NAI "noob@eap-noob.net" in the |
| | Initial Exchange. The server then assigns an |
| | identifier to the peer and sends it in the |
| | first server-to-peer request of the Initial |
| | Exchange. The assigned identifier is ephemeral |
| | until a successful Completion Exchange takes |
| | place. Thereafter, the peer identifier becomes |
| | permanent until the user resets the peer and |
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| | the server. Resetting the server means |
| | deleting the association for the peer from the |
| | server database. The peer identifier MUST |
| | follow the syntax of the utf8-username |
| | specified in [RFC7542]; however, it MUST NOT |
| | exceed 60 bytes in length and MUST NOT contain |
| | the character '+'. The server MUST generate |
| | the identifiers in such a way that they do not |
| | repeat and cannot be guessed by the peer or |
| | third parties beforehand. One way to generate |
| | the identifiers is to choose a random 40-digit |
| | lower-case hexadecimal string. |
| | |
| Peer State "+sX" | This part of the NAI informs the server about |
| | the peer state. The server uses this |
| | information together with the server state to |
| | decide on the type of the exchange and, thus, |
| | of the type of the next EAP-Request. The peer |
| | appends the peer state to the PeerId to form |
| | the username part of the NAI. (Sending it in |
| | the EAP-Response/Identity message avoids an |
| | additional round trip for querying the peer |
| | state.) If the peer is in state 0, it MUST use |
| | the NAI "noob@eap-noob.net"; otherwise, the |
| | peer MUST create the NAI as the concatenation |
| | of the PeerId, the string "+s", a single digit |
| | indicating the state of the peer, and the |
| | string "@eap-noob.net". |
| | |
| Type | EAP-NOOB message type. The type is an integer |
| | in the range 0..6. EAP-NOOB requests and the |
| | corresponding responses share the same type |
| | value. |
| | |
| PKs, PKp | The public components of the ECDH keys of the |
| | server and peer. PKs and PKp are sent in the |
| | JSON Web Key (JWK) format [RFC7517]. |
| | |
| Cryptosuites, | The identifiers of cryptosuites supported by |
| Cryptosuitep | the server and of the cryptosuite selected by |
| | the peer. The format is specified in Section |
| | 4.1. |
| | |
| Dirs, Dirp | OOB channel directions supported by the server |
| | and the peer. The possible values are 1=peer- |
| | to-server, 2=server-to-peer, 3=both |
| | directions. Endpoints that are general-purpose |
| | computers or online services SHOULD support |
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| | both directions. IoT devices with a limited |
| | user interface will mostly support only one |
| | direction. If the negotiated value is 3, the |
| | user may be presented with two OOB messages, |
| | one for each direction, even though the user |
| | needs to deliver only one of them. Since this |
| | can be confusing to the user, it RECOMMENDED |
| | that the peer selects value 1 or 2. The EAP- |
| | NOOB protocol itself is designed to cope also |
| | with selected value 3, in which case it uses |
| | the first delivered OOB message. In the |
| | unlikely case of simultaneously delivered OOB |
| | messages, the protocol prioritizes the server- |
| | to-peer direction. |
| | |
| Ns, Np | Nonces for the Initial Exchange. |
| | |
| ServerInfo | This field contains information about the |
| | server to be passed from the EAP method to the |
| | application layer in the peer. The content of |
| | this field is specific to the application. It |
| | could include, for example, the network name |
| | and server name or a Uniform Resource Locator |
| | (URL) [RFC1738] or some other information that |
| | helps the user to deliver the OOB message to |
| | the server through the out-of-band channel. |
| | |
| PeerInfo | This field contains information about the peer |
| | to be passed from the EAP method to the |
| | application layer in the server. The content |
| | of this field is specific to the application. |
| | It could include, for example, the peer make, |
| | model and serial number that helps the user to |
| | distinguish between devices and to deliver the |
| | OOB message to the correct peer through the |
| | out-of-band channel. |
| | |
| MinSleep | The number of seconds for which peer MUST NOT |
| | start a new execution of the EAP-NOOB method |
| | with the authenticator, unless the peer is |
| | reset by the user. The server can use this |
| | field to limit the rate at which peers probe |
| | it for the completion of the OOB Step. |
| | MinSleep is an unsigned integer in the range |
| | 0..3600. |
| | |
| Noob | Secret nonce sent through the OOB channel and |
| | used for the session key derivation. The party |
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| | that received the OOB message uses this secret |
| | in the Completion Exchange to authenticate the |
| | exchanged key to the party that sent the OOB |
| | message. |
| | |
| Hoob | Cryptographic fingerprint (i.e. hash value) |
| | computed from all the parameters exchanged in |
| | the Initial Exchange and in the OOB message. |
| | Receiving this fingerprint over the OOB |
| | channel guarantees the integrity of the key |
| | exchange and parameter negotiation. Hence, it |
| | authenticates the exchanged key to the party |
| | that receives the OOB message. |
| | |
| Ns2, Np2 | Nonces for the Reconnect Exchange. |
| | |
| MACs, MACp | Message authentication codes for mutual |
| | authentication, key confirmation, and |
| | integrity check on the exchanged information. |
| | The input to the HMAC is defined below, and |
| | the key for the HMAC is defined in Section |
| | 3.5. |
| | |
| PKs2, PKp2 | The public components of the ECDH keys of the |
| | server and peer. These MUST be present if a |
| | new cryptosuite was negotiated. Otherwise, |
| | either party may omit the field. PKs2 and PKp2 |
| | are sent in the JSON Web Key (JWK) format |
| | [RFC7517]. |
| | |
| MACs2, MACp2 | Message authentication codes for mutual |
| | authentication, key confirmation, and |
| | integrity check on the Reconnect Exchange. The |
| | input to the HMAC is defined below, and the |
| | key for the HMAC is defined in Section 3.5. |
| | |
+------------------+------------------------------------------------+
Table 1: Message data items
All nonces (Ns, Np, Ns2, Np2, Noob) are 16-byte fresh random byte
strings generated by the party that sends the message.
The fingerprint Hoob is computed with the hash function specified in
the negotiated cryptosuite and truncated to the 16 leftmost bytes of
the output. The message authentication codes (MACs, MACp, MACs2,
MACp2) are computed with the HMAC function [RFC2104] based on the
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same cryptographic hash function and truncated to the 16 leftmost
bytes of the output.
The inputs to the hash function for computing the fingerprint Hoob
and to the HMAC for computing MACs, MACp, MACs2 and MACp2 are JSON
arrays containing a fixed number (15) of members. The array member
values MUST be copied to the array verbatim from the in-band
messages, and space characters or whitespace MUST NOT be added
anywhere in the JSON structure.
The inputs for computing the fingerprint and message authentication
codes are the following:
Hoob = H(Dir,Vers,Verp,PeerId,Cryptosuites,Dirs,ServerInfo,Cryptos
uitepp,Dirp,PeerInfo,PKs,Ns,PKp,Np,Noob).
MACs = HMAC(Kms; 2,Vers,Verp,PeerId,Cryptosuites,Dirs,ServerInfo,C
ryptosuitep,Dirp,PeerInfo,PKs,Ns,PKp,Np,Noob).
MACp = HMAC(Kmp; 1,Vers,Verp,PeerId,Cryptosuites,Dirs,ServerInfo,C
ryptosuitep,Dirp,PeerInfo,PKs,Ns,PKp,Np,Noob).
MACs2 = HMAC(Kms2; 2,Vers,Verp,PeerId,Cryptosuites,"",[ServerInfo]
,Cryptosuitep,"",[PeerInfo],[PKs2],Ns2,[PKp2],Np2,"")
MACp2 = HMAC(Kmp2; 1,Vers,Verp,PeerId,Cryptosuites,"",[ServerInfo]
,Cryptosuitep,"",[PeerInfo],[PKs2],Ns2,[PKp2],Np2,"")
Missing input values are represented by empty strings "" in the
array. The values indicated with "" are always empty strings. The
values in brackets MUST be included if they were exchanged in the
same Reconnect Exchange; otherwise these values are replaced by empty
strings "".
The parameter Dir indicates the direction in which the OOB message
containing the Noob value is being sent (1=peer-to-server, 2=server-
to-peer). This field in needed to prevent the user from accidentally
delivering the OOB message back to its originator in the rare cases
where both OOB directions have been negotiated. The keys for the
HMACs are defined in the following section.
The nonces (Ns, Np, Ns2, Np2) and message authentication codes (MACs,
MACp, MACs2, MACp2) in the in-band messages and in the cryptographic
function inputs MUST be base64url encoded [RFC4648]. The values Noob
and Hoob in the OOB channel MAY also be base64url encoded, if that is
appropriate for the application and the used OOB channel.
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The ServerInfo and PeerInfo are JSON object with UFT-8 encoding. The
length of each encoded object as a byte arrays MUST NOT exceed 500
bytes. The format and semantics of these objects MUST be defined by
the application that uses the EAP-NOOB method.
3.4. Fast reconnect and rekeying
EAP-NOOB implements Fast Reconnect ([RFC3748], section 7.2.1) that
avoids repeated use of the user-assisted OOB channel. For this
reason, the EAP server and peer store the session state in persistent
memory after a successful Completion Exchange. This persistent data,
called "persistent EAP-NOOB association", MUST include at least the
following data: PeerId, negotiated cryptosuite, Kms, Kmp, and Kz.
The last three are shared keys used internally by EAP-NOOB for
rekeying in the Reconnect Exchange. When a persistent EAP-NOOB
association exists, the EAP server and peer are in the Registered
state (4) or Reconnecting state (3), as shown in Figure 1.
The rekeying and Reconnect Exchange may be needed for several
reasons. A timeout, software or hardware failure, or user action may
cause the EAP server, peer or authenticator to lose its non-
persistent state data such as master keys. Change in the supported
cryptosuites in the EAP server or peer may also cause the need for a
new key exchange. When the EAP server or peer detects such an event,
it MUST change from the Registered to Reconnecting state. The EAP-
NOOB method will then perform the Reconnect Exchange next time when
EAP is triggered. Thus, the difference between the Registered state
and Reconnecting state is that, in the Reconnecting state, some of
the non-persistent data related to the EAP-NOOB association between
the EAP server and peer may be lost or stale, and a new key exchange
is needed.
3.4.1. Reconnect Exchange
The server chooses the Reconnect Exchange when peer is in the
Reconnecting (3) state and the server itself is in the Registered (4)
or Reconnecting (3) state. The peer MUST NOT initiate EAP-NOOB when
the peer is in Registered state.
The Reconnect Exchange comprises two EAP-NOOB request-response pairs,
one for algorithm and parameter negotiation and the other for the key
exchange. In the first request and response (Type=5) the server and
peer negotiate a cryptosuite in the same way as in the Initial
Exchange. The messages MAY also contain PeerInfo and ServerInfo
objects. In the second request and response (Type=6), the server and
peer exchange the public components of ECDH keys and nonces
(PKs2,Ns2,PKp2,Np2). The server ECDH key MUST be fresh, i.e. not
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previously used with the same peer, and the client ECDH key SHOULD be
fresh, i.e. not previously used.
However, if the negotiated cryptosuite is the same as previously, the
server MAY refuse to perform a new ECDH exchange by omitting PKs2,
and the peer MAY refuse by omitting PKp2. If the server omits PKs2,
it is RECOMMENDED that the peer also omits PKp2, as it will not be
used in any case. When one or both public keys are not present, the
new master keys are derived from the fresh nonces and the previously
established shared key Kz, as defined in Section 3.5. The security
property lost by refusing the ECDH exchange is forward secrecy.
The server and client MAY send updated ServerInfo and PeerInfo
objects in the Reconnect Exchange. If there is no update to the
values, they SHOULD omit this information from the messages.
Both sides MUST compare the received message authentication code with
a locally computed value. If the EAP server finds that it has
received the correct value of MACp2, the Reconnect Exchange ends in
EAP-Success; otherwise, in EAP-Failure.
After successful Reconnect Exchange, both the server and the peer
move to the Registered (4) state. If a new ECHD key exchange was
performed, they also update the persistent EAP-NOOB association with
the changed values.
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EAP Peer EAP Server
| |
|<----------- EAP-Request/Identity -| |
| |
| |
|------------ EAP-Response/Identity -------------->|
| (NAI=PeerId+s3@eap-noob.net) |
| |
| |
|<----------- EAP-Request/EAP-NOOB ----------------|
| (Type=5,PeerId,Cryptosuites,[ServerInfo]) |
| |
| |
|------------ EAP-Response/EAP-NOOB -------------->|
| (Type=5,PeerId,Cryptosuitep,[PeerInfo]) |
| |
|<----------- EAP-Request/EAP-NOOB ----------------|
| (Type=6,PeerId,[PKs2,]Ns2,MACs2) |
| |
| |
|------------ EAP-Response/EAP-NOOB -------------->|
| (Type=6,PeerId,[PKp2,]Np2,MACp2) |
| |
| |
|<----------- EAP-Success -------------------------|
| |
Figure 7: Reconnect Exchange
3.4.2. User reset
As shown in the association state machine in Figure 1, the only
specified way for the association to return from the Registered state
(4) to the Unregistered state (0) is through user-initiated reset.
After the reset, a new OOB message will be needed to establish a new
association between the EAP server and peer. Typical situations in
which the user reset is required are when the other side has
accidentally lost the persistent EAP-NOOB association data, or when
the peer device is decommissioned.
The server could detect that the peer is in the Registered or
Reconnecting state but the server itself is in one of the ephemeral
states 0..2 (including situations where the server does not recognize
the PeerId). In this case, effort should be made to recover the
persistent server state, for example, from a backup storage -
especially if many peer devices are similarly affected. If that is
not possible, the EAP server SHOULD log the error or notify an
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administrator. The only way to continue from such a situation is by
having the user reset the peer device.
On the other hand, if the peer is in any of the ephemeral states
0..2, including the Unregistered state, the server will treat the
peer as a new peer device and allocate a new PeerId to it. The
PeerInfo can be used by the administrator as a clue to which physical
device has lost its state. However, there is no secure way of
matching the "new" peer with the old PeerId without repeating the OOB
step. This situation will be resolved when the user performs the OOB
step and, thus, identifies the physical peer device. The server user
interface SHOULD support situations where the "new" peer is actually
a previously registered peer that has been reset by a user or has
otherwise lost the persistent EAP-NOOB association data and needs to
be merged with the old peer data in the server.
3.5. Key derivation
The EAP output values MSK and EMSK are derived with the Elliptic
Curve Diffie-Hellman (ECDH) algorithm. In the terminology of the
NIST specification [NIST-DH], we use a C(2, 0, ECC CDH) scheme, i.e.
two ephemeral keys and no static keys. The server and peer compute
the ECDH shared secret Z as defined in section 6.1.2.2 and the secret
keying material as defined in section 5.8.1 of the NIST
specification. The hash function H for the Concatenation Key
Derivation Function is taken from the negotiated cryptosuite.
The Concatenation Key Derivation Function in the NIST specification
requires some additional input: AlgorithmID, PartyUInfo, PartyVInfo,
SuppPubInfo, and SuppPrivInfo. In EAP_NOOB, the AlgorithmID is the
fixed-length 8-byte ASCII string "EAP-NOOB". When keys are derived
in the Completion Exchange, PartyUInfo is the nonce Np as a 16-byte
byte string, and PartyVInfo is the nonce Ns as a 16-byte byte string.
SuppPubInfo is not allowed in EAP-NOOB; that is, it is not included
in the input of the key derivation function. In the Completion
Exchange, SuppPrivInfo is the nonce Noob as a 16-byte byte string.
When keys are derived in the Reconnect Exchange, the key derivation
process is the same except for the following differences: PartyUInfo
is the nonce Np2 as a 16-byte byte string, and PartyVInfo is the
nonce Ns2 as a 16-byte byte string, and neither SuppPubInfo nor
SuppPrivInfo is allowed.
After a successful Completion Exchange, the outputs of the EAP method
are the following: MSK and EMSK are the bytes 0..63 and 64..127,
respectively, of the output of the Concatenation Key Derivation
Function. The 16-byte keys Kms and Kmp and the 32-byte key Kz used
internally by EAP-NOOB for computing HMAC values are the bytes
128..143, 144..159, and 160..191, respectively, of the output of the
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Concatenation Key Derivation Function. EAP server and peer store the
values Kms, Kmp and Kz in the persistent EAP-NOOB association.
After a successful Reconnect Exchange, there are two methods for
deriving the new master keys. The first method is used when ECDH
public keys were exchanged in the Reconnect Exchange. In this
method, the outputs of the EAP method are the following: MSK and EMSK
are the bytes 0..63 and 64..127, respectively, of the output of the
Concatenation Key Derivation Function. The 32-byte key Kms2 is
created by concatenating the stored 16-byte Kms value with the bytes
128..143 of the output of the Concatenation Key Derivation Function.
The 32-byte key Kmp2 is similarly created by concatenating the stored
16-byte Kmp value with the bytes 144..159 of the output of the
Concatenation Key Derivation Function. A new 32-byte key Kz is
obtained by taking bytes 160..191 of the output of the Concatenation
Key Derivation Function. EAP server and peer update the value of Kz
in the persistent EAP-NOOB association.
The second method is used when no ECDH public keys were exchanged in
the Reconnect Exchange (or if only one party sent its public key).
In this method, input Z to the Concatenation Key Derivation Function
is replaced with the 32-byte key Kz from the persistent EAP-NOOB
association. This method achieves rekeying without the computational
cost of the ECDH exchange, but does not provide forward secrecy. In
this second method, no updates are made to the persistent EAP-NOOB
association.
3.6. Error handling
Various error conditions in EAP-NOOB are handled by sending an error
notification message (type=0) instead of the expected next EAP
request or response message. Both the EAP server and the peer may
send the error notification, as shown in Figure 8 and Figure 9.
After sending or receiving an error notification, the server MUST
send an EAP-Failure message. The notification MAY contain an
ErrorInfo field, which is a UTF-8 encoded text string with a maximum
length of 500 bytes. It is used for sending descriptive information
about the error, which may be useful for logging and debugging.
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EAP Peer EAP Server
... ...
| |
|<----------- EAP-Request/EAP-NOOB ----------------|
| (Type=0,[PeerId],ErrorCode,[ErrorInfo]) |
| |
| |
|<----------- EAP-Failure -------------------------|
| |
Figure 8: Error notification from server to peer
EAP Peer EAP Server
... ...
| |
|------------ EAP-Response/EAP-NOOB -------------->|
| (Type=0,[PeerId],ErrorCode,[ErrorInfo]) |
| |
| |
|<----------- EAP-Failure -------------------------|
| |
Figure 9: Error notification from peer to server
3.6.1. Invalid messages
If the NAI structure is invalid, the server SHOULD send the error
code 1001 to the peer. The recipient of an EAP-NOOB request or
response SHOULD send the following error codes back to the sender:
1002 if it cannot parse the message as a JSON object or there are
missing or unrecognized members in the JSON object; 1003 if a data
field has an invalid value, such as an integer out of range; 1004 if
the received message type was unexpected; 1005 if the PeerId has an
unexpected value; and 1006 if the ECDH key is invalid.
3.6.2. Unwanted peer
The preferred way for the EAP server to rate limit EAP-NOOB
connections from a peer is to use the MinSleep parameter in the
Waiting Exchange. However, if the EAP server receives repeated EAP-
NOOB connections from a peer which apparently should not connect to
this server, the server MAY indicate that the connections are
unwanted by sending the error code 2001. The peer MAY refrain from
reconnecting to the same EAP server and, if possible, both the EAP
server and peer SHOULD indicate this error condition to the user.
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However, in order to avoid persistent denial-of-service, the peer is
not required to stop entirely from reconnecting to the server.
3.6.3. State mismatch
In the states indicated by "UA" in Figure 10 in Appendix A, user
action is required to reset the association state or to recover it,
for example, from backup storage. In those case, the server sends
the error code 2002 to the peer. If possible, both the EAP server
and peer SHOULD indicate this error condition to the user.
3.6.4. Negotiation failure
If there is no matching protocol version, the peer sends the error
code 3001 to the server. If there is no matching cryptosuite, the
peer sends the error code 3002 to the server. If there is no
matching OOB direction, the peer sends the error code 3003 to the
server. In practice, there is no way of recovering from these errors
without software or hardware changes. If possible, both the EAP
server and peer SHOULD indicate these error conditions to the user.
3.6.5. Cryptographic verification failure
If the EAP server or peer detect an unrecognized PeerId or incorrect
fingerprint (Hoob) in the OOB message, the recipient SHOULD indicate
the failure to accept the OOB message to the user. The recipient
MUST remain in the Waiting for OOB state (1) as if no OOB message was
received.
Note that if the OOB message was delivered from the server to the
peer and the peer does not recognize the PeerId, the likely cause is
that the user has unintentionally delivered the OOB message to the
wrong destination. If possible, the peer SHOULD indicate this to the
user; however, the peer device may not have capability for many
different error indications and it MAY use the same method or error
indication as in the case of an incorrect fingerprint.
The rationale for the above is that the invalid OOB message could
have been presented to the recipient by mistake or intentionally by a
malicious party and, thus, it should be ignored in the hope that the
honest user will soon deliver a correct OOB message.
If the EAP server or peer detects an incorrect message authentication
code (MACs, MACp, MACs2, MACp2), it sends the error code 4001 to the
other side. If this error occurred in the Completion Exchange, both
sides must remain in the old state as if the failed Completion
Exchange did not take place. On the other hand, if the error
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occurred in the Reconnect Exchange, both sides MUST go to the
Reconnecting state (3).
The rationale for the above is that the invalid cryptographic
messages may have been spoofed by a malicious party and, thus, it
should be ignored. In particular, a spoofed message on the network
should not force the honest user to perform the OOB step again. In
practice, however, the error may be caused by other failures, such as
software errors. For this reason, the EAP server MAY limit the rate
of peer connections after the above error. Also, there MUST be a way
for the user to reset the EAP server and peer to the Unregistered
state (0), so that the OOB step can be repeated.
4. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the EAP-
NOOB protocol, in accordance with [RFC2434].
The EAP Method Type number for EAP-NOOB needs to be assigned.
This memo also requires IANA to create new registries as defined in
the following subsections.
4.1. Cryptosuites
An EAP server MUST supply one or more suggestions for cryptosuites as
the Cryptosuites value in the Initial Exchange. They are formatted
as a JSON array of the identifier integers. Each suite MUST appear
only once in the array. The cryptosuites MUST be supplied in order
of priority. Peers MUST supply exactly one suite in the Cryptosuitep
value, formatted as an identifier integer. The following suites are
defined by EAP-NOOB:
+-------------+-----------------------------------------+
| Cryptosuite | Algorithms |
+-------------+-----------------------------------------+
| 1 | Curve25519 [RFC7748], SHA-256 [RFC6234] |
+-------------+-----------------------------------------+
Table 2: EAP-NOOB cryptosuites
Assignment of new values for new cryptosuites MUST be done through
IANA with "Specification Required" and "IESG Approval" as defined in
[RFC2434].
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4.2. Error codes
The error codes defined by EAP-NOOB are listed in Table 3.
+------------+----------------------------------------+
| Error code | Purpose |
+------------+----------------------------------------+
| 1001 | Invalid NAI or peer state |
| 1002 | Invalid message structure |
| 1003 | Invalid data |
| 1004 | Unexpected message type |
| 1005 | Unexpected peer identifier |
| 1006 | Invalid ECDH key |
| 2001 | Unwanted peer |
| 2002 | State mismatch, user action required |
| 3001 | No mutually supported protocol version |
| 3002 | No mutually supported cryptosuite |
| 3003 | No mutually supported OOB direction |
| 4001 | MAC verification failure |
+------------+----------------------------------------+
Table 3: EAP-NOOB error codes
Assignment of new error codes MUST be done through IANA with
"Specification Required" and "IESG Approval" as defined in [RFC2434].
4.3. Domain name reservation considerations
"eap-noob.net" should be registered as a special-use domain. The
considerations required by [RFC6761] for registering this special use
domain name are as follows:
o Users: Non-admin users are not expected to encounter this name or
recognize it as special. AAA administrators may need to recognize
the name.
o Application Software: Application software is not expected to
recognize this domain name as special.
o Name Resolution APIs and Libraries: Name resolution APIs and
libraries are not expected to recognize this domain name as
special.
o Caching DNS Servers: Caching servers are not expected to recognize
this domain name as special.
o Authoritative DNS Servers: Authoritative DNS servers MUST respond
to queries for eap-noob.net with NXDOMAIN.
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o DNS Server Operators: Except for the authoritative DNS server,
there are no special requirements for the operators.
o DNS Registries/Registrars: There are no special requirements for
DNS registrars.
5. Security considerations
EAP-NOOB is an authentication and key derivation protocol and, thus,
security considerations can be found in most sections of this
specification. In the following, we explain the protocol design and
highlight some other special considerations.
5.1. Authentication principle
The mutual authentication in EAP-NOOB is based on two separate
features, both conveyed in the OOB message. The first authentication
feature is the secret nonce Noob. The peer and server use this
secret in the Completion Exchange to mutually authenticate the
session key previously created with ECDH. The message authentication
codes computed with the secret nonce Noob are alone sufficient for
authenticating the key exchange. The OOB channel might, however, be
vulnerable to eavesdropping of the OOB channel, which could lead to
compromise of the secret nonce, which will then enable a man-in-the-
middle attack on the in-band channel. This is why we include, as a
second authentication feature, the integrity-protecting fingerprint
Hoob in the OOB message. It is typically more difficult to spoof or
alter messages on the human-assisted OOB channel, such as bar code,
sound burst or user-transferred URL, than it is to spy on them.
The security provided by the cryptographic fingerprint is somewhat
intricate to understand. The party that receives the OOB message
uses Hoob to verify the integrity of the ECDH exchange. Thus, that
party can detect man-in-the-middle attacks on the in-band channel.
The other party, however, is not equally protected because the OOB
message and fingerprint are sent only in one direction. Some
protection to the OOB sender is afforded by the fact that the user
may notice the failure of the association at the OOB receiver and
therefore reset the OOB sender. Indeed, other device-pairing
protocols have solved a similar situation by requiring the user to
confirm to the OOB sender that the association was accepted by the
OOB-receiver, e.g. by pressing an "accept" button on the sender.
Since EAP-NOOB was designed to work strictly with one-directional OOB
communication, it does not rely on such input to the OOB sender.
To summarize, EAP-NOOB uses the combined protection of the secret
nonce Noob and the cryptographic fingerprint Hoob, both conveyed in
the OOB message. The secret nonce Noob alone is sufficient for
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mutual authentication, unless the attacker can eavesdrop it from the
OOB channel. If an attacker is able to eavesdrop the secret nonce
and performs a man-in-the-middle attack on in-band channel, the
mismatching fingerprint will alert the OOB receiver, which will
reject the OOB message. In this case, the association will appear to
be complete only on the OOB sender side. The user in many
applications will detect this apparently one-sided association
because the peer device does not appear registered on the server or
network.
The expected use cases for EAP-NOOB are ones where it replaces a
user-entered access credentials. In wireless network access for IoT
devices, the user-entered credential is often a passphrase, which is
shared by all the network stations. Like any other EAP-based
solution, EAP-NOOB establishes a different master secret for each
peer device, which is obviously more resilient to device compromise
than a common master secret. Additionally, it is possible to revoke
the security association for an individual device on the server side.
Forward secrecy in EAP-NOOB is optional. The Reconnect exchange in
EAP-NOOB provides forward secrecy only if both the server and peer
send their fresh ECDH keys. This allows both the server and the peer
to limit the frequency of the costly computation that is required for
forward secrecy. The server should make its decision primarily based
on what it knows about the peer's computational capabilities.
5.2. Identifying and naming peer devices
EAP-NOOB relies on physical possession or identification of the peer
device and secure communication between the user and the server. The
main remaining threat against EAP-NOOB is that the attacker performs
a man-in-the-middle attack on the in-band channel and, during the
protocol execution, tricks the user to deliver the OOB message to or
from the wrong peer. The server will now be associated with that
wrong peer. Similarly, the attacker could try to trick the user to
accessing the wrong server in the OOB step. This reliance on user in
identifying the correct parties is an inherent property of out-of-
band authentication.
One mechanism that can be used to mitigate user mistakes is
certification of trusted servers and peer devices. For example, if
used together with EAP-NOOB, vendor certificates could prevent
accidental association with a rogue peer device. Compared to a fully
certificate-based authentication, EAP-NOOB does not depend on trusted
third parties and does not require the user to know the identifier of
the peer device; physical access is sufficient.
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The user could also accidentally deliver the OOB message to more than
one peer device. This could, for example, occur if the OOB message
is a bar code and the peer is a camera: the user could by mistake
show the bar code first to the wrong camera. Such accidents in EAP-
NOOB will not enable the wrong camera to compute the master key or to
opportunistically eavesdrop the communication. This is because the
wrong peer device would need to have performed a man-in-the middle
attack on the in-band channel before the accident. In comparison,
simpler solutions where the master key is transferred to the device
via the OOB channel would be vulnerable to opportunistic attacks if
the user mistakenly delivers the master key to more than one device.
After completion of EAP-NOOB, the server may store the PeerInfo data,
and the user may use it to identify the peer and its properties, such
as make and model or serial number. A compromised peer could lie
about this information in the PeerInfo that it sends to the server.
If the server stores any information about the peer, it is important
that this information is approved by the user during or after the OOB
step. Without rigorous user checking, the PeerInfo is not
authenticated information and should not be relied on. Therefore, it
is better to include only minimal information about the peer in
PeerInfo and to ask the user to name the peer devices. In many
applications, such as OOB authentication for ad-hoc wireless network
access, it may be unnecessary to store any names for the peer device.
Since the user delivering the OOB message will often communicate with
the server over an authenticated channel, e.g. logging into a secure
web page, the user identity and user-given name can in those cases be
reliably stored for the peer device. It is these user identities and
user-given names that should be later used for access control and
revocation.
Another reason to include only minimal information in the PeerInfo is
potential privacy issues. The PeerInfo field is typically
transmitted in plaintext between the peer and the authenticator.
Although the PeerInfo sent by a new, unregistered device will not
leak any information specifically about the user, it could reveal
device identifiers and information about other device properties,
which the user may want to avoid leaking at this point.
The PeerId value in the protocol is a server-allocated identifier for
its association with the peer and SHOULD NOT be shown to the user
because its value is initially ephemeral. Since the PeerId is
allocated by the server and the scope of the identifier is the single
server, the so-called identifier squatting attacks, where a malicious
peer could reserve another peer's identifier, are not possible in
EAP-NOOB. The server SHOULD assign a random or pseudo-random PeerId
to each new peer. It SHOULD NOT select the PeerId based on any peer
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characteristics that it may know, such as the peer's lower-layer
address.
5.3. Downgrading threats
The fingerprint Hoob protects all the information exchanged in the
Initial Exchange, including the cryptosuite negotiation. The message
authentication codes MACs and MACp also protect the same information.
The message authentication codes MACs2 and MACp2 protect information
exchanged during key renegotiation in the Reconnect Exchange. This
prevents downgrade attacks to weaker cryptosuites as long as the
possible attacks take more time than the maximum time allowed for the
EAP-NOOB completion. This is typically the case for recently
discovered cryptanalytic attacks.
As an additional precaution, the EAP server and peer SHOULD check for
downgrading attacks in the Reconnect Exchange. As long as the server
or peer saves any information about the other party, it SHOULD also
remember the previously negotiated cryptosuite and not accept
renegotiation of any cryptosuite that is known to be weaker than the
previous one (e.g. a deprecated cryptosuite or the same ECDH field
with a shorter key).
Integrity of the direction negotiation cannot be verified in the same
way as the integrity of the cryptosuite negotiation. That is, if the
OOB channel used in an application is critically insecure in one
direction, a man-in-the-middle attacker could modify the negotiation
messages and thereby cause that direction to be used. Applications
that support OOB messages in both directions SHOULD therefore ensure
that the OOB channel has sufficiently strong security in both
directions. While this is a theoretical vulnerability, it could
arise in practice if EAP-NOOB is deployed in unexpected applications.
However, most devices acting as the peer are likely to support only
one direction of exchange, in which case interfering with the
direction negotiation can only prevent the completion of the
protocol.
5.4. EAP security claims
EAP security claims are defined in section 7.2.1 of [RFC3748]. EAP-
NOOB makes the following security claims:
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+----------------+--------------------------------------------------+
| Security | EAP-NOOB claim |
| property | |
+----------------+--------------------------------------------------+
| | |
| Authentication | ECDH key exchange with out-of-band |
| mechanism | authentication |
| | |
| Protected | yes |
| cryptosuite | |
| negotiation | |
| | |
| Mutual | yes |
| authentication | |
| | |
| Integrity | yes |
| protection | |
| | |
| Replay | yes |
| protection | |
| | |
| Key derivation | yes |
| | |
| Key strength | The specified cryptosuites provide key strength |
| | of at least 128 bits. |
| | |
| Dictionary | not applicable |
| attack | |
| protection | |
| | |
| Fast reconnect | yes |
| | |
| Cryptographic | not applicable |
| binding | |
| | |
| Session | yes |
| independence | |
| | |
| Fragmentation | no (The largest EAP-NOOB packet is at most TBD |
| | bytes long.) |
| | |
| Channel | yes (The ServerInfo and PeerInfo can be used to |
| binding | convey integrity-protected channel properties |
| | such as peer MAC address.) |
+----------------+--------------------------------------------------+
Table 4: Security claims
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6. References
6.1. Normative references
[IEEE-802.1X]
Institute of Electrical and Electronics Engineers, "Local
and Metropolitan Area Networks: Port-Based Network Access
Control", IEEE Standard 802.1X-2004. , December 2004.
[NIST-DH] Barker, E., Johnson, D., and M. Smid, "Recommendation for
Pair-Wise Key Establishment Schemes Using Discrete
Logarithm Cryptography", NIST Special Publication 800-56A
Revision 1 , March 2007,
<http://dx.doi.org/10.1145/2632048.2632049>.
[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, DOI 10.17487/RFC1738,
December 1994, <http://www.rfc-editor.org/info/rfc1738>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>.
[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>.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 2434,
DOI 10.17487/RFC2434, October 1998,
<http://www.rfc-editor.org/info/rfc2434>.
[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,
<http://www.rfc-editor.org/info/rfc3748>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
March 2008, <http://www.rfc-editor.org/info/rfc5216>.
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[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<http://www.rfc-editor.org/info/rfc6234>.
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC6761, February 2013,
<http://www.rfc-editor.org/info/rfc6761>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7517] Jones, M., "JSON Web Key (JWK)", RFC 7517,
DOI 10.17487/RFC7517, May 2015,
<http://www.rfc-editor.org/info/rfc7517>.
[RFC7542] DeKok, A., "The Network Access Identifier", RFC 7542,
DOI 10.17487/RFC7542, May 2015,
<http://www.rfc-editor.org/info/rfc7542>.
[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>.
6.2. Informative references
[Sethi14] Sethi, M., Oat, E., Di Francesco, M., and T. Aura, "Secure
Bootstrapping of Cloud-Managed Ubiquitous Displays",
Proceedings of ACM International Joint Conference on
Pervasive and Ubiquitous Computing (UbiComp 2014), pp.
739-750, Seattle, USA , September 2014,
<http://dx.doi.org/10.1145/2632048.2632049>.
[SimplePairing]
Bluetooth, SIG, "Simple pairing whitepaper", Technical
report , 2007.
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Appendix A. Exchanges and events per state
Figure 10 shows how the EAP server chooses the exchange type
depending on the server and peer states. The table also indicates
other possible events that may lead to a state change. In the fields
marked with asterisk (*), the direction of the OOB message is further
limited by the negotiated OOB directions (Dirp). Therefore, these
OOB events are not always possible. Additionally, user may reset the
association state at the EAP server or peer at any time. The fields
with "UA" indicate state combinations where user action is required
to recover or reset the association state.
+--------+--------------------------+-------------------------------+
| pier state: Unregistered (0) |
+--------+--------------------------+-------------------------------+
| server | possible exchanges | next peer and |
| state | and events | server states |
+--------+--------------------------+-------------------------------+
| 0 | Initial Exchange | both 1 (0 on error) |
| | | |
| 1 | Initial Exchange | both 1 (0 on error) |
| | | |
| 2 | Initial Exchange | both 1 (0 on error) |
| | | |
| 3 | UA | affected side 0 or 3 |
| | | |
| 4 | UA | affected side 0 or 3 |
+--------+--------------------------+-------------------------------+
| pier state: Waiting for OOB (1) |
+--------+--------------------------+-------------------------------+
| 0 | Initial Exchange | both 1 (0 on error) |
| | | |
| 1 | Waiting Exchange | both 1 |
| | OOB from server to peer* | peer 2 (1 on failure) |
| | OOB from peer to server* | server 2 (1 on failure) |
| | | |
| 2 | Completion Exchange | both 4 (no change on failure) |
| | OOB from server to peer* | peer 2 (1 on failure) |
| | | |
| 3 | UA | affected side 0 or 3 |
| | | |
| 4 | UA | affected side 0 or 3 |
+--------+--------------------------+-------------------------------+
| pier state: OOB Received (2) |
+--------+--------------------------+-------------------------------+
| 0 | Initial Exchange | both 1 (0 on error) |
| | | |
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| 1 | Completion Exchange | both 4 (no change on failure) |
| | OOB from peer to server* | server 2 (1 on failure) |
| | | |
| 2 | Completion Exchange | both 4 (no change on failure) |
| | | |
| 3 | UA | affected side 0 or 3 |
| | | |
| 4 | UA | affected side 0 or 3 |
+--------+--------------------------+-------------------------------+
| pier state: Reconnecting (3) |
+--------+--------------------------+-------------------------------+
| 0 | UA | affected side 0 or 3 |
| | | |
| 1 | UA | affected side 0 or 3 |
| | | |
| 2 | UA | affected side 0 or 3 |
| | | |
| 3 | Reconnect Exchange | both 4 (3 on failure) |
| | | |
| 4 | Reconnect Exchange | both 4 (3 on failure) |
+--------+--------------------------+-------------------------------+
| pier state: Registered (4) |
+--------+--------------------------+-------------------------------+
| 0 | UA | affected side 0 or 3 |
| | | |
| 1 | UA | affected side 0 or 3 |
| | | |
| 2 | UA | affected side 0 or 3 |
| | | |
| 3 | Reconnect Exchange | both 4 (3 on failure) |
| | | |
| 4 | Timeout/Failure | one or both 3 |
| | UA | affected side 0 or 3 |
+--------+--------------------------+-------------------------------+
Figure 10: Exchanges and events possible in each state
Appendix B. TODO list
o Check maximum lengths of all messages to ensure no fragmentation.
o Update Kms and Kmp in the persistent EAP_NOOb association after
ECDH rekeying. This will add to security but is somewhat tricky.
o Clarify the relation of Unregistered state and no association
stored.
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o Consider less disruptive ways for handling protocol errors in
state 1, compared to the current solution of returning to state 0.
o Add examples of all exchanges and messages.
Authors' Addresses
Tuomas Aura
Aalto University
Aalto 00076
Finland
EMail: tuomas.aura@aalto.fi
Mohit Sethi
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: mohit@piuha.net
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